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NEUROSURGERY VOLUME 61 | NUMBER 4 | OCTOBER 2007 SUPPLEMENT 4 | S4-1

INTRODUCTION

ANATOMY AND SURGICAL APPROACHES OF THE

TEMPORAL BONE AND ADJACENT AREAS

Neurosurgery 61:S4-1, 2007 DOI: 10.1227/01.NEU.0000280027.92382.2B www.neurosurgery-online.com

The temporal bone is the most complicated osseous struc-ture in the human body. Five parts participate in its for-mation. The facial and carotid canals in the temporal bone

are the longest canals of passage of any cranial nerve or arterythrough the cranium. This complexity is further increased bythe genus, and bends in the facial nerve and carotid arterywithin the temporal bone. The presence of the delicate cochlearand vestibular membranes within the temporal bone makes itthe only bone that houses the end organs of a cranial nerve. Italso houses the complicated mechanism for transmittingsounds from the outward world to the inner ear. The fact thatthe temporal bone faces the middle and posterior cranial fossaand also has lateral and lower surfaces yields the potential formultiple complex surgical routes to the temporal bone, andthrough it to deeper areas. It is the focus of more surgical routesand approaches than any other bone in the cranium. The deli-cate neural, vascular, and transmission systems within the boneadd to the complexity of these surgical approaches and providea special challenge in dealing with lesions in the area. Thiswork, done with many of our research fellows, representsknowledge gained from nearly five decades of the study of microsurgical anatomy. We hope that the illustrations in threedimensions will aid all who deal with this complicated and

delicate anatomy.Our previous article (Chapter 1) on the temporal bone wasincluded as a starting point because it provides an up-to-datetwo-dimensional description of the anatomy and approachesto the temporal bone (1). Additional information on the sur-rounding area will be found in the volume Cranial Anatomy andOperative Approaches published by NEUROSURGERY (2). Thethree-dimensional (3D) part begins with a review of the osseousrelationships and proceeds through the anatomy and surgicalapproaches directed to and through the temporal bone fromthe middle and posterior fossa and laterally through the mas-toid. This is followed by sections related to the exposuresdirected along the margins of the temporal bone, which includethe far lateral and transcondylar approaches and the approaches

to the jugular foramen and fourth ventricle. Each of the lattersections is preceded by a short description of the approach.The pages with the 3D illustrations are to be viewed with the

colored glasses. On the lower right, below the large 3D illustra-tion, is a two-dimensional illustration with labels guiding theviewer to the important structures in the area. Each illustrationis followed by a short legend. The 3D illustrations are to beviewed with the blue lens of the colored glasses placed in front

of the right eye and the red lens in front of the left eye. Some of the cardboard glasses can be bent so the red and blue arereversed with a resulting loss of 3D viewing.

This volume is dedicated to the fellows who have labored inour microsurgery laboratory beginning more than 40 years ago.Each fellow has been challenged to improve and build uponthe efforts of the previous fellow. Several dozen fellows, begin-ning with Shigeaki Kobayashi in 1966, have made contribu-tions to our knowledge of the temporal bone and surroundingareas. Special thanks go to Robin Barry, who has worked withus for more than two decades and who aligned all the picturesfor 3D images, and to Laura Dickinson, who has labored overthis manuscript.

Capturing 3D images of the quality presented here requirescareful preparation of the specimen and meticulous dissection,followed by the even greater challenge of obtaining satisfactory3D images. Obtaining excellent specimens, completing meticu-lous specimen preparation, and combining that with surgicallyprecise dissections, and outstanding two- and 3D photographyis a rare achievement. We hope this work will enhance thereader’s understanding of this complex area and that it willresult in accurate, gentle, and safe operative procedures forpatients requiring surgery in this delicate area.

This work is dedicated to the microsurgery fellows at theUniversity of Florida who, for more than 40 years, have taughtme so much about microsurgical anatomy and the temporal bone.

Hiroshi Abe, JapanHajime Arai, JapanAllen S. Boyd, Jr., TennesseeRobert Buza, OregonAlvaro Campero, ArgentinaAlberto C. Cardoso, BrazilChristopher C. Carver, CaliforniaPatrick Chaynes, FranceChanyoung Choi, KoreaEvandro de Oliveira, BrazilHatem El Khouly, Egypt

W. Frank Emmons, Washington J. Paul Ferguson, Georgia Juan C. Fernandez-Miranda, SpainAndrew D. Fine, FloridaBrandon Fradd, FloridaKiyotaka Fujii, JapanYutaka Fukushima, JapanAdriano Garcia-Scaff, Brazil



RHOTON

Hirohiko Gibo, Japan John L. Grant, VirginiaKristinn Gudmundsson, IcelandDavid G. Hardy, EnglandFrank S. Harris, TexasTsutomu Hitotsumatsu, JapanTakuya Inoue, Japan

Tooru Inoue, JapanYukinari Kakizawa, JapanToshiro Katsuta, JapanMasatou Kawashima, JapanChang Jin Kim, South KoreaRobert S. Knego, FloridaShigeaki Kobayashi, JapanChae Heuck Lee, South KoreaXiao-Yong Li, ChinaWilliam Lineaweaver, California

J. Richard Lister, FloridaQing Liang Liu, China

Jack E. Maniscalco, FloridaRichard G. Martin, Alabama

Carolina Martins, BrazilHaruo Matsuno, JapanToshio Matsushima, Japan

J. Robert Mozingo, deceasedHiroshi Muratani, JapanAntonio C.M. Mussi, BrazilShinji Nagata, JapanYoshihiro Natori, JapanKazunari Oka, JapanMichio Ono, Japan

Shigeyuki Osawa, JapanT. Glenn Pait, ArkansasWayne S. Paullus, TexasDavid Perlmutter, FloridaMark Renfro, TexasWade H. Renn, GeorgiaSaran S. Rosner, New York

Pablo Rubino, ArgentinaNaokatsu Saeki, JapanShuji Sakata, JapanEduardo R. Seoane, ArgentinaXiang-en Shi, ChinaSatoru Shimizu, JapanRyusui Tanaka, JapanNecmettin Tanriover, TurkeyHelder Tedeschi, BrazilErdener Timurkaynak, TurkeyXiaoguang Tong, ChinaSatoshi Tsutsumi, Japan

Jay Ulm, FloridaHung T. Wen, BrazilC.J. Whang, South KoreaIsao Yamamoto, JapanAlexandre Yasuda, BrazilNobutaka Yoshioka, JapanArnold A. Zeal, Florida

1. Rhoton AL Jr: The temporal bone and transtemporal approaches.Neurosurgery 47 [Suppl 3]:S211–S265, 2000.

2. Rhoton AL Jr: Cranial Anatomy and Surgical Approaches. Baltimore,

Lippincott Williams & Wilkins, 2003.

From Pernkopf E, Ferner H: Atlas of Topographical and Applied Human Anatomy. Philadelphia, W.B. Saunders Company, 1963.



Only a broad collection of superlatives can begin to describemy impression of this “meisterwerk” from Professor

Rhoton. As is the case with the majority of Rhoton’s works, thisexhibition of the temporal bone and its relationships to the sur-rounding neuroanatomy must be viewed as required study bythose aspiring to master the region’s complex construction.

For many of our colleagues, the temporal bone has essen-tially been the purview of our otological colleagues. Owing toits intricate construction, a higher level of dedication to its sur-gery and pathology has been mandated. Despite the proficiencyof our otological partners in lateral cranial base surgery, it isequally important for the neurosurgeon to share this anatomicalexpertise. Such mastery on the part of the neurosurgeon isessential in helping our colleagues help us by providing optimalexposure from our perspective as the surgeon largely responsi-

ble for dissection in and around critical brain and cranial nervestructures, as well as the cerebral vasculature. Presentation of this material in three-dimensional (3D) format is an importantadjunct to our educational corpus as it brings everyone theopportunity to see the material in a fashion available to onlythose otherwise granted personal access to these phenomenalanatomical preparations. I can personally attest to the difficultyof reproducing such exquisite preparations. No other laboratoryhas produced relevant dissections in such painstaking, exquisitedetail. This work represents a true legacy in the continuum of neurosurgical education. Neurosurgeons young and matureowe Dr. Rhoton a debt of gratitude for his contributions to ourmost relevant science as surgeons, surgical neuroanatomy.

J. Diaz DayBurlington, Massachusetts

This supplement continues Professor Rhoton’s wonderfulinstruction of the anatomy of the head and neck. This work is

devoted to the temporal bone and adjacent areas. In addition to

anatomical dissections, the supplement contains details of surgi-cal approaches. Production in 3D adds tremendously to its value.The 3D portion of the supplement is divided into 12 sections.

The osseous relationships are described first. Following this,attention is directed to the middle cranial fossa anatomy, dis-cussing relationships of the temporal bone as viewed from above.The next chapter details the surgical anatomy of the middle cra-nial fossa approach. Chapter 4 is strictly an anatomic dissectionof the temporal bone from an anterior view. This greatly helpsone to appreciate the relationships of the various structures froman approach not normally seen. Next, the temporal bone is dis-sected laterally; again demonstrating the intricate anatomy of this structure. Chapter 6 illustrates the relationships of the soft tis-sue of the neck and face to the temporal bone. These structures

are frequently encountered in lateral cranial base surgery.The following six chapters discuss surgical approaches.Chapter 7 details the retrolabyrinthine and transcochlearapproaches. Stepwise dissections detail these approaches. Thepresigmoid approach is illustrated in Chapter 8 and the ret-rosigmoid approach in Chapter 9. The detailed anatomic rela-tionships of structures in the cerebellopontine angle and poste-rior fossa are beautifully demonstrated. Chapter 10 details the


COMMENTS

telovelar approach to the fourth ventricle. As in all of ProfessorRhoton’s work, the illustrations beautifully demonstrate thisanatomy. The far lateral and transcondylar approaches are illus-trated in Chapter 11. This anatomy is less frequently encoun-tered by the cranial base surgeon, and readers will find it partic-ularly helpful. The same may be said for Chapter 12, whichillustrates the complex anatomy of the jugular foramen. Therelationships of the lower cranial nerves are particularly welldemonstrated.

Production of Professor Rhoton’s marvelous dissections in 3Dliterally and figuratively adds a whole new dimension to thevalue of this wonderful work. 3D illustrations add significantly tounderstanding the complex anatomy of this region. The surgicalapproaches in particular are much better understood in the 3Dformat. We should all be grateful to Professor Rhoton for his ded-ication in producing these outstanding teaching tools. Everyoneshould utilize these materials not only in learning theseapproaches but for periodic review of this complex anatomy.

Derald E. BrackmannNeuro-otologist

Los Angeles, California

Dr. Rhoton´s description of the microanatomy and operativeapproaches to the temporal bone and adjacent areas is a very

worthwhile contribution to neurosurgery, particularly for theeducation of those interested in cranial base approaches. Thepresentation of these complex anatomical structures in such a 3Dmode helps to provide an overview of all relevant structureswithin the chosen surgical corridor.

This work is unique in the history of neurosurgery. It greatlyfacilitates the detailed understanding of the microanatomy of the temporal bone and its surrounding structures. A variety of very important cranial nerves and vessels runs through this rel-atively small area. Both the normal anatomy and its variations

are presented in detail. The reader has to consider these anatom-ical variations that can create complications during surgery (e.g.,as discussed in the supplement, 15% of the greater petrosalnerves are located directly under the dura in the middle cranialfossa). Thus, a facial palsy may occur in the case of elevation ortransection of the dura.

For younger and less experienced neurosurgeons, thisextraordinary anatomical demonstration is highly valuable forlearning the functional importance of these structures and thespatial relationships between them. Intraoperative neurovascu-lar injury can lead to significant surgical morbidity. The opera-tive approaches should be selected so that only minimal expo-sure of important structures is required. In my 40 years of experience with cranial base surgery during which I operated

on more than 8000 cases, I gradually developed my neurosurgi-cal strategy. In the beginning, we thought we needed to exposeall cranial nerves and vessels around the lesion in order toachieve complete removal of large tumors and to preserve theanatomical structures. The increased experience and knowledgeof microsurgical anatomy have highlighted two importantaspects. First, the pathological lesion displaces the structures indifferent directions. Therefore, the normal anatomy does not



always completely correspond to the pathological anatomy. Theknowledge of the spatial orientation and relations of space-occupying lesions helps the precise planning of surgeries. Thesecond aspect is that tumor removal does not necessitate a largeapproach and exposure of all surrounding structures. The artand quality of neurosurgery relate to the ability to select thesimplest trajectory to the lesion that does not involve or compro-mise structures with functional importance. Another veryimportant and still underestimated point is the avoidance of venous occlusion, which could cause excessive brain edema orintracerebral hematomas in certain cases. Furthermore, the vari-ability in venous anatomy among different individuals is aston-ishing. The beautiful and precise description of the variousvenous drainage patterns performed by Dr. Rhoton will defi-nitely help to focus attention on this topic.

The philosophy of simple non-risky approaches to the pathol-ogy in the temporal bone, middle and/or posterior fossae can

only be accomplished with a profound knowledge of themicroanatomy, as well as of all possible approaches to this area.Dr. Rhoton´s study of the temporal bone anatomy contains 13chapters, beginning with the description of the osseous relation-ships, and includes chapters on the anatomical view of the cranial

base. It is valuable for its presentation of the structures viewedthrough the most frequently utilized surgical approaches, includ-ing the retrosigmoid, the telovelar, the far lateral, and the differ-ent petrosal approaches. The excellent 3D views of the structuresallow the reader the possibility to appreciate the depth and spa-tial relationships, making this a great educational contribution.Finally, I would like to personally thank Dr. Rhoton for his com-mitment and effort and to congratulate him for this outstandingachievement in modern neurosurgery.

Madjid Samii Hannover, Germany

COMMENTS

S4-4 | VOLUME 61 | NUMBER 4 | OCTOBER 2007 SUPPLEMENT 4 www.neurosurgery-online.com

From Pernkopf E, Ferner H: Atlas of Topographical and Applied Human Anatomy. Philadelphia, W.B. Saunders Company, 1963.



PART 1

OVERVIEW OF THE

TEMPORAL BONE IN

TWO DIMENSIONS




CHAPTER 1

Albert L. Rhoton, Jr., M.D.

Department of Neurosurgery,University of Florida,

Gainesville, Florida

Reprint requests:Albert L. Rhoton, Jr., M.D.,

University of Florida,Department of Neurological Surgery,McKnight Brain Institute,

P.O. Box 100265,Gainesville, FL 32610–0265.Email: [email protected]

Osseous Relationships

The temporal bone is divided into squa-mosal, petrous, mastoid, tympanic, andstyloid parts (Figs. 1-1 and 1-2 ). The

squamosal part helps enclose the brain. Themastoid part is trabeculated and pneumatizedto a variable degree and contains the mastoidantrum. The petrous part is compact andencloses the cochlea, the vestibule, and thesemicircular, facial, and carotid canals (Fig.1-3). The tympanic part forms part of the wallof the tympanic cavity and the externalacoustic meatus. The styloid projects down-ward and serves as the site of attachment of several muscles. This section examines theseparts in greater detail and defines the ana-tomic basis of the approaches directedthrough the temporal bone to the posteriorfossa and petroclival region. The approachesexamined are the middle fossa, translab-yrinthine, transcochlear, combined supra- andinfratentorial presigmoid, subtemporal ante-

rior transpetrosal, subtemporal preauricularinfratemporal, and the postauricular trans-temporal approaches.

The approaches directed through the sur-face of the temporal bone forming the middlefossa floor include 1) the very limited middlefossa exposure of the internal acoustic mea-tus; 2) the anterior petrosectomy approachdirected medial to the internal acoustic mea-tus through the petrous apex to access theupper anterior part of the posterior fossa andclivus; 3) the extended middle fossa ap-proach, which may include not only resectionof the roof of the internal acoustic meatus and

petrous apex, but is extended lateral to theinternal acoustic meatus to include resection,as needed, of the semicircular canals, vesti-

bule, roof of the mastoid antrum and tym-panic cavity, and the posterior face of thetemporal bone; and 4) the subtemporal pre-auricular infratemporal fossa approach inwhich the middle fossa exposure is combined

with exposure of the infratemporal fossa and,if needed, the petrous carotid, petrous apex,pterygopalatine fossae, and orbit.

The approaches directed through the mas-toid in front of the sigmoid sinus vary in theamount of temporal bone resected. Theyinclude 1) the minimal mastoidectomy vari-ant in which only enough presigmoid dura isexposed to open the dura in front of the sig-moid without exposing the labyrinth; 2) theretrolabyrinthine approach, which exposes the

bony capsule of the labyrinth; 3) the partiallabyrinthectomy, which includes removal of one or more of the semicircular canals; 4) thetranslabyrinthine approach, which includesresection of the semicircular canals andvestibule; and 5) the transcochlear modifica-tion, which includes removal of all thelabyrinth, including the cochlear and possiblythe petrous apex. These variants of the trans-mastoid approaches can all be combined, asneeded, with the supra- and infratentorial pre-

sigmoid approaches to the middle and poste-rior fossa.

The final approach to be reviewed is thepostauricular transtemporal approach, whichallows lesions involving the mastoid, tym-panic cavity, petrous apex, and jugular fora-men to be followed backward to the areasexposed by the retrosigmoid and far-lateralapproaches and forward to the infratemporal,pterygopalatine and middle fossae, lateralmaxilla, and orbit. Selecting an approachdirected through the temporal bone requiresan understanding of its complex anatomy andits relationship to the petroclival region, theinfratemporal fossa, and parapharyngealspace. Protecting and preserving the facialnerve, the petrous carotid artery, and the sen-sory organs of the inner ear that are containedwithin the temporal bone are important ele-ments in operative approaches directedthrough the lateral aspect of the cranial base.

OVERVIEW OF TEMPORAL BONE

KEY WORDS: Cranial base, Cranial nerves, Facial nerve, Internal carotid artery, Microsurgical anatomy, Skull

base, Skull base neoplasm, Surgical approach, Temporal bone

Neurosurgery 61:S4-7–S4-60, 2007 DOI: 10.1227/01.NEU.0000280024.07630.65 www.neurosurgery-online.com




RHOTON

FIGURE 1-1. Temporal bone. A and B, infe-rior views. A, the temporal bone has asquamosal part, which forms some of the floorand lateral wall of the middle cranial fossa. Itis also the site of the mandibular fossa inwhich the mandibular condyle sits. The tym-

panic part forms the anterior, lower, and part

of the posterior wall of the external canal, partof the wall of the tympanic cavity, the osseous portion of the eustachian tube, and the poste-rior wall of the mandibular fossa. The mastoid

portion contains the mastoid air cells andmastoid antrum. The petrous part is the siteof the auditory and vestibular labyrinth, thecarotid canal, the internal acoustic meatus,and the facial canal. The petrous part also

forms the anterior wall and the dome of the jugular fossa. The styloid part projects down-ward and serves as the site of attachment of three muscles. B, inferior view of the temporaland surrounding bones. The squamosal and

petrous parts articulate anteriorly with the greater wing of the sphenoid. The petrous

apex faces the foramen lacerum and is sepa-rated from the clival part of the occipital boneby the petroclival fissure. The occipital bone

joins with the petrous part of the temporalbone to form the jugular foramen. Themandibular fossa is located between the ante-rior and posterior roots of the zygomatic

process. C and D, superior views. C, themedial part of the upper surface is the site of the trigeminal impression in which Meckel’scave sits. Farther laterally is the prominenceof the arcuate eminence overlying the superiorsemicircular canal. Anterolateral to the arcu-ate eminences is the tegmen, a thin plate of bone overlying the mastoid antrum and epi-

tympanic area. The temporal bone articulatesanteriorly with the sphenoid bone, above withthe parietal bone, and posteriorly with theoccipital bone. The zygomatic process of thesquamosal part has an anterior and a poste-rior root between which, on the lower surface,is located the mandibular canal. D, temporaland surrounding bones. The squamosal partof the temporal bone joins anteriorly with thesphenoid bone to form the floor of the middlecranial fossa. Posteriorly, it articulates withthe occipital bone to form a portion of theanterior wall of the posterior fossa. Medially, it articulates with the clival por-tion of the occipital bone at the petroclival fissure. The sigmoid sulcus descendsalong the posterior surface of the mastoid portion and turns forward to enter the

jugular foramen. The foramen lacerum is located at the junction of the tempo-ral, sphenoid, and occipital bones. The porus of the internal acoustic meatus islocated in the central part of the posterior surface. Ac., acoustic; Ant., anterior;

Arc., arcuate; Car., carotid; Cond., condyle; Digast., digastric; Emin., emi-nence; For., foramen; Gr., greater; Impress., impression; Int., internal; Jug.,

jugular; Mandib., mandibular; N., nerve; Occip., occipital; Pet., petrosal;

Post., posterior; Proc., process; Sig., sigmoid; Stylomast., stylomastoid; Trig.,trigeminal; Tymp., tympanic.



THE TEMPORAL BONE ANDTRANSTEMPORAL APPROACHES

Lateral Surface

When the skull and temporal bone are viewed from a lateral

perspective, some landmarks useful in performing approachesdirected around and through the temporal bone can be identi-fied (Fig. 1-2). The posterior end of the superior temporal linecontinues inferiorly as the supramastoid crest and blends intothe upper edge of the zygomatic arch. The supramastoid crestis located at the level of the floor of the middle fossa. The junc-tion of the supramastoid crest with the squamous suture islocated at the lateral end of the petrous ridge. The meeting

point of the parietomastoid and squamous sutures is located afew millimeters below the lateral end of the petrous ridge. Thesuperior edge of the junction of the sigmoid and transversesinuses is located at the junction of the squamous and pari-etomastoid suture.

The mastoid antrum, a pneumatized space opening into the

tympanic cavity, is located about 1.5 cm deep to the suprameataltriangle, a depression in the mastoid surface located betweenthe posterosuperior edge of the external meatus, the supramas-toid crest, and the vertical tangent along the posterior edge of the meatus. The suprameatal spine of Henle is located at theouter end of the posterosuperior edge of the external canalalong the anterior edge of the suprameatal triangle and corre-sponds to the level of the lateral semicircular canal and tym-



FIGURE 1-2. Temporal bone. A, posterior view of a right temporal bone. Thesquamosal part forms part of the floor and lateral wall of the middle fossa. Thesigmoid sulcus descends along the posterior surface of the mastoid portion. The

internal acoustic meatus enters the central portion of the petrous part of thebone. The trigeminal impression and arcuate eminence are located on the uppersurface of the petrous part. The vestibular aqueduct connects the vestibule inthe petrous part with the endolymphatic sac, which sits on the posterior petroussurface inferolateral to the internal acoustic meatus. B, enlarged view. Thetransverse crest separates the meatal fundus into a superior part where the

facial canal and superior vestibular areas are situated, and an inferior part

where the cochlear and inferior vestibular areas are located. The vertical crestseparates the facial and superior vestibular areas. C, enlarged view of anotherinternal acoustic meatus. The transverse crest divides the meatal fundus into

superior and inferior parts. The anterior part above the transverse crest is thesite of the facial canal and the posterior part is the site of the superior vestibu-lar area. Below the transverse crest, the cochlear area is anterior and the infe-rior vestibular area is posterior. D, another internal acoustic meatus. The viewis directed to expose the singular foramen, for the singular branch of the inferiorvestibular nerve that innervates the posterior ampullae. The inferior vestibularnerve also has a saccular and, occasionally, a utricular branch. (Continues)



panic segment of the facial nerve at a depth of approximately1.5 cm. Several landmarks are also helpful in identifying thelocation of the junction of the transverse and sigmoid sinuses atthe posterior aspect of the mastoid. The asterion located at the

junction of the lambdoid, occipitomastoid, and parietomastoidsutures is usually located over the junction of the lower part of

the transverse and sigmoid sinuses. A burr-hole placed at thissite will usually expose the lower edge of this junction. A burr-hole located at the junction of the supramastoid crest and thesquamosal suture will be located at the posterior part of themiddle fossa floor just above and anterior to the upper edge of the junction of the transverse and sigmoid sinuses.


RHOTON

FIGURE 1-2. (Continued) E , lateral view of the temporal bone. The squamosal part forms part of the lateral wall of the middle fossa, the posterior part of the zygomatic arch, and the upper part of the mandibular fossa. The tympanic part forms the posterior wall of the mandibular fossa and almost all of the wallof the external canal. The styloid process is ensheathed at its base by the tym-

panic part and projects downward, serving as the attachment of several mus-cles. The mastoid part is located posteriorly and contains the mastoid air cellsthat coalesce at the mastoid antrum. F, enlarged view of the external auditorycanal. The spine of Henley, an excellent landmark for locating the deep site of the lateral canal and tympanic segment of the facial nerve, is located along the

posterosuperior margin of the external canal. The mastoid antrum is locateddeep to the depressed area, called the suprameatal triangle, located behind thespine of Henley. The view into the canal exposes the tympanic cavity, which hasthe promontory overlying the basal turn of the cochlea and the oval and roundwindows in its medial wall. G, lateral surface of the temporal bone in theintact skull. The tympanic part forms the anterior and lower and part of the

posterior wall of the external canal. The mandibular fossa is formed above andanteriorly by the squamosal part and behind by the tympanic part. The mastoidantrum is located posterosuperior to the spine of Henley, between the spine of

Henley and the anterior part of the supramastoid crest. The asterion, the junc-tion of the lambdoid, parietomastoid, and occipital mastoid sutures, is usuallylocated over the lower half of the junction of the sigmoid and transverse sinuses.The midpoint of the parietal mastoid suture is usually located at the anteriormargin of the junction of the transverse and sigmoid sinuses, and the lateraledge of the petrous ridge is located at the junction of the squamosal suture andthe supramastoid crest. H, the supra- and infratentorial areas have been exposedwhile preserving the bone at the site of the sutures. The asterion, located at the

junction of the lambdoid, occipitomastoid, and parietomastoid sutures, overliesthe lower half of the junction of the transverse and sigmoid sinuses. The junc-tion of the supramastoid crest and the squamosal suture is located at the pos-terior edge of the middle fossa and slightly anterior and above the junction of the transverse and sigmoid sinuses. Ac., acoustic; Arc., arcuate; CN, cranialnerve; Coch., cochlear; Emin., eminence; Ext., external; For., foramen;Impress., impression; Inf., inferior; Int., internal; Mandib., mandibular;Occipitomast., occipitomastoid; Parietomast., parietomastoid; Proc., process;Sig., sigmoid; Sp., spine; Sup., superior; Supramast., supramastoid; Trans.,transverse; Trig., trigeminal; Vert., vertebral; Vest., vestibular.





FIGURE 1-3. A–D, posterior surface of the temporal bone. A, the internalmeatus is located near the center and the jugular foramen at the lower edgeof the posterior surface. The sigmoid sinus descends along the posterior sur-

face of the mastoid and turns forward on the occipital bone to pass throughthe sigmoid part of the jugular foramen. The inferior petrosal sinus descendsalong the petroclival fissure and passes through the petrosal part of the jugu-lar foramen. The subarcuate fossa is located superolateral and the ostium forthe vestibular aqueduct lateral to the internal acoustic meatus. The trigemi-nal impression is a shallow trough on the upper surface of the temporal bonebehind the foramen ovale. The arcuate eminence overlies the superior semicir-cular canals. B, temporal bone with the nerves preserved. The abducens nerveascends to enter Dorello’s canal. The trigeminal nerve passes above the

petrous apex to enter the porus of Meckel’s cave. The facial and vestibulo-cochlear nerves enter the internal acoustic meatus, and the glossopharyngeal,

vagus, and accessory nerves enter the jugular foramen. The posterior andsuperior semicircular canals have been exposed. C, enlarged view. The upperend of the posterior canal and the posterior end of the superior canal share thecommon crus. The endolymphatic duct extends downward from the vestibuleand opens into the endolymphatic sac located beneath the dura inferolateralto the meatus. The endolymphatic ridge, the bridge of bone forming the pos-terior lip of the vestibular aqueduct, has been preserved. The jugular bulb canbe seen through the thin bone below the internal meatus. D, enlarged view of the fundus of the meatus after removal of the posterior wall. The upper edgeof the porus has been preserved. The subarcuate artery enters the subarcuate

fossa. The inferior vestibular nerve gives rise to the singular branch to the post erior ampullae, plus utricular and saccular branches . The superiorvestibular nerve innervates the ampullae of the superior and lateral semicir-cular canals and commonly gives rise to a utricular branch. (Continues)

The Tympanic Part

The tympanic part of the temporal bone is a curved plate

anterior to the mastoid process (Figs. 1-1, 1-2, and 1-4). It formspart of the wall of the external acoustic meatus, tympanic cav-ity, and osseous part of the Eustachian tube. Its concave poste-rior surface forms the anterior wall, floor, and part of the pos-terior wall of the external acoustic meatus. The roof and upperposterior wall are formed by the squamosal part. Its surfacecontains a portion of the tympanic sulcus for attachment of thetympanic membrane, which closes the medial end of the exter-

nal canal. The anterior surface, which is concave, forms theposterior wall of the mandibular fossa. Its lateral border forms

most of the margin of the external acoustic meatus. Medially, it joins the petrous part at the petrotympanic fissure throughwhich the chorda tympani passes. The carotid canal and the

jugular foramen are located medial to the tympanic part.The styloid process, a slender spicule ensheathed by the infe-

rior border of the tympanic bone, projects into the infratempo-ral fossa and is the site of attachment for the styloglossus, sty-lopharyngeus, and stylohyoid muscles (Fig. 1-5). It is located



immediately anterior to the emergence of the facial nerve fromthe stylomastoid foramen and is covered laterally by theparotid gland. The stylomastoid foramen, the external end of the facial canal, opens between the styloid and mastoidprocesses. The facial nerve crosses the lateral surface of the sty-loid process, and the external carotid artery crosses the tip.Resecting the styloid process and reflecting the attached mus-

cles downward exposes the internal jugular vein as it exits the jugular foramen and the carotid artery as it enters the carotidcanal medial to the tympanic bone.

The Squamous Part

The externally convex surface of the squamosal part givesattachment to the temporalis muscle (Figs. 1-1, 1-2, and 1-5).


RHOTON

FIGURE 1-3. (Continued) E–H, posterior surface of the temporal bone. E, the petrous apex medial to the internal acoustic meatus has been removed to exposethe petrous carotid. The lateral genu of the petrous carotid, located at the junc-tion of the vertical and horizontal segments of the petrous carotid, is situated

below and medial to the cochlea. The jugular bulb extends upward toward thevestibule and semicircular canals adjacent to the posterior meatal wall. The infe-rior petrosal sinus courses along the petroclival fissure and enters the petrosal

part of the jugular foramen, and the sigmoid sinus descends in the sigmoid groove and enters the sigmoid part of the foramen. The glossopharyngeal,vagus, and accessory nerves pass through the central or intrajugular part of the

foramen between the sigmoid and petrosal parts. F, bone has been removedalong the anterior margin of the meatal fundus to open the cochlea, and alongthe posterior margin to expose the vestibule. The jugular bulb extends upwardtoward the semicircular canals and vestibule. G, enlarged view. The cochlearnerve penetrates the modiolus of the cochlea where its fibers are distributed tothe turns of the cochlear duct. The basal turn of the cochlea communicates belowthe modiolus with the vestibule. H, enlarged view of the vestibule and cochlea.

The stapes has been removed from the oval window. The promontory in themedial wall of the tympanic cavity is located lateral to the basal turn of thecochlea. A silver fiber has been introduced into the superior canal, a red fiberinto the lateral canal, and a blue fiber into the posterior canal. The ampullated

ends are located at the bulbous ends of the three fibers. The common crus of thesuperior and posterior canals is located at the site where the tips of the blue andsilver fibers overlap. The superior vestibular nerve passes to the ampullae of thesuperior and lateral canals. The singular branch of the inferior vestibular nerveinnervates the posterior ampullae. A small black fiber has been introduced intothe opening of the endolymphatic duct into the vestibule. A., artery; Ac.,acoustic; Arc., arcuate; Car., carotid; CN, cranial nerve; Coch., cochlear;Emin., eminence; Endolymph., endolymphatic; Fiss., fissure; For., foramen;Hypogl., hypoglossal; Impress., impression; Inf., inferior; Int., internal;Intermed., intermedius; Jug., jugular; Lat., lateral; N., nerve; Nerv., nervus;Pet., petrosal, petrous; Petrocliv., petroclival; Post., posterior; Semicirc., semi-circular; Sig., sigmoid; Subarc., subarcuate; Sup., superior; Trig., trigeminal;Vest., vestibular.



The supramastoid crest extends backward across its posteriorpart, giving attachment to the temporalis muscle and fascia.The suprameatal triangle, a depressed area, located below theanterior part of the crest and behind the posterosuperior mar-gin of the external meatus, marks the deep location of the mas-toid antrum. The cerebral surface of the squamosal part is con-cave, accommodating the temporal lobe and joining the greaterwing of the sphenoid anteriorly. The zygomatic process of thesquamosal part projects forward and with the zygomatic bonecompletes the zygomatic arch. The attachment of the zygo-matic process to the squama is wide giving it anterior and pos-terior edges, referred to as the anterior and posterior roots. Thetemporalis fascia attaches to the superior border of the archand the masseter attaches to the lower border. The posteriorroot of the zygomatic process blends posteriorly into thesuprameatal crest. The anterior root is located at the anteriormargin of the temporomandibular joint, with the joint forminga rounded fossa on the lower margin of the zygomatic process

between the anterior and posterior roots. The upper margin of the zygomatic process between the two roots gives attachmentto the posterior part of the temporalis muscle. The mandibularfossa, located on the lower margin of the process between thetwo roots, is delimited in front by the articular tubercle andposteriorly by the postglenoid tubercle adjacent to its junctionwith the tympanic bone. The squamotympanic fissure islocated between the medial part of the squamosal part of themandibular fossa and the medial part of the tympanic bone.The petrotympanic fissure is situated between the tympanicplate and the petrosal part and leads into the tympanic cavity;it contains the anterior ligament of the malleus and the anteriortympanic branch of the maxillary artery. The anterior canalicu-lus for the chorda tympani exits the tympanic cavity in thepetrotympanic fissure. The rootlets of the temporal branch of the facial nerve cross the lateral aspect of the zygomatic archand course through the subcutaneous tissues on the superficiallayer of the temporal fascia. During resection of the zygomaticarch, the superficial temporalis fascia should be carefully dis-sected from the underlying deep fascia, starting as close as pos-

sible to the tragal cartilage, and carried forward, reflecting thesuperficial fascia anteriorly to avoid damage to the filaments of the temporal branch to the frontalis muscle, which crosses theouter surface of the superficial fascia.

The Mastoid Part

The mastoid is the posterior part of the temporal bone (Figs.1-1, 1-2, and 1-4). It projects downward to form the process

that is the site of attachment, from superficial to deep, of thesternocleidomastoid, splenius capitis and longissimus capitismuscles, and the posterior belly of the digastric muscle (Fig.1-5). The lower surface medial to the mastoid process isgrooved by the mastoid notch to which the posterior belly of the digastric attaches. Medial to the notch, the occipital groovegives passage to the occipital artery. The fascia covering theanterior margin of the posterior belly of the digastric is contin-uous anteriorly with the connective tissue surrounding theemergence of the mastoid segment of the facial nerve from thestylomastoid foramen and can be used as a landmark for iden-tifying the initial extracranial segment of the nerve. After exit-ing the stylomastoid foramen, the nerve divides in the sub-stance of the parotid gland into temporal, zygomatic, buccal,marginal mandibular, and cervical branches (Fig. 1-5). The tem-poral and zygomatic branches cross the zygomatic arch and theouter surface of the superficial fascia of the temporalis muscle.Keeping the connective tissue surrounding the nerve at the sty-lomastoid foramen intact during mobilization of the facialnerve will reduce the risk of facial nerve damage. The posterior

border of the mastoid process is perforated by one or moreforamina through which an emissary vein to the sigmoid sinusand a dural branch from the occipital artery pass.

The medial aspect of the mastoid process is grooved by thesigmoid sinus (Figs. 1-1–1-4). The sinus represents the posteriorlimit of the mastoid cavity. The sinus meets the roof of the cav-ity at the level of the petrous ridge. The angle between thesuperior petrosal and sigmoid sinuses and the middle fossadura delimits a dural space called the sinodural angle. The sin-odural angle is an important landmark when exposing the con-tents of the mastoid. Inferiorly, the sigmoid sinus curves medi-ally and forward, crossing the occipital bone to enter the

jugular foramen. The superior aspect of the jugular foramencorresponds to the apex of the jugular bulb and constitutes theinferior limit of the mastoid cavity.

The medial limit of the mastoid cavity is formed by the block of solid bone, the otic capsule, containing the bonylabyrinth (Figs. 1-4 and 1-6). The area of posterior fossa dura

mater that can be exposed through the mastoid cavity betweenthe sigmoid and superior petrosal sinuses, the otic capsule,and the jugular bulb is called Trautman’s triangle. The size of this dural triangle is important in surgical procedures in whichthe dura delimited by the triangle must be opened medial tothe sigmoid sinus. The distance from the anterior margin of the sigmoid sinus to the otic capsule at the level of the poste-rior semicircular canal averages 8 mm (range, 6–9 mm) on the


RHOTON

FIGURE 1-4. (Continued) superficial to the basal turn of the cochlea. Thelabyrinth and fundus of the internal meatus are located medial to the tympaniccavity. A line directed medially through the skull along the long axis of theexternal meatus will also approximate the site of the long axis of the internalmeatus on the medial side of the promontory and acousticovestibular labyrinth.

F, the stapes has been removed from the oval window. The handle of the malleusattaches to the tympanic membrane, the neck is crossed by the chorda tympani,

and the head articulates with the incus, which has been removed. The tendon

of the tensor tympani attaches to the upper part of the handle of the malleus.The stapedial muscle is housed within the pyramidal eminence and its tendoninserts on the stapedial neck. Chor., chorda; CN, cranial nerve; Emin., emi-nence; Endolymph., endolymphatic; Epitymp., epitympanic; Eust.,eustachian; Jug., jugular; Lat., lateral; Long., longus; M., muscle; Mast.,mastoid; Memb., membrane; N., nerve; Post., posterior; Proc., process; Seg.,segment; Sig., sigmoid; Sp., spine; Squamomast., squamomastoid; Temp.,

temporal; Tymp., tympani, tympanic; Tympanomast., tympanomastoid.





FIGURE 1-5. A–F, muscular and osseous relationships. A, the skin and sub-cutaneous tissues have been removed to expose the parotid gland and the facial

nerve branches that course deep to the parotid gland on their way to the facialmuscles. The masseter muscle has two heads: a more superficial anterior head,which passes downward to the lateral surface of the angle of the jaw, and adeeper posterior head, which arises from the medial surface of the zygomaticarch and passes to the mandibular body. The sternocleidomastoid attaches to thelateral part of the superior nuchal line and mastoid process, descends in ananterior direction, and is crossed by the greater auricular nerve. The temporalis

fascia attaches to the upper surface of the zygomatic arch. The trapezius mus-cle attaches to the medial part of the superior nuchal line. The posterior trian-

gle of the neck, located between the sternocleidomastoid and trapezius, has thesemispinalis capitis, splenius capitis, and levator scapulae in its floor. The ter-

minal branches of the occipital artery and the greater occipital nerve reach thesubcutaneous tissues by passing between the attachment of the trapezius andsternocleidomastoid muscles to the superior nuchal line. B, enlarged view. The

facial nerve branches are exposed along the anterior edge of the parotid gland.C, the parotid gland has been removed to expose the facial nerve and itsbranches distal to the stylomastoid foramen. The nerve passes lateral to the sty-loid process, the external carotid artery, and mandibular neck. The superficialand deep heads of the masseter muscle are exposed. This lower end of the ster-nocleidomastoid muscle has been reflected posteriorly by dividing (Continues)




RHOTON

FIGURE 1-5. (Continued) its attachment to the clavicle and sternum. Thesuperficial temporal artery ascends in front of the ear. D, the upper part of themandibular ramus and the lower part of the temporalis muscle and its attach-ment to the coronoid process have been removed while preserving the inferioralveolar nerve. The infratemporal fossa is located medial to the mandible and onthe deep side of the temporalis muscle. The upper and lower heads of the lateral

pterygoid, which insert along the temporomandibular joint, and the superficialhead of the medial pterygoid, which extends from the lateral pterygoid plate tothe angle of the jaw, have been exposed. The structures in the infratemporal

fossa include the pterygoid muscles, branches of the mandibular nerve, themaxillary artery, and the pterygoid venous plexus. The sternocleidomastoidmuscle has been reflected out of the exposure to expose the splenius capitis mus-

cle. E, posterolateral view. The splenius capitis has been reflected downward toexpose the longissimus capitis, superior oblique, and semispinalis capitis. Theoccipital artery passes along the occipital groove on the medial side of the digas-tric groove. F, the longissimus capitis has been reflected downward to expose therectus capitis posterior minor and major, which descend from the occipital boneto attach to the spinous process of C1 and C2, respectively; the superior oblique,which passes from the occipital bone to the transverse process of C1; and theinferior oblique, which extends from the spinous process of C2 to the transverse

process of C1. The vertebral artery, in its ascent from C2 to C1, is exposedmedial to the attachment of the levator scapulae to the C1 transverse process.The C1 transverse process is situated immediately behind the internal jugularvein and a short distance below and behind the jugular foramen. (Continues)



right side, and 7 mm (range, 4–9 mm) on the left (44). The dis-tance between the apex of the jugular bulb and the superiorpetrosal sinus is also an important determinate of the size of exposure that can be achieved by opening Trautman’s triangle.This distance is reduced if there is a high jugular bulb. The

jugular bulb usually lies inferior to the ampulla of the poste-rior semicircular canal, but it can project superiorly as far asthe level of the lateral semicircular canal (27). The average dis-tance from the jugular bulb to the superior petrosal sinus is1446p10.5mm (range, 10–19 mm) on the right side, and 16 mm(range, 11–21 mm) on the left (44).

The mastoid interior is composed of trabeculated bone,which coalesces to form a cavity, the mastoid antrum, that com-municates through an opening, the aditus, that leads forward

to the epitympanic part of the tympanic cavity (Figs. 1-4 and1-6). The lateral semicircular canal is medial to the epitympanicrecess. The medial wall of the antrum faces the posterior semi-circular canal. The roof is formed by the tegmen in the floor of the middle cranial fossa. The mastoid segment of the facialcanal courses adjacent to the anteroinferior margin of theantrum. The lateral wall of the mastoid antrum, through whichit is usually approached surgically, is formed by the postmeatalpart of the squamous temporal bone. The lateral wall of theantrum is located deep to the suprameatal triangle, which isdemarcated superiorly by the suprameatal crest, located at thelevel of the floor of the middle fossa; anteroinferior by the pos-terosuperior margin of the acoustic meatus, which indicatesapproximately the position of the descending or mastoid part

of the facial canal; and posteriorly by a posterior vertical tan-gent to the posterior margin of the external meatus. The aircells in the mastoid may extend behind the sigmoid sinus andinto the squamosal part of the temporal bone, the posteriorroot of the zygomatic process, the osseous roof of the externalacoustic meatus, the floor of the tympanic cavity near the jugu-lar bulb, and the petrous apex surrounding the carotid canal,eustachian tube, and labyrinth.

The tympanic cavity is a narrow air-filled space between thetympanic membrane laterally and the promontory containingthe auditory and vestibular labyrinth medially (Figs. 1-4, 1-6,and 1-7). It communicates posteriorly with the mastoid antrumand anteriorly through the eustachian tube with the nasophar-ynx. It contains the malleus, incus, and stapes. The tympaniccavity opens upward into the epitympanic recess, which con-tains the head of the malleus and body of the incus. The roof of the tympanic cavity is formed by a thin plate, the tegmen tym-pani, which separates the middle fossa and tympanic cavities,and also roofs the mastoid antrum and the tensor tympani.The thin floor of the tympanic cavity separates the cavity fromthe jugular bulb. The medial part of the floor is perforated byan opening for the tympanic branch of the glossopharyngeal

nerve. The lateral wall is formed by the tympanic membraneand the osseous ring to which the membrane attaches. The ringis deficient above near the openings of the anterior and poste-rior canaliculi for the chorda tympani (Figs. 1-4 and 1-6). Theposterior canaliculus for the chorda tympani arises from thefacial canal a few millimeters above the stylomastoid foramenand ascends in front of the facial canal to open into the tym-panic cavity at the level of the upper part of the handle of themalleus. The chorda tympani passes in close relation to thetympanic membrane and the upper part of the handle of themalleus and forward to enter its anterior canaliculus at themedial aspect of the petrotympanic fissure, and descends ver-tically medial to the sphenoid spine and lateral pterygoid mus-cle to join the lingual nerve.

The medial wall of the tympanic cavity, which forms the lat-eral boundary of the inner ear and the petrosal part of the tem-poral bone, is the site of the promontory, the oval and roundwindows, and the prominence over the facial nerve (Figs. 1-2and 1-4). The tympanic nerve plexus grooves the promontoryoverlying the lateral bulge of the basal turn of the cochlea. Theapex of the cochlea lies near the medial wall of the cavity ante-rior to the promontory. The oval window is posterosuperior to



FIGURE 1-5. (Continued) G–L, muscular and osseous relationships. G, themandibular condyle and ramus have been removed to expose the styloid processand attached muscles. The pterygoid muscles and some branches of the mandibu-lar nerve have been removed to expose the auriculotemporal nerve, which splitsinto two roots that surround the middle meningeal artery. The levator veli pala-tini, which attaches the lower margin of the eustachian tube, is in the medial partof the exposure. The longus capitis is exposed medial to the internal carotid

artery in the retropharyngeal area. H, the muscles that attach to the styloid process have been divided at their origin. The facial nerve crosses the lateral surface of the styloid process. The attachment of the tensor veli palatine to the skullbase extends between the foramen ovale and the eustachian tube. I, the externalauditory canal has been removed, but the tympanic membrane and cavity havebeen preserved. The levator veli palatine and part of the tensor veli palatine havebeen removed and the membranous part of the eustachian tube opened. Theeustachian tube crosses anterior to and is separated from the petrous carotid bya thin shell of bone. The jugular bulb and lateral bend of the petrous carotid arelocated below the osseous labyrinth. The pterygopalatine fossa is exposed ante-riorly. J, the Eustachian tube has been resected and the mandibular nerve dividedat the foramen ovale to expose the petrous carotid. This exposes the longus capi-

tis and rectus capitis anterior, both of which are located behind the posterior pha-ryngeal wall. K, the petrous carotid has been reflected forward out of the carotidcanal to expose the petrous apex medial to the carotid canal. L, the petrous apexand upper clivus have been drilled and the dura opened to expose the anterolat-eral aspect of the pons below the trigeminal nerve. The sigmoid sinus and the

jugular bulb have been removed to expose the nerves exiting the jugular fora-men. A., artery; Alv., alveolar; Ant., anterior; Aur., auricular; Brs., branches;

Cap., capitis; Car., carotid; CN, cranial nerve; Cond., condyle; Constr., con-strictor; Eust., eustachian; Ext., external; Gl., gland; Gr., greater; Inf ., inferior;Int., internal; Jug., jugular; Lat., lateral; Lev., levator; Long., longus; Longiss.,longissimus; M., muscle; Maj., major; Mandib., mandibular; Max., maxillary;Med., medial; Memb., membrane; Min., minor; N., nerve; Obl., oblique;Occip., occipital; Pal., palatini; Parapharyng., parapharyngeal; Pet., petrosal;Post., posterior; Proc., process; Pteryg., pterygoid; Pterygopal., pterygopala-tine; Rec., rectus; Scap., scapula; Semispin., semispinalis; Splen., splenius;Sternocleidomast., sternocleidomastoid; Suboccip., suboccipital; Sup., supe-rior; Superf ., superficial; Temp., temporal, temporalis; Tens., tensor; TM., tem-

poromandibular; Trans., transverse; Tymp., tympanic; V., vein; Veli./Vel.,veli; Vert., vertebral.



the promontory and connects the tympanic cavity to thevestibule, and is occupied by the footplate of the stapes. The

round window is posteroinferior to the oval window andopens under the overhanging edge of the promontory. Theprominence of the facial canal is located above the oval win-dow. The posterior wall of the tympanic cavity is mainly thesite of the aditus, the opening of the tympanic cavity, into themastoid antrum. The medial wall of the aditus has a roundprominence overlying the lateral semicircular canal. Thepyramidal eminence, which houses the stapedial muscle, is

located just behind the oval window and anterior to the mas-toid part of the facial canal. The stapedius extends forward

from the eminence to attach to the neck of the stapes. The fossaincudis is a small depression low and posterior in the epitym-panic recess; it contains the short process of the incus, which isfixed to the fossa by ligamentous fibers.

The anterior wall of the tympanic cavity narrows and leadsinto the eustachian tube, which communicates the nasopharynxwith the tympanic cavity (Figs.1-4, 1-7, and 1-8). It has bonyand cartilaginous parts. The bony part begins in the anterior


RHOTON

FIGURE 1-6. A–D, translabyrinthine exposure. A, the insert shows the siteof the exposure directed through the mastoid. The spine of Henley at the pos-terosuperior margin of the external meatus is a superficial landmark that

approximates the deep site of the lateral semicircular canal and the tympanicsegment of the facial nerve. The mastoidectomy has been completed. Thesuperior petrosal and sigmoid sinuses, the jugular bulb, and the facial nerveare usually skeletonized in the approach, leaving a thin layer of bone overthem. The semicircular canals, which are located in the cortical bone medialto the cancellous mastoid and the mastoid antrum, have been exposed. Thedura between the sigmoid and superior petrosal sinuses, the jugular bulb, andthe labyrinth, which faces the cerebellopontine angle, is referred to asTrautman’s triangle. B, the mastoid antrum opens through the aditus into theepitympanic part of the tympanic cavity, which contains the upper part of themalleus and incus. The tympanic segment of the facial nerve passes between

the lateral canal and the stapes in the oval window and then turns down-ward as the mastoid segment. The chorda tympani arises from the mastoidsegment of the facial nerve and passes upward and forward along the deep

surface of the tympanic membrane crossing the neck of the malleus. Theincus, the head of which is located in the epitympanic area, has a long processthat attaches to the stapes. C, the semicircular canals and vestibule have beenremoved and the dura lining the internal acoustic meatus has been opened toexpose the vestibulocochlear nerve. D, the dura has been opened to expose the

petrosal cerebellar surface and the structures in the cerebellopontine angle. Anatomic variants that limit the exposure include an anterior position of thesigmoid sinus, a high jugular bulb, or a low middle fossa plate. The jugularbulb may extend upward into the posterior wall of the internal acoustic mea-tus and be encountered as the posterior meatal wall is being removed byeither the translabyrinthine or retrosigmoid approaches. (Continues)



part of the tympanic cavity and is directed anteriorly and medi-ally. It joins the cartilaginous part at the junction of the squa-mous and petrous parts of the temporal bone. The cartilaginous

part of the tube is attached to the lower margin of thesphenopetrosal groove, which is situated between the petrous bone and the greater wing of the sphenoid bone, and its baselies directly under the mucous membrane of the lateral wall of the nasaopharynx. Both the petrous carotid and eustachiantube are directed anteromedially, with the Eustachian tube

being located along the anterior margin of the carotid canal(Figs. 1-7 and 1-8). The tensor tympani muscle and its bony

semicanal are located above the eustachian tube, parallel to thehorizontal segment of the petrous carotid. The canals for thetensor tympani superiorly and the osseous part of the

eustachian tube inferiorly open into the upper part of the ante-rior wall of the tympanic cavity. These canals are inclineddownward, anteriorly, and medially; they open into the angle

between the squamous and petrous parts of the temporal boneand are separated by a thin, bony septum. The canal for the ten-sor tympani extends posterolaterally on the medial wall of thetympanic cavity, to end above the oval window where the pos-terior end of the canal curves laterally to form a pulley, the



FIGURE 1-6. (Continued) E–H , trans-labyrinthine exposure. E, enlarged view of theexposure in the cerebellopontine angle. In thiscase, the glossopharyngeal and vagus nervescan be seen, although, in the translabyrinthineexposure, the jugular bulb often obstructs theview of the nerves entering the jugular fora-men. F, the vestibulocochlear nerve has beenelevated to expose the facial nerve. G, thelabyrinthine, tympanic, and mastoid segmentsof the facial nerve have been exposed in prepa-ration for transposition of the nerve for atranscochlear approach. H, the facial nerve hasbeen transposed backward and the bone ante-rior to the meatal fundus has been removed to

expose the cochlea for a transcochlear approachin which the cochlea is removed to gain accessto the side of the clivus and front of the brain-stem. The cochlear nerve has been divided. Thecochlear fibers innervating the cochlear duct

pass thro ugh the modiol us. Ac. , acoustic;A.I.C.A., anteroinferior cerebellar artery;Chor., chorda; CN, cranial nerve; Coch.,cochlear; In f . , inferior; Int . , internal ;Intermed., intermedius; Jug., jugular; Laby.,labyrinthine; Lat., lateral; Mast., mastoid; N.,nerve; Nerv., nervus; Pet., petrosal; P.I.C.A.,

posteroinferior cerebellar artery; Post., poste-rior; Seg., segment; Sig., sigmoid; Sup., supe-rior; Tymp., tympani, tympanic; V., vein;

Vest., vestibular.



trochleariform process, around which the tensor tympani ten-don turns laterally to attach to the handle of the malleus.

The Petrous Part

The petrous part of the temporal bone is wedged between

the sphenoid and occipital bones (Figs. 1-1 and 1-3). It containsthe acoustic and vestibular labyrinth and is the site of the jugu-lar fossa and the facial and carotid canals (Figs. 1-3, 1-4, and1-7). It has a base, apex, three surfaces and margins. The apexis located in the angle between the greater wing of the sphe-noid and the occipital bone and is the site of the carotid canalsmedial opening. It forms the posterolateral limit of the foramenlacerum. The anterior surface faces the floor of the middle cra-

nial fossa and its surface is grooved by the trigeminal impres-sion for the trigeminal ganglion; anterolateral to this, it formsthe roof of the carotid canal (Figs. 1-1 and 1-7). Lateral to thetrigeminal impression is a shallow depression, which partiallyroofs the internal acoustic meatus and is limited laterally by the

arcuate eminence, which overlies the superior semicircularcanal. The posterior slope of the arcuate eminence overlies theposterior and lateral semicircular canals. Farther laterally, theroof covers the vestibule and part of the facial canal. Thetegmen extends laterally from here and roofs the mastoidantrum and tympanic cavities and the canal for the tensor tym-pani. Opening the tegmen from above exposes the heads of the malleus, incus, the tympanic segment of the facial nerve,


RHOTON

FIGURE 1-7. A–D, middle fossa exposure of the temporal bone. A, super-olateral view. The tentorium, except the edge, has been removed. The durahas been removed from the middle fossa floor and cavernous sinus wall toexpose the greater petrosal nerve, middle meningeal artery, and the nerves inthe sinus wall. B, the middle fossa floor has been opened to expose thecochlea, semicircular canals, petrous carotid artery, and the facial, cochlear,and superior vestibular nerves in the meatus. The superior canal bulgesupward into the middle fossa below the arcuate eminence. The cochlear nerve

passes below the facial nerve to enter the cochlea, which is located above thelateral genu of the petrous carotid in the angle between the pregeniculate

facial and greater petrosal nerves. C, another temporal bone drilled to expose

the internal acoustic meatus, cochlea, vestibule, semicircular canals, tympanic cavity, and external meatus. The vestibule is located posterolateral andthe cochlea is anteromedial to the fundus of the internal meatus. Thevestibule communicates below the meatal fundus with the cochlea. The ten-sor tympani muscle and eustachian tube are layered along, but are separated

from, the anterior surface of the petrous carotid by a thin layer of bone. Thetegmen has been opened to expose the head of the incus and malleus in theepitympanic area. The internal acoustic meatus lies directly medial to, but isseparated from, the external meatus by the tympanic cavity and thelabyrinth. D, the nerves in the meatus have been separated to expose thesuperior and inferior vestibular, facial, and cochlear nerves. (Continues)



and the superior and lateral semicircular canals (Fig. 1-7). Thetympanic segment of the facial nerve begins at the geniculateganglion and ends at the level of the stapes, where the nerve

turns downward below the lateral semicircular canal. Thetegmen anteriorly is grooved by the greater petrosal nerveextending anterior and medial from the area in front of thearcuate imminence and crossing the floor of the middle fossatoward the foramen lacerum (Figs. 1-7 and 1-8). The greaterpetrosal nerve can be identified medial to the arcuate emi-nence as it leaves the geniculate ganglion by passing throughthe facial hiatus to reach the middle fossa floor. It runs beneath

the dura of the middle fossa in the sphenopetrosal grooveformed by the junction of the petrous and sphenoid bones,immediately superior and anterolateral to the horizontal seg-

ment of the petrous carotid. In a previous study, we foundthat bone of the middle cranial fossa was absent over thegeniculate ganglion in 16% of the specimens, thus exposing thefacial nerve and geniculate ganglion to the danger of injuryduring elevation of the dura from the floor of the middle fossa(31). Facial nerve injury can also result from damaging the

branch of the middle meningeal artery, which passes throughthe facial hiatus to supply the nerve, or from traction applied



FIGURE 1-7. (Continued) E–H, middle fossa exposure of the temporal bone.

E, enlarged view. The vestibule, into which the semicircular canals open, com-municates below the meatal fundus with the cochlea. The vertical crest, oftencalled Bill’s bar, separates the superior vestibular and facial nerves at the

meatal fundus. The tendon of the tensor tympani makes a right-angle turnaround the trochleariform process in the medial margin of the tympanic cav-ity to insert on the malleus. F, enlarged view. The superior canal projectsupward in the floor of the middle fossa. The lateral canal is situated above thetympanic segment of the facial nerve in the posteromedial part of the epitym-

panic area, and the posterior canal is located lateral to the posterior wall of theinternal acoustic meatus. G, bone has been removed below the greater petrosalnerve to expose the petrous carotid. The tensor tympani muscle above and theeustachian tube below are layered along the anterior surface of the petrouscarotid. H, enlarged view. Suture has been placed in the three semicircular

canals. The anterior end of the superior and lateral canals and the lower endof the posterior canal are the site of the ampullae. The posterior end of the supe-rior canal and the upper end of the posterior canal join to form a common crus.The facial and superior vestibular nerves have been removed to expose the

cochlear and inferior vestibular nerves. The singular branch of the inferiorvestibular nerve innervates the posterior ampullae. The superior vestibularnerve innervates the superior and lateral ampullae. A., artery; Ac., acoustic;A.I.C.A., anteroinferior cerebellar artery; Car., carotid; CN, cranial nerve;Coch., cochlear; Eust., eustachian; Ext., external; Gang., ganglion; Genic.,

genicula te; Gr., greater; Inf ., inferior; Lat ., lateral; M., muscle; Men.,meningeal; Mid., middle; N., nerve; Pet., petrosal, petrous; Post., posterior;S.C.A., superior cerebellar artery; Sup., superior; Tens., tensor; Tent., tento-rial; Tymp., tympani, tympanic; Vert., vertebral; Vest., vestibular.



to the ganglion whenmanipulating the greaterpetrosal nerve (30).

The l esser pet r osa lnerve from the tympanicplexus passes through thetympanic canaliculus,which is located anteriorto the facial hiatus andcourses in an anterome-dial direction parallel tothe greater petrosal nerve(Fig. 1-8). The cochlea lies

be lo w th e fl oo r of th emiddle fossa in the angle

between the labyrinthinesegment of the facia lnerve and the greater pet-rosal nerve, just medial tothe geniculate ganglion,anterior to the fundus of the internal acoustic mea-tus, and posterosuperiorto the lateral genu of thepetrous carotid artery.The cochlea is separatedfrom the petrous carotid

b y a 2 . 1 m m ( r a n g e ,0.6–10.0 mm) thickness of

bone and can be injuredduring exposure of thepetrous carotid. The mid-dle meningeal artery,an important landmark

when approaching thestructures of the middle fossa, enters the cranial cavitythrough the foramen spinosum of the sphenoid bone. Theforamen spino-sum is an average of 4.5 mm (range, 3–6 mm)anterolateral to the carotid canal and 14.0 mm (range,11.0–17.0 mm) anterolateral to the geniculate ganglion (44).

The posterior surface of the petrosal part faces the poste-rior cranial fossa and cerebellopontine angle and is continu-ous with the mastoid surface (Figs. 1-1–1-.3). The openingfor the internal auditory meatus is situated midway betweenthe base and the apex on the posterior surface. The lateralend of the meatus is divided into superior and inferior halves

by the transverse crest. The area above the transverse crest isfurther divided by the vertical crest, also called Bill’s bar,

which separates the anteriorly located facial canal from theposteriorly located superior vestibular area (29). The cochleaand inferior vestibular nerves penetrate the lateral end of themeatus below the transverse crest, with the cochlear nerve

being located anteriorly. The posterior wall of the meatus,lateral to the porus is the site of a small bony opening, thesubarcuate fossa, which gives passage to the subarcuateartery, a branch of the anteroinferior cerebellar artery (AICA),

which usually ends blindly in the region of the superiorsemicircular canal. Inferolateral to the porus of the meatus isthe opening for the vestibular aqueduct, which transmits theendolymphatic duct that opens below into the endolym-phatic sac located between the dural layers. The opening of the cochlear aqueduct, also called the cochlear canaliculusand occupied by the perilymphatic duct, is situated inferiorto the porus of the internal meatus at the anteromedial edgeof the jugular foramen, just superior and lateral to where theglossopharyngeal nerve enters the intrajugular part of the

jugular foramen.The inferior surface is very irregular. The apex is connected

medially to the clivus by fibrocartilage and gives attachment to

the levator veli palatini and the cartilaginous portion of theeustachian tube (Figs. 1-1 and 1-9). Behind this is the opening of the carotid canal, behind which is the jugular fossa that con-tains the jugular bulb. The small foramen for the tympanic

branch of the glossopharyngeal nerve is located on the ridge between the carotid canal and jugular foramen. On the lateralwall of the jugular bulb is the mastoid canaliculus for the auric-ular branch of the vagus nerve.


RHOTON

FIGURE 1-8. A, superior view of the temporal bone and infratemporal fossa and orbit. The floor of the middle fossa has beenremoved to expose the temporalis muscle in the temporal fossa and the pterygoid muscles and branches of the third trigemi-nal division in the infratemporal fossa. The posterior part of the middle fossa forming the upper surface of the temporomandibu-

lar joint has been removed to expose the mandibular condyle. The internal acoustic meatus extends laterally from the poste-rior surface of the temporal bone. The mastoid is located behind the external canal and lateral to the semicircular canals andvestibule. B, enlarged view. The trigeminal nerve has been reflected forward and bone has been removed over the eustachiantube, tensor tympani muscle, petrous carotid, and internal acoustic meatus. Dura has been removed from the lateral wall of the cavernous sinus to expose the trochlear, trigeminal, and oculomotor nerves in the sinus wall and the abducens nerve pass-ing below the petrosphenoid ligament and through Dorello’s canal. The greater petrosal nerve is joined by the deep petrosalbranches of the carotid sympathetic plexus to form the vidian nerve, which passes forward in the vidian canal, which has beenunroofed. The lesser petrosal nerve arises from the tympanic branch of the glossopharyngeal nerve, which passes across the

promontory in the tympanic nerve plexus and regroups to cross the floor of the middle fossa, exiting the skull to provide parasympathetic innervation through the otic ganglion to the parotid gland. The tensor tympani muscle and eustachian arelayered along, but are separated from, the anterior surface of the petrous carotid by a thin layer of bone. A., artery; Car.,carotid; Cav., cavernous; Chor., chorda; CN, cranial nerve; Cond., condyle; Eust., eustachian; Gang., ganglion; Gen., genic-ulate; Gr., greater; Lat., lateral; Less., lesser; Lig., ligament; M., muscle; Mandib., mandibular; Max., maxillary; N.,nerve; Ophth., ophthalmic; Pet., petrosal, petrous; Pteryg., pterygoid; Semicirc., semicircular; Sphen., sphenoid; Temp.,temporal; Tens., tensor; Tymp., tympani, tympanic.



The superior border, located along the petrous ridge, isgrooved by the superior petrosal sinus and serves as the attach-ment of the tentorium cerebelli, except medially where it is

crossed by the posterior trigeminal root. The lower posterior border, located along the petroclival fissure, is the site of agroove in which resides the inferior petrosal sinus that connects



FIGURE 1-9. Inferior views of an axial sec-tion of the skull base. A, the infratemporal

fossa is surrounded by the maxillary sinusanteriorly, the mandible laterally, the sphe-noid pterygoid process anteromedially, andthe parapharyngeal space posteromedially,and contains the mandibular nerve and max-

illary artery and their branches, the medialand lateral pterygoid muscles, and the ptery-

goid venous plexus. B, part of the lateral pterygoid muscle has been removed to exposethe branches of the trigeminal nerve coursingin the infratemporal fossa below the greatersphenoid wing. The pterygopalatine fossa islocated between the posterior maxillary wallanteriorly, the sphenoid pterygoid process

posteriorly, the nasal cavity medially, and theinfratemporal fossa laterally. The pharyngealrecess (fossa of Rosenmüller) projects later-ally from the posterolateral corner of thenasopharynx with its lateral apex facing theinternal carotid artery laterally and the fora-men lacerum above. The posterior nasopha-ryngeal wall is separated from the lowerclivus and the upper cervical vertebra by thelongus capitis, and the nasopharyngeal roof rests against the upper clivus and the poste-rior part of the sphenoid sinus floor. C, thesphenoid pterygoid process has been removedto expose the maxillary nerve passing throughthe foramen rotundum to enter the ptery-

gopala tine fossa where it gives rise to theinfraorbital nerve, which courses in the roof of the maxillary sinus. The maxillary nervewithin the pterygopalatine fossa gives off communicating rami to the pterygopalatine

ganglion. The vidian nerve, formed by theunion of the deep petrosal nerve from the

carotid sympathetic plexus and the greater petrosal nerve, courses forward through thevidian canal to join the pterygopalatine gan-

glion. The terminal part of the petrous carotidis exposed above the foramen lacerum. D,enlarged view with highlighting of the pre-(red) and poststyloid (yellow) compartmentsof the parapharyngeal space. The styloiddiaphragm, formed by the anterior part of thecarotid sheath, separates the parapharyngealspace into pre- and poststyloid parts. The

pres ty lo id co mpar tment, a na rrow fa t-containing space between the medial ptery-

goid and tensor veli palatini, separates the infratemporal fossa from the medi-ally located lateral nasopharyngeal region containing the tensor and levator veli

palatini and the eustachian tube. The oststyloid compartment, located behindthe prestyloid part, contains the internal carotid artery, internal jugular vein,and the cranial nerves IX through XII. A., artery; Cap., capitis; Car., carotid;CN, cranial nerve; Cond., condyle; Eust., eustachian; For., foramen; Gl.,

gland; Gr., greater; Infraorb., infraorbital; Infratemp., infratemporal; Int.,internal; Jug., jugular; Lat., lateral, lateralis; Lev., levator; Long., longus; M.,

muscle; Mandib., mandibular; Max., maxillary; N., nerve; Nasolac., naso-lacrimal; Occip., occipital; Pal., palatini; Parapharyng., parapharyngeal;Proc., process; Pteryg., pterygoid; Pterygopal., pterygopalatine; Rec., rec-tus; Tens., tensor; V., vein; Vel., veli.



the cavernous sinus and the medial wall of the jugular bulb.Behind this, the jugular fossa of the temporal bone joins withthe jugular notch on the jugular process of the occipital bone toform the margins of the jugular foramen.

The jugular foramen is located at the lower end of the petro-occipital fissure and is divided into a larger lateral opening,the sigmoid part, that receives the drainage of the sigmoidsinus, and a small medial part, the petrosal part, that transmitsthe inferior petrosal sinus (Fig. 1-1). The intrajugular part,located between the sigmoid and petrosal parts, transmits theglossopharyngeal, vagus, and accessory nerves. The anterior

border is joined laterally to the temporal squama at the pet-rosquamosal suture and medially articulates with the sphe-noid’s greater wing.

The bony labyrinth consists of three parts: the vestibule, thesemicircular canals, and the cochlea. The vestibule, located inthe central part of the bony labyrinth, is a small cavity at theconfluence of the ampullate and nonampullated ends of thesemicircular canals. It is situated lateral to the meatal fundus,medial to the tympanic cavity, posterior to the cochlea, andsuperior to the apex of the jugular bulb (Figs. 1-3, 1-4, and 1-7).

The floor of the vestibule is separated from the apex of the jugular bulb by a thickness of bone that averages 6 mm (range,4–8 mm) on the right side and 8 mm (range, 4–10 mm) on theleft side (44). This distance is particularly important duringtranslabyrinthine approaches since the height of the jugular

bulb is a major determinant of the size of the exposure of thecerebellopontine angle that can be achieved with this approach.A high-placed jugular bulb may be the source of troublesome

bleeding and air emboli if it is opened during exposure of thelabyrinth or internal acoustic meatus.

The semicircular canals are situated posterosuperior to thevestibule (Figs. 1-3, 1-4, and 1-7). The anterior part of the lat-eral semicircular canal is situated above the tympanic seg-

ment of the facial nerve and can be used as a guide to locat-ing that segment of the nerve. The posterior semicircularcanal lies parallel to and in close proximity with the posteriorsurface of the petrous bone in the area just behind and lateralto the lateral end of the internal acoustic meatus. The superiorsemicircular canal projects toward the floor of the middlefossa, usually in close relation to the arcuate eminence. Eachcanal has an ampullated and a nonampullated end that opensinto the vestibule. The anterior end of the lateral and superiorcanals and the inferior end of the posterior canal are the siteof the ampullae, which are innervated by the vestibularnerves. The posterior ends of the superior and posteriorcanals, the ends opposite the ampullae, join to form a com-mon crus that opens into the vestibule. The superior vestibu-

lar nerve innervates the ampullae of the superior and lateralcanals, and the singular branch of the inferior vestibular nerveinnervates the posterior ampulla. The vestibular nerves alsohave branches to the utricle and saccule located within thevestibule. The internal auditory meatus can be found medialto the arcuate eminence at an angle of about 60 degreesmedial from the long axis of the superior semicircular canal.The superior canal is the most susceptible to damage in com-

pleting the middle fossa approach to the internal acousticmeatus. The posterior canal may be damaged in removingthe posterior wall to expose the meatal contents by the ret-rosigmoid approach (Fig. 1-3).

During surgical approaches to the cerebellopontine anglein which the posterior meatal lip is removed, care should betaken to avoid opening the vestibular aqueduct, vestibule,

posterior semicircular canal, or the common crus (Figs. 1-2and 1-3). In our studies, we observed that there is a constantset of relationships among the structures around the poste-rior meatal lip. The common crus of the posterior and supe-rior semicircular canals is located lateral to the entrance of the subarcuate artery into the subarcuate fossa. The vestibu-lar aqueduct has an oblique orientation. It leaves thevestibule and runs in a posterior direction to open beneaththe dura mater at a level corresponding to that of the poste-rior semicircular canal. The average distance between theposterior semicircular canal, at the level with the junction of the common crus, and the lateral edge of the porus was 7 mm(range, 5–9 mm) (44).

The carotid artery, at the point where it enters the carotidcanal, is surrounded by a strong layer of connective tissue thatmakes it difficult to mobilize the artery at this point ( Figs. 1-9and 1-10) (38, 39). The vertical segment of the artery passesupward in the canal toward the genu, where it curves antero-medially to form the horizontal segment. The Eustachian tubeand the tensor tympani muscle are located parallel to and alongthe anterior margin of the horizontal segment, where they areseparated from the artery by a thin layer of bone.

The trigeminal ganglion and the adjacent part of the poste-rior root and their surrounding dural and arachnoidal cavern,called Meckel’s cave, sit in an impression on the upper surfaceof the petrous apex above the medial part of the petrous carotid(Figs. 1-1, 1-7, and 1-8). The length of the horizontal segment of the petrous carotid that can be exposed by removing bone lat-eral to the trigeminal ganglion averages 1-1 mm (range,4.0–11.0 mm) (44). The length that can be exposed can beincreased if the mandibular branch of the trigeminal nerve isretracted or divided, after which the average length that can beexposed increases to 20.1 mm (range, 17.5–21-0 mm) (Figs. 1-7and 1-8) (10, 17). Gaining this added exposure can be particu-larly helpful during surgical procedures that are directedthrough the petrous apex to complete a vascular anastomosis,to occlude the artery for control of bleeding, and to allow formobilization of the vertical and horizontal segments of theartery (40). Avenous plexus of variable size, an extension of thecavernous sinus within the periosteal covering of the distalpart of the canal, surrounds the artery.

The facial nerve in the temporal bone, which often blocksaccess to lesions within and deep to the temporal bone, isdivided into three segments (Figs. 1-4, 1-5, and 1-7). The first,or labyrinthine segment, which is located in the petrous part,extends from the meatal fundus to the geniculate ganglionand is situated between the cochlea anteromedially and thesemicircular canals posterolaterally. The labyrinthine segmentends at the site at which the greater superficial petrosal nerve


RHOTON



arises from the facial nerve at the level of the geniculate gan-glion. From there, the nerve in its canal turns laterally andposteriorly along the medial surface of the tympanic cavity,thus giving the name tympanic segment to that part of thenerve. The tympanic segment runs between the lateral semi-circular canal above and the oval window below. As the nervepasses below the midpoint of the lateral semicircular canal, itturns vertically downward and courses through the petrouspart adjacent to the mastoid part of the temporal bone; thusthe third segment, which ends at the stylomastoid foramen, iscalled the mastoid or vertical segment.

Petroclival Region

These transtemporal operative approaches are often directedto the petroclival region located where the posterior surface of

the petrous temporal bone meets the clival part of the occipital bone along the petroclival fissure. The junction of the two bones forms a line that extends from the jugular foramen to thepetrous apex (Fig. 1-1). From a surgical standpoint, theintradural compartments of the petroclival region are dividedalong this petroclival line into 1) an inferior space related to themedulla and to the structures around the region of the foramenmagnum; 2) a middle space related to the pons and to the struc-



FIGURE 1-10. A–D, preauricular subtemporal -infratemporal fossa approach. A, thescalp flap has been reflected forward. The flapis positioned so that a neck dissection as wellas a frontotemporal craniotomy can be com-

pleted. The scalp flap has been reflected for-ward while protecting the facial nerve and itsbranches. The neck dissection has been com-

plet ed below the parotid gland. The facialnerve branches passing deep to the parotidhave been preserved. B, the dissection hasbeen carried around the parotid gland to

expose the branches of the facial nerve. Theinternal jugular vein and internal carotidartery are exposed below the gland. C, the

parotid gland has been removed to expose thebranches of the facial nerve distal to the sty-lomastoid foramen. D, a segment of themandibular ramus has been removed, leavingthe mandibular condyle in the mandibular

fos sa, to expose the max ill ary artery and pterygoid muscles in the infratemporal fossa.Branches of the third trigeminal division passbetween the lateral and medial pterygoidmuscles. The inferior alveolar nerve descendsto enter the inferior alveolar foramen andcanal. (Continues)



tures in the prepontine and cerebellopontine angle; and 3) asuperior space related to the contents of the interpeduncularcistern, and to the sellar and parasellar regions.

The Inferior Petroclival Space

The inferior petroclival space corresponds to the anteriorsurface of the medulla and adjacent part of the clivus and ante-rior margin of the foramen magnum (4). The neurovascularstructures in this region are those contained in the pre-medullary cistern. The superior limit is the junction of the ponsand medulla. The inferior limit is the rostral margin of the firstcervical nerve root, the site of the junction of the spinal cordand the medulla. The inferior petroclival space includes thelower four cranial nerves, lower part of the cerebellum, thevertebral artery and its branches, and the structures aroundthe occipital condyle.

The Middle Petroclival Space

The middle petroclival space corresponds to the anterolateralsurface of the pons and cerebellum. Its superior limit is at thepontomesencephalic sulcus and the lower limit is at the pon-tomedullary sulcus. The lateral limits are formed by the poste-rior surface of the petrous bone and by the contents of the cere-

bellopontine angle including the trigeminal, abducens, facial,

and vestibulocochlear nerves, the basilar artery, and the AICAand the superior petrosal veins.

The Superior Petroclival Space

The superior petroclival space is located anterior to the midbrain and corresponds to the anterior part of the tentorialincisura. It extends anteriorly and laterally to the sellar andparasellar regions. Its roof is formed by the diencephalic struc-

tures forming the floor of the third ventricle. The posterior limitis formed by the cerebral peduncles and the posterior perfo-rated substance. The inferior limit is situated above the originof the trigeminal nerve at the pontomesencephalic sulcus. Itincludes the intradural segment of the oculomotor andtrochlear nerves, the basilar artery and its branching into theposterior cerebral artery (PCA) and superior cerebellar artery(SCA), and the cavernous carotid and its intracavernous

branches to the dura of the upper clivus. The medial edge of the tentorium divides the superior petroclival space into infra-and supratentorial compartments.

Adjacent Structures

The structures important in accessing the temporal bonefrom posteriorly and laterally have already been reviewed. Thissection reviews the structures located in front of the temporal

bone that are important in reaching lesions that involve the bone or involve both the bone and areas anterior to it. Theyinclude several muscles, like the temporalis and masseter, theinfratemporal fossa, and the parapharyngeal spaces.

The temporalis muscle, along with the deep temporal ves-sels, passes between the gap formed by the zygomatic archand the floor of the temporal fossa (Fig. 1-5). The muscleattaches to the coronoid process of the mandible. The superfi-

cial and the deep temporalis fasciae attach, respectively, to thelateral and medial aspects of the upper border of the zygo-matic arch. Inferiorly, the parotid fascia invests the parotidgland and the masseter muscle and attaches to the lower bor-der of the zygomatic arch. The masseter muscle has two super-imposed layers. A superficial layer which attaches to the zygo-matic process of the maxilla and anterior part of the lower

border of the zygomatic arch and a deep layer which attaches


RHOTON

FIGURE 1-10. (Continued) E, a frontotemporal craniotomy has been completed and the dura of the lateral wall of the cavernous s inus has been ele-vated. In addition, the lateral orbital wall has been removed to expose the

globe, extraocular muscles, and lacrimal gland. F, enlarged view of theregion of the cavernous sinus. The PCA and SCA have been exposed cours-

ing above and below the oculomotor and trochlear nerves, respectively. Theoptic nerve is exposed above the internal carotid artery. An opening has been

made into the lateral wall of the sphenoid sinus between the first and seconddivisions. The maxillary nerve passes forward to join the terminal branchesof the maxillary artery in the pterygopalatine fossa. The maxillary nerve con-tinues forward along the floor of the orbit as the infraorbital nerve. Thesuperior ophthalmic vein descends across the origin of the lateral rectus

muscle and enters the anterior portion of the cavernous sinus. (Continues)



to the medial aspect of the whole zygomatic arch. Inferiorly itinserts onto the angle and ramus of the mandible.

The parotid gland, the parotid duct, and the branches of thefacial nerve are located superficial to the masseter muscle(Figs. 1-5, 1-9, and 1-10). In surgical procedures in which the

mandibular condyle is resected or displaced inferiorly, theparotid gland, along with the branches of the facial nerve,can be dissected from the underlying masseter to avoid exces-sive traction on the facial nerve and to reduce the risk of facialpalsy (33).

Muscles commonly encountered in operative approaches tothe region of the temporal bone include the posterior belly of the digastric muscle and the muscles attached to the styloid

process. The posterior digastric belly originates in the digastricgroove, lateral to the occipital groove in which the occipitalartery courses, and inserts onto the hyoid bone. The musclesattached to the styloid process, the stylohyoid, styloglossus,and stylopharyngeus muscles, extend to the hyoid bone,

tongue, and pharyngeal wall, respectively.Infratemporal Fossa

The infratemporal fossa, a route through which some tem-poral bone lesions can be reached, is a not uncommon site of involvement by lesions that also involve the temporal bone(11). The osseous boundaries of the infratemporal fossa arethe posterolateral maxillary surface anteriorly, the lateral



FIGURE 1-10. (Continued) G–J, preauricu-lar subtemporalinfratemporal fossa approach.

G, the floor of the middle fossa has beenresected back to the level of the tensor tym-

pani muscle and eustachian tube, and the petrous carotid artery. The nerves exiting the jugular foramen and hypoglossal canal passlaterally between the internal carotid arteryand internal jugular vein to reach their endorgans. H, the eustachian tube and tensortympani have been resected and the bone lat-eral to the foramen ovale removed. Thisexposes the full length of the petrous carotid. I,the petrous carotid has been reflected forwardout of the carotid canal to expose the petrousapex medial to the jugular foramen and lateralwall of the clivus. J, the petrous apex and adja-cent part of the clivus medial to the jugular

foramen and cochlea have been removed and

the dura opened to expose the junction of thevertebral and basilar arteries and the origin of the AICA. A., artery; A.I.C.A., anteroinfe-rior cerebellar artery; Alv., alveolar; Bas.,basilar; Brs., branches; Cap., capitis; Car.,carotid; Cav., cavernous; CN, cranial nerve;Ext., external; Front., frontal; Gl., gland;Inf ., inferior; Infraorb., infraorbital; Int.,internal; Jug., jugular; Lac., lacrimal; Lat.,lateral; Long., longus; M., muscle; Max.,maxillary; Med., medial; N., nerve; Ophth.,ophthalmic; P.C.A., posterior cerebral artery;Pet., petrosal, petrous; Pteryg., pterygoid;Pterygopal., pterygopalatine; Rec., rectus;S.C.A., superior cerebellar artery; Sphen.,

sphenoid; Submandib., submandibular;Sup., superior; Temp., temporal; Tens., ten-sor; TM., temporomandibular; Tymp., tym-

pani; V., vein; Vert., vertebral.




RHOTON

pterygoid plate anteromedially, the mandibular ramus later-ally, and the tympanic part of the temporal bone and the sty-loid process posteriorly. The fossa is domed anteriorly by theinfratemporal surface of the greater sphenoid wing, the site of the foramina ovale and spinosum, and posteriorly by thesquamous part of the temporal bone (Figs. 1-8-1-10). The infe-rior, posteromedial, and superolateral aspects are open with-out bony walls.

The structures located in the infratemporal fossa are thepterygoid muscles and venous plexus and the branches of themaxillary artery and mandibular nerve. The lateral pterygoidmuscle crosses the upper part of the infratemporal fossa, orig-inating from the upper and lower heads; the upper head arisesfrom the infratemporal surface of the greater sphenoid wing,and the lower head originates from the lateral pterygoid plate(Figs. 1-8-1-10). Both heads pass posterolaterally and insert onthe neck of the mandibular condylar process and the articulardisc of the temporomandibular joint. The medial pterygoidmuscle crosses the lower part of the infratemporal fossa andarises with superficial and deep heads; the superficial headarises from the lateral aspect of the palatine pyramidal processand the maxillary tuberosity and passes superficial to the lowerhead of the lateral pterygoid; and the deep head originatesfrom the medial surface of the lateral pterygoid plate and thepterygoid fossa between the two pterygoid plates and passesdeep to the lower head of the lateral pterygoid. Both headsdescend backward and laterally to attach to the medial surfaceof the mandibular ramus below the mandibular foramen. Thesphenomandibular ligament, located medial to the mandibularcondylar process, descends from the sphenoid spine to attachto the lingula of the mandibular foramen. The structureslocated or passing between the sphenomandibular ligamentand the mandible are the lateral pterygoid and the auriculotem-poral nerve superiorly, and the inferior alveolar nerve, the

parotid gland, the maxillary artery and its inferior alveolar branch inferiorly.The maxillary artery is divided into three segments:

mandibular, pterygoid, and pterygopalatine (Figs. 1-8-1-10).The mandibular segment arises from the external carotid arterynear the posterior border of the condylar process, passes

between the process and the sphenomandibular ligament,along the inferior border of the lower head of the lateral ptery-goid, and gives rise to the deep auricular, anterior tympanic,middle and accessory meningeal, and the inferior alveolararteries. The middle meningeal ascends medial to the lateralpterygoid to enter the foramen spinosum, the accessorymeningeal arises from the maxillary or middle meningeal toenter the foramen ovale, and the inferior alveolar descends to

enter the mandibular foramen. The pterygoid segment usuallycourses lateral to, but occasionally medial to, the lower head of the lateral pterygoid and gives rise to the deep temporal, ptery-goid, masseteric, and buccal arteries. The pterygopalatine seg-ment courses between the two heads of the lateral pterygoidand enters the pterygopalatine fossa by passing through thepterygomaxillary fissure. Its branching will be described withthe pterygopalatine fossa.

The pterygoid venous plexus is located in the infratemporalfossa and has two parts: a superficial part located between thetemporalis and lateral pterygoid; and a deep part situated

between the lateral and medial pterygoids anteriorly, and between the lateral pterygoid and the parapharyngeal spaceposteriorly. The deep part is more prominent and connects withthe cavernous sinus by emissary veins passing through the

foramina ovale and spinosum, and occasionally through thesphenoidal emissary foramen (foramen of Vesalius). The maindrainage of the pterygoid plexus is through the maxillary veinto the internal jugular vein.

The mandibular nerve enters the infratemporal fossa bypassing through the foramen ovale on the lateral side of theparapharyngeal space, where it gives rise to several smaller

branches, and then divides into a smaller anterior trunk and alarger posterior trunk (Figs. 1-8-1-10). The anterior trunk givesrise to the deep temporal and masseteric nerves, which supplythe temporalis and the masseter, respectively, and the nerve tothe lateral pterygoid. The buccal nerve, which conveys sensoryfibers, passes anterolaterally between the two heads of the lat-eral pterygoid, and descends lateral to the lower head to reachthe buccinator and the buccal mucosa. The posterior trunkgives off the lingual, inferior alveolar, and auriculotemporalnerves, which descend medial to the lateral pterygoid. The lin-gual and inferior alveolar nerves, the former coursing anteriorto the latter, pass between the lateral and medial pterygoids.The auriculotemporal nerve usually splits to encircle the mid-dle meningeal artery and passes posterolaterally between themandibular ramus and the sphenomandibular ligament. Thechorda tympani nerve, which contains the taste fibers from theanterior two-thirds of the tongue and the parasympatheticsecretomotor fibers to the submandibular and sublingual sali-vary glands, enters the infratemporal fossa through thepetrotympanic fissure, descends medial to the auriculotempo-ral and inferior alveolar nerves, and joins the lingual nerve.The otic ganglion is situated immediately below the foramenovale on the medial side of the mandibular nerve. The ganglionreceives the lesser petrosal nerve, which crosses the floor of the middle fossa anterolateral to the greater petrosal nerve toexit through the foramen ovale or the more posteriorly situatedcanaliculus innominatus and conveys parasympathetic secreto-motor fibers to the parotid gland via the auriculotemporalnerve. The medial pterygoid nerve arises from the medialaspect of the mandibular nerve close to the otic ganglion anddescends to supply the medial pterygoid and tensor veli pala-tini. The nervus spinosus, a meningeal branch, also arises nearthe otic ganglion and ascends through the foramen spinosumto innervate the middle fossa dura.

Parapharyngeal Space

The parapharyngeal space is located in the lateral pharyngealwall and is shaped like an inverted pyramid, with its base onthe skull base superiorly and its apex at the hyoid bone inferi-orly. The parapharyngeal space is subdivided into prestyloidand poststyloid compartments by the styloid diaphragm, afibrous sheet that also constitutes the anterior part of the



carotid sheath (Figs. 1-5 and 1-9). The prestyloid part, situatedanteriorly between the fascia covering the opposing surfaces of the medial pterygoid and tensor veli palatini, is a thin fat-filledcompartment separating the structures in the infratemporalfossa from the eustachian tube and the tensor and levator velipalatini muscles in the lateral nasopharyngeal wall. The upperportion of the prestyloid part is situated between two fascialsheets, which are oriented in a sagittal plane. The lateral sheetarises from the medial surface of the medial pterygoid, passesupward, backward, and medial to the mandibular nerve andthe middle meningeal artery, incorporating the spheno-mandibular ligament posteriorly, and reaching the retro-mandibular deep lobe of the parotid gland. The medial sheet isformed by the fascia overlying the lateral surface of the tensorveli palatini and is continuous inferiorly with the fascia overthe superior pharyngeal constrictor and posteriorly with thethick styloid diaphragm, which envelopes the stylopharyngeus,styloglossus, and stylohyoid and blends into the carotid sheath.The superior border is located where the two fascial sheets fusetogether and insert in the skull base along a line extending

backward from the pterygoid process lateral to the origin of thetensor veli palatini, medial to the foramina ovale and spin-osum to the sphenoid spine and the posterior margin of the gle-noid fossa. The sharply angled inferior boundary is situated atthe junction of the posterior digastric belly and the greaterhyoid cornu. The poststyloid part, which contains the internalcarotid artery, internal jugular vein, and the initial extracranialsegment of cranial nerves IX through XII, is separated from theinfratemporal fossa by the posterolateral portion of the presty-loid part. The glossopharyngeal nerve exits the skull throughthe intrajugular part of the jugular foramen, anterior to thevagus and accessory nerves, and passes forward, medial to thestyloid process in close relationship to the lateral surface of thecarotid artery as the artery enters the carotid canal (Fig. 1-9).

Care is required to avoid injury to the glossopharyngeal nerveif the artery is to be mobilized at the carotid canal. The vagusnerve leaves the skull through the anteromedial edge of theintrajugular part of the foramen and courses deep within thecarotid sheath, between the internal carotid artery and the

jugular vein. The accessory nerve exits the intrajugular partand runs backward, lateral to the jugular vein and medial tothe styloid process and the posterior belly of the digastric mus-cle, to innervate the sternocleidomastoid muscle.

The hypoglossal nerve exits through the hypoglossal canal,deep to the jugular vein and to the nerves emerging from the

jugular foramen, and runs downward, between the carotidartery and the jugular vein (Figs. 1-9 and 1-10). It becomessuperficial at the level of the angle of the jaw where it crosses

the internal and external carotid arteries, close to the level of the common carotid bifurcation, to innervate the tongue.

Pterygopalatine Fossa

The pterygopalatine fossa, which opens laterally into themedial part of the infratemporal fossa, is bounded posteriorly

by the sphenoid pterygoid process, medially by the palatineperpendicular plate, that bridges the interval between the max-

illa and pterygoid process, and opens superiorly through themedial part of the inferior orbital fissure into the orbital apex(Figs. 1-5, 1-9, and 1-10) (11). The fossa contains the maxillarynerve, pterygopalatine ganglion, maxillary artery, and their

branches, all embedded in fat tissue. Its lateral boundary, thepterygomaxillary fissure, opens into the infratemporal fossaand allows passage of the maxillary artery from the infratem-poral into the pterygopalatine fossa, where the artery gives riseto its terminal branches. The lower part of the fossa is funnel-shaped, with its inferior apex opening into the greater andlesser palatine canals, which transmit the greater and lesserpalatine nerves and vessels, and communicate with the oralcavity. The sphenopalatine foramen, located in the upper partof the fossa’s medial wall, conveys the sphenopalatine nerveand vessels, and opens into the superior nasal meatus justabove the root of the middle nasal concha. The foramen rotun-dum opens just below the superior orbital fissure through thesuperior part of the posterior wall of the fossa. The pterygoidcanal opens through the sphenoid pterygoid process inferome-dial to the foramen rotundum and conveys the vidian nervecarrying autonomic fibers to the pterygopalatine ganglion. Themaxillary nerve, after entering the fossa, gives off ganglionic

branches to the pterygopalatine ganglion. It then deviates lat-erally just beneath the inferior orbital fissure, giving rise to, inorder, the zygomatic and posterosuperior alveolar nerves out-side of the periorbita. It then turns medially as the infraorbitalnerve, passing through the inferior orbital fissure to enter theinfraorbital groove, where the anterior and middle superioralveolar nerves arise. Finally, it exits the infraorbital foramen toterminate on the cheek. The pterygopalatine ganglion, locatedin front of the pterygoid canal and inferomedial to the maxil-lary nerve, receives communicating rami from the maxillarynerve and gives rise to the greater and lesser palatine nervesfrom the lower surface of the ganglion, the sphenopalatine

nerve and pharyngeal branch from the medial surface, and theorbital branch from the superior surface. The vidian nerve isformed by the union of the greater petrosal nerve, which con-veys parasympathetic fibers arising from the facial nerve at thelevel of the geniculate ganglion, and the deep petrosal nerve,which conveys sympathetic fibers from the carotid plexus, toreach the lacrimal gland and nasal mucosa. The parasympa-thetic fibers synapse in the pterygopalatine ganglion, whereasthe sympathetic fibers do not. The sympathetic fibers synapsein the superior cervical sympathetic ganglion.

The third or pterygopalatine segment of the maxillaryartery enters the pterygopalatine fossa by passing throughthe pterygomaxillary fissure. This segment courses in ananterior, medial, and superior direction and gives rise to the

infraorbital artery, which passes through the inferior orbitalfissure and courses with the infraorbital nerve; the posterosu-perior alveolar artery, which descends to pierce the postero-lateral wall of the maxilla; the recurrent meningeal branches,which pass through the foramen rotundum; and the greaterand lesser palatine arteries, which descend through thegreater and lesser palatine canals; the vidian artery to thepterygoid canal; the pharyngeal branch to the palatovaginal





canal; and finally the sphenopalatine artery, which passesthrough the sphenopalatine foramen to reach the nasal cav-ity and is considered to be the terminal branch of the maxil-lary artery because of its large diameter. The arterial struc-tures in the pterygopalatine fossa are located anterior to theneural structures.

Arterial RelationshipsThe arteries that may be involved in pathological abnormal-

ities involving the temporal bone include the upper cervicaland petrous portions of the internal carotid artery, the posteri-orly directed branches of the external carotid artery, and theupper portion of the vertebral artery.

Common Carotid Artery

The common carotid artery bifurcates into the internal andexternal carotid arteries at the level of the upper border of thethyroid cartilage. The internal carotid artery initially ascendsrelatively superficial in the carotid triangle of the neck, butassumes a much deeper position after passing medial to theposterior belly of the digastric (Figs. 1-9 and 1-10). Below thedigastric, it is crossed by the hypoglossal nerve and the ansacervicalis, and by the lingual and facial veins. Medial to thedigastric, it is crossed by the stylohyoid muscle and the occip-ital and posterior auricular arteries. Superior to the digastric,the internal carotid artery is separated from the external carotidartery by the styloid process and the muscles attached to it. Atthe entrance into the carotid canal, the artery is involved by adense sheath of connective tissue and is separated from theinternal jugular vein by the hypoglossal nerve and by thenerves exiting from the jugular foramen.

The internal carotid artery passes, almost straightly upward,posterior to the external carotid artery and anteromedial to theinternal jugular vein to reach the carotid canal. At the level of

the skull base, the internal jugular vein courses just posterior tothe internal carotid artery, being separated from it by thecarotid ridge. Between them, the glossopharyngeal nerve islocated laterally and the vagus, accessory, and hypoglossalnerves medially.

After the internal carotid artery enters the carotid canal withthe carotid sympathetic nerves and surrounding venousplexus, it ascends a short distance (the vertical segment), reach-ing the area below and slightly behind the cochlea, where itturns anteromedially at a right angle (the site of the lateral

bend) and courses horizontally (the horizontal segment)toward the petrous apex (Figs. 1-8-1-10). At the medial edge of the foramen lacerum, it turns sharply upward at the site of themedial bend to enter the posterior part of the cavernous sinus.

The petrolingual ligament, which extends from the lingualprocess of the sphenoid bone to the petrous apex, crosses abovethe junction of the petrous and cavernous carotid.

External Carotid Artery

The external carotid artery ascends anterior to the internalcarotid artery on the posteromedial margin of the parotid glandand medial to the digastric and stylohyoid muscles. Proximal

to its terminal bifurcation into the maxillary and the superficialtemporal arteries, it gives rise to six branches that can bedivided into anterior and posterior groups according to theirdirections. The latter group is related to the region of the tem-poral bone.

The ascending pharyngeal artery, the first branch of the pos-terior group, often provides the most prominent supply to themeninges around the jugular foramen (18). It arises either at the

bifurcation or from the lowest part of the external or internalcarotid arteries. Rarely, it arises from the origin of the occipitalartery. It courses upward between the internal and the externalcarotid arteries, giving rise to numerous branches to neighbor-ing muscles, nerves, and lymph nodes. Its meningeal branchespass through the foramen lacerum to be distributed to the duralining the middle fossa and through the jugular foramen or thehypoglossal canal to supply the surrounding dura of the pos-terior cranial fossa. The ascending pharyngeal artery also givesrise to the inferior tympanic artery, which reaches the tympaniccavity by way of the tympanic canaliculus along with the tym-panic branch of the glossopharyngeal nerve.

The occipital artery, the second and largest branch of theposterior group, arises from the posterior surface of the exter-nal carotid artery and courses obliquely upward between theposterior belly of the digastric muscle and the internal jugularvein, and then medial to the mastoid process and either super-ficial or deep to the longissimus capitis muscle (Fig. 1-5). Itcourses deep to the latter muscle if it courses in the occipitalgroove of the mastoid bone, which is located medial to thedigastric groove. After passing the longissimus capitis muscle,the occipital artery courses deep to the splenius capitis muscle,finally reaching a subcutaneous location by piercing the fascia

between the attachment of the sternocleidomastoid and thetrapezius muscles to the superior nuchal line. The occipitalartery gives rise to several muscular and meningeal branches,

anastomoses with other branches of the external carotid includ-ing the ascending pharyngeal and superficial temporal andalso with branches of the vertebral artery. Its meningeal

branches, which enter the posterior fossa through the jugularforamen or the condylar canal, may make a significant contri-

bution to tumors of the jugular foramen.The posterior auricular artery, the last branch in the posterior

group, arises above the posterior belly of the digastric muscleand travels between the parotid gland and the styloid process.At the anterior margin of the mastoid process, it divides intoauricular and occipital branches, which are distributed to thepostauricular and the occipital regions, respectively. The stylo-mastoid branch, which arises below the stylomastoid foramen,enters the stylomastoid foramen to supply the facial nerve. Its

loss can lead to a facial palsy, even though it anastomoses withthe petrosal branch of the middle meningeal artery. The poste-rior auricular branch may share a common trunk with theoccipital artery, or sometimes it is absent, in which case, theoccipital artery gives rise to the stylomastoid artery. Membersof the anterior group, whose origins may be visualized inexposing lesions in the region, include the superior thyroid,lingual, and facial arteries.


RHOTON



The superficial temporal artery arises from the externalcarotid artery in the substance of the parotid gland behind theneck of the mandible where it is crossed by the temporal andzygomatic branches of the facial nerve (Fig. 1-5). It ascendsover the posterior root of the zygoma and divides into anteriorand posterior branches that run with the superficial temporalvein and the auriculotemporal nerve over the superficial tem-poralis fascia.

Vertebral Artery

The vertebral artery, above the transverse foramen of theaxis, veers laterallt to reach the transverse foramen of the atlas,which is situated further lateral than the transverse foramen of the axis. The artery, after ascending through the transverseprocess of the atlas, is located on the medial side of the rectuscapitis lateralis muscle. From here, it turns medially behindthe lateral mass of the atlas and the atlanto-occipital joint andis pressed into the groove on the upper surface of the posteriorarch of the atlas. The first cervical nerve courses on the lowersurface of the artery between the artery and the posterior arch

of the atlas. After passing medially above the lateral part of theposterior arch of the atlas, the artery enters the vertebral canal by passing below the lower, arched border of the posterioratlanto-occipital membrane, which transforms the sulcus inwhich the artery courses on the upper edge of the posteriorarch of the atlas into an osseofibrous casing that may ossify,transforming it into a complete or incomplete bony canal sur-rounding the artery.

Opening the dura exposes the intradural segment of the ver-tebral artery. As the artery pierces the dura, it is encased in afibrous tunnel that binds the posterior spinal artery, dentateligament, first cervical nerve, and the spinal accessory nerve tothe vertebral artery. Care should be taken to preserve the pos-terior spinal artery during the dural opening and mobilization

of the vertebral artery because it may be incorporated into thedural cuff around the vertebral artery. The intradural segmentof the vertebral artery, after emerging from the fibrous duraltunnel, ascends in front of the rootlets of the hypoglossal nerveto reach the front of the medulla. Oblongata where it unitesnear the junction of pons and medulla with its mate to form the

basilar artery. Before reaching the lower border of pons, thevertebral artery gives off the PICA, which courses backwardaround the lateral surface of the medulla and between therootlets of glossopharyngeal, vagus, and accessory nerves.

Venous Relationships

The venous drainage of the structures of the skull base is

through the internal jugular veins, the sinuses in the duramater, and a series of emissary veins communicating the intra-and extracranial compartments (25). The superior petrosalsinus sits on the petrous ridge and connects the cavernousand transverse sinuses. It receives tributaries from the inferiorsurface of the temporal lobe and from the petrosal veins thatdrain the cerebellum and brainstem. The inferior petrosal sinuscourses along the petro-occipital fissure and drains the clival

area. It consists of one or more channels that, at its lower end,course rostral or caudal to or between the nerves passingthrough the jugular foramen. It enters the medial wall of the

jugular bulb just anterior to where the cranial nerves descendin the anteromedial wall of the jugular bulb (18). It joins thecavernous sinus at its upper margin. The transverse sinus

begins at the level of the internal occipital protuberance andpasses laterally and forward to the posterolateral part of thetemporal bone where it joins the superior petrosal sinus andcontinues as the sigmoid sinus. It receives drainage from thetentorial surface of the cerebellum through the tentorialsinuses and from the temporal lobe through the vein of Labbé.The basilar venous plexus consists of multiple interconnectingchannels situated between the layers of dura mater on theclivus. It forms the largest communication between the pairedcavernous sinus and communicates through the inferior pet-rosal sinuses with the sinuses in the region of the foramenmagnum (10).

SURGICAL APPROACHES

The suboccipital retrosigmoid and far lateral approaches tointradural pathologies arising in the region of the cerebello-pontine angle, lower clivus, and foramen magnum, arereviewed later in this volume. The approaches reviewed hereare those directed through the temporal bone.

Middle Fossa Approach

The middle fossa approach to the internal acoustic meatusis usually selected for small tumors that are located predom-inantly within the internal acoustic meatus in which there isan opportunity to preserve hearing. With this approach, themeatus is approached from above, through a temporal cran-iotomy located above the ear and zygoma (Figs. 1-7 and 1-11)(2). The dura under the temporal lobe is elevated from thefloor of the middle cranial fossa until the arcuate eminenceand the greater petrosal nerve are identified. The distancefrom the inner table of the skull to the facial hiatus, throughwhich the greater petrosal nerve passes, ranges from 1.3 to 2.3cm (average, 1.7 cm) (42). When separating the dura from thefloor of the middle fossa, one should remember that bonemay be absent over all or part of the geniculate ganglion. Inour previous study of 100 temporal bones, all or part of thegeniculate ganglion and the genu of the facial nerve werefound to be exposed in the floor of the middle fossa in 15

bones (15%) (31). In 15 other specimens, the geniculate gan-glion was completely covered, but no bone extended over the

greater petrosal nerve. The greatest length of greater petrosalnerve covered by bone was 6.0 mm. More than 50% of thespecimens had less than 2.5 mm of greater petrosal nerve cov-ered. It also is important to remember that the petrous seg-ment of the carotid artery may be exposed without a coveringof bone in the floor of the middle fossa deep to the greaterpetrosal nerve (17) In a previous study, we found that a 7-mmlength of petrous carotid artery may be exposed without a






RHOTON

FIGURE 1-11. Middle fossa approach to the internal acoustic meatus. A, thevertical line shows the site of the scalp incision and the stippled area outlines

the bone flap bordering the middle fossa floor. B, the dura has been elevatedto expose the middle meningeal artery, the greater petrosal nerve, and the

arcuate eminence. C, bone has been removed to expose the junction of the greater petrosal nerve and the geniculate ganglion. A portion of the upperwall of the internal meatus has been removed. The upper surface of the arcu-

ate eminence has been drilled to expose the superior semicircular canal. In themiddle fossa approach, for an acoustic neuroma, the cochlea and semicircular

canal are not opened, as seen in this dissection illustrating some of the impor-tant structures that are to be avoided in opening the meatus. D, enlargedview. The cochlea, located below the middle fossa floor in the angle between

the facial and greater petrosal nerves, has been opened in the area anterome-dial to the meatal fundus. The roof of the meatus has been opened to expose

the superior vestibular nerve, which innervates the ampullae of the superior

and lateral canals and the meatal segment of the facial nerve. E, the vestibule

and semicircular canals are located posterolateral and the cochlea is locatedanteromedial to the meatal fundus. The tensor tympani is layered along the

anterior edge and the greater petrosal nerve above the petrous carotid. F,enlarged view. The vertical crest (Bill’s bar) separates the facial and superiorvestibular nerves at the meatal fundus. The superior and inferior vestibular

nerves are located posteriorly and the facial and cochlear nerves anteriorly in

the meatus, with the cochlear nerve passing below the facial nerve to enter themodiolus. The labyrinthine segment of the facial nerve courses superolateralto the cochlea. A., artery; Ac., acoustic; Arc., arcuate; Car., carotid; CN, cra-nial nerve; Coch., cochlear; Emin., eminence; Gang., ganglion; Genic.,

geniculate; Gr., greater; Inf ., inferior; Int., internal; Laby., labyrinthine;

M., muscle; Meat., meatal; Men., meningeal; Mid., middle; N., nerve; Pet.,

petrosal, petrous; Post., posterior; Seg., segment; Sup., superior; Tens., ten-sor; Tymp., tympani; Vert., vertebral; Vest., vestibular.



bony covering in the area below where the greater petrosalnerve passes below the lateral margin of the trigeminal gan-glion to reach the vidian canal at the anterior margin of theanterior margin of the foramen lacerum (30, 31). The foramenspinosum and middle meningeal artery and the foramenovale and third trigeminal division are situated at the anteriormargin of the extradural exposure. The extradural exposurecan usually be completed without obliterating the middlemeningeal artery at the foramen spinosum.

Two different methods are used for exposing the internalacoustic meatus. One, the older method, is to remove boneover the greater petrosal nerve and to follow it to the genic-ulate ganglion and the genu of the facial nerve. From here,the labyrinthine portion of the facial nerve is followed tothe lateral end of the internal auditory canal, after whichthe canal is unroofed. The other or preferred method is

begun by drilling at the petrous ridge above the fundus of the meatus in the area medial to the arcuate eminence. Theangle between the long axis of the superior semicircularcanal or the greater petrosal nerve and the long axis of theinternal acoustic meatus is helpful in selecting the site fordrilling. The long axis of the central part of the internalacoustic meatus is located an average of 61 degrees behindthe long axis of the greater petrosal nerve and an average of 37 degrees medial to the long axis of the arcuate eminenceand superior semicircular canal. The drilling is directedanterolateral from the meatal porus to the meatal funduswhere the vertical crest is identified.

The lateral part of the bone removal near the meatal fundusis limited posteriorly by the superior semicircular canal andvestibule, which are located a few millimeters behind and ori-ented parallel to the labyrinthine segment of the facial nerve(Figs. 1-7 and 1-11). The anteromedial edge of the exposure islimited by the cochlea, which sits only a few millimeters ante-

rior to the site of bone removal, in the angle between thelabyrinthine portion of the facial nerve and the greater petrosalnerve. The cochlea and the semicircular canals should beavoided in this approach if hearing is to be preserved. The ver-tical crest, which is identified at the upper edge of the meatalfundus, provides a valuable landmark for identifying the facialnerve. In the final stage of bone removal, the upper wall of theinternal auditory canal is removed to expose the dura lining theentire superior surface of the internal auditory canal from thevertical crest to the porus. The dura is opened to expose thepathology.

The extended middle fossa approach used for the removal of larger acoustic neuromas includes wider opening of the poste-rior part of the petrous pyramid (21, 28, 42, 43). This approach

combines different degrees of resection of the bony labyrinthwith the subtemporal transtentorial routes (Fig. 1-12).Extending the resection of the petrous bone posteriorly over themastoid and the bony labyrinth exposes the whole intrapetrouscourse of the facial nerve, and provides access to the cerebello-pontine angle by a combination of subtemporal, trans-labyrinthine, and presigmoid routes, all directed through theposterior part of the floor of the middle fossa.

Subtemporal Anterior Transpetrosal Approach

This approach is made through a temporal or orbitozygo-matic craniotomy that extends down to the floor of the middlefossa (Figs. 1-12 and 1-13) (19). The dura is carefully elevatedfrom the floor of the middle fossa to expose the middlemeningeal artery, which may be obliterated and divided at the

foramen spinosum. Further elevation of the dura toward thepetrous ridge will expose the arcuate eminence and greaterpetrosal nerve posteriorly. The cochlea, which is to be pre-served, and the anterior wall of the internal auditory canal con-stitute the lateral limit of the exposure through the petrousapex. A portion of the bone layer above the superior wall of theinternal auditory canal, which averages 5 mm (range, 3–7 mm)in thickness, can be removed with a drill to improve the expo-sure (44). The petrous carotid forms the anterior limit of theexposure. The limit above the medial part of the bone resectionis the trigeminal nerve in Meckel’s cave. Drilling is directed

behind the petrous carotid, through the petrous apex medial tothe cochlea and under the trigeminal nerve. The petrous apexis removed and the bone removal is extended to the lateral

side of the clivus, exposing the inferior petrosal sinus at the lat-eral edge of the clivus. Care is required to prevent damage tothe abducens nerve as it passes through Dorello’s canal locatedat the upper edge of the petroclival fissure. The width of the

bone resection from the trigeminal impression to the posteriorwall of the internal auditory canal averages 13 mm (range,9–14 mm) (44). The depth of the exposure, from the trigeminalganglion to the petroclival fissure, averages 13 mm (range,9–17 mm). The cochlea lies below the floor of the middle fossanear the apex of the angle formed by the greater petrosal nerveanteriorly and the internal acoustic meatus posteriorly. Thecochlea is to be avoided if hearing is to be preserved.

After the bone removal is completed, the superior petrosalsinus is obliterated and divided in the area just lateral to the

trigeminal nerve, and the dural incision is extended across thetentorium. The dural leaflets of the tentorium are retracted withsutures and the dural incision is carried downward below thesuperior petrosal sinus to the lower margin of the openingthrough the petrous apex. The approach is then directed

between the lower margin of the trigeminal nerve above, andthe internal acoustic meatus inferiorly and laterally (20).

The exposure is small, as described above, and may requiresignificant temporal lobe retraction, especially if the goal is toreach the lower aspect of the brainstem. To reach the anterioraspect of the pons, the view must be directed from lateral tomedial above the internal auditory canal. The angles of viewthrough the area of the petrousectomy can be increased if thecranium is approached at a higher level through a frontotempo-ral craniotomy combined with zygomatic arch resection.

Translabyrinthine Approach

In the translabyrinthine approach, the internal acoustic mea-tus and cerebellopontine angle are approached through a mas-toidectomy and labyrinthectomy (Fig. 1-6) (16, 29, 38) Thereare two goals of bone removal in this approach. The first is to





nal auditory meatus. The vestibular aqueduct and theendolymphatic sac may be opened and removed during the

bone removal between the meatus and the jugular bulb. Thecochlear canaliculus will be seen deep to the vestibular aque-

duct as bone is removed in the area between the meatus andthe jugular bulb. The lower end of the cochlear canaliculus issituated just above the area where the glossopharyngeal nerveenters the medial half of the jugular foramen. The labyrinthec-tomy portion of the procedure involves removing the semicir-cular canals and the vestibule to expose the dura lining theinternal auditory canal. The lateral and posterior semicircularcanals are drilled away. As the bone removal proceeds medi-

ally, the ampullae of the lateral and superior semicircularcanals are exposed. At this point some bleeding can occur asthe subarcuate artery is encountered in the bone near the cen-ter of the superior semicircular canal. The vestibule is an oval-

shaped cavity located immediately lateral to the internalacoustic meatus, which forms the communication between thesemicircular canals and the cochlea. Bone is removed medialand posterior to the vestibule, completely exposing it anteriorand inferior to the facial nerve. Care is required to avoid injuryto the facial nerve as it courses below the lateral canal and theampullae of the posterior canal and around the superolateralmargin of the vestibule.



FIGURE 1-12. (Continued) E–H, anterior petrosectomy and extended mid-dle fossa approach. E, additional bone has been removed around the internalacoustic meatus and the dura opened to expose the facial and vestibulo-cochlear nerves. F, the exposure has been extended lateral to the internal

acoustic meatus. The tegmen has been opened to expose the head of the incusin the epitympanic area. The osseous capsule of the labyrinth has been openedto expose the semicircular canals. The presigmoid dura behind the labyrinthhas been exposed and opened. G, a translabyrinthine approach directedthrough the middle fossa has been completed by removing the semicircularcanals and vestibule. The dura has been opened to give an exposure throughthe middle fossa similar to that seen with the presigmoid approach. Thelabyrinthine, tympanic, and mastoid segments of the facial nerve have been

exposed. H, this extended middle fossa exposure extends from the lateral wallof the cavernous sinus, across the trigeminal nerve to the area lateral to theinternal acoustic meatus, and provides wide access to the anterior part of the

posterior fossa. A., artery; Ac., acoustic; A.I.C.A., anteroinferior cerebellar

artery; Bas., basilar; Car., carotid; Cav., cavernous; Chor., chorda; CN, cra-nial nerve; Ext., external; Gang., ganglion; Gen., geniculate; Genic., genic-ulate; Inf ., inferior; Int., internal; Laby., abyrinthine; Lat., lateral; M., mus-cle; Mast., mastoid; Men., meningeal; Mid., middle; N., nerve; P.C.A.,

posterior cerebral artery; Pet., petrosal, petrous; P.I.C.A., posteroinferiorcerebellar artery; Post., posterior; S.C.A., superior cerebellar artery; Seg.,segment; Sup., superior; Tens., tensor; Tymp., tympani; Tent., tentorial;Trig., trigeminal; Tymp., tympani, tympanic.




RHOTON

FIGURE 1-13. A–F, subtemporal exposure of the right middle, infratem-

poral, and posterior fossae. A, the insert shows the side of the scalp incision. A frontotemporal craniotomy has been completed and the dura has been ele-vated from the middle fossa floor and lateralwall of the cavernous sinus. B,

enlarged view. The bony roof over the geniculate ganglion and internalmeatus has been removed and the dura lining the meatus opened to expose

the facial and superior vestibular nerves. C, additional middle fossa floorhas been removed to expose the petrous carotid, the cochlea in the anglebetween the greater petrosal nerve and pregeniculate part of the facial nerve,

the semicircular canals and tympanic cavity. The tensor tympani muscleand eustachian tube are exposed in front of the petrous carotid artery. D, the

bone between the superior and posterior canals has been removed to expose

the vestibule with which both ends of the semicircular canals communicate.The vestibule contains the utricle and saccule and communicates below the

fundus of the meatus with the cochlea. The meatal segment of the facia lnerve courses in the internal acoustic meatus, the labyrinthine segment

between the semicircular canals and the cochlea, the tympanic segmentbetween the anterior margin of the lateral canal and the oval window on themedial side of the tympanic cavity, and the mastoid segment descends to

exit the stylomastoid foramen. E, the petrous apex, medial to the cochlea andextending under the trigeminal nerve, has been removed to expose the lat-

eral edge of the clivus and the posterior fossa dura. F, the medial tentorialedge has been divided behind the petrous ridge to expose the oculomotor,trochlear, and trigeminal nerves and the basilar artery. (Continues)





FIGURE 1-13. (Continued) G–L, subtemporal exposure of the right middle,infratemporal, and posterior fossae. G, the dural opening has been extendeddownward to expose the lateral edge of the clivus and the inferior petrosalsinus coursing along the petroclival fissure. The abducens nerve and the

AICA are in the lower margin of the exposure. H, an osteotomy of the zygo-matic arch and the floor of the middle fossa surrounding the mandibular

fossa has been completed to aid in exposing the infratemporal fossa. I, themandibular fossa and floor of the middle fossa, extending medially to the levelof the foramen ovale, have been removed. Branches of the mandibular nerveand maxillary artery are exposed in the infratemporal fossa. The greater pet-rosal nerve joins the deep petrosal nerve from the carotid sympathetic plexusto form the vidian nerve, which passes forward in the vidian canal to reachthe pterygopalatine fossa. J, the upper portion of the cervical carotid isexposed medial to the jugular foramen. The petrous carotid crosses behind theeustachian tube and tensor tympani. K, the eustachian tube and tensor tym-

pani have been resected, the petrous carotid reflected forward out of the carotidcanal, the petrous apex removed, and the posterior fossa dura opened to exposethe vertebral artery and the AICA. L, enlarged view. The right vertebralartery has been displaced forward to expose the left vertebral artery. The

AICA passes toward the nerves entering the internal acoustic meatus. A.,artery; A.I.C.A., anteroinferior cerebellar artery; Alv., alveolar; Ant., ante-

rior; Bas., basilar; Car., carotid; Chor., chorda, choroidal; CN, cranial nerve;Comm., communicating; Eust., eustachian; Gang., ganglion; Gen., genic-ulate; Genic., geniculate; Gr., greater; Inf ., inferior; Int., internal; Jug.,

jugular; Laby., labyrinthine; Lat., lateral; M., muscle; Mandib., mandibu-lar; Mast., mastoid; Max., maxillary; Meat., meatal; Men., meningeal; Mid.,middle; N., nerve; P.C.A., posterior cerebral artery; Pet., petrosal, petrous;Post., posterior; S.C.A., superior cerebellar artery; Seg., segment; Sup.,superior; Temp., temporal; Tens., tensor; Trig., trigeminal; Tymp., tym-

pani, tympanic; V., vein; Vert., vertebral; Vest., vestibular.



The internal auditory canal is located medial and anterior tothe tympanic segment of the facial nerve. The dura lining theinternal canal is exposed by drilling away the semicircularcanals and vestibule and the bone around the superior, poste-rior, and inferior margins of the internal canal. Further boneremoval at the lateral end of the meatus exposes the transverseand vertical crests (Fig. 1-2). The intrameatal portion of thefacial nerve is separated from the superior vestibular nerve atthe lateral end of the canal by the vertical crest, also calledBill’s bar, that can be used to positively identify the facial nerve(13, 16). The initial part of labyrinthine segment of the facialnerve, which lies just in front of the vertical crest, is exposed atthe meatal fundus. After identifying the facial nerve, the duralining the meatus is opened. The dural incision in Trautman’striangle is V-shaped with the apex of the “V” extending to theincision along the meatal dura. One limb of the “V” extends

below the superior petrosal sinus and the other limb extendsabove the jugular bulb. The dural flap is then reflected posteri-orly to expose the structures in the meatus and the cerebello-pontine angle. The subarcuate artery, or the AICA, may be

encountered in the dura of Trautman’s triangle. Usually, thesubarcuate artery arises from the AICA and passes through thedura on the upper posterior wall of the meatus as a fine stem.Occasionally, however, the subarcuate artery, along with its ori-gin from the AICA, may be incorporated into the dura on theposterior face of the temporal bone. The approach may includetransection of the external canal and obliteration of the middleear with packing of the eustachian tube at closure.

Transcochlear Approach

The transcochlear approach is primarily an anteromedialextension of the translabyrinthine approach (Fig. 1-6) (3, 15,16). It usually includes division and closure of the externalcanal, resection of at least the posterior part of the osseous

external canal, and the tympanic membrane and ossicles, andobliteration of the eustachian tube. After exposing the dura lin-ing the internal auditory canal, as described for thetranslabyrinthine approach, the incus is removed and the facialnerve is exposed from the geniculate ganglion to the stylomas-toid foramen. The greater superficial petrosal nerve is tran-sected and the facial nerve is transposed posteriorly. In thefinal stage, the bone removal is carried through the facial canal,after nerve transposition, and the cochlea and adjacent part of the petrous apex are drilled away (Fig. 1-6).

Medially, the bone removal extends to the edge of the clivus,exposing the inferior petrosal sinus from the jugular bulb

below to the superior petrosal sinus above. The ascending por-

tion of the petrous carotid is exposed at the anterior limit of thedissection. The bone removal, which now extends to the lateraledge of the clivus, could easily be carried medially into theclivus. Extending the dural opening in this area permits visu-alization of the abducent nerve medial to the internal acousticmeatus, the lower margin of the trigeminal nerve, the nervesentering the jugular foramen, a segment of the basilar artery,and the origin and initial segment of the AICA.

An alternative to transposing the facial nerve is to completean extensive bone removal in the hypotympanic and retrofacialareas extending forward to the carotid canal, thus skeletonizingthe mastoid segment of the facial nerve and leaving it sus-pended in a shell of bone, as described by Gantz and Fisch (7).In this approach, the external auditory canal is closed as a blindsac and the tympanic membrane, incus, and body of themalleus are removed (7). A mastoidectomy is performed,including the removal of the retrofacial, retrolabyrinthine, andsupralabyrinthine compartments. The facial nerve is identifiedat its tympanic segment and at the stylomastoid foramen. Theinferior part of the tympanic bone is removed to expose theinfralabyrinthine compartment, the jugular bulb, and theintrapetrous carotid artery. The retrofacial dissection is carriedmedially and superiorly, removing the semicircular canals andvestibule. The dissection of the posterior fossa dura is carriedinferiorly around the internal auditory canal and under thefacial canal. The cochlea is drilled away by working inferiorand anterior to the facial canal. The facial canal is then left as a

bridge over the operative fie ld and the dura is exposed

between the carotid artery and the jugular bulb.

Combined Supra- and InfratentorialPresigmoid Approach

The presigmoid approach combines the supra- and infraten-torial craniotomy centered on the mastoid and varying degreesof mastoid and labyrinthine resection (Fig. 1-14). The minimaldegree of mastoid resection, which we refer to as a minimalmastoidectomy, exposes only enough of the presigmoid dura toopen the dura in front of the sigmoid sinus for exposure of thecerebellopontine angle (Figs. 1-15 and 1-16). The next moreextensive degree of mastoid resection, the retrolabyrinthinemodification, is a more complete mastoidectomy exposing the

bony capsule of the semicircular canals and skeletonizing at

least a portion of the facial nerve. In the partial labyrinthec-tomy, one or two of the semicircular canals, commonly thesuperior and/or posterior canals, are resected with preserva-tion of the lateral canal. Removal of these canals may, but notalways, be associated with the loss of hearing (37). The poste-rior canal may be removed to increase access to the posteriorfossa, and removing the superior canal alone gives a moredirect access to the petrous apex along the middle fossa. Thenext more extensive modification is the translabyrinthineapproach, in which the semicircular canals and vestibule areresected uniformly, resulting in the loss of hearing. Thetranslabyrinthine approach provides excellent access to theinternal auditory canal. The next more extensive modification

is the transcochlear approach, in which the cochlea locatedanteromedial to the fundus of the meatus is removed, thus pro-viding access to the medial part of the petrous apex and theside of the clivus. Another modification, which we call theextended translabyrinthine approach, and is similar to thetranscochlear approach, involves drilling bone both anteriorand posterior to the facial nerve, leaving the facial nerve skele-tonized in a column of bone and working both anterior and


RHOTON



posterior to the facial nerve to remove the cochlea and accessthe side of the clivus. Gaining access for drilling the cochleaanterior to the facial nerve commonly requires that at least partof the posterior part of the external canal be removed, that thetympanic cavity be obliterated, and that the internal carotidartery be exposed below the promontory.

In evaluating these approaches in our laboratory, we havefound that the minimal mastoidectomy gives approximately thesame exposure as the retrolabyrinthine approach, but is done atreduced risk since the semicircular canals and facial nerve arenot skeletonized (Figs. 1-14 and 1-15). Removing the posteriorcanal increases access to the posterior fossa, but access is onlyslightly increased over that achieved with the retrolabyrinthineapproach. Removing the superior canal increases access to the

middle fossa and petrous apex and reduces the needed retrac-tion of the temporal lobe. The translabyrinthine approach doesnot significantly increase the access to the area medial to theporus of the internal acoustic meatus over that achieved withthe minimal mastoidectomy or retrolabyrinthine approach, butdoes provide access to the internal auditory canal. The

transcochlear modification, in which bone is removed up to theedge of the clivus, does significantly increase access to the frontof the brainstem and clivus over that achieved with the lesserdegrees of bony resection. The retrosigmoid, the presigmoidminimal mastoidectomy, and the retrolabyrinthine approacheswere compared and yielded nearly the same exposure of thecerebellopontine angle, but the retrosigmoid approach did notprovide the additional exposure of the middle fossa and petrous



FIGURE 1-14. A–D, presigmoid approach.A, the insert shows the temporo-occipital cran-iotomy and the mastoid exposure. The mas-toidectomy has been completed and the dense

cortical bone around the labyrinth has beenexposed. The tympanic segment of the facialnerve and the lateral canal are situated deep tothe spine of Henley. Trautman’s triangle, the

patch of dura in front of the sigmoid sinus, facesthe cerebellopontine angle. B, the presigmoiddura has been opened and the superior petrosalsinus and tentorium divided, taking care to pre-serve the vein of Labbé that joins the transversesinus, and the trochlear nerve that enters theanterior edge of the tentorium. The abducensand facial nerves are exposed medial to thevestibulocochlear nerve. The posteroinferiorcerebellar artery courses in the lower margin of the exposure with the glossopharyngeal andvagus nerves. The SCA passes below the oculo-motor and trochlear nerves and above thetrigeminal nerve. C, the semicircular canalshave been opened. The superior canal is locatedunder the middle fossa’s arcuate eminence andthe posterior canal is located immediately lat-eral to the posterior wall of the internal acousticmeatus. D, labyrinthine exposure in anotherspecimen. The tympanic segment of the facialnerve courses below the lateral canal and turnsdownward as the mastoid segment where it

gives or ig in to the chorda tym pan i, see nascending along the inner surface of the tym-

panic membrane and neck of the malleus. Thehead of the malleus and incus are located in theepitympanic area above the level of the tym-

panic membrane. The mastoid antrum commu-nicates through the aditus with the epitympanicarea and tympanic cavity. (Continues)



apex that could be achieved in the combined supra- andinfratentorial presigmoid approach.

The skin incision is started in the temporal region above the

zygoma, and extends above the ear and downward in the sub-occipital area medial to the mastoid process (Figs. 1-14, 1-15,and 1-17). The skin flap is reflected forward to the level of theexternal auditory canal. The temporal muscle is elevated andreflected anteriorly, and the muscles over the mastoid and sub-occipital areas are swept inferiorly. A temporooccipital cran-iotomy is performed and the transverse sinus is exposed. Afterthe bone flap is elevated, a mastoidectomy is carried out with-

out entering the labyrinth. The sigmoid sinus is skeletonizedfrom the sinodural angle to the jugular bulb. Bone is removedsuperiorly to expose the floor of the middle fossa and the supe-

rior petrosal sinus. Trautman’s triangle is exposed in the arealateral to the otic capsule.The dura mater is then incised along the base of the tempo-

ral craniotomy, while preserving the junction of the vein of Labbé with the transverse sinus. The posterior fossa dura isopened anterior to the sigmoid sinus in Trautman’s triangle.The dural incision is extended across the superior petrosalsinus to join the dural incision in the temporal dura. After divi-


RHOTON

FIGURE 1-14. (Continued) E–H, presigmoidapproach. E, the labyrinthectomy has beencompleted to expose the internal acoustic mea-tus. F, the dura lining the meatus has beenopened and the facial nerve has been trans-

posed posteriorly. The facial segments are thecisternal segment located in the cistern medial

to the meatal porus, the meatal segment thatextends laterally from the porus to the meatal fundu s, the labyrinth ine segme nt tha t islocated between the fundus and the geniculate

ganglion, the tympanic segment that arises atthe ganglion and the sharp turn, the genu, and

passes between the lateral semicircular canaland the oval window, and the mastoid segmentthat descends to exit the stylomastoid foramen.The labyrinthine segment courses between thesemicircular canals and vestibule on its pos-terolateral side and the cochlea on its anterome-dial margin. The superior and inferior vestibu-lar nerves have lost their end organs with thedrilling of the semicircular canals andvestibule. The cochlear nerve passes laterally toenter the cochlea, which is still preserved inthe bone anteromedial to the fundus of the mea-tus. G, the cochlear nerve has been divided andreflected and bone removed to expose thecochlea. H, the transcochlear exposure, com-

pleted by removing the cochlea and surround-ing petrous apex, provides access to the front of the brainstem and vertebrobasilar junction, butat the cost of loss of hearing due to thelabyrinthectomy and almost certain temporaryor permanent facial weakness associated withthe posterior transposition of the facial nerve.A., artery; Ac., acoustic; A.I.C.A., anteroinfe-rior cerebellar artery; Bas., basilar; Br., branch;Chor., chorda; Cist., cisternal; CN, cranial

nerve; Coch., cochlear; Gang., ganglion;Genic., geniculate; Inf ., inferior; Int., inter-nal; Jug., jugular; Laby., labyrinthine; Lat.,lateral; Marg., margin; Mast., mastoid; Meat.,meatal; Memb., membrane; N., nerve; Pet.,

petrosal; P.I.C.A., posteroinferior cerebellarartery; Post., posterior; S.C.A., superior cere-bellar artery; Seg., segment; Sp., spine; Sup.,superior; Tymp., tympani, tympanic; V., vein;Vert., vertebral; Vest., vestibular.



sion of the superior petrosal sinus,the tentorium is incised parallel toand just behind the petrous ridgeand superior petrosal sinus. Thisdural incision is extended from thesite of division of the superior pet-rosal sinus through the medial edgeof the tentorium to the incisura

behind where the trochlear nerveenters the tentorial edge. Care istaken to avoid injury to the IVth cra-nial nerve in its course near the ten-torial margin. The posterior portionof the temporal lobe is elevated andthe sigmoid sinus is displaced poste-riorly along with the cerebellar hemi-sphere while preserving the junctionof the vein of Labbé with the sig-moid sinus. The sigmoid sinus limitsthe ability for superior retraction of the temporal lobe and can be ligatedto improve the exposure if bilateralvenous angiography show adequatecommunication through the torcularto the opposite side (24). The petro-clival region can be exposed from themiddle fossa and tentorial incisurato near the foramen magnum,although access to the lower petrocli-val region may be limited by the

jugular bulb. The presigmoid expo-sure provides a shorter working dis-tance to the petroclival area and pro-vides multiple angles for dissection.

The major arteries in the posteriorfossa are easily accessible. The expo-sure can also be combined with a far-lateral approach (Fig. 1-17).

Subtemporal PreauricularInfratemporal Fossa Approach

The subtemporal preauricularinfratemporal approach is directedthrough the infratemporal and mid-dle fossae to the part of the anteriorsurface of the petrous bone locatedmedial to the cochlea and to thepetroclival region (Figs. 1-10, 1-13,

and 1-18). This description outlinesthe full extent of the anatomic expo-sure available through this approach,

but it can often be ta ilored to asmaller, more limited, approach. Acurvilinear incision starting in thefrontal region turns downward infront of the ear into the cervical



FIGURE 1-15. A–D, comparison of the retrosigmoid

approach and the minimal mastoidectomy, retro-labyrinthine, translabyrinthine, and transcochlearapproach modifications of the presigmoid approach.

A, retrosigmoid approach. The left cerebellum hasbeen elevated to expose the cranial nerves V through

XI in the cerebellopontine angle. The illustrations

from each step are to be compared with the views from the other modifications of the approach. B, the

facial and vestibulocochlear nerves and the flocculus have been retracted to expose the side of the basilar artery.

C, for the minimal mastoidectomy, only enough bone is removed in front of the sigmoid sinus to open the pre-

sigmoid dura and divide the superior petrosal sinus and tentorium. D, the presigmoid dura has been openedand the sigmoid sinus has been retracted posteriorly. The view is approximately the same as that seen withthe retrosigmoid exposure. The retrosigmoid approach provides a better view of the nerves entering the jugu-

lar foramen. (Continues)



region. The incision may be extended downward only to thearea just below the tragus if only the petrous apex and upperpart of the infratemporal fossa are to be exposed, but it can beextended onto the upper neck if a neck dissection is needed.The skin flap is separated from the underlying tissues and

reflected forward. The facial nerve and its major branches areidentified distal to the stylomastoid foramen and followed tothe parotid gland. The parotid gland is separated from the mas-seteric fascia to avoid excessive stretching of the facial nerve atthe stylomastoid foramen (33, 38, 39). The superficial tempo-ralis fascia in which the upper facial branches course is sepa-rated from the temporalis muscle and is reflected forward toprevent damage to the branch of the facial nerve to the frontalis

muscle as the zygomatic arch is exposed. The zygomatic arch isdivided at its anterior and posterior ends, and the temporalismuscle, with the overlying segment of the zygomatic arch, isreflected downward. The mandibular condyle and the capsuleof the temporomandibular joint are either dislocated down-

ward or excised. The temporomandibular joint can be removedin a single piece for later replacement by dividing the mandibu-lar neck below the condyle and osteotomizing the middle fossafloor around the mandibular fossa (Fig. 1-18). The internalcarotid artery, the internal jugular vein, and the vagus, acces-sory, and hypoglossal nerves may be exposed in the neck if needed. The posterior belly of the digastric muscle may bedivided and the styloid process resected.


RHOTON

FIGURE 1-15. (Continued) E–H, compari-son of the retrosigmoid approach and the min-imal mastoidectomy, retrolabyrinthine, trans-labyrinthine, and transcochlear approachmodifications of the presigmoid approach. E,the bony capsule around the semicircularcanals and the facial nerve have been exposed

for the retrolabyrinthine variant of the presig-moid approach. F, the exposure with the retro-labyrinthine version does not differ signifi-cantly from that achieved with the minimalmastoidectomy. G, the semicircular canals andvestibule have been removed and the dura lin-ing the internal acoustic meatus has beenopened to complete the translabyrinthine expo-sure. This yields an exposure of the internalacoustic meatus but provides only minimalimprovement in the exposure of the structuresmedial to the porus of the meatus. H, thenerves have been separated beginning laterallyat the fundus of the meatus and extending thecleavage plane medially toward the brainstem.The superior vestibular nerve is behind the

facial nerve and the inferior vestibular nerve isbehind the cochlear nerve. (Continues)



A frontotemporal craniotomy is then performed. The dura iselevated from the floor of the middle fossa to expose and oblit-erate the middle meningeal artery at the foramen spinosumand to expose the arcuate eminence, the third trigeminal divi-sion at the foramen ovale, and the greater petrosal nerve. Thegreater petrosal nerve is transected if necessary to avoid trac-tion on the facial nerve. The floor of the middle fossa, includ-

ing the lateral and inferior aspects of the superior orbital fis-sure, and the lateral margin of the foramina ovale may beremoved to expose the structures in the infratemporal fossa.

If needed, bone can be removed medial to the mandibularfossa to expose the eustachian tube and the tensor tympanimuscle, both of which may be resected (Figs. 1-10, 1-13, and1-18). The bone removal is continued inferiorly, exposing theascending portion of the petrous carotid. In this segment, thecarotid artery is surrounded by a periosteal sheath, whichencloses a periarterial venous plexus that is an extension of thecavernous sinus. At the entrance of the carotid canal, a densefibrocartilaginous ring encircles the artery. If mobilizationof theartery is required, care must be taken when dividing the ringnot to damage the IXth cranial nerve that is in close proximity

to the carotid canal as it exits the jugular foramen. After mobi-lizing the carotid artery and displacing it forward, the petrousapex and the clival region to the level of the foramen magnumcan be approached medial to and behind the artery. Duringdrilling, the very hard cortical bone along the petrous apexgives place to a crumbly cancellous bone in the region of theclivus, as the dura of the anterior and lateral aspects of theposterior fossa is being exposed. The area exposed is limited by

Meckel’s cave superiorly, by the cochlea and internal auditorycanal laterally, by the abducens nerve in its course through theDorello’s canal medially, and by the hypoglossal canal inferi-orly. If the dura is opened, the structures along the lateral andanterior aspects of the upper medulla and lower two-thirds of the pons will be exposed (41). The tentorium can be divided togive access to the upper clival region.

Dividing the third trigeminal division above the foramenovale will permit exposure of the junction of the petrous andcavernous carotid along with the structures in the inferolateralportion of the cavernous sinus (17, 39). The pterygopalatinefossa, parapharyngeal space, lateral maxilla, and orbit can beexposed farther anteriorly. The lateral aspect of the sphenoid

bone and the sphenoid sinus can also be approached by remov-ing bone medial to the maxillary nerve at the root of the ptery-goid process.

Postauricular Transtemporal Approach

The postauricular transtemporal approach is most commonlyselected for lesions that involve the mastoid and tympaniccavities and track along the nerves and arteries to reach the

middle and infratemporal fossa (Figs. 1-19 and 1-20). It can,however, be tailored at its posterior margin to include a ret-rosigmoid, far-lateral, or presigmoid exposure of the posteriorfossa or, at its anterior limits, to include exposure of the ptery-gopalatine fossa and lateral parts of the maxillary orbit or ante-rior cranial fossa.

A question mark incision is started behind the hairline in thetemporal region, extending behind the ear over the mastoid



FIGURE 1-15. (Continued) I and J, compar-ison of the retrosigmoid approach and theminimal mastoidectomy, retrolabyrinthine,translabyrinthine, and transcochlear approachmodifications of the presigmoid approach. I,the labyrinthine, tympanic, and mastoid seg-

ments of the facial nerve have been exposed in preparation for the posterior transposition of the nerve needed to complete the transcochlearexposure. J, the facial nerve has been trans-

po sed an d th e co ch le a an d pe trous ap exremoved to complete the transcochlear expo-sure of the anterior aspect of the brainstemand the basilar artery. A., artery; A.I.C.A.,anteroinferior cerebellar artery; Bas., basilar;Cist., cisternal; CN, cranial nerve; Coch.,cochlear; Flocc., flocculus; Inf ., inferior;Laby., labyrinthine; Lat., lateral; Mast., mas-toid; Meat., meatal; N., nerve; Pet., petrosal;P.I.C.A., posteroinferior cerebellar artery;Post., posterior; Presig., presigmoid; S.C.A.,

superior cerebellar artery; Seg., segment; Sig.,sigmoid; Suboccip., suboccipital; Sup., supe-rior; Tymp ., tympanic; V., vein; Vest.,vestibular.




RHOTON

FIGURE 1-16. A–F, comparison of the retrosigmoid and the various modifi-

cations of the presigmoid exposure. The modifications of the presigmoidapproach include the minimal mastoidectomy, retrolabyrinthine, partiallabyrinthine, translabyrinthine, modified transcochlear, and the fulltranscochlear approach with facial nerve transposition. A, the scalp incision(insert) is positioned for a supra- and infratentorial exposure through a tem-

poro-occipital craniotomy. A temporo-occipital craniotomy has been completedand the dura opened to expose the temporal lobe and the retrosigmoid area. Thetransverse and sigmoid sinuses have been preserved. The cerebellum has been

retracted to expose the nerves in the cerebellopontine angle. B, enlarged view of

the retrosigmoid exposure to compare with the exposure obtained with the var-ious modification of the presigmoid approach. C, in the retrosigmoid exposurethe vestibulocochlear nerve has been elevated and the glossopharyngeal nervedepressed to expose the basilar artery at the origin of the AICA. D, subtempo-ral exposure. The temporal lobe has been elevated to expose the optic tract andoculomotor nerve and the PCA, internal carotid, and anterior choroidal arter-ies. E, the tentorium has been opened while preserving the trochlear nerve. TheSCA courses below and the PCA above the oculomotor and (Continues)



process and continuing inferiorly in front of the sternocleido-mastoid muscle onto the neck. The skin flap is then reflectedforward and the external auditory canal is divided at the bone-cartilage junction and closed as a blind sac. The sternocleido-mastoid muscle is detached from the mastoid process andreflected inferiorly. The periosteum and posterior portion of the temporalis muscle are reflected anteriorly, thus exposingthe temporal, mastoid, and retromastoid areas. The posterior

belly of the digastric muscle is divided and reflected inferiorly.At this point, the facial nerve is identified distal to the stylo-mastoid foramen and is followed, along with its major

branches, into the substance of the parotid gland (5). The inter-nal jugular vein, the carotid bifurcation, and the glossopharyn-

geal, vagus, accessory, and hypoglossal nerves are exposedand isolated in the neck. This allows for proximal control of the internal carotid artery and ligation of the main feedingvessels from the external carotid artery to a neoplasm early inthe procedure.

After this, temporal and/or retromastoid craniotomies may be performed with a simple mastoidectomy. The remainingskin of the external auditory canal, the tympanic membrane,the malleus, incus, and stapes arch (leaving the footplate) areremoved. The facial nerve is completely skeletonized from thegeniculate ganglion to the stylomastoid foramen.

If exposure of the jugular foramen and lower clival region isdesired, a new facial canal is created by drilling a groove in the

bone of the anterior attic wall, between the geniculate ganglion

and the root of the zygoma. The facial nerve is carefully freed atthe stylomastoid foramen, while leaving some of the surround-ing connective tissue attached to the nerve, and the nerve istransposed anteriorly into the new bony groove of the epitympa-num and imbedded for its protection into the parotid tissue (5).

The dura of the middle fossa and the sigmoid sinus from thesinodural angle to the jugular bulb is skeletonized. Then thesigmoid sinus and the jugular vein are ligated in this sequence,

and the sigmoid sinus divided. Part of the wall of the sinus, bulb, and/or vein may be excised to increase the exposure.This allows for dissection of the lower cranial nerves at the

jugular foramen, as well as for their mobilization and posteriordisplacement if necessary. The posterior mobilization of thelower cranial nerves allows for a direct exposure of the struc-tures along the lateral and anterior aspects of the medulla andlower pons without the necessity for brain retraction.Dissection in the area of the jugular foramen has proven to beextremely difficult, as the lower cranial nerves are particularlyfragile and difficult to isolate from the surrounding tissues.

Exposure of the middle clival structures requires removal of the bony labyrinth, as described for the translabyrinthine

approach. The internal auditory canal is exposed, the facial nerveidentified, and the cochlear and vestibular nerves divided. Thegreater superficial petrosal nerve is sectioned at its origin fromthe geniculate ganglion. The facial nerve is freed from all itsattachments in the temporal bone and reflected posteriorly. The

bony portion of the external auditory canal and the tympanic bone are drilled away, exposing the ascending portion of theintrapetrous carotid artery medial to the eustachian tube.

The dissection is continued by drilling away the cochlea,starting at its basal turn, to expose part of the horizontal seg-ment of the petrous carotid artery. Anterior displacement of thecarotid artery and removal of the cochlea provides a wide expo-sure of the lateral and anterior portions of the pons andmedulla. This exposure extends from the inferior aspect of the

trigeminal ganglion to the foramen magnum. The exposure may be carried medially into the clivus and retropharyngeal spaceand anteriorly to expose the mucosa of the sphenoid sinus.

If the approach is to be extended to the parasellar and paras-phenoidal areas, the zygomatic arch is divided and reflectedinferiorly with the masseter muscle. The temporalis muscle isseparated from its attachment to the coronoid process of themandible and reflected anteriorly and superiorly. A temporal


RHOTON

FIGURE 1-16. (Continued) trochlear nerves. F, minimal mastoidectomymodification of the presigmoid approach. The minimal mastoidectomyapproach is completed by removing only enough bone in the front of the sig-moid sinus so that the presigmoid dura can be opened to expose the posteriorcranial fossa. The bony capsule of the labyrinth is not exposed in the minimalmastoidectomy as it is in the retrolabyrinthine approach. The exposure shownwith the minimal mastoidectomy in this figure is to be compared with the ret-

rosigmoid exposure shown in B. G–N, comparison of the retrosigmoid andthe various modifications of the presigmoid exposure. G, deep exposure withthe minimal mastoidectomy with retraction of the vestibulocochlear and glos-sopharyngeal nerves, to be compared with the retrosigmoid approach shownin C. The exposure is similar to that obtained with the retrosigmoid approach.H, retrolabyrinthine approach in which more extensive drilling of the mastoidhas been completed to expose the osseous capsule of the semicircular canals.I, the dura has been folded forward after completing the retrolabyrinthineexposure. The exposure differs little from that obtained with the minimalmastoidectomy exposure shown in F and G. J, the exposure with the poste-rior canal partial labyrinthectomy is similar to that achieved with the mini-mal mastoidectomy. K, the partial labyrinthectomy has been extended byremoving the superior canal in addition to removal of the posterior canal. L,

the infratentorial exposure does not differ significantly from that achievedwith the minimal mastoidectomy, as shown in F and G. Removal of the supe-rior canal reduces the required temporal lobe retraction and aids in the expo-sure along the middle fossa floor and petrous apex. M, translabyrinthineexposure in which the semicircular canals and the vestibule have beenremoved. This adds the internal auditory canal to the exposure, but does notimprove the exposure of the structures medial to the meatus, as compared with

the minimal mastoidectomy or even the retrosigmoid approach. N, the facialnerve has been transposed posteriorly out of the field and the cochlea has beenremoved to complete the transcochlear approach. This approach greatlyimproves access to the front of the brainstem, clivus, and basilar artery, butis done at the cost of a temporary or permanent facial paralysis and loss of hearing. A., artery; Ac., acoustic; A.I.C.A., anteroinferior cerebellar artery;Ant., anterior; Bas., basilar; Car., carotid; Chor., choroidal; CN, cranialnerve; Comm., communicating; Inf ., inferior; Int., internal; Lat., lateral;Mast., mastoid; P.C.A., posterior cerebral artery; Ped., peduncle; Pet., pet-rosal; P.I.C.A., posteroinferior cerebellar artery; Post., posterior; S.C.A.,superior cerebellar artery; Seg., segment; Sig ., sigmoid; Sup., superior;Temp., temporal; Tent., tentorial; Tr., trunk; Trans., transverse; V., vein;Vert., vertebral.



craniotomy is then performed, and extensive bone is removedalong the whole lateral aspect of the middle cranial fossa. The

ascending ramus of the mandible is either displaced anteriorlyor resected, and the petrous carotid is exposed distally to theproximal portion of the intracavernous segment after removingthe cartilaginous portion of the Eustachian tube. The cavernoussinus can be approached and the intracavernous carotid arteryexposed by dividing the mandibular segment of the trigeminalnerve. The approach can also be extended to the retrosigmoidarea and down the vertebral artery to the C1 to C2 level, or to

the suboccipital triangle for a far-lateral or transcondylar expo-sure. The lateral orbit and pterygopalatine fossa can be

accessed at the anterior limit of the exposure.

DISCUSSION

Pathologies can arise anywhere within the petroclivalregion and frequently are not restricted to a single anatomiccompartment of the cranial base. Involvement of multiple cra-nial nerves and arteries occurs because cranial base tumors



FIGURE 1-17. A–D, combined presigmoidand far-lateral approach. A, the insert showsthe site of the scalp incision and mastoid tip.The scalp flap has been reflected forward. Themastoidectomy exposes the dense cortical bonehousing the semicircular canals. The bone flapis outlined. The occipital artery courses back-ward between the digastric and superior

oblique. B, enlarged view. The tympanic seg-ment of the facial nerve courses below the lat-eral canal. The chorda tympani arises from themastoid segment of the facial nerve. The mas-toid antrum, which has been drilled away,opens through the aditus into the epitympanic

part of the tympanic cavity. C, the presigmoidand temporal dural incisions have been out-lined. D, the temporal and presigmoid durahas been opened. One goal of the procedure isto preserve the vein of Labbe, which emptiesinto the transverse sinus. (Continues)



tend to achieve considerable size before producing clinicalmanifestation (32). The distinction between the benign ormalignant tumors in this area is not rigid because many

benign tumors can have a very invasive characteristic. Theselection of the best surgical approach depends on the loca-

tion, extension, size, and nature of the pathology. An advan-tage of these approaches directed through the temporal boneto the petroclival area is that they reach the area through tis-sue planes outside the oropharynx. They provide anotherroute by which anterior intradural lesions situated medial tothe nerves entering the internal acoustic meatus and jugularforamen can be approached without entering the nasophar-ynx. They also provide an avenue of exposure for lesions that

involve the temporal and sphenoid bones in addition to theclivus. One or a combination of the lateral approaches is fre-quently used to expose intra- or extradural clival lesions thatalso involve the temporal and sphenoid bones. They also pro-vide access to the anterior aspect of the midbrain, pons, and

medulla and to the cerebellopontine angle and nerves in theposterior fossa. They may also provide better access to thetemporal bone, jugular foramen, and petrous segment of theinternal carotid artery than the other anterior or posteriorapproaches. The area may be approached from directly lateralthrough the mastoid, labyrinth, and cochlea, as in thetranslabyrinthine and transcochlear approaches; from abovethrough a subtemporal middle fossa route; from behind in


RHOTON

FIGURE 1-17. (Continued) E–H, combined presigmoid and far-lateral approach. E, the duralincision has been extended through Trautman’striangle and across the superior petrosal sinusand tentorium, taking care to preserve the veinof Labbe and the trochlear nerve. The semicircu-lar canals have been opened. F, enlarged view.The posterior canal faces the posterior fossa lat-eral to the internal acoustic meatus. The supe-rior canal projects upward, below the arcuateeminence, toward the floor of the middle fossa.The lateral canal is a useful landmark for iden-tifying the tympanic segment of the facial nerve,

which courses between the canal and the stapessitting in the oval window. The epitympanicarea opens through the aditus into the mastoidantrum. G, the labyrinthectomy has been com-

pleted and the dura lining the meatus opened toexpose the cisternal, meatal, labyrinthine, tym-

panic, and mastoid segments of the facial nerve.The SCA courses above the trigeminal nerve. H,enlarged view along the opened tentorialincisura. The oculomotor and trochlear nervescourse between the PCA and SCA. The SCArests against the upper surface of the trigeminalnerve. (Continues)



the retrosigmoid suboccipital approach; or from multipledirections using such combined supra- and infratentorialapproaches as the presigmoid approach, to which atranslabyrinthine or transcochlear approach may be added.Alternative or extended approaches, most of which includesome route through the mastoid and petrous parts, include

the anterior transpetrosal, the subtemporal preauricularinfratemporal, and the far-lateral transcondylar approach.The retrosigmoid suboccipital approach, described in the

chapter on the cerebellopontine angle, offers a wide view of the cerebellopontine angle and of the intradural structures

behind the ipsilateral lower clivus, but the dural surface of thepetrous apex, upper clivus, and tentorial incisura are not wellseen from this exposure (26, 35, 36, 46) (Figs. 1-15 and 1-16).

Removal of posterior wall of the internal auditory canalthrough the retrosigmoid provides access to the contents of the meatus as far lateral as the vertical and transverse crests.The vestibule can be opened if needed to remove a tumorextending into the labyrinth. Care is required to avoid injuryto the posterior semicircular canal and common crus if there

is the possibility of preserving hearing (29). The retrosigmoidapproach provides easy access to the intradural part of cranialnerves V, VII, VIII, and IX through XII. It also provides accessto the nerve-related segments of the arteries of the posteriorcirculation. The vertebrobasilar junction can be exposed insome cases, although the lower cranial nerves and the jugulartubercle are frequent obstacles. Retraction of the pons andworking between the cranial nerves is necessary to reach the



FIGURE 1-17. (Continued) I–L, combined presigmoid and far-lateralapproach. I, the insert shows the site of the additional skin incision neededto add a retrosigmoid craniotomy and far-lateral approach. The scalp flap hasbeen reflected to expose the suboccipital triangle located between the superiorand inferior oblique and the rectus capitis posterior major and in the depthsof which the vertebral artery courses with a dense venous plexus. J, thevenous plexus has been removed to expose the margins of the suboccipital tri-

angle. K, the rectus capitis posterior major and the inferior oblique have beenreflected medially and the superior oblique laterally to expose the vertebralartery and surrounding venous plexus behind the atlanto-occipital joint. L,the venous plexus has been removed to expose the vertebral artery coursingwith the C1 nerve behind the atlanto-occipital joint and across the upperedge of the posterior atlantal arch. M and N, combined presigmoid and far-lateral approach. (Continues)





FIGURE 1-18. Preauricular subtemporal-infratemporal fossa approach. A, the scalp incision is posi-tioned so that a frontotemporal craniotomy can becompleted. The operation is often completed with anincision that extends downward only to the level of the tragus. However, it can be extended if a neck dis-section is needed. The scalp flap has been reflected

forward, taking care to protect the branches of the fac ial nerve. B, the temporalis muscle has beenrefracted forward and the craniotomy completed. Themandibular condyle and fossa and a portion of the

zygomatic arch were removed in a single piece, asshown in the insert, and the middle fossa floorremoved. C, exposure after removal of the middle

fossa floor lateral to the foramen ovale and beforeresection of the tensor tympani muscle. The lowerorifice of the carotid canal is located in front of the

jugular foramen. The eustachian tube, which passesacross the front of the petrous carotid, has beenopened. D, the tensor tympani and Eustachian tubehave been resected to expose the horizontal segment of the petrous carotid. E, the internal carotid artery hasbeen reflected forward and the petrous apex drilled toexpose the posterior fossa dura and the inferior pet-rosal sinus coursing along the petroclival fissure. F,the dura facing the petrous apex has been opened andthe vertebral arteries and AICA exposed. This expo-sure is directed through the petrous apex medial to the

cochlea and jugular foramen and does not risk loss of facial nerve function or hearing, as do the approachesdirected through the petrous apex that require facialnerve transposition and resection of the labyrinth.A., artery; A.I.C.A., anteroinferior cerebellar artery;Brs., branches; Car., carotid; CN, cranial nerve;Eust., eustachian; Gang., ganglion; Gl., gland; Gr.,

greater; Inf ., inferior; Int., internal; Jug., jugular;M., muscle; Max., maxillary; Men ., meningeal;Mid ., middle; N., nerve; Pet ., petrosal, petrous;Post., posterior; Temp., temporal; Tens., tensor;TM., temporomandibular; Trig., trigeminal; Tymp.,tympani; V., vein; Vert., vertebral; Zygo., zygomatic.



riorly. A significant degree of temporal lobe retraction may berequired. This may be reduced by using a frontotemporalcraniotomy with zygomatic resection. Although only a smallwindow in the petrous bone is provided, exposure can beexpanded by dividing the adjacent part of the tentorium. The

lateral and anterior surfaces of the pons and the upper clivusand adjacent part of the cavernous sinus can be approachedthrough this route (Fig. 1-13). The facial, vestibulocochlear,trigeminal, and abducens nerves can be identified. Thepetrous carotid may limit the surgeon’s line of vision andrestrict access to the inferior part of the petroclival region, butthis restriction may be overcome with anterior mobilizationof the artery (39, 41). The approach provides access to the

anterior aspect of the brainstem and basilar artery in the area between the tri geminal nerve above and the facia l andvestibulocochlear nerves below. In approaching the basilarartery through this route, the size and location of the lesionin relation to the petrous ridge is critical. The trigeminal

nerve can be mobilized to improve the exposure, althoughthis may result in postoperative facial hypesthesia (19, 20).The anterior transpetrosal approach can be used alone forextradural pathologies restricted to the petrous apex or as asurgical step to approaching intradural pathologies in thepetroclival region. It provides a route for resecting extradurallesions that extend from the level of the trigeminal nerve tothe foramen magnum.


RHOTON

FIGURE 1-19. A–D, anatomic basis of the postauric-ular transtemporal approach. A, the incision sweepswidely around the posterior margin of the ear so that aretrosigmoid, presigmoid, and far-lateral exposure canbe obtained behind the ear, and a subtemporal, infratem-

poral , pterygopalatine, and orbital exposure can beobtained in front of the ear. B, the scalp flap has beenreflected forward, the external canal transected, and the

parotid gland and superficial branches of the facialnerve exposed. C, the sternocleidomastoid muscle has

been reflected. The neck dissection exposes the internal jugular vein, C1 transverse process, and the glossopha-ryngeal, vagus, accessory, and hypoglossal nerves. Theaccessory nerve is retracted forward. D, the parotid

gland has been removed to expose the temporofacial andcervicofacial trunks of the facial nerve and the temporo-mandibular joint. The splenius capitis muscle has beenreflected downward to expose the superior and inferioroblique muscles, which insert on the transverse processof C1 and border the suboccipital triangle in which thevertebral artery courses. (Continues)



Removal of the posterior part of the petrous pyramid has been used for acoustic neuroma removal as part of extendedapproaches directed through the middle fossa (21, 28, 42, 43)(Fig. 1-12). The extended approaches combine different degreesof resection of the bony labyrinth with the subtemporaltranstentorial routes. Extending the resection of the petrous

bone posteriorly over the mastoid and the bony labyrinthexposes the whole intrapetrous course of the facial nerve, andprovides access to the cerebellopontine angle by a combina-tion of subtemporal, translabyrinthine, and presigmoid routes(Figs. 1-12 and 1-13) (9).

The subtemporal preauricular infratemporal approachreaches the skull base from an anterolateral direction (Figs.1-10, 1-13, and 1-18). Division of the zygomatic arch, resection

or displacement of the mandibular condyle, and extensiveresection of the lateral part of the middle fossa floor exposesthe infratemporal fossa, the nasopharynx, the para- andretropharyngeal areas, and the ethmoid, sphenoid, and maxil-lary sinuses. The approach also provides access to the uppercervical and petrous carotid. The cavernous sinus also can be

approached through its lateral and basal aspects. Anterior dis-placement of the petrous carotid allows direct access to theclivus and for extensive resection of the petrous bone medialto the cochlea. This exposes the extradural clival region fromthe level of the trigeminal nerve to the foramen magnum (33,36, 38, 39). The approach can also provide access to theintradural space ventral to the brainstem (41). The exposure of the cerebellopontine angle and foramen magnum is limited



FIGURE 1-19. (Continued) E–H, anatomic basis of the postauricular transtemporal approach. E, a segmentof the mandibular ramus has been removed to expose the

upper and lower head of the lateral pterygoid and themaxillary artery in the infratemporal fossa. The inferioralveolar canal and nerve have been exposed. F, themandibular ramus, in front of the inferior alveolarcanal, has been removed to provide a wider exposure of the inferotemporal fossa. The upper head of the lateral

pterygoid muscle passes backward from the inferotempo-ral surface of the greater sphenoid wing and the lowerhead passes upward from the lateral pterygoid plate.Both heads insert on the mandibular neck and the jointcapsule. The superficial head of the medial pterygoidmuscle passes from the maxillary tuberosity and ptery-

goid plate to the mandibular angle. The deep head of themedial pterygoid arises from the pterygoid fossa betweenthe pterygoid plates. G, enlarged view of the infratem-

poral area after removal of the mandibular condyle andlateral pterygoid muscles. The branches of the mandibu-lar nerve are exposed below the foramen ovale. Thelargest branches are the lingual and superior alveolarnerves, which are predominantly sensory. The auricu-lotemporal nerve arises as two roots, which often passaround the middle meningeal artery before joining. H,the pterygoid muscles, a segment of the maxillaryartery, and the mandibular and facial nerve brancheshave been reflected or removed to expose the internal

jugular vein exiting the jugular foramen on the medialside of the stylomastoid foramen, the internal carotidartery ascending to enter the carotid canal, the tensorand levator veli palatini descending from their originbordering the eustachian tube, and the terminal seg-

ment of the maxillary artery entering the pterygopala-tine fossa. (Continues)



because the approach is carried anterior and medial to cranialnerves VII through XII and the cochlea is not resected (36).Anterior transposition of the petrous carotid artery allowsunhindered exposure of the origin of the AICA and the verte-

brobasilar junction. The approach could be used as an alterna-tive lateral route to vascular lesions of the midbasilar artery or

at the vertebrobasilar junction, when these lesions cannot beexposed through either the retromastoid or subtemporaltranstentorial approaches.

The postauricular transtemporal approach, which combinesa transcochlear exposure with an infratemporal approach,may be used as an alternative to the preauricular infratempo-ral approach when the pathology involves the mastoid andthe infratemporal fossa and extends to the facial recess, hypo-

tympanic area, and jugular bulb (5, 6, 34) (Figs. 1-19 and 1-20).The structures of the lower and middle clivus can be exposedwithout the need for brain retraction. The facial nerve is dis-placed anterosuperiorly and the sigmoid sinus ligated anddivided. Displacement of the facial nerve from its bony canalseriously interferes with its vascular supply and temporary or

permanent loss of function is to be expected (33). Resection of the jugular bulb allows for exposure of the lower cranialnerves in the jugular foramen. Mobilization of the nerves inthe medial part of the jugular foramen is extremely difficultand nerve damage is likely to occur if it is attempted. The lat-eral and anterior surfaces of the lower pons, medulla, andcervicomedullary junction are well exposed. The extent of exposure of the major arteries is dependent on the different


RHOTON

FIGURE 1-19. (Continued) I–L, anatomic basis of the

postauricular transtemporal approach. I, a mastoidec-tomy has been completed to expose the semicircularcanals and the mastoid segment of the facial canal. Theendolymphatic sac sits under the presigmoid dura. J,the external canal has been resected to expose the struc-tures in the tympanic cavity. The tympanic segment of the facial nerve courses between the lateral semicircularcanal and the stapes sitting in the oval window. Thechorda tympani arises from the mastoid segment of the

facial nerve, passes forward along the inner surface of the tympanic membrane and the neck of the malleus toenter its anterior canaliculus, exits the skull along the

petrotympanic suture, and joins the lingual nerve in theinfratemporal fossa. The promontory overlies the basalturn of the cochlea. The tendon of the tensor tympani

muscle makes a right-angle turn around the trochlei- form process to insert on the malleus. K, the incus andmalleus have been removed while preserving the stapesand the tensor tympani muscle. The petrous carotid hasbeen exposed. The nerves exiting the jugular foramenhave been retracted forward to expose the hypoglossalnerve exiting the hypoglossal canal. L, a frontotempo-ral craniotomy has been completed and the floor of themiddle cranial fossa removed. The semicircular canalshave been exposed above the jugular bulb and the stapeshas been removed from the oval window. The maxillarynerve has been exposed in the pterygopalatine fossa.The membranous wall of the Eustachian tube has beenopened to expose the tube’s opening into the nasophar-

ynx. (Continues)




RHOTON

FIGURE 1-19. (Continued) S–X , anatomic basis of the postauriculartranstemporal approach. S, enlarged view of the medial wall of the tym-

panic cavity before mobil izing the facial nerve. The stapedial muscle passes forward f rom the pyramidal eminence below the facial nerve and attaches onthe neck of the stapes. The tensor tympani muscle passes backward and lat-erally, giving rise to a narrow tendon that makes a sharp turn around thetrochleariform process at the lateral end of its semicanal to insert on the han-dle of the malleus. The basal turn of the cochlea is located deep to the

promontory. The tympanic segment of the facial nerve courses above thestapes. T, enlarged view of the labyrinth. The semicircular canals have beenunroofed and the stapes has been removed from the oval window. The roundwindow is located below and behind the oval window. U, the facial nerve hasbeen reflected forward out of the facial canal and the vestibule has beenopened. The ampullae of the superior and the lateral canal open into the

vestibule anteriorly and are innervated by the superior vestibular nerve.Only the upper edge of the superior canal was preserved in opening thevestibule. The ampullae of the posterior canal is located at its lower end andis innervated by the singular branch of the inferior vestibular nerve. V, a

probe is directed through the vestibule to the inner surface of the membranecovering the round window, which is located behind and below the ovalwindow. W, enlarged view of the labyrinth after opening the promontory toexpose the cochlea. The jugular bulb is located below the vestibule and semi-circular canals and the lateral genu of the internal carotid artery in positionbelow the cochlea. The cochlea wraps around the modiolus through which thebranches of the cochlear nerve are distributed to the cochlear duct. X, thetemporal lobe has been elevated to expose the internal carotid, PCA, andSCA in the basal cisterns. The dura has been elevated from the lateral wallof the cavernous sinus. (Continues)



anatomic variants and direction of displacement of the ves-sels. Exposure of the structures of the middle clivus requiresposterior facial nerve displacement and drilling of thelabyrinth with consequent destruction of any residual hear-ing. The lateral and part of the anterior surfaces of the ponscan be exposed up to the point of emergence of the trigeminalnerve. Exposure of the superior petroclival space requires that

the transtemporal exposure be combined with a subtemporalexposure. The transtemporal approach can easily be extendedto the infratemporal fossa, and the same exposure provided

by the preauricular approach can be achieved. When thisapproach is combined with an infratemporal fossa exposureand anterior displacement of the intrapetrous carotid artery,the petrous part of the temporal bone can be completelyremoved, providing the widest possible exposure of the petro-

clival region (Figs. 1-19 and 1-20). The retrosigmoid, far-lateral, and transcondylar exposures can be obtained at theposterior margin of the exposure, and the anterior limit can beextended to include the pterygopalatine fossa and lateral partof the maxilla, orbit, and anterior cranial fossa.

Extensive removal of lesions involving the skull base fre-quently require reconstruction of the resultant bony, neural,

and dural defects (Fig. 1-21). The presence of cerebrospinalfluid leaks and the close proximity to contaminated spacesof the oro- or nasopharynx increases the risks of meningitis.Opened sinuses should be obliterated, dural incisions andopenings should be sutured and sealed, nerves should bereanastomosed or grafted, and devascularized grafts of boneor dura should be covered with vascularized tissue when-ever possible.



FIGURE 1-19. (Continued) Y and Z, anatomicbasis of the postauricular transtemporal approach.

Y, overview before opening the dura. The postau-ricular approach offers the potential for providingretrosigmoid, presigmoid, and farlateral exposuresand can be used to access the infratemporal and

pterygopalatine fossae, the orbit, and the subtemporal areas. In this case, the exposure extends fromthe retrosigmoid area forward to the orbit. Themaxillary sinus has been opened below the orbital

floor. Z, overview of exposure after opening thedura. A., artery; Alv., alveolar; Aur., auricular;Br., branch; Brs., branches; Cap., capitis; Car.,carotid; Cerv., cervical; Chor., chorda, choroid;CN , cranial nerve; Coch., cochlear; Cond.,condyle; Endolymph., endolymphatic; Eust.,eustachian; Ext., external; Fac., facial; Gang.,

gangl ion; Genic., geniculate; Gl., gland; Gr., grea ter ; Hypogl., hypoglossal; Inf ., inferior;Infraorb., infraorbital; Infratemp., infratempo-ral ; In t . , internal ; Ju g . , jugular ; Laby .,

labyrinthine; Lat., lateral; Lev., levator; M., mus-cle; Mandib., mandibular; Mast., mastoid; Max.,maxillary; Med., medial; N., nerve; Obl., oblique;Occip., occipital; Pal., palatini; P.C.A., posteriorcerebral artery; Ped., peduncle; Pet., petrosal,

petrous; P.I.C.A., posteroinferior cerebellar artery;Plex., plexus; Post., posterior; Proc., process;Pteryg., pterygoid; Pterygopal., pterygopalatine;Rec., rectus; S.C.A., superior cerebellar artery;Scap., scapula; Seg., segment; Semicirc., semi-circular; Sig., sigmoid; Sphen., sphenoid; Splen.,splenus; Sternocleidomast., sternocleidomastoid;Sup., superior; Superf ., superficial; Symp., sym-

pathetic; Temp., temporal; Tens., tensor; TM.,temporomandibular; Trans., transverse; Tymp.,

tympani, tympanic; V., vein; Vel., veli; Vert., ver-tebral; Vest., vestibular.




RHOTON

FIGURE 1-20. A–F, postauricular transtemporal approach. This exposureincludes the transtemporal and infratemporal approaches in combination

with a craniotomy. A, the scalp flap has been reflected forward to expose thesternocleidomastoid, parotid gland, and the greater auricular nerve. B, theexternal canal has been divided to reflect the flap forward for a parotid andneck dissection that exposes the facial nerve and its trunks, the posteriordigastric belly, and the internal jugular vein. C, the mastoidectomy hasbeen completed to expose the presigmoid dura, the sigmoid sinus, and thesemicircular canals. The mandibular condyle has been resected to provideaccess to the infratemporal fossa. D, a temporo-occipital craniotomy has

been completed, the zygomatic arch opened, and the temporalis musclereflected to expose the maxillary artery and pterygoid muscles in the

infratemporal fossa. E, enlarged view of the temporal and infratemporalexposures. The posterior wall of the external canal has been removed. Theauriculotemporal branch of the mandibular nerve is often split into tworootlets by the middle meningeal artery. F, enlarged view of the tympaniccavity. The anterior part of the lateral semicircular canal is located above thetympanic segment of the facial nerve. The promontory overlies the basalcochlear turn. (Continues)





FIGURE 1-20. (Continued) G–L, postauricular transtemporal approach. G,the external canal has been resected in preparation for exposing the petrouscarotid. H, the junction of the cervical and petrous carotid has been exposedin the area below the promontory. The lateral margin of the stylomastoid and

jugular foramina have been removed to expose the jugular bulb below thesemicircular canals. I, the mandibular nerve has been exposed below the fora-men ovale. A more extensive exposure of the petrous carotid has been com-

pleted so that the artery can be ref lected forward out of the carotid canal to provide access for drill ing of the petrous apex. J, the petrous carotid has beenreflected forward and the petrous apex removed to expose the clivus and infe-rior petrosal sinus. K, the facial nerve has been moved out of the facial canal,and a total labyrinth and petrous apicectomy have been completed. L, a seg-

ment of the sigmoid sinus and the jugular bulb have been removed to exposethe nerves passing through the jugular foramen. The dura has been openedand the facial nerve displaced posteriorly. The temporal lobe has been elevatedto expose the subtemporal area while preserving the vein of Labbe. A., artery;Ac., acoustic; Aur., auricular; Bas., basilar; Car., carotid; Chor., chorda;CN, cranial nerve; Cond., condyle; Ext., external; Gl., gland; Gr., greater;Inf ., inferior; Int., internal; Jug., jugular; Lat., lateral; M., muscle; Mandib.,mandibular; Mast., mastoid; Max ., maxillary; Mid., middle; Men.,meningeal; N., nerve; Pet., petrosal, petrous; Proc., process; Seg., segment;Semicirc., semicircular; Sig., sigmoid; Sternocleidomast ., sternocleidomas-toid; Sup., superior; Temp., temporal; Trans., transverse; Tymp., tympani,tympanic; V., vein; Vert., vertebral.



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Janecka IP (eds): Surgery of Cranial Base Tumors. New York, Raven Press, 1993,pp 351–365.

4. de Oliveira E, Rhoton AL Jr, Peace D: Microsurgical anatomy of the region of

the foramen magnum. Surg Neurol 24:293–352, 1985.5. Fisch U, Pillsbury HC: Infratemporal fossa approach to lesions in the tempo-

ral bone and base of the skull. Arch Otolaryngol 105: 99–107, 1979.

6. Fisch U, Fagan P, Valavanis A: The infratemporal fossa approach for the lat-eral skull base. Otolaryngol Clin North Am 7:513–552, 1984.

7. Gantz BJ, Fisch U: Modified transotic approach to the cerebellopontine angle.Arch Otolaryngol 109:252–256, 1983.

8. Hakuba A, Nishimura S, Inoue Y: Transpetrosal-transtentorial approach andits application in the therapy of retrochiasmatic craniopharyngiomas. Surg

Neurol 24:405–415, 1985.9. Hakuba A, Nishimura S, Jang BJ: A combined retroauricular and preauricu-

lar transpetrosal-transtentorial approach to clivus meningiomas. Surg Neurol

30:108–116, 1981-10. Harris FS, Rhoton AL Jr: Anatomy of the cavernous sinus: A microsurgical

study. J Neurosurg 45:169–180, 1976.

11. Hitotsumatsu T, Rhoton ALJr: Unilateral upper and lower subtotal maxillec-tomy approaches to the skull base: Microsurgical anatomy. Neurosurgery

46:1416–1453, 2000.

12. Hitselberger WE, House WF: Surgical approaches to acoustic tumors. Arch

Otolaryngol 84:286–291, 1966.13. House WF: Middle cranial fossa approach to the petrous pyramid. Arch

Otolaryngol 78:460–469, 1963.14. House WF: Evolution of transtemporal bone removal of acoustic tumors.

Arch Otolaryngol 80:731–742, 1964.15. House WF, Hitselberger WE: The transcochlear approach to the skull base.

Arch Otolaryngol 102:334–342, 1976.

16. House WF, De la Cruz A, Hitselberger WE: Surgery of the skull base:Transcochlear approach to the petrous apex and clivus. Otolaryngology

86:770–779, 1971.

17. Inoue T, Rhoton AL Jr, Theele D, Barry ME: Surgical approaches to the cav-ernous sinus: A microsurgical study. Neurosurgery 26:903–932, 1990.

18. Katsuta T, Rhoton AL Jr, Matsushima T: The jugular foramen: Microsurgical

anatomy and operative approaches. Neurosurgery 41:149–202, 1997.19. Kawase T, Shiobara R, Toya S: Anterior transpetrosal-transtentorial approach

for sphenopetroclival meningiomas: Surgical method and results in 10

patients. Neurosurgery 28:869–876, 1991.20. Kawase T, Toya S, Shiobara R, Mine T: Transpetrosal approach for aneurysms

of the lower basilar artery. J Neurosurg 63:857–861, 1985.

21. King TT, Morrison AW: Translabyrinthine and transtentorial removal of acoustic nerve tumors: Results in 150 cases. J Neurosurg 52:210–216, 1980.

22. King TT, Morrison AW: Translabyrinthine operation for the removal

of acoustic nerve tumors, in Schmidek HH, Sweet WH (eds): Operative

Neurosurgical Techniques. New York, W.B. Saunders Co., 1988, vol 1, pp 685–704.23. Lister JR, Rhoton AL Jr, Matsushima T, Peace D: Microsurgical anatomy of the

posterior inferior cerebellar artery. Neurosurgery 10:170–199, 1982.24. Malis LI: Surgical resection of tumors of the skull base, in Wilkins RH,

Rengachary SS (eds): Neurosurgery. New York, McGraw-Hill, 1985, vol 1, pp

1011–1021.

25. Matsushima T, Rhoton AL Jr, de Oliveira E, Peace D: Microsurgical anatomy

of the veins of the posterior fossa. J Neurosurg 59:63–105, 1983.

26. Mayberg MR, Symon L: Meningiomas of the clivus and apical petrous bone:

Report of 35 cases. J Neurosurg 65:160–167, 1986.

27. McElveen JT: The translabyrinthine approach to the cerebellopontine angle

tumors, in Wilkins RH, Rengachary SS (eds): Neurosurgery Update I: Diagnosis,

Operative Technique, and Neurooncology. New York, McGraw-Hill, 1990, pp

415–423.

28. Morrison AW, King TT: Experiences with a translabyrinthinetranstentorialapproach to the cerebellopontine angle. J Neurosurg 38:382–390, 1973.

29. Pait TG, Harris FS, Paullus WS, Rhoton AL Jr: Microsurgical anatomy and

dissection of the temporal bone. Surg Neurol 8:363–391, 1977.

30. Paullus WS, Pait TG, Rhoton AL Jr: Microsurgical exposure of the petrous

portion of the carotid artery. J Neurosurg 47:713–726, 1977.

31. Rhoton ALJr, Pulec JL, Hall GM, Boyd AS Jr: Absence of bone over the genic-

ulate ganglion. J Neurosurg 28:48–53, 1961-

32. Samii M, Ammirati M, Mahran A, Bini W, Sepehrnia A: Surgery of petrocli-

val meningiomas: Report of 24 cases. Neurosurgery 24:12–17, 1989.

33. Schramm VL Jr: Infratemporal fossa surgery, in Sekhar LN, Schramm VL Jr

(eds): Tumors of the Cranial Base: Diagnosis and Treatment. Mount Kisco, Futura

Pub. Co., 1987, pp 421–437.

34. Sekhar LN, Estonillo R: Transtemporal approach to the skull base: An

anatomical study. Neurosurgery 19:799–808, 1986.

35. Sekhar LN, Jannetta PJ: Cerebellopontine angle meningiomas: Microsurgical

excision and follow up results. J Neurosurg 60:500–505, 1984.

36. Sekhar LN, Jannetta PJ, Burkhart LE, Janosky JE: Meningiomas involving the

clivus:Asix-year experience with 41 patients. Neurosurgery 27:764–781, 1990.

37. Sekhar LN, Schessel DA, Bucur SD, Raso JL, Wright DC: Partial labyrinthec-

tomy petrous apicectomy approach to neoplastic and vascular lesions of the

petroclival area. Neurosurgery 44:537–550, 1999.

38. Sekhar LN, Schramm VL Jr, Jones NF: Operative management of large neo-

plasms of the lateral and posterior cranial base, in Sekhar LN, Schramm VL

Jr (eds): Tumors of the Cranial Base: Diagnosis and Treatment . Mount Kisco,

Futura Publishing Co, 1987, pp 655–682.

39. Sekhar LN, Schramm VL Jr, Jones NF: Subtemporal-preauricular infratempo-

ral fossa approach to large lateral and posterior cranial base neoplasms.

J Neurosurg 67:488–499, 1987.

40. Sekhar LN, Schramm VL Jr, Jones NF, Yonas H, Horton J, Latchaw RE,

Curtain H: Operative exposure and management of the petrous and upper

cervical internal carotid artery. Neurosurgery 19:967–982, 1986.

41. Sen CN, Sekhar LN: The subtemporal and preauricular infratemporal

approach to intradural structures ventral to the brain stem. J Neurosurg73:345–354, 1990.

42. Shiobara R, Ohira T, Kanzaki J, Toya S: A modified extended middle cranial

fossa approach for acoustic nerve tumors. J Neurosurg 68:358–365, 1981.

43. Tator CH, Nedzelski JM: Facial nerve preservation in patients with large

acoustic neuromas treated by a combined middle fossa transtentorial

translabyrinthine approach. J Neurosurg 57:1–7, 1982.

44. Tedeschi H, Rhoton AL Jr: Lateral approaches to the petroclival region. Surg

Neurol 41:180–216, 1994.

45. Wen HT, Rhoton ALJr, Katsuta T, de Oliveira E: Microsurgical anatomy of the

transcondylar, supracondylar, and paracondylar extensions of the far-lateral

approach. J Neurosurg 87:555–585, 1997.

46. Yasargil MG, Mortara RW, Curcic M: Meningiomas of basal posterior cranial

fossa, in Krayenbühl H (ed): Advances and Technical Standards in Neurosurgery.

Vienna, Springer-Verlag, 1980, vol 17, pp 3–115.


RHOTON




OSSEOUS RELATIONSHIPS

FIGURE 1-3. Separate temporal and sphenoid bones have been fitted together along the

squamosal suture. The greater wing of the sphenoid bone forms the anterior wall and the

anterior part of the floor of the middle fossa. The posterior part of the floor of the middle

fossa is formed by the petrous and mastoid parts of the temporal bone. The foramen ovale

and spinosum in the greater sphenoid wing are positioned anterior to the petrous apex. The

trigeminal impression is located on the middle fossa surface of the petrous apex. The arcu-

ate eminence overlies the superior semicircular canal. The tegmen is the site of a paper-thinlayer of bone that roofs the mastoid antrum, the external acoustic meatus, and the tympanic

cavity. The mastoid part of the temporal bone is the site of the mastoid air cells and mas-

toid antrum. The sigmoid sulcus descends along the inner surface of the mastoid part. The

lingual process of the sphenoid bone extends posteriorly toward the petrous apex and par-

tially surrounds the junction of the petrous and cavernous segments of the internal carotid

artery. The petrolingual ligament extends from the lingual process to the petrous apex

above the junction of the petrous and cavernous segments of the internal carotid artery.





FIGURE 1-5. Oblique view of the temporal

and surrounding bones. The petrous and mas-

toid parts of the temporal bone articulate poste-

riorly with the occipital bone to form the lateral

part of the anterior wall of the posterior fossa.

Medially, the petrous part of the temporal bone

articulates along the petroclival fissure with the

clival portion of the occipital bone and the body

of the sphenoid bone to form the medial part of

the anterior wall of the posterior fossa. The

jugular foramen is posi tioned between the

occipital and temporal bone at the inferolateral

edge of the petroclival fissure. The petrous part

of the temporal bone forms the anterior edge,

and the condylar part of the occipital bone forms

the posterior edge of the jugular foramen. The

jugular foramen has three parts: a laterally placed sigmoid part, through which the sigmoid

sinus drains; a smaller medial part, the petrosal

part, through which the inferior petrosal sinus

drains; and an intermediate part, the intrajugu-

lar part, through which the glossopharyngeal,

vagus, and accessory nerves pass.



FIGURE 1-6. Inferior view of both temporal bones and the occipital bone. The

petrous apex fits against the clival part of the occipital bone along the petro-

clival fissure. The jugular foramen is located between the lower ends of the

petroclival fissure and the occipitomastoid suture. The jugular fossa, in which

the jugular bulb resides, is on the lower surface of the petrous part of the tem-

poral bone. The stylomastoid foramen is positioned directly lateral to the jugu-

lar foramen. The external orifice of the carotid canal is located anterior to the

jugular foramen. The right jugular foramen is larger than the left, as is com-

mon. The mandibular fossa, in which the mandibular condyle sits, is located

medial to the root of the zygomatic process.


RHOTON





FIGURE 1-7. Inferior view of the middle

and posterior parts of the cranial base

formed by the sphenoid, temporal , andoccipital bones. The squamosal part of the

temporal bone forms the posterior part of

the floor and lateral wall of the middle

cranial fossa, the roof of the mandibular

fossa in which the mandibular condyle

sits, and the posterior part of the zygo-

matic arch. The tympanic part of the tem-

poral bone forms the anterior, lower, and

part of the posterior wall of the external

canal, part of the osseous floor of the tym-

panic cavity and Eustachian tube, and

the posterior wall of the mandibular fossa.

The mastoid part contains the mastoid air

cells and mastoid antrum. The petrous

part is th e si te of th e au di to ry andvestibular labyrinth, the carotid and facial

canals, and the internal acoustic meatus.

The external orifice of the carotid canal

opens anterior to the jugular foramen.

The jugular fossa, in which the jugular

bulb sits, is located on the lower surface of the petrous part. The stylomastoid

foramen opens between the anterior edge of the digastric groove and the sty-

loid process. The styloid part projects downward behind the tympanic part and

serves as the site of attachment of the three styloid muscles.



FIGURE 1-8. Oblique enlarged infe-

rior view of the right temporal, sphe-

noid, and occipital bones. The tym-

panic part of the temporal bone is positioned between the squamosal part

anteriorly and the petrous and mas-

toid parts posteriorly. The petrous part

of the trigeminal bone is wedged

between the sphenoid and occipital

bones. The petrous apex faces the fora-

men lacerum and is separated from the

clival part of the occipital bone by the

petrocl ival fissure . The dome of the

jugular fossa, in which the jugular

bulb sits, is on the lower surface of the

petrous part. The carotid canal is posi-

tioned anterior to the jugular foramen.

The tympanic canaliculus, located

between the jugular fossa and carotid

canal, is the opening through which

Jacobson’s branch of the glossopharyn-

geal nerve passes to reach the tym-

panic cavity and, finally, the middle

fossa, where it becomes the lesser pet-

rosal nerve.


RHOTON





FIGURE 1-9. Anterior view of a right temporal bone. The squamosal part of the temporal bone

forms the lateral wall and floor of the middle fossa, the posterior part of the zygomatic arch, and

the roof of the mandibular fossa in which the mandibular condyle sits. The zygomatic process of

the squamosal part projects forward to join the zygomatic bone in completing the zygomatic arch.

The tympanic part forms the posterior wall of the mandibular fossa, the anterior, lower, and partof the posterior wall of the external auditory canal and part of the floor of the tympanic cavity

and osseous part of the Eustachian tube. The petrous part, located medial to the squamosal, tym-

panic, and mastoid parts, is the site of the internal acoustic meatus, the acoustic and vestibular

labyrinth, and the facial and carotid canals. The mastoid part is located behind the lateral part

of the tympanic and squamosal parts and is the site of the mastoid air cells and mastoid antrum.



FIGURE 1-10. Lateral view of a right temporal bone. The squamosal

part forms part of the lateral wall and floor of the middle fossa, the pos-

terior part of the zygomatic arch, and the upper surface of the mandibu-

lar fossa. The tympanic part forms the posterior wall of the mandibular

fossa; the anterior wall, lower wall, and part of the posterior wall of the

external canal; and the floor of the tympanic cavity and adjacent osseous

portion of the eustachian tube. The styloid process is ensheathed at its

base by the tympanic part and projects downward, serving as the attach-

ment of three styloid muscles. The mastoid part is located posterior to the

external acoustic meatus and contains the mastoid air cells, which coa-

lesce into a large cavity at the mastoid antrum. The retrolabyrinthine,

translabyrinthine, and transcochlear approaches are directed throughthe mastoid part. The digastric muscle attaches medial to the mastoid tip

in the digastric groove. The oval window is exposed in the medial wall

of the tympanic cavity.


RHOTON





FIGURE 1-11. Enlarged view of the

right external acoustic meatus. Thespine of Henle, located along the

posterosuperior margin of the exter-

nal canal, is positioned superficial

to the deep site of the lateral semicir-

cular canal and the junction of the

tympanic and mastoid segments of

the facial nerve. The mastoid

antrum is positioned deep to the

superficial depressed area, called the

suprameatal triangle, located above

and behind the spine of Henle. The

medial wall of the tympanic cavity

is the site of the promontory, which

overlies the basal turn of the cochlea

and the oval and round windows.The footplate of the stapes sits in the

oval window. The round window is

separated from the cochlea by a thin

membrane.



FIGURE 1-12. Superior view of the

left and right temporal bones. The

medial part of the upper surface is the

site of the trigeminal impression in

which the trigeminal nerve and gan-

glion and Meckel’s cave sits. Fartherlaterally is the prominence of the

arcuate eminence overlying the supe-

rior semicircular canal. Lateral to the

arcuate eminences is the tegmen, a

thin plate of bone roofing the mastoid

antrum, epitympanic area, and exter-

nal acoustic meatus. The temporal

bone articulates anteriorly with the

sphenoid bone, above with the pari-

etal bone, and posteriorly with the

occipital bone. The zygomatic process

of the squamosal part has an anterior

and a posterior root, between which,

on the lower surface, is located the

mandibular condyle.


RHOTON





FIGURE 1-13. The floor of the right

middle fossa has been drilled to

remove bone and air cells and expose

the osseous capsule of the cochlea,

semicircular canals, and internal

acoustic meatus. The cochlea is

located anteromedial to the fundus of

the meatus. The superior, lateral, and

posterior semicircular canals are sit-

uated posterolateral to the fundus of

the meatus. The transverse crest sep-

arates the fundus of the meatus into

upper and lower parts. The facial and

superior vestibular nerves course

above and the cochlear and inferiorvestibular nerves course below the

transverse crest.



FIGURE 1-14. Posterior view of a right temporal bone. The squamosal part forms part of the

floor and lateral wall of the middle fossa. The sigmoid sulcus descends along the inner surface of

the mastoid portion. The porus of the internal acoustic meatus opens onto the central portion of

the posterior surface of the petrous part. The trigeminal impression, trigeminal prominence,

meatal depression, and arcuate eminence are located on the upper surface of the petrous part. The

endolymphatic duct connects the vestibule in the petrous part with the endolymphatic sac, which

sits on the posterior petrous surface inferolateral to the internal acoustic meatus. The intrajugu-

lar process separates the petrosal and sigmoid parts of the jugular foramen.


RHOTON





FIGURE 1-15. Enlarged view of Figure

1-14. The upper edge of the petrous ridge

is the site of two shallow depressions, the

trigeminal impression and the meataldepression, and two elevations, the

trigeminal prominence and the arcuate

eminence. The trigeminal impression, in

which Meckel’s cave and the enclosed part

of the trigeminal nerve sits, is located on

the medial part of the upper surface. The

trigeminal prominence is positioned at the

lateral edge of the trigeminal impression.

The area between the trigeminal promi-

nence and the arcuate eminence is the site

of another shallow depression, the meatal

depression, which is positioned above the

internal acoustic meatus. The tegmen, a

pap er- thi n area of bone tha t roo fs the

external canal, mastoid antrum, and tym-

panic cavity, is positioned on the upper

surface lateral to the arcuate eminence.

The subarcuate fossa, through which the

subarcuate branch of the anterior inferior

cerebellar artery passes, is located supero-

lateral to and the hiatus of the endolymphatic duct is positioned inferolateral to the internal acoustic meatus.



FIGURE 1-16. The posterior surface of

the right temporal bone has been drilled

while preserving the bone around the

internal acoustic meatus, semicircular

canals, and cochlea. The superior canal protrudes upward toward the arcuate

eminence in the floor of the middle fossa.

The posterior canal is exposed on the lat-

eral side of the superior canal. The poste-

rior end of the superior canal and the

upper end of the posterior canal join to

form the common crus, which opens into

the vestibule. The mastoid air cells have

been removed from the petrous apex. The

cochlea bulges upward anteromedial to

the fundus of the internal acoustic mea-

tus. Some of the mastoid air cells medial

to the sigmoid sulcus have been removed.

The cochlear aqueduct, which ends just

above the petrosal part of the jugular fora-men, has been preserved. The endolym-

phatic sac sits on the posterior surface of

the temporal bone below the superior and

lateral canals. The intrajugular processes

of the temporal and occipital bones separates the petrosal and sigmoid parts of

the jugular foramen. The glossopharyngeal, vagus, and accessory nerves exit

the cranium through the intrajugular part of the jugular foramen located

between the petrosal and sigmoid parts.


RHOTON



FIGURE 1-18. View of the fundus

of the right internal acoustic meatus.The transverse crest divides the

meatal fundus into superior and

inferior parts. Above the transverse

crest, the facial canal is anterior and

the superior vestibular area is poste-

rior. The facial canal and the supe-

rior vestibular area are separated by

the vertical crest (Bill’s Bar). Below

the transverse crest, the cochlear area

is anterior and the inferior vestibular

area is posterior. The singular fora-

men, through which the singular

branch of the inferior vestibular

nerve passes to innervate the poste-

rior canal ampullae, is located poste-

rior to the inferior vestibular area.

The inferior vestibular nerve also has

a saccular and, occasionally, a utric-

ular branch.


RHOTON



MIDDLE FOSSA: ANATOMICVIEW



CHAPTER 2

FIGURE 2-1. Middle fossa, anatomic view. Middle fossa surface of the temporal bone. The dura has been elevated from the floor of the middle fossa. Thetentorium, except for the attachment along the petrous ridge and superior

petrosal sinus, has been removed. The petrosphenoid ligament (Gruber’s lig-ament) forms the roof of Dorello’s canal, through which the abducens nerve

passes on the medial side of the first trigeminal division. The trigeminalnerve sits in a depression on the upper surface of the petrous part. At the lat-eral edge of the trigeminal impression, the floor of the middle fossa, adjacentto the sphenoid ridge, rises upward to form the trigeminal prominence. The

posterior part of the floor then settles into another depression between the

trigeminal prominence and the arcuate eminence. The depression between thetrigeminal prominence and the arcuate eminence, the meatal depression,roofs the majority of the internal acoustic meatus. The bone in the area lat-eral to the arcuate eminence, referred to as the tegmen, is usually paper-thin.The tegmen forms part of the roof of the external auditory canal, tympaniccavity, and mastoid antrum and air cells. The greater petrosal nerve isexposed directly under the dura of the middle fossa. In this case, the termi-nal part of the petrous carotid artery is also exposed under the dura andbelow the greater petrosal nerve, as occurs in approximately 15% of tempo-

ral bones. The petrous carotid artery is usually covered by bone up to the lat-eral side of the third trigeminal division.




RHOTON

FIGURE 2-2. The bone above the internal acousticmeatus has been removed. The meatus is situatedbelow the meatal depression. In the past, the drilling to

expose the internal acoustic meatus by the middle fossa approach was begun above the greater petrosalnerve, the geniculate ganglion, and the distal part of the labyrinthine segment. From there, the drilling wasdirected proximally from the fundus to the porus of themeatus. It is now common practice to begin thedrilling above the porus at the level of the petrousridge. It is in this area that the meatus is the widestand easiest to identify. From there, the drilling isdirected distally toward the fundus of the meatus andthe labyrinthine segment of the facial nerve. The ante-rior wall of the meatus is usually located 6 to 9 mmlateral to the point the trigeminal nerve crosses the

petrous ridge. Another method used to identify theapproximate site of the porus is to measure the angle

between the arcuate eminence or superior semicircularcanal and the greater petrosal nerve, and to begin thedrilling at the point that a line bisecting that anglewould cross the petrous apex. The superior semicircu-lar canal underlies the arcuate eminence, although, asseen here, it may not sit directly under the most prominent area and, in somecases, it may be separated from the floor of the middle fossa by a several mil-limeter layer of mastoid air cells. The canal is positioned slightly medial to

the arcuate eminence, as shown here, more often than it is situated directlyunder or lateral to the most prominent part of the arcuate eminence.





FIGURE 2-5. Add it ional bone has beenremoved anterior to the greater petrosal nerveto expose the tensor tympani and eustachian

tube, which are separated from each otherand from the terminal part of petrous carotidby a thin shell of bone. The cochlea is exposedin the angle between the labyrinthine seg-ment of the facial nerve and the greater pet-rosal nerve. The vestibule into which thesemicircular canals open is positioned behindthe fundus of the meatus. The tensor tym-

pani is a long slender musc le. Its tendonturns sharply laterally around the trochlear-iform process to attach to the upper part of the handle of the malleus. The temporal bone,when viewed from above, is organized similarto the letter “Y.” The lower single limb of theY is located along the external canal. The

upper two limbs of the Y are directed alongthe internal acoustic meatus posteriorly andthe eustachian tube anteriorly. Thus, thein t erna l and ex t e rna l meat i and th eeustachian tube together, when viewed fromabove, have a configuration similar to the let-ter “Y.” The labyrinth, which wraps around the fundus of the meatus and the tympanic cavity, is located at the junction of the three limbs of the Y.





FIGURE 2-7. Superior view of the

floor of the middle fossa after expos-ing the cochlea and vestibule. Thevestibule has been exposed below theanterior ends of the superior and lat-eral canals. The vestibule communi-cates, below the fundus of the mea-tus, with the cochlea. The cochlea islocated below the floor of the middle

fo ss a in th e an gl e be tw ee n th elabyrinthine segment of the facialnerve and the greater petrosal nerve.The vertical segment of the petrouscarotid turns medially to form thehorizontal segment at its lateralbend, which is positioned below the

cochlea. Fibers from the cervicalsympathetic ganglia ascend on thesurface of the carotid artery. Thearticulation of the lenticular processof the incus with the head of thestapes is seen below the anterior por-tion of the lateral semicircular canal.




RHOTON

FIGURE 2-8. Superolateral view. Thecisternal segment of the facial nerve hasbeen retracted to expose the nervus inter-medius. The petrous apex, which extendsbelow the trigeminal nerve and up to theside of the clivus, has been preserved. Thetympanic segment of the facial nerve

passes below the latera l semicircularcanal and turns downward to form themastoid (vertical or descending) seg-

ment.





FIGURE 2-9. Superolateral view. The pe trous apex medi al to the in te rnalacoustic meatus and posterior to the

petrous segment of the internal carotidartery has been removed, as would be per-

fo rmed in an anteri or petros ec tomyapproach. The exposure extends to the lat-eral edge of the clivus and inferior pet-rosal sinus. The abducens nerve passesabove the anterior inferior cerebellarartery and through the inferior petrosalsinus. The oculomotor nerve enters theroof of the cavernous sinus just below theorigin of the posterior communicating

artery from the internal carotid artery.The trochlear nerve passes along thelower margin of the tentorial edge. Thesuperior cerebellar artery sits on theupper edge of the trigeminal nerve.





FIGURE 2-11. Superolateral view of the right middle fossa in another speci-men. The bone has been removed toexpose the nerves in the internalacoustic meatus. The cochlea is enclosedin the bone in the angle between thelabyrinthine segment of the facial nerveand the greater petrosal nerve. Theincus and malleus are exposed in theepitympanic area. The superior semicir-cular canal, which sits below the medial

side of the arcuate eminence, has beenunroofed.





FIGURE 2-13. The trigeminal nerve has beenremoved to expose the cavernous sinus. The greater

petrosal nerve passes forward and medially, where itis joined by the deep petrosal branch of the carotid plexus to form the Vidian nerve. The abducens nerve passes under the petrosphenoid ligament to enter thecavernous sinus. A portion of the petrous apex belowthe trigeminal nerve has been removed. Removal of the floor of the middle fossa exposes the pterygoidmuscles and venous complex and branches of themandibular nerve and maxillary artery in theinfratemporal fossa. The temporalis muscle fills thetemporal fossa. A., artery; A.I.C.A., anterior inferiorcerebellar artery; Ac., acoustic; Arc., arcuate; Bas.,basilar; Car., carotid; Chor., chorda; Cist., cisternal;CN, cranial nerve; Coch., cochlear; Emin., eminence;Eust., eustachian; Ext., external; Gang., ganglion;Gen., geniculate; Gr., greater; Inf., inferior, infero;

Infratemp., infratemporal; Intermed., intermedius;Laby., labyrinthine; Lat., lateral; Lig., ligament;Ling., lingual; M., muscle; Mast., mastoid; Meat.,meatal; Memb., membrane; Men., meningeal; Mid.,middle; N., nerve; Nerv., nervus; Pet., petro, pet-rosal, petrous; Post., posterior; Proc., process; Prom.,

prominence; S.C.A., superior cerebellar artery; Seg., segment; Semicirc.,semicircular; Sig., sigmoid; Sphen., sphenoid; Sup., superior; Temp., tempo-

ral; Tens., tensor; Tent., tentorial; Trig., trigeminal; Troch., trochleariform;Tymp., tympani, tympanic; Vert., vertical; Vest., vestibular.




MIDDLE FOSSA SURGICALAPPROACH

FIGURE 3-4. Superior view of the auditoryossicles. The auditory ossicles are connectedto the walls of the tympanic cavity by liga-

ments, three for the malleus and one each forthe incus and stapes. The anterior ligament of the malleus is attached at one end to the neck and at the other end to the anterior wall of thetympanic cavity close to the petrotympanic

fissure. The lateral ligament of the malleus(not shown) is a triangular band passing

from the posterior part of the border of thetympanic incisura to the head of the malleus.The superior ligament of the malleus (notshown) attaches the head of the malleus tothe roof of the epitympanic area. The posteriorligament of the incus connects the end of theshort process of the fossa incudis, a shallowdepression in which the tip of the short

process sits . The superior ligament of theincus is a small fold of mucus membrane

passing from the body of the incus to the roof of the epitympanic area. The circumference of the base of the stapes is attached to the mar-

gin of the oval window by a ring of elastic fibers termed the anular ligament of the base of the stapes. The tendon of thetensor makes a right angle turn around the trochleariform process to attach

to the upper part of the handle of the malleus. The stapedial tendon attachesto the neck of the stapes.





FIGURE 3-8. Enlarged view. The geni cu la te gang li on an d th e di st allabyrinthine and proximal tympanicsegments of the facial nerve are exposeddirectly under the dura. The ganglion isexposed without a bony covering inapproximately 15% of temporal bones.Trauma to the ganglion during eleva-tion of the dura may cause a facial

pal sy. The middl e menin gea l art erysends a small branch along the greater

petrosal nerve to the geniculate gan-

gl ion and adjacent segments of the fa ci al ne rv e. Oc cl ud in g th is sm al lartery may cause facial paralysis.





FIGURE 3-10. The dura lining the internal acoustic meatus hasbeen opened to expose the cisternal, meatal, labyrinthine, and tym-

panic segments of the facial nerve; the super ior, inferior, andcochlear nerves; two rootlets of the nervus intermedius; and the

geniculate ganglion.





FIGURE 3-11. The dri l l ing has beenextended to expose the cochlea along theanteromedial edge of the fundus of the meatusand the vestibule and semicircular canalsalong the posterolateral margin of the meatal

fundus. Care must be taken in the middle fossa approach to the internal acoustic meatusto avoid entering the cochlea and vestibule. If either the cochlea or vestibule is entered, hear-ing will be lost. The transverse crest dividesthe fundus of the meatus into an upper and alower compartment. The facial and superiorvestibular nerves course above the transversecrest and the cochlear and inferior vestibularnerves pass below the crest. The facial nerve

passes through the anterior-superior quad-rant, the inferior vestibular nerve passesthrough the posterior-superior quadrant, the

cochlear nerve passes through the anterior-inferior quadrant, and the inferior vestibularnerve passes through the posterior-inferiorquadrant of the meatal fundus. The verticalcrest (Bill’s bar) separates the facial and supe-rior vestibular nerves.





FIGURE 3-12. The anterior petro-sectomy directed through the areamedial to the internal acoustic mea-tus has been completed. The exposureis directed posterior to the greater

petrosal nerve and below the trigem-inal nerve to the lateral edge of theclivus adjacent to the inferior petrosalsinus, and to the posterior fossa durathat faces the brainstem.





FIGURE 3-14. Drilling has been completed andthe dura opened. The exposure extends below thetrigeminal nerve and to the anterolateral surfaceof the pons and lateral edge of the clivus. Theanterior inferior cerebellar artery passes next tothe abducens nerve in the lower part of the expo-sure. The abducens nerve ascends and passesbelow the petrosphenoid (Gruber’s) ligament toreach the cavernous sinus between the cavernouscarotid artery and the first trigeminal division.The exposure extends superiorly to the level of the oculomotor nerve. This approach can be

directed to a basilar bifurcation located below thelevel of the dorsum sellae or to selected lesionsalong the trunk of the basilar artery or anterolat-eral brainstem. The approach also provides a

pathway to the clivus that avoids going throughthe oral cavity.





FIGURE 3-17. Enlarged superior view of the junction of the eustachian tube and the internaland external meati. The mastoid air cells on the

lateral side of the semicircular canals have beenremoved. The superior vestibular, inferiorvestibular, facial, and cochlear nerves areexposed at the fundus of the meatus. Thecochlear nerve enters the modiolus of the cochlea.The vertical crest (Bill’s Bar) separates the facialand superior vestibular nerves at the fundus of the meatus. The superior vestibular nerve inner-vates the ampullated anterior ends of the supe-rior and lateral canals. The inferior vestibularnerve innervates the lower or ampullated end of the posterior canal. The chorda tympani crossesthe tympanic membrane and the upper part of the handle of the malleus. The greater petrosalnerve passes medially above the petrous carotid.The cochlea sits in the angle between thelabyrinthine segment of the facial nerve and the

greater petrosal nerve. The lesser petrosal nervecrosses above the tensor tympani and exits thecranium near the foramen ovale to reach the otic

ganglion and innervate the parotid gland.





FIGURE 3-18. The mastoid antrumand air cells, which are positionedbehind the external canal and lateralto the semicircular canals andvestibule, have been removed. Notethat the internal and external audi-tory canals are located in a straightline from each other. The cochlea isexposed below the floor of the middle

fossa in the angle between the greater petrosal nerve and the labyrinthinesegment of the facial nerve. The

petrous carotid is exposed below the greater petrosal nerve and behind theeustachian tube. The chorda tympanicrosses the inner surface of the tym-

panic membrane. The facial andsuperior vestibular nerves pass above

and the cochlear and inferior vestibu-lar nerves pass below the transversecrest. The vertical crest (Bill’s Bar)separates the facial and superiorvestibular nerves at the fundus of themeatus. The lower segment of thesigmoid sinus has been preserved.





FIGURE 3-20.Lateral view. The anterior surface of the temporalbone faces the posterior part of the infratemporal fossa, which con-

tains the branches of the maxillary artery and mandibular nerveand the pterygoid muscles and venous plexus. The infratemporal

fossa opens medially into the pterygopalatine fossa. The maxillarynerve passes through the foramen rotundum to enter the ptery-

gopalatine fossa and send branches along the orbital floor. Theophthalmic nerve passes through the superior orbital fissure andsends branches along the orbital roof. Bone has been removed toexpose the sphenoid sinus above and below the maxillary nerve,and the vidian nerve below the maxillary nerve. A., artery;A.I.C.A., anterior inferior cerebellar artery; Ac., acoustic; Ant.,anterior; Arc., arcuate; Bas., basilar; Car., carotid; Cav., cav-ernous; Chor., chorda; CN, cranial nerve; Coch., cochlear; Cond.,condyle; Emin., eminence; Eust., eustachian; Ext., external; Fiss.,

fissure; Flocc., flocculus; Gang., ganglion; Gen., geniculate; Gr., greater; Inf., inferior; Int., internal; Intermed., intermedius;Laby., labyrinthine; Lat., lateral; Lent., lenticular; Less., lesser;Lig., ligament; Ling., lingual; M., muscle; Mandib., mandibular;Max., maxillary; Meat., meatal; Memb., membrane; Men.,meningeal; Mid., middle; N., nerve; Nerv., nervus; Ophth., ophthalmic;Orb., orbital; Pet., petro, petrosal, petrous; Post., posterior; Proc., process;Prom., prominence; Pteryg., pterygoid; Pterygopal., pterygopalatine; S.C.A.,superior cerebellar artery; Seg., segment; Semicirc., semicircular; Sig., sig-

moid; Sphen., sphenoid; Stap., stapedial; Sup., superior; Temp., temporal,temporalis; Tens., tensor; Tent., tentorial, tentorium; Transv., transverse;Trig., trigeminal; Troch., trochleariform; Tymp., tympani, tympanic; Vert.,vertical; Vest., vestibular; Zygo., zygomatic.




CHAPTER 4

ANTERIORVIEW

FIGURE 4-1. Anterior view of a stepwise dissection of a cross section throughthe anterior part of the temporal bone. The coronal section crosses the tempo-ral lobe and floor of the middle fossa just anterior to the external canal andtympanic part of the temporal bone. The mandibular condyle has been removed

from the mandibular fossa. The posterior margin of the mandibular fossa is formed by the tympanic part of the temporal bone, which also forms the lower

and anterior wall and part of the posterior wall of the external canal. Threemuscles arise from the styloid process, which projects downward, and isensheathed at its base by the tympanic part of the temporal bone. The internalcarotid artery ascends medial and slightly posterior to the styloid process toenter the carotid canal. The facial nerve exits the stylomastoid foramen postero-lateral to the styloid process.

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RHOTON

FIGURE 4-5. Enlarged view. The tendon of the stapedial muscleextends forward from the apex of the pyramidal eminence to insert onthe neck of the stapes. The chorda tympani crosses the upper part of thetympanic membrane and the handle of the malleus in the area lateralto the long process of the incus. The footplate of the stapes sits in theoval window. The promontory in the medial wall of the tympanic cav-

ity overlies the basal turn of the cochlea. The niche leading to the roundwindow is located below the promontory. The lenticular process of theincus articulates with the head of the stapes.



FIGURE 4-6. Additional bone hasbeen removed behind the epitym-

panic area and head of the malleus

and body of the incus to expose thesuperior semicircular canal. The prom on to ry ha s be en dr il le d toexpose the basal turn of the cochlea.The osseous spiral lamina projectsinto the area between the scala tym-

pani and the scala vestibuli. The del-icate membranes that surround andsupport the cochlear duct attach tothe spiral lamina and the outer wallof the cochlea. The scala tympani isseparated from the tympanic cavityby a thin membrane across the roundwindow. The oval window, in whichthe footplate of the stapes sits, leads

from the tym panic cavity to the

vestibule of the inner ear. The stapeshas been removed from the oval win-dow. The pyramidal eminence isexposed below the segment of thechorda tympani ascending to reachthe upper part of the handle of themalleus.


ANTERIORVIEW




CHAPTER 5

LATERAL VIEW

FIGURE 5-1. Lateral view of the temporal bone. Stepwise dissec-tion. The tympanic part of the temporal bone forms the anteriorand lower walls and part of the posterior wall of the externalcanal, part of the floor of the tympanic cavity, and the osseous por-tion of the eustachian tube. The facial nerve exits the craniumthrough the stylomastoid foramen medial to the lower part of thetympanomastoid suture at the anterior end of the digastric groove.The spine of Henle, at the junction of the upper and posterioredge of the external canal, approximates the deep site of the tym-

panic segment of the facial nerve and the lateral semicircularcanal. The suprameatal triangle, a depressed area posterosuperiorto the external canal and behind the spine of Henle, is locatedsuperficial to the mastoid antrum. The antrum is located superfi-cial to the semicircular canals. The squamosal and tympanic partsof the temporal bone form the upper and posterior surfaces of themandibular fossa.

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LATERALVIEW

FIGURE 5-2. A mastoidectomy hasbeen completed to expose the osseouscapsule of the posterior and lateralcanals, the sigmoid sinus, and thedura of the middle fossa. The jugularbulb is medial to the cortical boneabove the digastric groove. Thechorda tympani crosses the innersurface of the tympanic membrane.



FIGURE 5-6. The malleus and tympanic membrane have been removed.The lateral portion of the lateral semi-circular canal has been removed andthe vestibule is exposed deep in thearea of the ampullated end of the lat-eral canal. A portion of the superiorand lateral canals have also beenremoved. The posterior end of thesuperior canal and the upper end of the posterior canal join deep to thebridge of bone outlined by the arrowsto form the common crus.


LATERALVIEW




LATERAL CRANIAL BASE

FIGURE 6-3. The parotid gland has beenremoved to expose the facial nerve and itsbranches distal to the stylomastoid foramen.The nerve passes lateral to the styloid process,the external carotid artery, and mandibularneck. The masseter muscle has two heads: amore superficial anterior head, which passesdownward to the lateral surface of the angle of the jaw, and a deeper posterior head, whicharises from the medial surface of the zygomaticarch and passes to the mandibular body. Thislower end of the sternocleidomastoid muscle

has been reflected backward by dividing itsattachment to the clavicle and sternum. Thesuperficial temporal artery ascends in front of the ear.




RHOTON

FIGURE 6-4. The upper part of themandibular ramus and the lower

part of the temporalis muscle and itsattachment to the coronoid processhave been removed while preservingthe inferior alveolar and lingualnerves. The infratemporal fossa islocated medial to the mandible on thedeep side of the temporalis muscle.The exposure includes the upper andlower heads of the lateral pterygoidmuscle, which insert along the tem-

poromandibular joint, and the superficial head of the medial pterygoid,which extends from the lateral ptery-

goid plate to the angle of the jaw. Thestructures in the infratemporal fossainclude the pterygoid muscles,branches of the mandibular nerve,th e max i l lary ar t e ry and i t sbranches, and the pterygoid venous

plexus. Pathology involving the temporal bone may extend to involve theinfratemporal fossa.



FIGURE 6-5. The lateral pterygoidmuscles have been removed toexpose the branches of the trigemi-nal nerve passing through the fora-

men ovale to convey sensation f romthe mandibular area and to supplythe muscles in the infratemporal

fossa. The middle meningeal artery passes between two rootlets of theauriculotemporal nerve to reach the

foramen spinosum. The lingual andinferior alveolar branches of thethird division have been preserved.Removal of the remaining part of the ramus of the mandible exposesthe styloid process and adjacentmuscles. The deep temporal arteriesand nerves course along to the

periosteal surface of the sphenoidand temporal bones to reach thedeep surface of the temporalis mus-cle. Preserving the temporalis mus-cle requires that the muscle be ele-vated using careful subperiostealdissection because its nerve andvascular supply course on the deep

periosteal surface. Loss of the deep temporal nerve and arteries will result in temporalis atrophy and a poor cosmetic result after surgery.





FIGURE 6-7. The longissimus capitis hasbeen reflected downward to expose the rectuscapitis posterior minor and major, whichdescend from the occipital bone to attach tothe spinous process of C1 and C2, respectively;the superior oblique, which passes from theoccipital bone to the transverse process of C1;and the inferior oblique, which extends fromthe spinous process of C2 to the transverse

process of C1. The site of passage of vertebralartery behind the atlanto-occipital joint islocated deep in the suboccipital trianglelocated between the superior and inferior

oblique and the rectus capitis posterior majormuscles. The C1 transverse process is situatedimmediately behind the internal jugular veinand a short distance below and behind the

jugular foramen.






RHOTON

FIGURE 6-8. The superior and infe-rior oblique and rectus capitus poste-rior major muscles have been removedto expose the vertebral artery passingbehind the atlanto-occipital joint. Therectus capitus lateralis muscle extendsupward from the transverse process of C1 to attach to the occipital bone behindthe jugular foramen. The vertebralartery courses in the depths of the sub-occipital triangle located between theinferior and superior oblique muscles

and the rectus capitus posterior major,all of which have been removed.



FIGURE 6-11. The external audi-tory canal has been removed but thetympanic membrane and cavity havebeen preserved. The levator and ten-sor veli palatini muscles have beenremoved and the membranous part of the eustachian tube has been opened.The eustachian tube crosses anteriorto and is separated from the petrouscarotid by a thin shell of bone. The

jugular bulb and lateral bend of the petrous carotid are located below theacousticovestibular labyrinth. Thesecond trigeminal division exits the

foramen rotundum and enters the pterygopalatine fossa. The third divi-sion exits the foramen ovale to reachthe infratemporal fossa.






RHOTON

FIGURES 6-16–6-18. Another specimen. Thecortical bone surrounding the osseous part of the external canal, the semicircular canals, andthe incus and malleus has been preserved. The

petrous apex in front of the labyrinth has beenremoved to expose the petrous carotid. A shortsegment of the tensor tympani muscle has been

preserved. The fac ial and vest ibulocochl earnerves arise at the junction of the pons andmedulla. The labyrinthine, tympanic, and mas-toid segments of the facial nerve and thebranches of the mandibular nerve in theinfratemporal fossa have been preserved.




CHAPTER 7

RETROLABYRINTHINE, TRANSLABYRINTHINE,AND TRANSCOCHLEARAPPROACHES

FIGURE 7-1. Mastoidectomy and retrolabyrinthine, partial labyrinthine,translabyrinthine, and transcochlear approaches. Right mastoid. The retroau-ricular flap and the sternocleidomastoid muscle have been reflected forwardand the trapezius and underlying splenius capitus have been reflected back-ward to expose the mastoid and attachment of the longissimus capitus muscle.The posterior belly of the digastric muscle originates medial to the mastoid tipalong the digastric groove. The spine of Henle is positioned at the posteriorsuperior margin of the external meatus. The spine is positioned superficial to

the deep site of the lateral semicircular canal and junction of the tympanic andmastoid segments of the facial nerve. The supramastoid crest, a continuationof the superior temporal line, is positioned at approximately the level of theupper margin of the transverse and sigmoid sinuses. The area below the ante-rior part of the supramastoid crest and behind the spine of Henle, called thesuprameatal triangle, is positioned superficial to the mastoid antrum. Thesemicircular canals are positioned deep to the mastoid antrum.

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RHOTON

FIGURE 7-2. The superficial mastoid air cellshave been removed. The air cells coalesce in thearea deep to the suprameatal triangle to form themastoid antrum, which is positioned lateral tothe bone enclosing the semicircular canals.




TRANSLABYRINTHINE APPROACHES

FIGURE 7-3. The drilling has beenextended through the mastoid antrum toexpose the epitympanic area where the bodyof the incus and head of the malleus reside.The facial recess, located between the mas-toid segment of the facial nerve and thechorda tympani, has been opened to exposethe long process of the incus and the articu-lation of the lenticular process of the incuswith the head of the stapes. The bridge of bone, positioned posterior to the tip of theshort process of the incus, between the epi-tympanic area and facial recess, is referred

to as the “buttress.” The chorda tympaniarises from the lower portion of the mastoidsegment of the facial nerve, ascends ante-rior to the facial recess, and crosses the innersurface of the tympanic membrane.



FIGURE 7-5. Enlarged view. The short process of the incus points toward the

tympanic segment of the facial nerve passing between the lateral semicircularcanal and the stapes sitting in the ovalwindow. The superior and lateral canalampullae, located at the anterior end of these canals, are innervated by the supe-rior vestibular nerve. The posterior canalampulla, located at the lower end of the

posterior canal, is innervated by the singular branch of the inferior vestibularnerve. In the translabyrinthine approach,drilling through the anterior (ampul-lated) ends of the superior and lateralcanals exposes the superior vestibulararea and nerve at the fundus of the mea-

tus. Drilling the lower (ampullated) endof the posterior canal exposes the infe-rior vestibular area and nerve at the fun-dus of the meatus. The posterior end of the superior canal and the upper end of the posterior canal join to form a singlechannel, the common crus, which opensinto the vestibule.





FIGURE 7-7. A partial labyrinthec-tomy has been completed by remov-ing the posterior and superior semi-circular canals. Silver and black sutures mark the previous position of the superior and posterior canals.The lateral canal has been preserved.Removing these two canals may notresult in a total loss of hearing. Thechance of preserving some hearingafter this type of partial labyrinthec-tomy is improved if the drilled endsof the two canals are obliterated withbone dust or wax or other material.The upper end of the posterior canaland the posterior end of the superiorcanal join to form the common crus.






RHOTON

FIGURE 7-10. The dura has beenopened to expose the Cranial Nerves inthe cerebellopontine angle. The vestibulo-cochlear nerve has been depressed toexpose the facial nerve and the nervusintermedius. The motor root of thetrigeminal nerve has been exposed supe-rior and medial to the main sensory root.The glossopharyngeal and vagus nervesare at the lower margin of the exposure

just above the jugular bulb. The flocculusand choroid plexus protrude from the

foramen of Luschka behind the vestibulo-cochlear nerve. The anterior inferior cere-

bellar artery loops laterally between the facial and vestibulocochlear nerves. Asmall branch of the posterior inferiorcerebellar artery descends posterior to the

glossopharyngeal and vagus nerves.




RHOTON

FIGURE 7-14. Enlarged view. Theanterior inferior cerebellar arteryloops laterally to the fundus of themeatus. The cochlear nerve pene-trates the modiolus at the fundus of the meatus. The scala tympani andvestibuli and the osseous spiral crestin the cochlea are exposed.



FIGURE 7-15. Removal of thecochlea opens the channel for remov-ing the remainder of the petrousapex. The exposure, directed belowthe trigeminal nerve, extends medi-ally to the front of the pons andmedulla and to the lateral side of thebasilar artery. The abducens nerveascends lateral to the basilar artery.The tympanic membrane has beenremoved. Removing the cochlea and

petrous apex exposes a short segmentof the petrous carotid artery.






RHOTON

FIGURE 7-16. Enlarged view of the completed transcochlear approach. The expo-sure extends to the lateral edge of the clivusand the inferior petrosal sinus. The basilarartery and anterior surface of the pons areat the deep end of the exposure. A high

jugular bulb may block access to the areabelow the glossopharyngeal nerve. Theabducens nerve passes behind the anteriorinferior cerebellar artery and lateral to thebasilar artery. A., artery; A.I.C.A., ante-rior inferior cerebellar artery; Asc., ascend-ing; Bas., basilar; Br., branch; Cap., capi-tis; Car., carotid; Chor., chorda; CN ,cranial nerve; Coch., cochlear; Comm.,common; Endolymph., endolymphatic;Epitymp., epitympanic; Flocc., flocculus;Gang., ganglion; Gen., geniculate; Inf.,inferior; Intermed., intermedius; Jug. ,

jugular; Laby., labyrinthine; Lat., lateral;Lent., lenticular; Longiss., longissimus;M., muscle; Mast., mastoid; Meat., meatal;Memb., membrane; Mid., middle; N.,nerve; Nerv., nervus; Pet., petrosal,

petrous; Pharyng., pharyngeal; Post., pos-terior; Proc., process; Seg., segment; Semicirc., semicircular; Sig., sigmoid;Sp ., spine; Sternocleidomast., sternocleidomastoid; Sup., superior;

Suprameat., suprameatal; Triang., triangle; Tymp., tympani, tympanic;Vest., vestibular.




CHAPTER 8

PRESIGMOID APPROACH

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FIGURE 8-1. Presigmoidapproach, left presigmoidapproach. The scalp incisionis shown on the lower left.The mastoidectomy hasbeen completed and thedense cortical bone aroundthe semicircular canals hasbeen exposed. The tympanicsegment of the facial nerveand the lateral canal are situated deep to the spine of Henle.

Trautmann’s triangle, the patch of dura in front of the sigmoid sinus, faces the cerebellopontine angle. The endolymphatic sac sits beneaththe dura in Trautmann’s triangle.




RHOTON

FIGURE 8-2. Enlarged view. The presigmoid dura is opened anterior tothe sigmoid sinus. The incision, out-lined with strips of suture, shouldcross the superior petrosal sinus a fewmillimeters forward of the area shownin this illustration. The temporaldura is opened and the tentorium isdivided, taking care to preserve thevein of Labbé that joins the transversesinus and the trochlear nerve thatenters the anterior margin of the ten-torium.




PRESIGMOIDAPPROACH

FIGURE 8-3. Retrolabyrinthine presigmoid exposure in which thesemicircular canals have been pre-served. The presigmoid dura hasbeen opened and the superior pet-rosal sinus and tentorium divided.The abducens and facial nerves areexposed medial to the vestibulo-cochlear nerve. The anterior inferiorcerebellar artery passes below thevestibulocochlear nerve. The supe-rior cerebellar artery passes abovethe trigeminal nerve. The posteroin-

ferior cerebellar artery courses in thelower margin of the exposure with

the glossopharyngeal and vagusnerves. Choroid plexus protrudesinto the cerebellopontine anglebehind the glossopharyngeal andvagus nerves.




RHOTON

FIGURE 8-4. The vestibulocochlearnerve has been retracted downwardto expose the nervus intermedius and

facial nerve. The trigeminal nerve isat the upper margin of the exposure.The motor root of the trigeminalnerve arises as a series of rootlets

positioned superomedial to the mainsensory root. The glossopharyngeal,vagus, and accessory nerves are at thelower margin of the exposure. The

flocculus protrudes into the cerebel-

lopontine angle behind the glossopha-ryngeal nerve.




PRESIGMOIDAPPROACH

FIGURE 8-5. The semicircular canals and vestibulehave been removed to complete the translabyrinthineapproach to the internal acoustic meatus and cerebel-lopontine angle and to expose the vestibulocochlearand facial nerves in the internal acoustic meatus.The temporal lobe has been elevated. The segment of the trochlear nerve that passes below the medial edgeof the tentorium and the junction of the vein of Labbé with the transverse sinus have been preserved. The

jugular bulb is at the lower margin of the exposure.




RHOTON

FIGURE 8-6. The cochlea has been removedto complete a transcochlear exposure thatextends to the lateral margin of the clivus

and the inferior petrosal sinus. The verticalsegment of the petrous carotid is exposedanterior to the jugular bulb. The lateral sideof the basilar artery and the anterior surfaceof the pons are in the depths of the exposure.The superior cerebellar artery passes belowthe trochlear nerve and above the trigeminalnerve. A., artery; A.I.C.A., anterior inferiorcerebellar artery; Bas., basilar; Car., carotid;Chor., chorda, choroid; CN, cranial nerve;Endolymph., endolymphatic; Flocc., floc-culus; Inf., inferior; Intermed., inter-medius; Jug., jugular; Lat., lateral; Mast.,mastoid; Meat., meatal; Memb., membrane;Mid., middle; N., nerve; Nerv., nervus;P.I.C.A., posterior inferior cerebellar artery;

Pet., petrosal, petrous; Plex., plexus; Post., posterior; S.C.A., superior cerebellar artery;Seg., segment; Semicirc., semicircular; Sig.,sigmoid; Sup., superior; Temp., temporal;Triang., triangle; Tymp., tympani, tym-

panic; V., vein.




CHAPTER 9

CEREBELLOPONTINE ANGLE AND RETROSIGMOID APPROACH


CEREBELLOPONTINE ANGLE ANDRETROSIGMOID APPROACH

Cerebellopontine Angle

The cerebellopontine angle is located between the superiorand inferior limbs of the cerebellopontine fissure, an angularcleft formed by the petrosal cerebellar surface folding aroundthe pons and middle cerebellar peduncle (1). The cerebellopon-tine fissure faces the posterior surface of the temporal boneand has superior and inferior limbs that meet at a lateral apex.Cranial Nerves IV through XI are located near or within the

angular space between the two limbs commonly referred to asthe cerebellopontine angle. The trochlear and trigeminal nervesare located near the fissure’s superior limb, and the glossopha-ryngeal, vagus, and accessory nerves are located near the infe-rior limb. The facial and acousticovestibular nerve rises nearthe central part of the fissure. The abducens nerve is locatednear the base of the fissure, along a line connecting the anteriorends of the superior and inferior limbs.

Optimizing operative approaches to the cerebellopontineangle requires an understanding of the relationship of the cere-

bellar arteries to the cranial nerves, brainstem, cerebellarpeduncles, fissures between the cerebellum and brainstem, andthe cerebellar surfaces. When examining these relationships,three neurovascular complexes are defined: an upper complexrelated to the superior cerebellar artery (SCA), a middle com-plex related to the anterior inferior cerebellar artery (AICA),and a lower complex related to the posterior inferior cerebellarartery (PICA).

Other structures, in addition to the three cerebellar arteries,occurring in sets of three in the posterior fossa that bear a con-sistent relationship to the SCA, AICA, and PICA are the partsof the brainstem (midbrain, pons, and medulla), the cerebellarpeduncles (superior, middle, and inferior), the fissures betweenthe brainstem and the cerebellum (cerebellomesencephalic,cerebellopontine, and cerebellomedullary), and the surfaces of the cerebellum (tentorial, petrosal, and suboccipital). Each neu-rovascular complex includes one of the three parts of the brain-

stem, one of the three surfaces of the cerebellum, one of thethree cerebellar peduncles, and one of the three major fissures between the cerebellum and the brainstem. In addition, eachneurovascular complex contains a group of cranial nerves. Theupper complex includes the oculomotor, trochlear, and trigem-inal nerves that are related to the SCA. The middle complexincludes the abducens, facial, and vestibulocochlear nerves thatare related to the AICA. The lower complex includes the glos-

sopharyngeal, vagus, accessory, and hypoglossal nerves thatare related to the PICA.

In summary, the upper complex includes the SCA, midbrain,cerebellomesencephalic fissure, superior cerebellar peduncle,tentorial surface of the cerebellum, and the oculomotor,trochlear, and trigeminal nerves. The SCA arises in front of themidbrain, and passes below the oculomotor and trochlearnerves and above the trigeminal nerve to reach the cerebel-lomesencephalic fissure, where it runs on the superior cerebel-lar peduncle and terminates by supplying the tentorial surfaceof the cerebellum.

The middle complex includes the AICA, pons, middle cere-

bellar peduncle, cerebellopontine fissure, petrosal surface of the cerebellum, and the abducens, facial, and vestibulocochlearnerves. The AICA arises at the pontine level, courses in rela-tionship to the abducens, facial, and vestibulocochlear nervesto reach the surface of the middle cerebellar peduncle, where itcourses along the cerebellopontine fissure and terminates bysupplying the petrosal surface of the cerebellum.

The lower complex includes the PICA, medulla, inferior cerebellar peduncle, cerebellomedullary fissure, suboccipital sur-face of the cerebellum, and the glossopharyngeal, vagus, acces-sory, and hypoglossal nerves. The PICA arises at the medullarylevel, encircles the medulla, passing in relationship to the glos-sopharyngeal, vagus, accessory, and hypoglossal nerves toreach the surface of the inferior cerebellar peduncle, where itdips into the cerebellomedullary fissure and terminates by sup-plying the suboccipital surface of the cerebellum.

Retrosigmoid Approach

The most common operation directed to the upper neurovas-cular complex is the exposure of the posterior root of the trigem-inal nerve for a vascular decompression procedure for trigemi-nal neuralgia. For a vascular decompression operation, thisupper neurovascular complex is approached using a verticalscalp incision crossing the asterion, which usually overlies thelower half of the junction of the transverse and sigmoid sinuses.The bone opening, a small craniotomy, located behind the upperhalf of the sigmoid sinus, exposes the edge of the junction of the

transverse and sigmoid sinuses in its superolateral margin. Themost common finding at a vascular decompression operationfor trigeminal neuralgia is that a segment of the SCA com-presses the trigeminal nerve. The AICA or basilar artery is lesscommonly the compressing vessel. The most common venouscompression is by a tributary of a superior petrosal vein.

Operations directed to the middle complex are for theremoval of acoustic neuromas and other tumors and for the



The operation for hemifacial spasm is directed along theinferolateral margin of the cerebellum. The craniotomy islocated medial to the lower half of the sigmoid sinus. It is

not necessary to extend the bone opening downward tothe foramen magnum or upward to the transverse sinus.The inferolateral margin of the cerebellum is elevated witha small brain spatula and the arachnoid behind the glos-sopharyngeal and vagus nerves is opened. This willexpose the tuft of choroid plexus protruding from the fora-men of Luschka, and sitting on the posterior surface of theglossopharyngeal and vagus nerves. Commonly, the floc-culus is seen protruding behind the nerves and blocks theirvisualization at the junction with the brainstem. It mayalso be difficult to see the facial nerve that is hidden infront of the vestibulocochlear nerve. At this time in theoperation, it is important to recall that the facial nerve rootexits the brainstem 2 to 3 mm rostral to the point at which

the glossopharyngeal nerve enters the brainstem. Toexpose the nerve’s exit from the brainstem, the choroidplexus is gently separated from the posterior margin of the glossopharyngeal nerve so that its junction with the

brainstem can be seen. The brain spatula is advancedupward to elevate the choroid plexus away from the pos-terior margin of the glossopharyngeal nerve. The expo-sure is then directed several millimeters above the glos-sopharyngeal nerve to where the facial nerve will be seen

joining the brainstem below and in front of the vestibulo-cochlear nerve. At this point, it usually becomes obviouswhich vessel is compressing the nerve.

Our most common operation directed to the lowercomplex is for glossopharyngeal neuralgia. We have usu-

ally treated glossopharyngeal neuralgia by dividing theglossopharyngeal nerve and the upper quarter of thevagal rootlets. It is suggested that fewer of the rostralrootlets of the vagus nerve be cut if the diameters of theupper rootlets are large rather than small. Vascular de-compression is an option for treating glossopharyngealneuralgia, although we had excellent results with glos-sopharyngeal and upper vagal neurectomy. A detaileddescription of these operations and others dealing withpathologies in the cerebellopontine angle can be foundelsewhere (1).

REFERENCES

1. Rhoton AL Jr: The cerebellopontine angle and posterior fossa cranialnerves by the retrosigmoid approach. Neurosurgery 47 [Suppl3]:S93–S129, 2000.

relief of hemifacial spasm. The considerations related toacoustic neuromas will be dealt with first. The retrosig-moid approach to an acoustic neuroma is directed through

a vertical scalp incision that crosses the asterion. A burrhole is placed below the asterion and a craniotomy is per-formed, exposing the lower margin of the transverse sinussuperiorly, the posterior margin of the sigmoid sinus later-ally, and the inferior portion of the squamous part of theoccipital bone inferiorly. The nerves in the lateral part of the internal acoustic meatus are the facial, cochlear, andinferior and superior vestibular nerves. The position of thenerves is most constant in the lateral portion of the meatus,which is divided into a superior and an inferior portion bya horizontal ridge, called either the transverse or the falci-form crest. The facial and the superior vestibular nervesare superior to the crest. The facial nerve is anterior to thesuperior vestibular nerve and is separated from it at the

lateral end of the meatus by a vertical ridge of bone, calledthe vertical crest. The vertical crest is also called “Bill’s bar” in recognition of William House’s role in focusing onthe importance of this crest in identifying the facial nerveat the lateral end of the meatus. The cochlear and inferiorvestibular nerves run below the transverse crest, with thecochlear nerve located anteriorly. Thus, the lateral meatuscan be considered to be divided into four portions, withthe facial nerve being anterosuperior, the cochlear nerveanteroinferior, the superior vestibular nerve posterosupe-rior, and the inferior vestibular nerve posteroinferior. Thefacial nerve is commonly identified, even with a largetumor, in the anterosuperior quadrant at the lateral end of the meatus after removing the posterior meatal lip. The

cochlear nerve is identified in the anteroinferior quadrantof the meatus.

There is also a consistent set of relationships on the brain-stem side of an acoustic neuroma that aids in identificationof the facial and cochlear nerves on the medial side of thetumor. The landmarks on the medial or brainstem side thatare helpful in guiding the surgeon to the junction of thefacial nerve with the brainstem are the pontomedullary sul-cus; the junction of the glossopharyngeal, vagus, and spinalaccessory nerves with the medulla; the foramen of Luschkaand its choroid plexus; and the flocculus. These facial andcochlear nerves, although distorted by the tumor, usuallycan be identified on the brainstem side of the tumor at thelateral end of the pontomedullary sulcus, just rostral to the

glossopharyngeal nerve and just anterosuperior to the fora-men of Luschka, the flocculus, and the choroid plexus pro-truding from the foramen of Luschka.

RHOTON





CEREBELLOPONTINE ANGLE

FIGURE 9-1. Cerebellopontine angle and the retrosigmoidapproach. The petrosal surface of the cerebellum faces forwardtoward the posterior surface of the temporal bone. It is the

surface that is elevated to reach the cerebellopontine angle.The cerebellopontine fissure, a V-shaped fissure formed by thecerebellum wrapping around the pons and middle cerebellar

peduncle, has superior and inferior l imbs that define the margins of the cerebellopontine angle. Cranial Nerves V to XI arise in, or near, the cerebellopontine fissure or angle. Thesuperior limb extends above the trigeminal nerve and the infe-rior limb passes below the flocculus and the nerves that passto the jugular foramen. The superior and inferior limbs meetlaterally at the apex located at the anterior end of the petrosal

fissure that divides the petrosal surface of the cerebellum intosuperior and inferior parts. The fourth ventricle is locatedbehind the pons and medulla. The midbrain and pons are sep-arated by the pontomesencephalic sulcus and the pons andmedulla by the pontomedullary sulcus. The trigeminal nerves

arise from the mid pons. The abducens nerve arises in themedial part of the pontomedullary sulcus, rostral to themedullary pyramids. The facial and vestibulocochlear nervesarise at the lateral end of the pontomedullary sulcus immedi-ately rostral to the foramen of Luschka. The hypoglossal nerves arise ante-rior to the olives and the glossopharyngeal and vagus nerves arise posteriorto the olives. The flocculus and choroid plexus protrude from the foramen of Luschka behind to the glossopharyngeal and vagus nerves. The foramen of

Luschka opens into the cerebellopontine angle below the junction of the facialand vestibulocochlear nerves with the lateral end of the pontomedullary sul-cus. The choroid plexus protrudes from the lateral recess and foramen of Luschka behind the glossopharyngeal, vagus, and accessory nerves.




RHOTON

FIGURE 9-2. Enlarged view of the rightcerebellopontine angle. The petrosal surfaceof the cerebellum faces forward toward the

petrous bone and is the surface that is ele-vated to expose the cerebellopontine angleand posterior surface of the temporal bone.The cerebellopontine fissure, which mightalso be referred to as the cerebellopontineangle, is a V-shaped fissure formed wherethe cerebellum wraps around the pons andmiddle cerebellar peduncle. Cranial NervesV through XI arise in or near the marginsof the cerebellopontine fissure. The flocculusand choroid plexus extend laterally from the

foramen of Magendie above the lower limbof the fissure. The abducens nerve arises inthe medial part of the pontomedullary sul-cus rostral to the medullary pyramids. The

facial and vest ibulocochlear nerves arise

just rostral to the foramen of Luschka nearthe flocculus at the lateral end of the pon-tomedullary sulcus. The hypoglossal nervesarise anterior to, and the glossopharyngeal,vagus, and accessory nerves arise posteriorto, the olives. The facial and vestibulocochlear nerves join the brainstem 2 or3 mm rostral to the glossopharyngeal nerve, along a line drawn dorsal to the

olive along the origin of the rootlets of the glossopharyngeal, vagus, andaccessory rootlets.





FIGURE 9-3. The supratentorialand infratentorial areas have been

exposed while preserving the bone atthe site of the sutures. The asterion,located at the junction of the lamb-doid, occipitomastoid, and pari-etomastoid sutures, usually overliesthe lower half of the junction of thetransverse and sigmoid sinuses. Thevertical lateral suboccipital incision

for the retrosigmoid approach usuallycrosses the asterion. The burr hole forelevating a suboccipital bone flap isusually placed at the lower edge of theasterion. The junction of the supra-mastoid crest and the squamosalsuture is located at the posterior edge

of the middle fossa and slightly ante-rior to and above the junction of thetransverse and sigmoid sinuses. Thesupramastoid crest is an inferior con-tinuation of the superior temporalline.




RHOTON

FIGURE 9-4. The cerebellum hasbeen removed in this specimen toexpose the dura covering the part of the posterior fossa that faces the cere-bellopontine angle and is exposed inthe retrosigmoid approach. The fourthventricle sits on the posterior surfaceof the pons and medulla. The floccu-lus projects laterally into the cerebel-lopontine angle. An inferior petrosalvein passes from the right side of themedulla to the jugular bulb. The glos-sopharyngeal, vagus, and accessorynerves enter the jugular foramen. TheSCA is at the upper margin of theexposure. The PICA courses aroundthe glossopharyngeal, vagus, andaccessory nerves. The endolymphaticsac sits beneath the dura inferolateralto the acoustic meatus.





FIGURE 9-5. Retrosigmoid exposure of the right cerebellopontine angle. The facial nerve has been elevated and the vestibulocochlear nerve depressed toexpose both nerves entering the internal acoustic meatus. The AICA passesbetween the facial and vestibulocochlear nerves and turns medially to courseabove the flocculus and along the middle cerebellar peduncle and cerebellopon-tine fissure. A large superior petrosal vein passes behind the trigeminal nerve.The flocculus hides the junction of the facial and vestibulocochlear nerves

with the brainstem. The PICA passes between the glossopharyngeal and vagusnerves. The posterior trigeminal root is deeper, by the retrosigmoid approach,than the facial and vestibulocochlear nerves.





FIGURE 9-7. The flocculus andchoroid plexus, which protrude from

the foramen of Luschka, have beenelevated to expose the junction of the

facial and vestibulocochlear nerveswith the brainstem. The facial nerveis exposed below the vestibulo-cochlear nerve. A branch of the AICA

gives rise to both the subarcuate andlabyrinthine arteries. The subarcuateartery enters the dura and bonesuperolateral to the meatus. The

junction of the facial nerve with thebrainstem is easier to expose frombelow rather than above the floccu-lus and vestibulocochlear nerve in anoperation for hemifacial spasm. This

approach for decompressing the facialnerve in hemifacial spasm is referredto as an “infrafloccular approach.” Alarge PICA loops upward behind thevestibulocochlear nerve.




RHOTON

FIGURE 9-8. The dura lining the posterior wall of the internal acousticmeatus has been removed and the

po st er io r me at al wa ll ha s be enopened to expose the dura lining themeatus. The subarcuate artery usu-ally has to be obliterated and dividedbefore removing the posterior meatalwall. Two bundles from the nervusintermedius are exposed above thevestibulocochlear nerve. Care is takento avoid entering the semicircularcanals and vestibule during drilling

of the posterior wall of the meatus if hearing is to be preserved.





FIGURE 9-9. The meatal dura hasbeen opened, the facial nerve hasbeen elevated, and the vestibulo-cochlear nerve has been depressed toexpose the facial nerve coursing inthe anterior-superior quadrant of the meatus. The nervus inter-medius, which arises along the ante-rior surface of the vestibulocochlearnerve and passes laterally to jointhe facial nerve, is composed of sev-eral rootlets, as is common. Thesuperior vestibular nerve passes

posterior to the facial nerve, and thecochlear nerve is hidden anterior tothe inferior vestibular nerve.




RHOTON

FIGURE 9-10. The cleavage p lanebetween the superior and inferior vestibularnerves has been developed. The superiorvestibular and facial nerves pass above thetransverse crest and the inferior vestibularand cochlear nerves pass below the trans-verse crest. The facial nerve courses anteriorto the superior vestibular nerve and thecochlear nerve is located anterior to the infe-rior vestibular nerve. The vertical crest sep-

arates the superior vestibular and facialnerves at the fundus of the meatus.





FIGURE 9-13. The trigeminalnerve passes above the petrous apexand through the porus of Meckel’scave. The facial and vestibulo-

cochlear nerves enter the internalacoustic meatus, and the glossopha-ryngeal, vagus, and accessorynerves enter the jugular foramen.The posterior and superior semicir-cular canals have been exposed. Thesuperior semicircular canal is posi-tioned below the medial edge of thearcuate eminence. The upper end of the posterior canal and the posteriorend of the superior canal join to

form a common channel, the com-mon crus, which opens into thevestibule. The endolymphatic ducte x t e n d s d o w n w a r d f r o m t h e

ves t ibu le and opens in to th eendolymphatic sac located beneaththe dura inferolateral to the internalacoustic meatus. The endolymphaticridge, the bridge of bone forming theupper lip of the endolymphatic duct,has been preserved. The jugular bulb can be seen through the thin bonebelow the internal meatus. Entering the posterior canal, common crus, pos-

terior portion of the superior canal, or the vestibule during drilling of the posterior meatal wall may result in a loss of hearing.




RHOTON

FIGURE 9-14. Enlarged view of the fundus of the meatus after removal of

the posterior wall. The upper edge of the porus has been preserved. The

facial nerve and nervus intermediusare exposed medial to the porus of themeatus. The subarcuate artery entersthe subarcuate fossa. The inferiorvestibular nerve gives rise to the sin-

gular branch to the posterior ampul-lae, plus utricular and saccularbranches. The superior vestibularnerve innervates the ampullae of thesuperior and lateral semicircularcanals and commonly gives rise to autricular branch. Care is taken to

preserve the superior and posterior

canals and the common crus, plus theendolymphatic sac in those cases inwhich there is the opportunity to pre-serve hearing when drilling the pos-terior wall of the meatus.





FIGURE 9-15. The petrous apexmedial to the internal acoustic mea-tus has been partially removed to

expose the petrous segment of theinternal carotid artery. The lateral

genu of the petrous carotid, located atthe junction of the vertical and hori-

zontal segments, is situated belowand medial to the cochlea. The jugu-lar bulb extends upward, adjacent tothe posterior meatal wall, toward thevestibule and semicircular canals.The inferior petrosal sinus coursesalong the petroclival fissure andenters the petrosal part of the jugular

foramen. The sigmoid sinus descendsin the sigmoid sulcus and enters thesigmoid part of the foramen. The

glossopharyngeal, vagus, and acces-sory nerves pass through the centralor intrajugular part of the jugular

foramen located between the sigmoidand petrosal parts.




RHOTON

FIGURE 9-16. Bone has been removed along the anterior marginof the meatal fundus to open the cochlea, and along the posteriormargin to expose the vestibule. The cochlear nerve penetrates themodiolus of the cochlea, where its fibers are distributed to the turns

of the cochlear duct. The basal turn of the cochlea communicatesbelow the modiolus with the vestibule. The stapes has been removed

from the oval window. The promontory in the medial wall of thetympanic cavity is located lateral to the basal turn of the cochlea. Asilver fiber has been introduced into the superior semicircular canal,a red fiber into the lateral canal, and a blue fiber into the posteriorcanal. The ampullated ends of the canals are located at the bulbousends of the three fibers. The common crus of the superior and pos-terior canals is located where the tips of the blue and silver fiberscross. The superior vestibular nerve passes to the ampullae of thesuperior and lateral canals. The singular branch of the inferiorvestibular nerve innervates the posterior ampullae. A small black

fiber has been introduced into the opening of the endolymphaticduct into the vestibule. A., artery; Ac., acoustic; Arc., arcuate;Atl., atlanto; Car., carotid; Cer. Mes., cerebellomesencephalic; Cer.,

cerebellar; Cer. Pon., cerebellopontine; Chor., choroid; CN, cranialnerve; Coch., cochlear; Comm., common; Cond., condyle; Emin.,eminence; Endolymph., endolymphatic; Fiss., fissure; Flocc., floc-culus; For., foramen; Hypogl., hypoglossal; Impress., impression; Inf., infe-rior; Int., internal; Intermed., intermedius; Intrajug., intrajugular; Jug., jugu-lar; Laby., labyrinthine; Lat., lateral; Med., medial; Mid., middle; N., nerve;Nerv., nervus; Occip., occipital; Occipitomast., occipitomastoid;Parietomast., parietomastoid; Ped., peduncle; Pet., petrosal, petrous;

Petrocliv., petroclival; Plex., plexus; Pon. Med., pontomedullary; Pon. Mes., pontomesencephalic; Pon. Trig., pontotrigeminal; Pon., pontine; Post., poste-rior; Semicirc., semicircular; Sig., sigmoid; Subarc., subarcuate; Sup., supe-rior; Supramast., supramastoid; Trans., transverse; Trig., trigeminal; V., vein;Vert., vertebral, vertical; Vest., vestibular.




CHAPTER 10

TELOVELAR APPROACH TO THE FOURTH VENTRICLE

Neurosurgery 61:S4-193–S4-210, 2007 DOI: 10.1227/01.NEU.0000280026.15254.1F www.neurosurgery-online.com

I

n the past, operative access to the fourth ventriclewas obtained by splitting the cerebellar vermis or

removing part of a cerebellar hemisphere (1). Inexamining the clefts and walls of the cerebel-lomedullary fissure, we found that the inferior half of the roof of the fourth ventricle was formed by telachoroidea in which the choroid plexus arises, and theinferior medullary velum, another paper-thin layer,which attaches to the upper edge of the tela andextends from the nodule of the vermis to the flocculus.We also found that opening the tela alone will provideadequate ventricular exposure, in most cases, withoutsplitting the vermis. The inferior medullary velum canalso be opened if opening the tela does not provide ade-quate exposure. Opening the tela alone provides accessto the full length of the floor and the entire ventricular

cavity except, possibly, the fastigium, superolateralrecess, and the superior half of the roof. Opening theinferior medullary velum accesses the latter areas,including the superior half of the roof. Extending theopening in the tela laterally toward the foramen of Luschka opens the lateral recess and exposes the sur-faces of the cerebellar peduncles bordering the recess.Tumors in the fourth ventricle may stretch and thinthese two semitranslucent membranes to a degree thatone may not be aware that they are being opened inexposing a fourth ventricular tumor.

There are no reports of deficits after isolate openingof the tela and velum. However, other structuresexposed in the ventricle walls at risk for producing

deficits include the dentate nuclei, cerebellar pedun-cles, the floor of the fourth ventricle, and the posteriorinferior cerebellar artery (PICA). During an operationon the caudal part of the roof, one should remember

that the dentate nuclei are located just rostral to thesuperior pole of the tonsils underlying the dentate

tubercles in the posterolateral part of the roof, wherethey are wrapped around the superolateral recessesnear the lateral edges of the inferior medullary velum.All of the cerebellar peduncles converge on the lateralwall and roof, where they may be damaged. The supe-rior cerebellar peduncle is more likely to be injured dur-ing operations on lesions involving the superior part of the roof above the level of the dentate tubercles; theinferior peduncle is most susceptible to damage inexposing lesions within the lateral recess; and the mid-dle cerebellar peduncle is susceptible to injury duringprocedures in the cerebellopontine angle, because themiddle peduncle forms a major part of the cisternal sur-face facing the cerebellopontine angle.

The PICA is frequently exposed in approachesdirected through the tela choroidea or inferior medullarvelum. Occlusion of the branches of the PICA distal tothe medullary branches at the level of roof of the fourthventricle avoids the syndrome of medullary infarction

but produces a syndrome resembling labyrinthitis,which includes rotatory dizziness, nausea, vomiting,inability to stand or walk unaided, and nystagmuswithout appendicular dysmetria (1). The main trunk of the anterior inferior cerebellar artery is infrequentlyexposed in opening the cerebellomedullary fissure, butit may also send choroidal branches to the tela andchoroid plexus in the lateral recess.

REFERENCES

1. Rhoton AL Jr: Cerebellum and fourth ventricle. Neurosurgery 47[Suppl 3]:S7–S27, 2000.




TELOVELAR APPROACH

FIGURE 10-2. Enlarged view of thecerebellomedullary fissure and infe-rior half of the fourth ventricularroof. The lower parts of the vermisbehind the ventricle are the pyramidand uvula. The uvula hangs down-ward between the tonsils, thus, mim-icking the situation in the orophar-

ynx. The tela choroidea, a paper-thinependymal membrane exposed belowthe uvula, forms the lower part of the

fourth ventricular roof. The choroid plexus arises on the inner surface of the tela and extends downward in themidline though the foramen of

Magendie and laterally through the foramen of Luschka behind the glos-sopharyngeal and vagus nerves.




RHOTON

FIGURE 10-3. The right tonsil has beenretracted to expose the lower half of theroof, which is formed by the inferiormedullary velum and tela choroidea. Thecerebellomedullary fissure extendsupward between the rostral pole of thetonsil on one side and the tela choroideaand inferior medullary velum on theopposite side. The segment of the PICA

passing through this fissure is called thetelovelotonsillar segment. The choroid

plexus arises on the inner surface of thetela and extends downward in the midlinethrough the foramen of Magendie and lat-

erally through the foramen of Luschka.The inferior medullary velum arises onthe surface of the nodule, drapes acrossthe superior pole of the tonsil, and blendsinto the flocculus laterally.



FIGURE 10-4. Both tonsils have beenremoved to expose the infer iormedullary velum and tela choroideabilaterally. The telovelar junction isthe junction between the velum andtela. The rhomboid lip is a sheet-likelayer of neural tissue attached to thelateral margin of the ventricular floor,which extends posterior to the glos-sopharyngeal and vagus nerves and

joins the tel a choro idea to form a pouch at the outer extremity of the lat-eral recess. The right half of the telahas been removed to expose the ventri-cle and the lateral recess. The inferiormedullary velum extends laterally to

form a peduncle, the peduncle of the flocculus, which blends into the floccu-lus at the outer margin of the lateralrecess.


TELOVELAR APPROACH




RHOTON

FIGURE 10-7. Both tonsils havebeen retracted laterally to expose theinferior medullary velum and telachoroidea that form the lower half of the ventricular roof. The nodule of the vermis, on which the inferiormedullary arises, is hidden deep tothe uvula.




RHOTON

FIGURE 10-9. The tela choroideahas been opened, extending from the

foramen of Magendie to the junctionwith the inferior medullary velum.The uvula has been displaced to theright side to provide this viewextending from the obex up to theaqueduct.



FIGURE 10-10. The left half of theinferior medullary velum has beendivided to expose the superolateralrecess and the ventricular surface

formed by the superior and inferior peduncles.


TELOVELAR APPROACH



FIGURE 10-12. The cerebellar tonsilhas been elevated to expose the tela

forming the lower part of the roof of the lateral recess.


TELOVELAR APPROACH




RHOTON

FIGURE 10-15. The peduncle of the tonsil has been divided and the tonsil hasbeen lifted out of the cerebellomedullary fissure to expose the caudal surface of the inferior medullary velum and the tela choroidea that form the lower half of the ventricular roof.



F IGURES 10 -16 and 10-17.

Another specimen. Figure 10-16.Both tonsils have been removed toexpose the inferior medullary velumand tela choroidea. The inferiormedullary velum extends from thenodule along the inferior half of theroof of the fourth and blends later-ally into the flocculus. The tela, inwhich the choroid plexus arises, hasbeen removed on the left side. A dis-sector has been placed inside thesuperolateral recess to show the

paper-thin inferior medullary velum.

Opening the velum will expose thesuperolateral recess. The dorsalcochlear nucleus sits in the floor of the lateral recess.


TELOVELAR APPROACH




RHOTON

FIGURE 10-17. The velum has been

removed on the left side. Opening thevelum or removing it gives excellentaccess to the cerebellar peduncles and tothe superolateral recess of the fourthventricle. The auditory tubercle is a

prominence in the lateral recess thatoverlies the dorsal cochlear nucleus. A.,artery; Bivent., biventral; Cer., cere-bellar; Cer. Med., cerebellomedullary;Cer. Mes., cerebellomesencephalic;Coch., cochlear; Chor., choroid; CN,cranial nerve; Dent., dentate; Dors.,dorsal; Fiss., fissure; Flocc., flocculus;For., foramen; Inf., inferior; Lat., lat-eral; Med., medullary; Mid., middle;

Nucl., nucleus; Ped., peduncle; Plex., ple xus ; S.C.A., superior cerebellarartery; Suboccip., suboccipital; Sup.,s u p e r i o r ; T e l o v e l . , t e l o v e l a r ;Tonsillobivent., tonsillobiventral; V.,vein; Ve., vermian; Vel., velum; Vent.,ventricle; Vert., vertebral.




RHOTON

FIGURE 11-3. The splenius capitis has been reflected downward to expose thelongissimus and the semispinalis capitis muscles. The occipital artery on the left

passes deep and the right passes superficial to the longissimus capitis. Thedeep cervical fascia has been preserved in the illustration on the lower left.




RHOTON

FIGURE 11-5. The right superior oblique muscle has been reflected laterally.The rectus capitis posterior major extends from the occipital bone to the C2spinous process. The rectus capitis posterior minor extends from the occipital

bone to the midline tubercle on the posterior arch of C1. The inferior oblique

muscle extends from the C2 spinous process to the transverse process of C1. Theoccipital artery passes medial to the digastric muscle. The dense venous plexusin the suboccipital triangle surrounds the vertebral artery as it passes behind

the atlanto-occipital joint. The lower left shows the right unilateral exposure.




FAR LATERAL APPROACHES

FIGURE 11-6. The rectus capitis posterior major and the adjacent partof the rectus capitis posterior minorhave been reflected inferior and medi-ally. The superior and inferior obliquemuscles have been reflected down-ward. The vertebral artery passesbehind the atlantal condyle, gives riseto a posterior meningeal branch, and

passes deep to the posterior atlanto-occipital membrane to enter the dura.The rectus capitis lateralis extends

from the transverse process of C1 to

the occipital bone behind the jugular foramen.



FIGURE 11-8. The vertebral artery g i v e s o r i g i n t o t h e po s t e r i o rmeningeal artery, which ascendsthrough the foramen magnum andalong the occipital dura. Severalmuscular branches of the vertebralartery have been divided. The C1nerve passes between the vertebralartery and the posterior arch of C1.






RHOTON

FIGURE 11-9. A suboccipi tal cran-

iotomy has been completed and the pos-terior arch of C1 has been removed. Thevertebral artery passes behind and par-tially hides the atlanto-occipital joint.The facial and vestibulocochlear nervesenter the internal acoustic meatus. The

glossopharyngeal, vagus, and accessorynerves enter the jugular foramen. Therootlets of the hypoglossal nerve arestretched around the posterior surface of the vertebral artery. The rectus capitislateralis muscle extends from the occip-ital bone behind the jugular bulb to thetransverse process of C1. The posteriorinferior cerebellar artery rises just out-

side the dura and penetrates the durawith the vertebral artery. The dentateligament and spinal accessory nerveascend through the foramen magnum.The rostral attachment of the dentateligament is at the level of the foramenmagnum.



FIGURE 11-12. Comparison of expo-sure with the far lateral and transcondy-lar approaches. The far lateral exposureon the right side extends to the posteriormargin of the atlantal and occipitalcondyles and the atlanto-occipital joint.The prominence of the condyles on theright side limits the exposure along theanterolateral margin of the foramenmagnum. On the left side, a transcondy-lar exposure has been completed byremoving the upper part of the occipitalcondyle. The dura can be reflected fur-ther laterally with the transcondylarapproach than with the far lateralapproach. The condylar drilling provides

an increased angle of view and addi-tional space for exposure and dissection.The dentate ligament and accessorynerve ascend through the foramen mag-num. The rostral attachment of the den-tate ligament is at the level of the fora-men magnum.






RHOTON

FIGURE 11-13. The part of the left occipital condyle above the atlanto-occipital joint has beendrilled to expose the hypoglossal nerve in the hypoglossal canal. The glossopharyngeal andvagus nerves descend behind the jugular tubercle. Drilling the condyle above and below thehypoglossal canal provides entry into the lower part of the clivus medial to the condyle. A cuff of dura has been left on the vertebral artery.




RHOTON

FIGURE 11-15. Another specimen with the brainstem removed. The boneabove the occipital condyle has been removed to expose the hypoglossal nervein the hypoglossal canal. The glossopharyngeal, vagus, and accessory nervescross the jugular tubercle. The jugular bulb is located lateral to the occipitalcondyle and can be exposed by drilling the occipital bone in the paracondylararea.



FIGURE 11-16. The medial part of the right occipital condyle and the poste-rior arch of C1 have been removed. The extradural segment of the right verte-bral artery, which normally courses above the C1 nerve root, has been retractedbelow the level of the C1 nerve root. The intradural segment of the right ver-tebral artery has been retracted posteriorly to provide access to the cervi-comedullary region. The contralateral vertebral artery is exposed anterior to

the medulla. The hypoglossal nerve passes behind the vertebral artery. Thedrilling has provided wide access to the lower clivus adjacent to the occipitalcondyle and also to the lateral and anterior aspects of the brainstem.






CHAPTER 12

JUGULAR FORAMEN

Neurosurgery 61:S4-229–S4-250, 2007 DOI: 10.1227/01.NEU.0000280041.55157.E0 www.neurosurgery-online.com

The jugular foramen is located between the temporal and the occip-ital bones. It can be regarded as a hiatus between the temporal and the

occipital bones (1). The right foramen is usually larger than the left. Theforamen is configured around the sigmoid and inferior petrosalsinuses. The jugular foramen is divided into three compartments: twovenous compartments and a neural or intrajugular compartment. Thevenous compartments consist of a larger posterolateral venous channel,the sigmoid part, which receives the flow of the sigmoid sinus, and asmaller anteromedial venous channel, the petrosal part, which receivesthe drainage of the inferior petrosal sinus. The petrosal part forms acharacteristic venous confluens by also receiving tributaries from thehypoglossal canal, petroclival fissure, and vertebral venous plexus. Thepetrosal part empties into the sigmoid part through an opening

between the glossopharyngeal and the vagus nerves in the medial wallof the jugular bulb. The intrajugular or neural part, through which theglossopharyngeal, vagus, and accessory nerves course, is located

between the sigmoid and petrosal parts. The junction of the sigmoidand petrosal parts of the foramen, when viewed from above, is the siteof bony prominences on the opposing surfaces of the temporal andoccipital bones, called the intrajugular processes, which are joined bya fibrous, or, less commonly, an osseous bridge, the intrajugular sep-tum, separating the sigmoid and petrosal part of the foramen. Theglossopharyngeal, vagus, and accessory nerves penetrate the dura onthe medial margin of the intrajugular process of the temporal bone toreach the medial wall of the jugular bulb and internal jugular vein.

The jugular foramen is difficult to access surgically. The difficultiesin exposing this foramen are created by its deep location and the sur-rounding structures, such as the carotid artery anteriorly, the facialnerve laterally, the hypoglossal nerve medially, and the vertebral arteryinferiorly, all of which block access to the foramen and require carefulmanagement.

The structures that traverse the jugular foramen are the sigmoidsinus and jugular bulb, the inferior petrosal sinus, meningeal branches

of the ascending pharyngeal and occipital arteries, the glossopharyn-geal, vagus, and accessory nerves with their ganglia, the tympanic branch of the glossopharyngeal nerve (Jacobson’s nerve), the auricular branch of the vagus nerve (Arnold’s nerve), and the cochlear aqueduct.Tumors involving the jugular foramen can extend as follows: 1) alongthe eustachian tube into the nasopharynx and through the foramina atthe base of the cranium, 2) along the carotid artery to the middle fossa,3) through the intracranial orifice of the jugular foramen or along thehypoglossal canal to the posterior fossa, 4) through the tegmen tym-pani to the floor of the middle fossa, 5) through the round window andthe internal acoustic meatus to the cerebellopontine angle, and 6)through the extracranial orifice of the jugular foramen to the upper cer-vical region.

Surgical Approaches

The most common operative approaches used to access variousaspects of the foramen and adjacent areas are the postauriculartranstemporal, retrosigmoid, and far lateral approaches.

Postauricular Transtemporal ApproachThe postauricular transtemporal approach, the most common

approach selected for a lesion in the jugular foramen, accesses theregion from laterally, through the mastoid, and from below, through theneck. A C-shaped postauricular skin incision provides the exposurefor a mastoidectomy and the neck dissection. The external auditory

canal is either preserved or transected, depending on the anteriorextent of the pathological abnormality. The neck dissection is com-

pleted initially to gain control of the major vessels and the branchessupplying the tumor. The internal carotid artery, branches of the exter-nal carotid artery, internal jugular vein, and lower cranial nerves areexposed in the carotid sheath. A mastoidectomy with extensive drillingof the infralabyrinthine region accesses the jugular bulb. A limited mas-toidectomy confined to the area behind the stylomastoid foramen andmastoid segment of the facial nerve, combined with removal of theadjacent part of the jugular process of the temporal bone, will provideaccess to the posterior and posterolateral aspect of the jugular foramen.Three obstacles to exposure of the full lateral half of the jugular fora-men, the facial nerve, styloid process, and rectus capitis lateralis mus-cle are dealt with by transposing the facial nerve, removing the styloidprocess, and dividing the rectus capitis lateralis muscle. Anterior exten-sions of the pathological abnormality are reached by sacrificing theexternal and the middle ear structures. Sensorineural hearing can bepreserved by maintaining the footplate of the stapes in the oval win-dow to avoid opening the labyrinth. Intracranial extensions of thelesion are reached by the retrosigmoid or presigmoid approaches afteradding a suboccipital craniectomy. Some lesions can be removed by atranstemporal infralabyrinthine approach directed through the tem-poral bone below the labyrinth without a neck dissection, if theextracranial extension of the lesion is not prominent. The exposure can

be extended by opening the otic capsule (translabyrinthine approach).

Retrosigmoid ApproachA lesion located predominantly intradurally above the jugular fora-

men can be resected by the retrosigmoid approach. A lateral suboccip-ital craniectomy exposes the dura behind the sigmoid sinus. The durais opened, and the cerebellum is gently elevated away from the poste-rior surface of the temporal bone to expose the cisterns in the cerebel-lopontine angle and the intracranial aspect of the cranial nerves enter-

ing the jugular foramen, hypoglossal canal, and internal acousticmeatus. Lesions can be followed into only the upper part of the fora-men by this approach.

Far Lateral ApproachAn extended modification of the retrosigmoid approach, the far lat-

eral approach, may be selected if the tumor extends down to the fora-men magnum in front of or lateral to the lower brainstem. In thisapproach, the jugular foramen is opened from behind by completing aparacondylar modification of the far lateral approach. In this modifica-tion, the rectus capitis lateralis is detached from the occipital bone atthe posterior margin of the foramen and the posterior margin isremoved. The dura is opened and the cerebellum elevated to exposethe intracranial extension of the pathological abnormality at the lowerclivus and at the foramen magnum. In another variant of the approach,depending on the location and extent of the pathological abnormality,

the jugular tubercle is removed extradurally to minimize the retractionof the brainstem needed to reach the area anterior to the medulla andpontomedullary junction. Most jugular foramen tumors cannot bereached by this route because they extend forward beyond the limits of this approach to the posterior part of the foramen.

REFERENCES

1. Rhoton AL Jr: Jugular foramen. Neurosurgery 47 [Suppl 3]:S267–S285, 2000.




RHOTON

FIGURE 12-1. Jugul ar foramen.Posterior view of the cranial base with

the cranial nerves and arteries pre-served. The jugular foramen is posi-tioned below the internal acousticmeatus and superolateral to thehypoglossal nerves entering thehypoglossal canal. The glossopharyn-

gea l, vagus, and accessory nervesenter the dural roof of the jugular

foramen. The superior cerebellar arter-ies arise at the midbrain level and passbelow the oculomotor and trochlearnerves and above the trigeminalnerve. The anterior inferior cerebellararteries arise at the pontine level andcourse by the abducens, facial, andvestibulocochlear nerves. The poste-rior inferior cerebellar arteries arise

fr om th e ve rt eb ra l ar te ry at th emedullary level and course near the

glossopharyngeal , vagus, accessory,and hypoglossal nerves.




JUGULAR FORAMEN

FIGURE 12-2. The dural roof of the left jugular foramen has been exposed below the facial andvestibulocochlear nerves. There is a dural septum between the glossopharyngeal and vagusnerves at the roof of the jugular foramen. The glossopharyngeal nerve is often adherent to therootlets of the vagus nerve in the cistern, however, at the roof of the jugular foramen, there isconsistently a dural septum separating the glossopharyngeal from the vagus nerve. The glos-sopharyngeal nerve enters a shallow meatus, the glossopharyngeal meatus, in the dural roof of the foramen. The glossopharyngeal dural fold passes above the glossopharyngeal nerve at theentrance to the glossopharyngeal meatus. The vagus nerve enters the vagal meatus, which is

broader than, but not as deep, as the glossopharyngeal meatus, at the roof of the jugular fora-men. There is also a dural fold around the upper and lateral margin of the vagal meatus. Theaccessory nerve ascends to enter the lower part of the vagal meatus.




RHOTON

FIGURE 12-3. The left sigmoid and infe-rior petrosal sinuses have been unroofed.The glossopharyngeal, vagus, and acces-sory nerves are exposed at the roof of the

jugular foramen. The jugular foramen hasthree parts: sigmoid, petrosal, and intra-

jugular. The sigmoid sinus descends andturns forward to pass through the sigmoid

part of the jugular foramen. The inferior pe tros al si nu s de sc en ds an d pa ss esthrough the petrosal part of the jugular

foramen. The glossopharyngeal, vagus,and accessory nerves exit the craniumthrough the intrajugular part of the fora-

men, which is located between the sigmoidand petrosal parts. Two bundles of hypoglossal rootlets enter a bifid hypoglos-sal canal above the occipital condyle and

join after exiting the hypoglossal canal.



FIGURE 12-4. The jugular bulb hasbeen removed to expose the jugular

fossa on the lower surface of the temporal bone. The glo sso pharyngea lnerve enters the jugular foramenabove and medial to the vagus nerve.The tympanic branch (Jacobson’snerve) of the glossopharyngeal nervearises in the medial part of the jugular

fossa, ascends to cross the promontoryin the tympanic cavity, and gives riseto the lesser petrosal nerve. The auric-ular branch (Arnold’s nerve) of thevagus nerve arises in the intrajugular

part of the foramen and passes later-ally across the anterior margin of the

jugular fos sa. The bone above the

hypoglossal canal has been drilled toexpose a bifid hypoglossal canal. Thetwo bundles of hypoglossal rootlets

join at the extracranial end of thehypoglossal canal and descend in thecarotid sheath with the glossopharyn-

geal, vagus, and accessory nerves.


JUGULAR FORAMEN




RHOTON

FIGURES 12-5 AND 12-6. Inferior view of the temporal boneand jugular foramen. Figure 12-5, the internal jugular vein isexposed below the jugular foramen and descends on the medialside of the facial nerve and styloid process. The glossopharyngeal,vagus, accessory, and hypoglossal nerves descend in the carotidsheath with the internal carotid artery and internal jugular vein.The occipital condyle has been drilled to expose the passage of thehypoglossal nerve behind the vertebral artery and through thehypoglossal canal. The mandibular head, which sits in themandibular fossa, is exposed anterolateral to the jugular foramen.The middle meningeal artery and branches of the third trigemi-nal division are exposed below the greater sphenoid wing in theinfratemporal fossa. Bone has been removed to expose theeustachian tube and the petrous segment of the internal carotidartery. The Vidian nerve, which arises from the union of the

greater and deep petrosal nerves, continues forward in the Vidiancanal. The rectus capitis lateralis muscle attaches to the occipitalbone behind the jugular foramen. The auriculotemporal branch of the third trigeminal division conveys autonomic fibers from thelesser petrosal nerve to the otic ganglion, which provides auto-nomic innervation to the parotid gland.



FIGURE 12-6. The rectus capitis lateralis muscle has been resectedand the part of the occipital bone forming the posterior margin of the

jugular foramen has been removed to expose the lower part of thesigmoid sinus as it hooks forward to form the jugular bulb. Thevenous plexus in the hypoglossal canal has been removed. The infe-rior petroclival vein, which courses along the extracranial surface of the petroclival fissure, has been removed to expose the petrous apex

articulating with the lateral edge of the clivus along the petroclival fissure.


JUGULAR FORAMEN




RHOTON

FIGURE 12-9. The floor of the mid-dle fossa and the tympanic ring havebeen removed to expose the jugularbulb and petrous carotid. The jugularbulb is positioned below the semicir-cular canals. The junction of the ver-tical and horizontal segments of the

petrous carotid is positioned belowthe cochlea. The malleus and medialwall of the tympanic cavity havebeen preserved. The eustachian tubeextends downward and medially

across the anterior surface of the petrous carotid. The third trigeminaldivision has been elevated out of the

foramen ovale.



FIGURE 12-10. A short segment of theEustachian tube has been removed to exposemore of the horizontal segment of the

petrous carotid. The greater petrosal nervecourses along the floor of the middle fossaon the upper surface of the petrous carotid.The deep petrosal nerves arise from the sym-

pathetic nerves accompanying the internalcarotid artery. The deep and greater petrosalnerves join to form the vidian nerve, which

passes forward through the vidian canal to join the maxillary nerve and pterygopala-tine ganglion in the pterygopalatine fossa.

The pharyngobasilar fascia has been openedto expose the upper part of the longus capi-tis muscle.


JUGULAR FORAMEN




RHOTON

FIGURE 12-13. Posterior view of the nerves in the jugular foramen with the venous struc-tures removed. The posterior wall of the jugular foramen and hypoglossal canal have beenopened. The glossopharyngeal nerve enters the jugular foramen caudal to the cochlear aqueduct.The vagus nerve enters the jugular foramen behind the glossopharyngeal nerve. The auricu-lar branch of the vagus nerve (Arnold’s nerve) arises at the level of the superior ganglion and

passes across the anterior wall of the jugular bulb. The accessory nerve is formed by multiplerootlets that arise from the medulla and cervical spinal cord and collect together to form a bun-dle that blends into the lower margin of the vagus nerve at the level of the jugular foramen. Thevagal and accessory rootlets cross the surface of the jugular tubercle. The glossopharyngealnerve expands at the site of the superior and inferior ganglia. The superior ganglion of thevagus nerve is located at the level of or just below the dural roof of the foramen, and the infe-rior ganglion is located below the foramen at the level of the atlanto-occipital joint.




RHOTON

FIGURE 12-17. The external audi-tory canal has been transected and themiddle ear structures have beenremoved, except the stapes, which hasbeen left in the oval window. The lat-eral edge of the jugular foramen hasbeen exposed by completing the mas-toidectomy, transposing the facialnerve anteriorly, and fracturing thestyloid process across its base andreflecting it caudally. The rectus capi-tis lateralis muscle has been detached

from the jugular process of the occip-

ital bone. The petrous carotid is sur-rounded in the carotid canal by avenous plexus.




RHOTON

FIGURE 12-19. A segment of thesigmoid sinus, jugular bulb, andinternal jugular vein have beenremoved. The lateral wall of the

jugular bulb has been removed while preserving the medial wall and theopening of the inferior petrosal sinusinto the lower part of the bulb. The

glossopharyngeal, vagus, accessory,and hypoglossal nerves are exposedbelow the jugular bulb. The likeli-hood of preserving these nerves inexposing a jugular foramen lesion is

gr ea tl y en ha nc ed if th e me di alvenous wall can be preserved. The

main inflow from the inferior pet-rosal sinus is directed between the

glossopharyngeal and vagus nerves.



FIGURE 12-20. The medial venous wall of the jugular bulb has been removed. Theintrajugular ridge extends forward from theintrajugular process of the temporal bonealong the medial side of the jugular bulb. The

glossopharyngeal , vagus, and accessorynerves enter the dura on the medial side of the intrajugular process, but only the glos-sopharyngeal nerve courses through the fora-

men entirely on the medial side of the intrajugular ridge.


JUGULAR FORAMEN



RHOTON

anatomy and surgical appraches of the temporal bone.pdf

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