management of temporomandibular disorders and oc

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MANAGEMENT OF TEMPOROMANDIBULAR DISORDERS ISBN: 978-0-323-04614-5AND OCCLUSION, EDITION 6Copyright 2008, 2003, 1998, 1993, 1989, 1985 by Mosby, Inc., an affiliate of Elsevier Inc.

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Notice

Knowledge and best practice in this field are constantly changing. As new research and experiencebroaden our knowledge, changes in practice, treatment, and drug therapy may become necessary orappropriate. Readers are advised to check the most current information provided (i) on procedures featured or (ii) by the manufacturer of each product to be administered, to verify the recommendeddose or formula, the method and duration of administration, and contraindications. It is the responsibility of the practitioner, relying on his or her own experience and knowledge of the patient,to make diagnoses, to determine dosages and the best treatment for each individual patient, and totake all appropriate safety precautions. To the fullest extent of the law, neither the Publisher nor theAuthor assumes any liability for any injury and/or damage to persons or property arising out orrelated to any use of the material contained in this book.

The Publisher


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This text is personally dedicated to my wife, Barbara,

for her continued unconditional love, support, and

understanding throughout my professional life,

and to my mother, Lois Okeson, and my father-in-law, Harold Boian,

for their many years of steadfast encouragement. I miss them greatly.

This text is professionally dedicated to all of our patients.

It is my hope that this text may in some

way help reduce their suffering.


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About the AuthorJEFFREY P. OKESON, DMD

Dr. Okeson is a 1972 graduate of the University ofKentucky College of Dentistry. After graduation hecompleted two years with the Public Health Service ina rotating dental internship and direction of an out-patient clinic. He joined the faculty at the University ofKentucky in 1974. At present he is Professor, Chairmanof the Department of Oral Health Science, and Director

of the College's Orofacial Pain Center, which he established in 1977.The Center represents a multidisciplinary effort in the management ofchronic orofacial pain problems. Dr. Okeson has developed several post-graduate training programs in the Center, including a Master of ScienceDegree in Orofacial Pain. Dr. Okeson has more than 200 professionalpublications in the areas of occlusion, temporomandibular disorders(TMDs), and orofacial pain in various national and international journals.Dr. Okeson's textbook, Management of Temporomandibular Disorders andOcclusion, is used in the majority of U.S. dental schools and has beentranslated into nine languages. In addition to this text, Dr. Okeson isalso the author of Bells Orofacial Pains. This text is also widely used inorofacial pain programs throughout the world.

Dr. Okeson is an active member of many TMD and orofacial painorganizations, holding many offices and serving on numerous commit-tees and boards. He is a past president and fellow of the AmericanAcademy of Orofacial Pain (AAOP). He is a founding diplomate andpast president of the American Board of Orofacial Pain. He has beenactive in the AAOP, developing treatment and curriculum guidelinesfor TMDs and orofacial pain. He is the editor of the AAOP guidelines,titled Orofacial Pain: Guidelines for Classification, Assessment, and Management,Third Edition, which are used as treatment standards throughout theworld. Dr. Okeson has presented more than 600 invited lectures on thesubject of TMDs and orofacial pain in 48 states and 42 countries. Atnational and international meetings he is frequently referred to as theworld ambassador for orofacial pain. Dr. Okeson has received severalteaching awards from his dental students, as well as the University ofKentucky Great Teacher Award. He is also the first-ever recipient of theDistinguished Alumni Award from the College of Dentistry.


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T he study of occlusion and its relationship to function of the masticatory system hasbeen a topic of interest in dentistry formany years. This relationship has proved to bequite complex. Tremendous interest in this areaaccompanied by lack of complete knowledge hasstimulated numerous concepts, theories, andtreatment methods. This, of course, has led tomuch confusion in an already complicated field ofstudy. Although the level of knowledge today isgreater than ever before, there is still much tolearn. Some of todays techniques will prove to beour most effective treatments in the future. Othermethods will be demonstrated as ineffective andwill have to be discarded. Competent and caringpractitioners must establish their treatment meth-ods based on both their present knowledge andtheir constant evaluation of information receivedfrom the massive amount of ongoing research.This is an enormous task. I hope that this text willassist students, teachers, and practitioners in mak-ing these important treatment decisions for theirpatients.

I began my teaching career at the University ofKentucky in 1974 in the area of occlusion. At thattime I believed there was a need for a teachingmanual that presented the topics of occlusion andtemporomandibular disorders (TMDs) in an organ-ized, logical, and scientific manner. In 1975 I devel-oped such a manual to assist in teaching my dentalstudents. Soon, several other dental schoolsrequested use of this manual for their teachingprograms. In 1983 the CV Mosby Publishing Companyapproached me with the concept of developingthis manual into a complete textbook. After twoyears of writing and editing, the first edition waspublished in 1985. I am very pleased and humbledto learn that this text is currently being used in mostof the dental schools in the United States and hasbeen translated into numerous foreign languagesfor use abroad. This is professionally very satisfying,

and it is my hope that the true benefit of this textis found in the improved quality of care we offerour patients.

It is a privilege to once again have the opportu-nity to update this text. I have tried to include themost significant scientific findings that have beenrevealed in the past 4 years. I believe the strength ofa textbook lies not in the authors words but in thescientific references that are offered to support theideas presented. Unreferenced ideas should beconsidered only opinions that require further scientific investigation to either verify or negatethem. It is extremely difficult to keep a textbookupdated, especially in a field in which so much ishappening so quickly. Twenty-eight years ago, inthe first edition of this text, I referenced approxi-mately 450 articles to support the statements andideas. The concepts in this edition are supportedby nearly 2200 scientific references. This reflectsthe significant scientific growth of this field. Itshould be acknowledged that as future truths areuncovered, the professional has the obligation toappropriately respond with changes that bestreflect the new information. These changes aresometimes difficult for the clinician because theymay reflect the need to change clinical protocol.However, the best care for our patients rests in themost scientifically supported information.

The purpose of this text is to present a logicaland practical approach to the study of occlusionand masticatory function. The text is divided intofour main sections: The first part consists of sixchapters that present the normal anatomic andphysiologic features of the masticatory system.Understanding normal occlusal relationships andmasticatory function is essential to understandingdysfunction. The second part consists of four chapters that present the etiology and identificationof common functional disturbances of the mastica-tory system. Significant supportive documentationhas been included in this edition. The third part

v

Preface

vi Preface

consists of six chapters that present rational treat-ments for these disorders according to the signifi-cant etiologic factors. Recent studies have beenadded to support existing treatments, as well asfor new considerations. The last part consists offour chapters that present specific considerationsfor permanent occlusal therapy.

The intent of this text is to develop an understand-ing of, and rational approach to, the study of masti-catory function and occlusion. To assist the reader,certain techniques have been presented. It should berecognized that the purpose of a technique is toachieve certain treatment goals. Accomplishingthese goals is the significant factor, not the techniqueitself. Any technique that achieves the treatmentgoals is acceptable as long as it does so in a reason-ably conservative, cost-effective manner, with thebest interests of the patient kept in mind.

ACKNOWLEDGMENTS

A text such as this is never accomplished by the workof one person, but rather represents the accumula-tion of many who have gone before. The efforts ofthese individuals have led to the present state ofknowledge in the field. To acknowledge each of thesewould be an impossible task. The multiple listing ofreferences at the end of each chapter begins torecognize the true work behind this text. There are,however, a few individuals whom I feel both obligatedand pleased to acknowledge. First and foremost isDr. Weldon E. Bell. Although we lost this giant in1990, he remains my mentor to this day. He was theepitome of an outstanding thinker, information sim-ulator, and teacher. Within the seven texts he wroteon TMD and orofacial pain lies enough informationto keep a normal man thinking forever. He was a veryspecial man, and I sorely miss him still.

I would like to thank Dr. Terry Tanaka of SanDiego, California, for generously sharing his knowl-edge with me. Over the years I have come to valueTerrys professional and personal friendship moreand more. His anatomic dissections have con-tributed greatly to the professions understandingof the functional anatomy of our complex mastica-tory system.

I would like to thank my colleague, CharlesCarlson, PhD, for all that he has taught me regard-ing the psychology of pain. Charley and I haveworked together for nearly 20 years in our OrofacialPain Center, and I have seen him develop and successfully document his concepts of physicalself-regulation. These techniques have helped manyof our chronic pain patients. He has generouslyshared his ideas and concepts in Chapter 11.

I would also like to thank the following individu-als for allowing me to use some of their professionalmaterials and insights in this text: Dr. Per-LennartWestesson, University of Rochester; the late Dr. JulioTurell, University of Montevideo, Uruguay; and Dr. Jay Mackman, Milwaukee, Wisconsin.

I need to also thank all of my residents at theUniversity of Kentucky, both past and present, forkeeping me alert, focused, and searching for thetruth.

Last, but by no means least, I wish to expressmy gratitude to my family for their constant love,support, encouragement, and sacrifice during mywritings. My mother and father inspired andencouraged me from the very beginning. My sonshave understood the time commitment, and mywife has given up many evenings to my computer.I have been blessed with a wonderful, loving wifefor 37 years, and her sacrifice has resulted in thistextbook.

Jeffrey P. Okeson


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IP A R T

Functional AnatomyThe masticatory system is extremely complex. It is made up primarilyof bones, muscles, ligaments, and teeth. Movement is regulated by anintricate neurologic control system composed of the brain, brainstem,and peripheral nervous system. Each movement is coordinated tomaximize function while minimizing damage to any structure. Precisemovement of the mandible by the musculature is required to move theteeth efficiently across each other during function. The mechanics andphysiology of this movement are basic to the study of masticatoryfunction. Part I consists of six chapters that discuss the normalanatomy, function, and mechanics of the masticatory system. Functionmust be understood before dysfunction can have meaning.


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21C H A P T E R

Nothing is more fundamental to treating patientsthan knowing the anatomy.

JPO

T he masticatory system is the functional unitof the body primarily responsible for chew-ing, speaking, and swallowing. Componentsalso play a major role in tasting and breathing. The system is made up of bones, joints, ligaments,teeth, and muscles. In addition, an intricate neuro-logic controlling system regulates and coordinatesall these structural components.

The masticatory system is a complex and highlyrefined unit. A sound understanding of its func-tional anatomy and biomechanics is essential to the study of occlusion. This chapter describes theanatomic features that are basic to an understand-ing of masticatory function. A more detailed descrip-tion can be found in the numerous texts devotedentirely to the anatomy of the head and neck.

FUNCTIONAL ANATOMY

The following anatomic components are discussedin this chapter: the dentition and supportive struc-tures, the skeletal components, the temporo-mandibular joints (TMJs), the ligaments, and the muscles. After the anatomic features aredescribed, the biomechanics of the TMJ are pre-sented. In Chapter 2, the complex neurologic con-trolling system is described and the physiology ofthe masticatory system is presented.

DENTITION AND SUPPORTIVESTRUCTURES

The human dentition is made up of 32 permanentteeth (Fig. 1-1). Each tooth can be divided into twobasic parts: the crown, which is visible above thegingival tissue, and the root, which is submergedin and surrounded by the alveolar bone. The root isattached to the alveolar bone by numerous fibersof connective tissue that span from the cementumsurface of the root to the bone. Most of thesefibers run obliquely from the cementum in a cervi-cal direction to the bone (Fig. 1-2). These fibers areknown collectively as the periodontal ligament. Theperiodontal ligament not only attaches the toothfirmly to its bony socket but also helps dissipatethe forces applied to the bone during functionalcontact of the teeth. In this sense it can be thoughtof as a natural shock absorber.

The 32 permanent teeth are distributed equallyin the alveolar bone of the maxillary and mandibu-lar arches: 16 maxillary teeth are aligned in thealveolar process of the maxilla, which is fixed tothe lower anterior portion of the skull; the remain-ing 16 teeth are aligned in the alveolar process ofthe mandible, which is the movable jaw. The max-illary arch is slightly larger than the mandibulararch, which usually causes the maxillary teeth tooverlap the mandibular teeth both vertically and horizontally when in occlusion (Fig. 1-3). Thissize discrepancy results primarily from the fact that(1) the maxillary anterior teeth are much widerthan the mandibular teeth, which creates a greater

Functional Anatomy andBiomechanics of theMasticatory System


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Functional Anatomy and Biomechanics of the Masticatory System 3

arch width, and (2) the maxillary anterior teeth havea greater facial angulation than the mandibularanterior teeth, which creates a horizontal and verti-cal overlapping.

The permanent teeth can be grouped into fourclassifications as follows according to the mor-phology of the crowns.

The teeth located in the most anterior region ofthe arches are called incisors. They have a character-istic shovel shape, with an incisal edge. Four max-illary incisors and four mandibular incisors exist.The maxillary incisors are generally much largerthan the mandibular incisors and, as previouslymentioned, commonly overlap them. The functionof the incisors is to incise or cut off food duringmastication.

Posterior (distal) to the incisors are the canines.The canines are located at the corners of the archesand are generally the longest of the permanentteeth, with a single cusp and root (Fig. 1-4). Theseteeth are prominent in other animals such as dogs,and hence the name canine. Two maxillary and twomandibular canines exist. In animals the primaryfunction of the canines is to rip and tear food. Inthe human dentition, however, the canines usuallyfunction as incisors and are used only occasionallyfor ripping and tearing.

Still more posterior in the arch are the premolars(see Fig. 1-4). Four maxillary and four mandibularpremolars exist. The premolars are also called bicus-pids because they generally have two cusps. Thepresence of two cusps greatly increases the biting

A B

Fig. 1-1 Anterior (A) and lateral (B) views of the dentition.

Gingival tissue

Alveolar bone

Periodontalligament

Root

Crown

Fig. 1-2 TOOTH AND PERIODONTAL SUP-PORTIVE STRUCTURES. The width of the perio-dontal ligament is greatly exaggerated for illustrative purposes.

Fig. 1-3 The maxillary teeth are positioned slightly facialto the mandibular throughout the arch.

4 Functional Anatomy

surfaces of these teeth. The maxillary andmandibular premolars occlude in such a mannerthat food can be caught and crushed betweenthem. The main function of the premolars is tobegin the effective breakdown of food substancesinto smaller particle sizes.

The last class of teeth, found posterior to thepremolars, is the molars (see Fig. 1-4). Six maxillarymolars and six mandibular molars exist. The crownof each molar has either four or five cusps. Thisprovides a large, broad surface on which breakingand grinding of food can occur. Molars functionprimarily in the later stages of chewing, when foodis broken down into particles small enough to beeasily swallowed.

As discussed, each tooth is highly specializedaccording to its function. The exact interarch andintraarch relationships of the teeth are extremelyimportant and greatly influence the health and function of the masticatory system. A detaileddiscussion of these relationships is presented inChapter 3.

SKELETAL COMPONENTS

The masticatory system comprises three majorskeletal components. Two support the teeth: themaxilla and mandible (Fig. 1-5). The third, the tem-poral bone, supports the mandible at its articulationwith the cranium.

MaxillaDevelopmentally, there are two maxillary bones,which are fused together at the midpalatal suture(Fig. 1-6). These bones make up the greater part ofthe upper facial skeleton. The border of the maxillaextends superiorly to form the floor of the nasal cav-ity, as well as the floor of each orbit. Inferiorly, themaxillary bones form the palate and the alveolarridges, which support the teeth. Because the maxil-lary bones are intricately fused to the surroundingbony components of the skull, the maxillary teethare considered to be a fixed part of the skull and therefore comprise the stationary componentof the masticatory system.

Fig. 1-4 Lateral view.Fig. 1-5 Skeletal components that make up the mastica-tory system: maxilla, mandible, and temporal bone.

A

Fig. 1-6 The midpalatal suture (A) results from the fusionof the two maxillary bones during development.

MandibleThe mandible is a U-shaped bone that supports thelower teeth and makes up the lower facial skeleton.It has no bony attachments to the skull. It is sus-pended below the maxilla by muscles, ligaments,and other soft tissues, which therefore provide themobility necessary to function with the maxilla.

The superior aspect of the arch-shaped mandibleconsists of the alveolar process and the teeth (Fig. 1-7). The body of the mandible extends pos-teroinferiorly to form the mandibular angle andposterosuperiorly to form the ascending ramus.The ascending ramus of the mandible is formed bya vertical plate of bone that extends upward as twoprocesses. The anterior of these is the coronoidprocess. The posterior is the condyle.

The condyle is the portion of the mandible that articulates with the cranium, around whichmovement occurs. From the anterior view it hasmedial and lateral projections, called poles (Fig. 1-8).The medial pole is generally more prominent thanthe lateral. From above, a line drawn through thecenters of the poles of the condyle will usuallyextend medially and posteriorly toward the anteriorborder of the foramen magnum (Fig. 1-9). The totalmediolateral length of the condyle is between 18 and 23 mm, and the anteroposterior width is between 8 and 10 mm. The actual articulating

A

A

B

B

Fig. 1-7 A,The ascending ramus extends upward to form the coronoid process (A) and thecondyle (B). B, Occlusal view.

MPLP

Fig. 1-8 CONDYLE (ANTERIOR VIEW). Themedial pole (MP) is more prominent than the lateral pole (LP).

Fig. 1-9 INFERIOR VIEW OF SURFACE OFCRANIUM AND MANDIBLE. The condyles seemto be slightly rotated such that an imaginary line drawnthrough the lateral and medial poles would extend mediallyand posteriorly toward the anterior border of the foramenmagnum.



surface of the condyle extends both anteriorly and posteriorly to the most superior aspect of the condyle (Fig. 1-10). The posterior articulatingsurface is greater than the anterior surface. The articulating surface of the condyle is quiteconvex anteroposteriorly and only slightly convexmediolaterally.

Temporal BoneThe mandibular condyle articulates at the base of thecranium with the squamous portion of the temporalbone. This portion of the temporal bone is madeup of a concave mandibular fossa, in which thecondyle is situated (Fig. 1-11) and which has also

been called the articular or glenoid fossa. Posterior tothe mandibular fossa is the squamotympanic fissure, which extends mediolaterally. As this fissure extends medially, it divides into the pet-rosquamous fissure anteriorly and the petrotym-panic fissure posteriorly. Immediately anterior tothe fossa is a convex bony prominence called thearticular eminence. The degree of convexity of thearticular eminence is highly variable but importantbecause the steepness of this surface dictates the pathway of the condyle when the mandible ispositioned anteriorly. The posterior roof of themandibula fossa is quite thin, indicating that this area of the temporal bone is not designed to

BA

Fig. 1-10 CONDYLE. A, Anterior view. B, Posterior view. A dotted line marks the border of the articular surface.The articular surface on the posterior aspect of the condyleis greater than on the anterior aspect.

Fig. 1-11 A, Bony structures of the temporomandibular joint (lateral view).B,Articular fossa(inferior view). AE, Articular eminence; MF, mandibular fossa; STF, squamotympanic fissure.

B

AE

MF

STFA

MF

AE


sustain heavy forces. The articular eminence, how-ever, consists of thick dense bone and is morelikely to tolerate such forces.

TEMPOROMANDIBULAR JOINT

The area where the mandible articulates with the cranium, the TMJ, is one of the most complexjoints in the body. It provides for hinging move-ment in one plane and therefore can be considereda ginglymoid joint. However, at the same time italso provides for gliding movements, which classi-fies it as an arthrodial joint. Thus it has been tech-nically considered a ginglymoarthrodial joint.

The TMJ is formed by the mandibular condylefitting into the mandibular fossa of the temporalbone. Separating these two bones from direct artic-ulation is the articular disc. The TMJ is classified asa compound joint. By definition, a compound jointrequires the presence of at least three bones, yetthe TMJ is made up of only two bones. Functionally,the articular disc serves as a nonossified bone that permits the complex movements of the joint.Because the articular disc functions as a third bone,the craniomandibular articulation is considered acompound joint. The function of the articular discas a nonossified bone is described in detail in thesection on the biomechanics of the TMJ later inthis chapter.

The articular disc is composed of dense fibrousconnective tissue, for the most part devoid of anyblood vessels or nerve fibers. The extreme periph-ery of the disc, however, is slightly innervated.1,2

In the sagittal plane it can be divided into threeregions according to thickness (Fig. 1-12). The cen-tral area is the thinnest and is called the intermediatezone. The disc becomes considerably thicker bothanterior and posterior to the intermediate zone.The posterior border is generally slightly thickerthan the anterior border. In the normal joint thearticular surface of the condyle is located on the intermediate zone of the disc, bordered by thethicker anterior and posterior regions.

From an anterior view, the disc is generallythicker medially than laterally, which correspondsto the increased space between the condyle and the articular fossa toward the medial of thejoint (Fig. 1-13). The precise shape of the disc is

PB IZ AB

Fig. 1-12 ARTICULAR DISC, FOSSA, ANDCONDYLE (LATERAL VIEW). The condyle is normally situated on the thinner intermediate zone (IZ) ofthe disc.The anterior border of the disc (AB) is considerablythicker than the intermediate zone, and the posterior border (PB) is even thicker.

LP

MP

Fig. 1-13 ARTICULAR DISC, FOSSA, ANDCONDYLE (ANTERIOR VIEW). The disc is slightly thicker medially than laterally. LP, Lateral pole;MP, medial pole.


determined by the morphology of the condyle andmandibular fossa. During movement the disc issomewhat flexible and can adapt to the functionaldemands of the articular surfaces. Flexibility andadaptability do not imply that the morphology ofthe disc is reversibly altered during function, how-ever. The disc maintains its morphology unlessdestructive forces or structural changes occur inthe joint. If these changes occur, the morphologyof the disc can be irreversibly altered, producing

biomechanical changes during function. Thesechanges are discussed in later chapters.

The articular disc is attached posteriorly to aregion of loose connective tissue that is highly vas-cularized and innervated (Fig. 1-14). This is knownas the retrodiscal tissue or posterior attachment.Superiorly, it is bordered by a lamina of connectivetissue that contains many elastic fibers, the supe-rior retrodiscal lamina. The superior retrodiscallamina attaches the articular disc posteriorly to

A

Disc

RT

Condyle

SLP

ILP

B

SRL SC AS ACL

ICRTIRL

SLP

ILP

ACL

Fig. 1-14 TEMPOROMANDIBULAR JOINT. A, Lateral view. B, Diagram show-ing the anatomic components. ACL, Anterior capsular ligament (collagenous); AS, articular surface; IC, inferior joint cavity; ILP, inferior lateral pterygoid muscles; IRL, inferior retrodiscallamina (collagenous); RT, retrodiscal tissues; SC, superior joint cavity; SLP, superior lateralpterygoid muscles; SRL, superior retrodiscal lamina (elastic). The discal (collateral) ligamenthas not been drawn. (A, Courtesy Dr. Julio Turell, University of Montevideo, Uruguay.)

the tympanic plate. At the lower border of theretrodiscal tissues is the inferior retrodiscal lamina, which attaches the inferior border of theposterior edge of the disc to the posterior marginof the articular surface of the condyle. The inferiorretrodiscal lamina is composed chiefly of collage-nous fibers, not elastic fibers like the superiorretrodiscal lamina. The remaining body of theretrodiscal tissue is attached posteriorly to a largevenous plexus, which fills with blood as the condylemoves forward.3,4 The superior and inferior attach-ments of the anterior region of the disc are to thecapsular ligament, which surrounds most of thejoint. The superior attachment is to the anteriormargin of the articular surface of the temporalbone. The inferior attachment is to the anteriormargin of the articular surface of the condyle. Both these anterior attachments are composed ofcollagenous fibers. Anteriorly, between the attach-ments of the capsular ligament, the disc is alsoattached by tendinous fibers to the superior lateralpterygoid muscle.

The articular disc is attached to the capsular lig-ament not only anteriorly and posteriorly but alsomedially and laterally. This divides the joint intotwo distinct cavities. The upper or superior cavityis bordered by the mandibular fossa and the supe-rior surface of the disc. The lower or inferior cavityis bordered by the mandibular condyle and theinferior surface of the disc. The internal surfaces of the cavities are surrounded by specializedendothelial cells that form a synovial lining. Thislining, along with a specialized synovial fringelocated at the anterior border of the retrodiscal tissues, produces synovial fluid, which fills bothjoint cavities. Thus the TMJ is referred to as a syn-ovial joint. This synovial fluid serves two purposes.Because the articular surfaces of the joint are nonvascular, the synovial fluid acts as a mediumfor providing metabolic requirements to these tis-sues. Free and rapid exchange exists between the vessels of the capsule, the synovial fluid, and thearticular tissues. The synovial fluid also serves as alubricant between articular surfaces during func-tion. The articular surfaces of the disc, condyle, andfossa are very smooth, so friction during movementis minimized. The synovial fluid helps to minimizethis friction further.

Synovial fluid lubricates the articular surfacesby way of two mechanisms. The first is calledboundary lubrication, which occurs when the joint ismoved and the synovial fluid is forced from onearea of the cavity into another. The synovial fluidlocated in the border or recess areas is forced onthe articular surface, thus providing lubrication.Boundary lubrication prevents friction in the mov-ing joint and is the primary mechanism of jointlubrication.

A second lubricating mechanism is called weep-ing lubrication. This refers to the ability of the artic-ular surfaces to absorb a small amount of synovialfluid.5 During function of a joint, forces are createdbetween the articular surfaces. These forces drive asmall amount of synovial fluid in and out of thearticular tissues. This is the mechanism by whichmetabolic exchange occurs. Under compressiveforces, therefore, a small amount of synovial fluidis released. This synovial fluid acts as a lubricantbetween articular tissues to prevent sticking.Weeping lubrication helps eliminate friction in thecompressed but not moving joint. Only a smallamount of friction is eliminated as a result of weep-ing lubrication; therefore prolonged compressiveforces to the articular surfaces will exhaust thissupply. The consequence of prolonged static loading of the joint structures is discussed in laterchapters.

Histology of the Articular SurfacesThe articular surfaces of the mandibular condyleand fossa are composed of four distinct layers or zones (Fig. 1-15). The most superficial layer iscalled the articular zone. It is found adjacent to thejoint cavity and forms the outermost functionalsurface. Unlike most other synovial joints, thisarticular layer is made of dense fibrous connectivetissue rather than hyaline cartilage. Most of thecollagen fibers are arranged in bundles and ori-ented nearly parallel to the articular surface.6,7 Thefibers are tightly packed and can withstand theforces of movement. It is thought that this fibrousconnective tissue affords the joint several advan-tages over hyaline cartilage. Because fibrous con-nective tissue is generally less susceptible thanhyaline cartilage to the effects of aging, it is lesslikely to break down over time. It also has a much



better ability to repair than does hyaline cartilage.8

The importance of these two factors is significantin TMJ function and dysfunction and is discussedmore completely in later chapters.

The second zone, the proliferative zone, is mainlycellular. It is in this area that undifferentiated mes-enchymal tissue is found. This tissue is responsi-ble for the proliferation of articular cartilage inresponse to the functional demands placed on thearticular surfaces during loading.

In the third zone, the fibrocartilaginous zone, thecollagen fibrils are arranged in bundles in a cross-ing pattern, although some of the collagen is seenin a radial orientation. The fibrocartilage appearsto be in a random orientation, providing a three-dimensional network that offers resistance againstcompressive and lateral forces.

The fourth and deepest zone is called the calcifiedcartilage zone. This zone comprises chondrocytesand chondroblasts distributed throughout the artic-ular cartilage. In this zone the chondrocytes becomehypertrophic, die, and have their cytoplasm

evacuated, forming bone cells from within themedullary cavity. The surface of the extracellularmatrix scaffolding provides an active site forremodeling activity while endosteal bone growthproceeds, as it does elsewhere in the body.

The articular cartilage is composed of chondro-cytes and intercellular matrix.9 The chondrocytesproduce the collagen, proteoglycans, glycoproteins,and enzymes that form the matrix. Proteoglycansare complex molecules composed of a protein coreand glycosaminoglycan chains. The proteoglycansare connected to a hyaluronic acid chain formingproteoglycan aggregates that make up a great pro-tein of the matrix (Fig. 1-16). These aggregates arevery hydrophilic and are intertwined throughoutthe collagen network. Because these aggregatestend to blind water, the matrix expands and the ten-sion in the collagen fibrils counteracts the swellingpressure of the proteoglycan aggregates.10 In thisway the interstitial fluid contributes to supportjoint loading. The external pressure resulting fromjoint loading is in equilibrium with the internalpressure of the articular cartilage. As joint loadingincreases, tissue fluid flows outward until a new

Subarticularbone

Proliferativezone

Articular disc

Articular zone

Fibrocartila-ginous zone

Calcifiedcartilage zone

Fig.1-15 Histologic section of a healthy mandibular condyleshowing the four zones: articular, proliferative, fibrocartilagi-nous, and calcified. (From Cohen B, Kramer IRH, editors:Scientific foundations of dentistry, London, 1976, WilliamHeinemann.).

Hyaluronic acidMonomer

Interstitial fluid

Collagen fibril

Attached monomer

40 nm

Fig. 1-16 Collagen network interacting with the proteo-glycan network in the extracellular matrix forming a fiberreinforced composite. (From Mow VC, Ratcliffe A: Cartilage anddiarthrodial joints as paradigms for hierarchical materials andstructures, Biomaterials 13:67-81, 1992.)


equilibrium is achieved. As loading is decreased,fluid is reabsorbed and the tissue regains its origi-nal volume. Joint cartilage is nourished predomi-nantly by diffusion of synovial fluid, which dependson this pumping action during normal activity.11

This pumping action is the basis for the weepinglubrication that was discussed previously and isthought to be important in maintaining healthyarticular cartilage.12

Innervation of the Temporomandibular JointAs with all joints, the TMJ is innervated by thesame nerve that provides motor and sensory innervation to the muscles that control it (thetrigeminal nerve). Branches of the mandibularnerve provide the afferent innervation. Most inner-vation is provided by the auriculotemporal nerveas it leaves the mandibular nerve behind the jointand ascends laterally and superiorly to wrap aroundthe posterior region of the joint.13 Additional inner-vation is provided by the deep temporal and mas-seteric nerves.

Vascularization of the Temporomandibular JointThe TMJ is richly supplied by a variety of vesselsthat surround it. The predominant vessels are thesuperficial temporal artery from the posterior; the middle meningeal artery from the anterior; andthe internal maxillary artery from the inferior. Otherimportant arteries are the deep auricular, anteriortympanic, and ascending pharyngeal arteries. The condyle receives its vascular supply through itsmarrow spaces by way of the inferior alveolar arteryand also receives vascular supply by way of feedervessels that enter directly into the condylar headboth anteriorly and posteriorly from the larger vessels.14

LIGAMENTS

As with any joint system, ligaments play an impor-tant role in protecting the structures. The liga-ments of the joint are composed of collagenousconnective tissues that have particular lengths.They do not stretch. However, if extensive forcesare applied to a ligament, whether suddenly orover a prolonged period of time, the ligament canbe elongated. When this occurs, the function of

the ligament is compromised, thereby alteringjoint function. This alteration is discussed in futurechapters that discuss pathology of the joint.

Ligaments do not enter actively into joint function but instead act as passive restrainingdevices to limit and restrict border movements.Three functional ligaments support the TMJ: (1) the collateral ligaments, (2) the capsular ligament, and (3) the temporomandibular (TM) ligament.Two accessory ligaments also exist: (4) the spheno-mandibular and (5) the stylomandibular.

Collateral (Discal) LigamentsThe collateral ligaments attach the medial and lateral borders of the articular disc to the poles ofthe condyle. They are commonly called the discalligaments, and there are two. The medial discal liga-ment attaches the medial edge of the disc to themedial pole of the condyle. The lateral discal liga-ment attaches the lateral edge of the disc to the lat-eral pole of the condyle (see Figs. 1-14 and 1-17).

SC

IC

LDL

CL

AD

CL

MDL

Fig. 1-17 TEMPOROMANDIBULAR JOINT(ANTERIOR VIEW). AD, Articular disc; CL, capsularligament; IC, inferior joint cavity; LDL, lateral discal ligament;MDL, medial discal ligament; SC, superior joint cavity.


These ligaments are responsible for dividing thejoint mediolaterally into the superior and inferiorjoint cavities. The discal ligaments are true liga-ments, composed of collagenous connective tissuefibers; therefore they do not stretch. They functionto restrict movement of the disc away from thecondyle. In other words, they allow the disc tomove passively with the condyle as it glides anteri-orly and posteriorly. The attachments of the discalligaments permit the disc to be rotated anteriorlyand posteriorly on the articular surface of thecondyle. Thus these ligaments are responsible forthe hinging movement of the TMJ, which occursbetween the condyle and the articular disc.

The discal ligaments have a vascular supply andare innervated. Their innervation provides informa-tion regarding joint position and movement. Strainon these ligaments produces pain.

Capsular LigamentAs previously mentioned, the entire TMJ is sur-rounded and encompassed by the capsular liga-ment (Fig. 1-18). The fibers of the capsular ligamentare attached superiorly to the temporal bone along the borders of the articular surfaces of themandibular fossa and articular eminence. Inferiorly,the fibers of the capsular ligament attach to theneck of the condyle. The capsular ligament acts toresist any medial, lateral, or inferior forces thattend to separate or dislocate the articular sur-faces. A significant function of the capsular liga-ment is to encompass the joint, thus retaining the

synovial fluid. The capsular ligament is well inner-vated and provides proprioceptive feedback regard-ing position and movement of the joint.

Temporomandibular LigamentThe lateral aspect of the capsular ligament is reinforced by strong, tight fibers that make up thelateral ligament, or TM ligament. The TM ligamentis composed of two parts, an outer oblique portionand an inner horizontal portion (Fig. 1-19). Theouter portion extends from the outer surface of thearticular tubercle and zygomatic process pos-teroinferiorly to the outer surface of the condylarneck. The inner horizontal portion extends fromthe outer surface of the articular tubercle and zygo-matic process posteriorly and horizontally to thelateral pole of the condyle and posterior part of thearticular disc.

The oblique portion of the TM ligament resistsexcessive dropping of the condyle, therefore limit-ing the extent of mouth opening. This portion ofthe ligament also influences the normal openingmovement of the mandible. During the initialphase of opening, the condyle can rotate around afixed point until the TM ligament becomes tight asits point of insertion on the neck of the condyle isrotated posteriorly. When the ligament is taut, theneck of the condyle cannot rotate further. If the

Fig. 1-18 CAPSULAR LIGAMENT (LATERALVIEW). Note that it extends anteriorly to include thearticular eminence and encompass the entire articular sur-face of the joint.

Fig. 1-19 TEMPOROMANDIBULAR LIGA-MENT (LATERAL VIEW). Two distinct parts areshown: the outer oblique portion (OOP) and the inner hor-izontal portion (IHP). The OOP limits normal rotationalopening movement; the IHP limits posterior movement ofthe condyle and disc. (Modified from Du Brul EL: Sichers oralanatomy, ed 7, St Louis, 1980, Mosby.)

IHP

OOP


mouth were to be opened wider, the condyle wouldneed to move downward and forward across thearticular eminence (Fig. 1-20). This effect can bedemonstrated clinically by closing the mouth andapplying mild posterior force to the chin. With thisforce applied, the patient should be asked to openthe mouth. The jaw will easily rotate open until theteeth are 20 to 25 mm apart. At this point, resist-ance will be felt when the jaw is opened wider. If the jaw is opened still wider, a distinct change in the opening movement will occur, representing the change from rotation of the condyle around a fixed point to movement forward and down the articular eminence. This change in openingmovement is brought about by the tightening ofthe TM ligament.

This unique feature of the TM ligament, whichlimits rotational opening, is found only in humans.In the erect postural position and with a verticallyplaced vertebral column, continued rotationalopening movement would cause the mandible toimpinge on the vital submandibular and retro-mandibular structures of the neck. The outeroblique portion of the TM ligament functions toresist this impingement.

The inner horizontal portion of the TM ligamentlimits posterior movement of the condyle and disc.When force applied to the mandible displaces thecondyle posteriorly, this portion of the ligamentbecomes tight and prevents the condyle from moving into the posterior region of the mandibu-lar fossa. The TM ligament therefore protects theretrodiscal tissues from trauma created by the pos-terior displacement of the condyle. The inner hori-zontal portion also protects the lateral pterygoidmuscle from overlengthening or extension. Theeffectiveness of this ligament is demonstrated dur-ing cases of extreme trauma to the mandible. Insuch cases, the neck of the condyle will be seen tofracture before the retrodiscal tissues are severedor the condyle enters the middle cranial fossa.

Sphenomandibular LigamentThe sphenomandibular ligament is one of two TMJaccessory ligaments (Fig. 1-21). It arises from thespine of the sphenoid bone and extends downwardto a small bony prominence on the medial surfaceof the ramus of the mandible, which is called thelingula. It does not have any significant limitingeffects on mandibular movement.

A B

AB

C

AB

Fig. 1-20 EFFECT OF THE OUTER OBLIQUE PORTION OF THE TEMPOROMANDIBULAR (TM) LIGAMENT. A,As the mouth opens, the teethcan be separated about 20 to 25 mm (from A to B) without the condyles moving from thefossae. B, TM ligaments are fully extended. As the mouth opens wider, they force thecondyles to move downward and forward out of the fossae. This creates a second arc ofopening (from B to C).


Stylomandibular LigamentThe second accessory ligament is the stylo-mandibular ligament (see Fig. 1-21). It arises fromthe styloid process and extends downward and forward to the angle and posterior border of theramus of the mandible. It becomes taut when themandible is protruded but is most relaxed whenthe mandible is opened. The stylomandibular liga-ment therefore limits excessive protrusive move-ments of the mandible.

MUSCLES OF MASTICATION

The skeletal components of the body are heldtogether and moved by the skeletal muscles. Theskeletal muscles provide for the locomotion neces-sary for the individual to survive. Muscles aremade of numerous fibers ranging from 10 to 80 min diameter. Each of these fibers in turn is made upof successively smaller subunits. In most musclesthe fibers extend the entire length of the muscle,except for about 2% of the fibers. Each fiber isinnervated by only one nerve ending, located nearthe middle of the fiber. The end of the muscle fiberfuses with a tendon fiber, and the tendon fibers in turn collect into bundles to form the muscle tendon that inserts into the bone. Each muscle

fiber contains several hundred to several thousandmyofibrils. Each myofibril in turn has, lying side byside, about 1500 myosin filaments and 3000 actinfilaments, which are large polymerized proteinmolecules that are responsible for muscle contrac-tion. For a more complete description of the phys-iology of muscle contraction, other publicationsshould be pursued.15

Muscle fibers can be characterized by typeaccording to the amount of myoglobin (a pigmentsimilar to hemoglobin). Fibers with higher concen-trations of myoglobin are deeper red in color andcapable of slow but sustained contraction. Thesefibers are called slow muscle fibers or type I muscle fibers.Slow fibers have a well-developed aerobic metab-olism and are therefore resistant to fatigue. Fiberswith lower concentrations of myoglobin are whiterand are called fast muscle fibers or type II fibers. Thesefibers have fewer mitochondria and rely more onanaerobic activity for function. Fast muscle fibersare capable of quick contraction but fatigue morerapidly.

All skeletal muscles contain a mixture of fast and slow fibers in varying proportions thatreflect the function of that muscle. Muscles thatare called on to respond quickly are made of pre-dominately white fibers. Muscles that are mainlyused for slow, continuous activity have higher concentrations of slow fibers.

Four pairs of muscles make up a group calledthe muscles of mastication: the masseter, temporalis,medial pterygoid, and lateral pterygoid. Althoughnot considered to be muscles of mastication, thedigastrics also play an important role in mandibu-lar function and therefore are discussed in this section. Each muscle is discussed according to its attachment, the direction of its fibers, and itsfunction.

MasseterThe masseter is a rectangular muscle that originatesfrom the zygomatic arch and extends downward tothe lateral aspect of the lower border of the ramusof the mandible (Fig. 1-22). Its insertion on themandible extends from the region of the secondmolar at the inferior border posteriorly to includethe angle. It has two portions, or heads: (1) The

Stylomandibularligament

Sphenomandibular ligament

Fig. 1-21 Mandible, temporomandibular joint, and acces-sory ligaments.


superficial portion consists of fibers that run down-ward and slightly backward, and (2) the deep por-tion consists of fibers that run in a predominantlyvertical direction.

As fibers of the masseter contract, the mandibleis elevated and the teeth are brought into contact.The masseter is a powerful muscle that provides theforce necessary to chew efficiently. Its superficialportion may also aid in protruding the mandible.When the mandible is protruded and biting force isapplied, the fibers of the deep portion stabilize thecondyle against the articular eminence.

TemporalisThe temporalis is a large, fan-shaped muscle thatoriginates from the temporal fossa and the lateralsurface of the skull. Its fibers come together asthey extend downward between the zygomatic archand the lateral surface of the skull to form a tendonthat inserts on the coronoid process and anteriorborder of the ascending ramus. It can be dividedinto three distinct areas according to fiber direc-tion and ultimate function (Fig. 1-23). The anteriorportion consists of fibers that are directed almostvertically. The middle portion contains fibers that run obliquely across the lateral aspect of the skull (slightly forward as they pass downward).The posterior portion consists of fibers that are

aligned almost horizontally, coming forward abovethe ear to join other temporalis fibers as they passunder the zygomatic arch.

When the temporal muscle contracts, it elevatesthe mandible and the teeth are brought into contact. If only portions contract, the mandible ismoved according to the direction of those fibers that are activated. When the anterior portioncontracts, the mandible is raised vertically.Contraction of the middle portion will elevate andretrude the mandible. Function of the posteriorportion is somewhat controversial. Although itwould appear that contraction of this portion willretrude the mandible, DuBrul16 suggests that thefibers below the root of the zygomatic process arethe only significant ones and therefore contractionwill cause elevation and only slight retrusion.Because the angulation of its muscle fibers varies,the temporalis is capable of coordinating closingmovements. Thus it is a significant positioningmuscle of the mandible.

Pterygoideus MedialisThe medial (internal) pterygoid originates from thepterygoid fossa and extends downward, backward,and outward to insert along the medial surface ofthe mandibular angle (Fig. 1-24). Along with themasseter, it forms a muscular sling that supports

BA

DP

SP

Fig. 1-22 A, Masseter muscle. DP, Deep portion; SP, superficial portion. B, Function:elevation of the mandible.


the mandible at the mandibular angle. When itsfibers contract, the mandible is elevated and theteeth are brought into contact. This muscle is alsoactive in protruding the mandible. Unilateral con-traction will bring about a mediotrusive movementof the mandible.

Pterygoideus LateralisFor many years the lateral (external) pterygoid wasdescribed as having two distinct portions or bellies:(1) an inferior and (2) a superior. Because the mus-cle appeared anatomically to be as one in structureand function, this description was acceptable until

BA

AP

MP

PP

Fig. 1-23 A, Temporal muscle. AP, Anterior portion; MP, middle portion; PP, posterior por-tion. B, Function: elevation of the mandible.The exact movement by the location of the fibersor portion being activated.

A B

Fig. 1-24 A, Medial pterygoid muscle. B, Function: elevation of the mandible.


studies proved differently.17,18 Now it is appreci-ated that the two bellies of the lateral pterygoidfunction quite differently. Therefore in this text thelateral pterygoid is divided and identified as twodistinct and different muscles, which is appropri-ate because their functions are nearly opposite.The muscles are described as the inferior lateraland the superior lateral pterygoid.

Inferior Lateral Pterygoid. The inferior lateral pterygoid originates at the outer surface ofthe lateral pterygoid plate and extends backward,upward, and outward to its insertion primarily on the neck of the condyle (Fig. 1-25). When theright and left inferior lateral pterygoids contractsimultaneously, the condyles are pulled down thearticular eminences and the mandible is protruded.Unilateral contraction creates a mediotrusive move-ment of that condyle and causes a lateral move-ment of the mandible to the opposite side. Whenthis muscle functions with the mandibular depres-sors, the mandible is lowered and the condylesglide forward and downward on the articular eminences.

Superior Lateral Pterygoid. The superior lateral pterygoid is considerably smaller than theinferior and originates at the infratemporal surfaceof the greater sphenoid wing, extending almosthorizontally, backward, and outward to insert onthe articular capsule, the disc, and the neck of the

condyle (see Figs. 1-14 and 1-25). The exact attach-ment of the superior lateral pterygoid to the disc issomewhat debatable. Although some authors19

suggest no attachment, most studies reveal thepresence of a muscle-disc attachment.14,20-24

The majority of the fibers of the superior lateralpterygoid (60% to 70%) attach to the neck of the condyle, with only 30% to 40% attaching to thedisc. Importantly, the attachments are more pre-dominant on the medial aspect than on the lateral.Approaching the joint structures from the lateralaspect would reveal little or no muscle attachment.This may explain the different findings in thesestudies.

Although the inferior lateral pterygoid is activeduring opening, the superior remains inactive,becoming active only in conjunction with the ele-vator muscles. The superior lateral pterygoid isespecially active during the power stroke and whenthe teeth are held together. The power stroke refers tomovements that involve closure of the mandibleagainst resistance, such as in chewing or clenchingthe teeth together. The functional significance ofthe superior lateral pterygoid is discussed in moredetail in the next section, which deals with the bio-mechanics of the TMJ.

The clinician should note that the pull of the lat-eral pterygoid on the disc and condyle is predomi-nantly in an anterior direction; however, it also has

BA

Superior lateralpterygoid muscle

Inferior lateralpterygoid muscle

Fig. 1-25 A, Inferior and superior lateral pterygoid muscles. B, Function of the inferior lat-eral pterygoid: protrusion of the mandible.


a significantly medial component (Fig. 1-26). Asthe condyle moves more forward, the medial angulation of the pull of these muscles becomeseven greater. In the wide-open mouth position, thedirection of the muscle pull is more medial thananterior.

Interestingly, approximately 80% of the fibersthat make up both lateral pterygoid muscles are slow muscle fibers (type I).25,26 This suggeststhat these muscles are relatively resistant to fatigueand may serve to brace the condyle for long periodsof time without difficulty.

DigastricusAlthough the digastric is not generally considereda muscle of mastication, it does have an importantinfluence on the function of the mandible. It isdivided into two portions, or bellies (Fig. 1-27):1. The posterior belly originates from the mastoid

notch, just medial to the mastoid process; itsfibers run forward, downward, and inward to theintermediate tendon attached to the hyoid bone.

2. The anterior belly originates at a fossa on the lin-gual surface of the mandible, just above thelower border and close to the midline; its fibersextend downward and backward to insert at thesame intermediate tendon as does the poste-rior belly.

When the right and left digastrics contract andthe hyoid bone is fixed by the suprahyoid andinfrahyoid muscles, the mandible is depressed and pulled backward and the teeth are brought outof contact. When the mandible is stabilized, the digastric muscles with the suprahyoid andinfrahyoid muscles elevate the hyoid bone, whichis a necessary function for swallowing.

The digastrics are one of many muscles thatdepress the mandible and raise the hyoid bone(Fig. 1-28). Generally muscles that are attachedfrom the mandible to the hyoid bone are calledsuprahyoid, and those attached from the hyoid boneto the clavicle and sternum are called infrahyoid.The suprahyoid and infrahyoid muscles play amajor role in coordinating mandibular function, asdo many of the other numerous muscles of thehead and neck. It can be quickly observed that astudy of mandibular function is not limited to the muscles of mastication. Other major muscles,such as the sternocleidomastoid and the posteriorcervical muscles, play major roles in stabilizing theskull and enabling controlled movements of themandible to be performed. A finely tuned dynamicbalance exists among all of the head and neckmuscles, and this must be appreciated for anunderstanding of the physiology of mandibularmovement to occur. As a person yawns, the head is

BA

Fig. 1-26 A, When the condyle is in a normal relationship in the fossa, the attachments ofthe superior and inferior lateral pterygoid muscles create a medial and anterior pull on thecondyle and disc (arrows). B, As the condyle moves anteriorly from the fossa, the pullbecomes more medially directed (arrows).


brought back by contraction of the posterior cervi-cal muscles, which raises the maxillary teeth. Thissimple example demonstrates that even normalfunctioning of the masticatory system uses manymore muscles than just those of mastication. Withan understanding of this relationship, one can seethat any effect on the function of the muscles ofmastication also has an effect on other head and neck muscles. A more detailed review of thephysiology of the entire masticatory system is presented in Chapter 2.

BIOMECHANICS OF THETEMPOROMANDIBULAR JOINT

The TMJ is an extremely complex joint system. The fact that two TMJs are connected to the samebone (the mandible) further complicates the func-tion of the entire masticatory system. Each jointcan simultaneously act separately and yet not com-pletely without influence from the other. A soundunderstanding of the biomechanics of the TMJ isessential and basic to the study of function anddysfunction in the masticatory system.

Posterior digastric muscleHyoid bone

Intermediate tendon

Anterior digastric muscle

A B

Fig. 1-27 A, Digastric muscle. B, Function: depression of the mandible.

Suprahyoidmuscles

Hyoid bone

Infrahyoid muscles

Sternocleidomastoidmuscle

Fig. 1-28 Movement of the head and neck is a result ofthe finely coordinated efforts of many muscles.The musclesof mastication represent only part of this complex system.


The TMJ is a compound joint. Its structure andfunction can be divided into two distinct systems:1. One joint system is the tissues that surround

the inferior synovial cavity (i.e., the condyle andthe articular disc). Because the disc is tightlybound to the condyle by the lateral and medialdiscal ligaments, the only physiologic move-ment that can occur between these surfaces isrotation of the disc on the articular surface ofthe condyle. The disc and its attachment to the condyle are called the condyle-disc complex;this joint system is responsible for rotationalmovement in the TMJ.

2. The second system is made up of the condyle-disc complex functioning against the surface ofthe mandibular fossa. Because the disc is nottightly attached to the articular fossa, free slid-ing movement is possible between these sur-faces in the superior cavity. This movementoccurs when the mandible is moved forward(referred to as translation). Translation occurs inthis superior joint cavity between the superiorsurface of the articular disc and the mandibularfossa (Fig. 1-29). Thus the articular disc acts as a nonossified bone contributing to bothjoint systems, and hence the function of thedisc justifies classifying the TMJ as a true compound joint.The articular disc has been referred to as a

meniscus. However, it is not a meniscus at all. By def-inition, a meniscus is a wedge-shaped crescent offibrocartilage attached on one side to the articularcapsule and unattached on the other side, extend-ing freely into the joint spaces. A meniscus does notdivide a joint cavity, isolating the synovial fluid, nor

does it serve as a determinant of joint movement.Instead, it functions passively to assist movementbetween the bony parts. Typical menisci are foundin the knee joint. In the TMJ the disc functions as a true articular surface in both joint systemsand is therefore more accurately termed an articular disc.

Now that the two individual joint systems havebeen described, the entire TMJ can be consideredagain. The articular surfaces of the joint have nostructural attachment or union, yet contact mustbe maintained constantly for joint stability.Stability of the joint is maintained by constantactivity of the muscles that pull across the joint,primarily the elevators. Even in the resting state,these muscles are in a mild state of contractioncalled tonus (this feature is discussed in Chapter 2).As muscle activity increases, the condyle isincreasingly forced against the disc and the discagainst the fossa, resulting in an increase in theinterarticular pressure* of these joint structures.27-29

In the absence of interarticular pressure, the articular surfaces will separate and the joint willtechnically dislocate.

The width of the articular disc space varies with interarticular pressure. When the pressure is low, as in the closed rest position, the disc space widens. When the pressure is high, as duringclenching of the teeth, the disc space narrows. Thecontour and movement of the disc permit constantcontact of the articular surfaces of the joint, which

Fig. 1-29 Normal movement of the condyle and disc during mouth opening. As the condyle moves out of the fossa, the disc rotates posteriorly on the condyle around theattachment of the discal collateral ligaments. Rotational movement occurs predominately inthe lower joint space, whereas translation occurs predominately in the superior joint space.

*Interarticular pressure is the pressure between the articularsurfaces of the joint.

is necessary for joint stability. As the interarticularpressure increases, the condyle seats itself on thethinner intermediate zone of the disc. When thepressure is decreased and the disc space iswidened, a thicker portion of the disc is rotated tofill the space. Because the anterior and posteriorbands of the disc are wider than the intermediatezone, technically the disc could be rotated eitheranteriorly or posteriorly to accomplish this task.The direction of the disc rotation is determinednot by chance, but by the structures attached tothe anterior and posterior borders of the disc.

Attached to the posterior border of the articulardisc are the retrodiscal tissues, sometimes referredto as the posterior attachment. As previously men-tioned, the superior retrodiscal lamina is composedof varying amounts of elastic connective tissue.Because this tissue has elastic properties andbecause in the closed mouth position it is some-what folded over itself, the condyle can easilymove out of the fossa without creating any damageto the superior retrodiscal lamina. When the mouthis closed (the closed joint position), the elastictraction on the disc is minimal to none. However,during mandibular opening, when the condyle ispulled forward down the articular eminence, thesuperior retrodiscal lamina becomes increasinglystretched, creating increased forces to retract thedisc. In the full forward position, the posteriorretractive force on the disc created by the tensionof the stretched superior retrodiscal lamina is at a maximum. The interarticular pressure and themorphology of the disc prevent the disc from beingoverretracted posteriorly. In other words, as themandible moves into a full forward position andduring its return, the retraction force of the supe-rior retrodiscal lamina holds the disc rotated as farposteriorly on the condyle as the width of the artic-ular disc space will permit. This is an importantprinciple in understanding joint function. Likewise,it is important to remember that the superiorretrodiscal lamina is the only structure capable ofretracting the disc posteriorly on the condyle,although this retractive force is only present duringwide opening movements.

Attached to the anterior border of the articulardisc is the superior lateral pterygoid muscle. Whenthis muscle is active, the fibers that are attached

to the disc pull anteriorly and medially. Thereforethe superior lateral pterygoid is technically a protractor of the disc. Remember, however, that thismuscle is also attached to the neck of the condyle.This dual attachment does not allow the muscle topull the disc through the discal space. Protractionof the disc, however, does not occur during jawopening. When the inferior lateral pterygoid is pro-tracting the condyle forward, the superior lateralpterygoid is inactive and therefore does not bringthe disc forward with the condyle. The superior lat-eral pterygoid is activated only in conjunction withactivity of the elevator muscles during mandibularclosure or a power stroke.

Understanding the features that cause the discto move forward with the condyle in the absence ofsuperior lateral pterygoid activity is important. The anterior capsular ligament attaches the disc tothe anterior margin of the articular surface of thecondyle (see Fig. 1-14). In addition, the inferiorretrodiscal lamina attaches the posterior edge ofthe disc to the posterior margin of the articularsurface of the condyle. Both ligaments are com-posed of collagenous fibers and will not stretch.Therefore a logical assumption is that they forcethe disc to translate forward with the condyle.Although logical, such an assumption is incorrect:These structures are not primarily responsible formovement of the disc with the condyle. Ligamentsdo not actively participate in normal joint functionbut only passively restrict extreme border move-ments. The mechanism by which the disc is main-tained with the translating condyle is dependenton the morphology of the disc and the interarticu-lar pressure. In the presence of a normallyshaped articular disc, the articulating surface ofthe condyle rests on the intermediate zone,between the two thicker portions. As the interartic-ular pressure is increased, the discal space nar-rows, which more positively seats the condyle onthe intermediate zone.

During translation, the combination of discmorphology and interarticular pressure maintainsthe condyle on the intermediate zone and the discis forced to translate forward with the condyle.Therefore the morphology of the disc is extremelyimportant in maintaining proper position duringfunction. Proper morphology plus interarticular



pressure results in an important self-positioningfeature of the disc. Only when the morphology ofthe disc has been greatly altered does the ligamen-tous attachment of the disc affect joint function.When this occurs, the biomechanics of the joint isaltered and dysfunctional signs begin. These con-ditions are discussed in detail in later chapters.

As with most muscles, the superior lateralpterygoid is constantly maintained in a mild state

of contraction or tonus, which exerts a slight ante-rior and medial force on the disc. In the restingclosed joint position, this anterior and medialforce will normally exceed the posterior elasticretraction force provided by the nonstretchedsuperior retrodiscal lamina. Therefore in the rest-ing closed joint position, when the interarticularpressure is low and the disc space widened, thedisc will occupy the most anterior rotary position

1

2

3

4

5

6

7

8

Fig. 1-30 Normal functional movement of the condyle and disc during the full range ofopening and closing.The disc is rotated posteriorly on the condyle as the condyle is trans-lated out of the fossa. The closing movement is the exact opposite of opening. The disc isalways maintained between the condyle and the fossa.

on the condyle permitted by the width of the space.In other words, at rest with the mouth closed, the condyle will be positioned in contact with theintermediate and posterior zones of the disc.

This disc relationship is maintained duringminor passive rotational and translatory mandibu-lar movements. As soon as the condyle is movedforward enough to cause the retractive force of thesuperior retrodiscal lamina to be greater than themuscle tonus force of the superior lateral ptery-goid, the disc is rotated posteriorly to the extentpermitted by the width of the articular disc space.When the condyle is returned to the resting closedjoint position, once again the tonus of the superiorlateral pterygoid becomes the predominant forceand the disc is repositioned forward as far as thedisc space will permit (Fig. 1-30).

The functional importance of the superior lateral pterygoid muscle becomes obvious whenobserving the effects of the power stroke duringunilateral chewing. When one bites down on a hardsubstance on one side (e.g., a tough steak), theTMJs are not equally loaded. This occurs becausethe force of closure is not applied to the joint butis instead applied to the food. The jaw is fulcrumedaround the hard food, causing an increase in inter-articular pressure in the contralateral joint and asudden decrease in interarticular pressure in theipsilateral (same side) joint.30,31 This can lead toseparation of the articular surfaces, resulting indislocation of the ipsilateral joint. To prevent thisdislocation, the superior lateral pterygoid becomesactive during the power stroke, rotating the discforward on the condyle so that the thicker poste-rior border of the disc maintains articular contact.Therefore joint stability is maintained during thepower stroke of chewing. As the teeth pass throughthe food and approach intercuspation, the interar-ticular pressure is increased. As the interarticularpressure is increased in the joint, the disc space isdecreased and the disc is mechanically rotatedposteriorly so that the thinner intermediate zonefills the space. When the force of closure is discon-tinued, the resting closed joint position is onceagain assumed (see Fig. 1-30).

Understanding these basic concepts in TMJ func-tion is essential to understanding joint dysfunction.Normal biomechanical function of the TMJ must

follow the orthopedic principles just presented.Clinicians should remember the following:1. Ligaments do not actively participate in

normal function of the TMJ. They act as guidewires, restricting certain joint movements whilepermitting others. They restrict joint move-ments both mechanically and through neuro-muscular reflex activity (see Chapter 2).

2. Ligaments do not stretch. If traction force isapplied, they can become elongated (i.e.,increase in length). (Stretch implies the ability toreturn to the original length.) Once ligamentshave been elongated, normal joint function isoften compromised.

3. The articular surfaces of the TMJs must bemaintained in constant contact. This contact isproduced by the muscles that pull across thejoints (the elevators: temporal, masseter, andmedial pterygoid).A sound understanding of these principles is

necessary for the evaluation and treatment of thevarious disorders that are presented throughoutthe remainder of this book.

References

1. Wink CS, St Onge M, Zimny ML: Neural elements in thehuman temporomandibular articular disc, J Oral MaxillofacSurg 50:334-337, 1992.

2. Ichikawa H, Wakisaka S, Matsuo S, Akai M: Peptidergicinnervation of the temporomandibular disk in the rat,Experientia 45:303-304, 1989.

3. Westesson PL, Kurita K, Eriksson L, Katzberg RH:Cryosectional observations of functional anatomy of thetemporomandibular joint, Oral Surg Oral Med Oral Pathol68:247-255, 1989.

4. Sahler LG, Morris TW, Katzberg RW, Tallents RH:Microangiography of the rabbit temporomandibular jointin the open and closed jaw positions, J Oral Maxillofac Surg48:831-834, 1990.

5. Shengyi T, Yinghua X: Biomechanical properties and collagen fiber orientation of TMJ discs in dogs: part 1. Gross anatomy and collagen fibers orientation of the disc, J Craniomandib Disord 5:28-34, 1991.

6. De Bont L, Liem R, Boering G: Ultrastructure of the articularcartilage of the mandibular condyle: aging and degeneration,Oral Surg Oral Med Oral Pathol 60:631-641, 1985.

7. De Bont L, Boering G, Havinga P, Leim RSB: Spatial arrange-ment of collagen fibrils in the articular cartilage of themandibular condyle: a light microscopic and scanning electronmicroscopic study, J Oral Maxillofac Surg 42:306-313, 1984.



8. Robinson PD: Articular cartilage of the temporo-mandibular joint: can it regenerate? Ann R Coll Surg Engl75:231-236, 1993.

9. Mow VC, Ratcliffe A, Poole AR: Cartilage and diarthrodialjoints as paradigms for hierarchical materials and struc-tures, Biomaterials 13:67-97, 1992.

10. Maroudas A: Balance between swelling pressure and colla-gen tension in normal and degenerate cartilage, Nature260:808-809, 1976.

11. Mow VC, Holmes MH, Lai WM: Fluid transport and mechan-ical properties of articular cartilage: a review, J Biomech17:377-394, 1984.

12. Stegenga B, de Bont LG, Boering G, van Willigen JD: Tissue responses to degenerative changes in the temporomandibular joint: a review, J Oral Maxillofac Surg49:1079-1088, 1991.

13. Fernandes PR, de Vasconsellos HA, Okeson JP, Bastos RL,Maia ML: The anatomical relationship between the posi-tion of the auriculotemporal nerve and mandibularcondyle, Cranio 21:165-171, 2003.

14. Tanaka TT: TMJ microanatomy: an approach to current controversies, Chula Vista, Calif, 1992, Clinical ResearchFoundation.

15. Guyton AC: Textbook of medical physiology, ed 8, Philadelphia,1991, Saunders, p 1013.

16. Du Brul EL: Sichers oral anatomy, ed 7, St Louis, 1980,Mosby.

17. McNamara JA: The independent functions of the twoheads of the lateral pterygoid muscle in the human tem-poromandibular joint, Am J Anat 138:197-205, 1973.

18. Mahan PE, Wilkinson TM, Gibbs CH, Mauderli A,Brannon LS: Superior and inferior bellies of the lateralpterygoid muscle EMG activity at basic jaw positions, J Prosthet Dent 50:710-718, 1983.

19. Wilkinson TM: The relationship between the disk and the lateral pterygoid muscle in the human temporo-mandibular joint, J Prosthet Dent 60:715-724, 1988.

20. Dusek TO, Kiely JP: Quantification of the superior lateral pterygoid insertion on TMJ components, J Dent Res70:421-427, 1991.

21. Carpentier P, Yung JP, Marguelles-Bonnet R, Meunissier M:Insertion of the lateral pterygoid: an anatomic study of thehuman temporomandibular joint, J Oral Maxillofac Surg46:477-782, 1988.

22. Marguelles-Bonnet R, Yung JP, Carpentier P, Meunissier M:Temporomandibular joint serial sections made withmandible in intercuspal position, J Craniomandib Pract7:97-106, 1989.

23. Tanaka TT: Advanced dissection of the temporomandibular joint,Chula Vista, Calif, 1989, Clinical Research Foundation.

24. Heylings DJ, Nielsen IL, McNeill C: Lateral pterygoid muscleand the temporomandibular disc, J Orofac Pain 9:9-16, 1995.

25. Ericksson PO: Special histochemical muscle-fiber charac-teristics of the human lateral pterygoid muscle, Arch OralBiol 26:495-501, 1981.

26. Mao J, Stein RB, Osborn JW: The size and distribution offiber types in jaw muscles: a review, J Craniomandib Disord6:192-201, 1992.

27. Boyd RL, Gibbs CH, Mahan PE, Richmond AF, Laskin JL:Temporomandibular joint forces measured at the condyleof Macaca arctoides, Am J Orthod Dentofacial Orthop 97:472-479, 1990.

28. Mansour RM, Reynik RJ: In vivo occlusal forces andmoments: I. Forces measured in terminal hinge positionand associated moments, J Dent Res 54:114-120, 1975.

29. Smith DM, McLachlan KR, McCall WD: A numericalmodel of temporomandibular joint loading, J Dent Res65:1046-1052, 1986.

30. Rassouli NM, Christensen LV: Experimental occlusal inter-ferences. Part III. Mandibular rotations induced by a rigidinterference, J Oral Rehabil 22:781-789, 1995.

31. Christensen LV, Rassouli NM: Experimental occlusal interferences. Part IV. Mandibular rotations induced by apliable interference, J Oral Rehabil 22:835-844, 1995.

2C H A P T E R

25

Functional Neuroanatomyand Physiology of the

Masticatory System

You cannot successfully treat dysfunction unless youunderstand function.

JPO

T he function of the masticatory system iscomplex. Discriminatory contraction ofthe various head and neck muscles is nec-essary to move the mandible precisely and alloweffective functioning. A highly refined neurologiccontrol system regulates and coordinates theactivities of the entire masticatory system. It con-sists primarily of nerves and muscles; hence theterm neuromuscular system. A basic understanding ofthe anatomy and function of the neuromuscularsystem is essential to understanding the influencethat tooth contacts, as well as other conditions,have on mandibular movement.

This chapter is divided into three sections. Thefirst section reviews in detail the basic neuroanatomyand function of the neuromuscular system. The second describes the basic physiologic activities ofmastication, swallowing, and speech. The third sec-tion reviews important concepts and mechanismsthat are necessary to understand orofacial pain.Grasping the concepts in these three sections shouldgreatly enhance the clinicians ability to understand apatients complaint and provide effective therapy.

ANATOMY AND FUNCTION OF THENEUROMUSCULAR SYSTEM

For purposes of discussion, the neuromuscular sys-tem is divided into two major components: (1) the

neurologic structures and (2) the muscles. Theanatomy and function of each of these componentsis reviewed separately, although in many instances itis difficult to separate function. With an understand-ing of these components, basic neuromuscular func-tion can be reviewed.

MUSCLES

Motor UnitThe basic component of the neuromuscular systemis the motor unit, which consists of a number ofmuscle fibers that are innervated by one motorneuron. Each neuron joins with the muscle fiber ata motor endplate. When the neuron is activated,the motor endplate is stimulated to release smallamounts of acetylcholine, which initiates depolar-ization of the muscle fibers. Depolarization causesthe muscle fibers to shorten or contract.

The number of muscle fibers innervated by onemotor neuron varies greatly according to the func-tion of the motor unit. The fewer the muscle fibersper motor neuron, the more precise the move-ment. A single motor neuron may innervate onlytwo or three muscle fibers, as in the ciliary muscles(which precisely control the lens of the eye).Conversely, one motor neuron may innervate hun-dreds of muscle fibers, as in any large muscle (e.g.,the rectus femoris in the leg). A similar variationexists in the number of muscle fibers per motorneuron within the muscles of mastication. Theinferior lateral pterygoid muscle has a relativelylow muscle fiber/motor neuron ratio; therefore it iscapable of the fine adjustments in length needed


to adapt to horizontal changes in the mandibularposition. In contrast, the masseter has a greaternumber of motor fibers per motor neuron, whichcorresponds to its more gross function of provid-ing the force necessary during mastication.

MuscleHundreds to thousands of motor units along withblood vessels and nerves are bundled together byconnective tissue and fascia to make up a muscle.The major muscles that control movement of themasticatory system were described in Chapter 1.To understand the effect these muscles have oneach other and their bony attachments, one mustobserve the basic skeletal relationships of thehead and neck. The skull is supported in positionby the cervical spine. However, the skull is not centrally located or balanced over the cervicalspine. In fact, if a dry skull were placed in its cor-rect position on the cervical spine, it would beoverbalanced to the anterior and quickly fall forward.Any balance becomes even more remote when theposition of the mandible hanging below the ante-rior portion of the skull is considered. It can be eas-ily seen that a balance of the skeletal componentsof the head and neck does not exist. Muscles arenecessary to overcome this weight and massimbalance. If the head is to be maintained in anupright position so that one can see forward, mus-cles that attach the posterior aspect of the skull tothe cervical spine and shoulder region must con-tract. Some of the muscles that serve this functionare the trapezius, sternocleidomastoideus, spleniuscapitis, and longus capitis. It is possible, however, forthese muscles to overcontract and direct the line ofvision too far upward. To counteract this action, anantagonistic group of muscles exists in the anteriorregion of the head: the masseter (joining themandible to the skull), the suprahyoids (joining themandible to the hyoid bone), and the infrahyoids(joining the hyoid bone to the sternum and clavi-cle). When these muscles contract, the head is low-ered. Thus a balance of muscular forces exists thatmaintains the head in a desired position (Fig. 2-1).These muscles, plus others, also maintain properside-to-side positioning and rotation of the head.

Muscle Function. The motor unit can carry out only one action: contraction or shortening.

The entire muscle, however, has three potentialfunctions:1. When a large number of motor units in the

muscle are stimulated, contraction or an overallshortening of the muscle occurs. This type ofshortening under a constant load is called isotonic contraction. Isotonic contraction occurs inthe masseter when the mandible is elevated,forcing the teeth through a bolus of food.

2. When a proper number of motor units contractopposing a given force, the resultant function ofthe muscle is to hold or stabilize the jaw. Thiscontraction without shortening is called isometriccontraction, and it occurs in the masseter when anobject is held between the teeth (e.g., a pipe orpencil).

3. A muscle can also function through controlledrelaxation. When stimulation of the motor unit isdiscontinued, the fibers of the motor unit relaxand return to their normal length. By control ofthis decrease in motor unit stimulation, a pre-cise muscle lengthening can occur that allowssmooth and deliberate movement. This type ofcontrolled relaxation is observed in the masseterwhen the mouth opens to accept a new bolus offood during mastication.Using these three functions, the muscles of the

head and neck maintain a constant desirable headposition. A balance exists between the musclesthat function to raise the head and those that func-tion to depress it. During even the slightest move-ment of the head, each muscle functions in harmonywith others to carry out the desired movement. Ifthe head is turned to the right, certain muscles mustshorten (isotonic contraction), others must relax(controlled relaxation), and still others must stabilizeor hold certain relationships (isometric contraction).A highly sophisticated control system is necessary tocoordinate this finely tuned muscle balance.

These three types of muscle activities are pres-ent during routine function of the head and neck.Another type of muscle activity, eccentric contraction,can occur during certain conditions. This type ofcontraction is often injurious to the muscle tissue.Eccentric contraction refers to the lengthening of a mus-cle at the same time that it is contracting. An exam-ple of eccentric contraction occurs with the tissuedamage associated during an extension-flexion

Functional Neuroanatomy and Physiology of the Masticatory System 27

injury (whiplash injury). At the precise moment of amotor vehicle accident, the cervical muscles con-tract to support the head and resist movement. If,however, the impact is great, the sudden change inthe inertia of the head causes it to move while themuscles contract trying to support it. The result isa sudden lengthening of the muscles while theyare contracting. This type of sudden lengthening ofmuscles while contracting often results in injuryand is discussed in later sections of the chapterdevoted to muscle pain.

NEUROLOGIC STRUCTURES

NeuronThe basic structural unit of the nervous system isthe neuron. The neuron is composed of a mass of

protoplasm termed the nerve cell body and proto-plasmic processes from the nerve cell body calleddendrites and axons. The nerve cell bodies located inthe spinal cord are found in the gray substance of the central nervous system (CNS). Cell bodiesfound outside the CNS are grouped together inganglia. The axon (from the Greek word axon, mean-ing axle or axis) is the central core that forms the essential conducting part of a neuron and is anextension of cytoplasm from a nerve cell. Manyneurons group together to form a nerve fiber. Theseneurons are capable of transferring electrical andchemical impulses along their axes, enabling infor-mation to pass both in and out of the CNS.Depending on their location and function, neuronsare designated by different terms. An afferent neu-ron conducts the nervous impulse toward the CNS,

A B

Fig. 2-1 Precise and complex balance of the head and neck muscles must exist to maintainproper head position and function. A, Muscle system. B, Each of the major muscles acts likean elastic band.The tension provided must precisely contribute to the balance that maintainsthe desired head position. If one elastic band breaks, the balance of the entire system is disruptedand the head position altered.


whereas an efferent neuron conducts it peripherally.Internuncial neurons, or interneurons, lie wholly withinthe CNS. Sensory or receptor neurons, afferent intype, receive and convey impulses from receptororgans. The first sensory neuron is called the primaryor first-order neuron. Second- and third-order sensoryneurons are internuncial. Motor or efferent neuronsconvey nervous impulses to produce muscular orsecretory effects.

Nervous impulses are transmitted from oneneuron to another only at a synaptic junction, orsynapse, where the processes of two neurons are inclose proximity. All afferent synapses are locatedwithin the gray substance of the CNS, so there are no anatomic peripheral connections betweensensory fibers. All connections are within the CNS,and the peripheral transmission of a sensoryimpulse from one fiber to another is abnormal.

Information from the tissues outside the CNSmust be transferred into the CNS and on to thehigher centers in the brainstem and cortex for inter-pretation and evaluation. Once this information is evaluated, appropriate action must be taken.

The higher centers then send impulses down thespinal cord and back out to the periphery to aneffe

management of temporomandibular disorders and oc

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