October 26, 2015

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Tympanomastoid Suture- An important landmark for transcanal endoscopicdissection of posterior wall of external auditory canal

Author:Dr. Sheikh Shawkat Kamal, MBBS FCPS

Consultant OtolaryngologistSurgiscope Hospital Limited

ChittagongBangladesh

Cell Phone: 880 1711406943E-mail: drjoyent@yahoo.com

Abstract:

Objective: To evaluate the feasibility of using tympanomastoid suture line as a landmark for safe transcanal endoscopic dissection of posteriorwall of external auditory canal.

Study design: Case series study from May’2014 to August’2015.

Setting: Private tertiary care hospital.

Cases: 5 dry cadaveric temporal bones and 11 patient’s ears among them 8 ears had cholesteatoma in atticoantral region and 3noncholesteatomas ears had suspected pathological shadow in their CT radiogram of mastoid antrum.

Interventions: In cholesteatomas ears, transcanal endoscopic bony dissection just above the tympanomastoid suture line was done to exposethe attic, aditus ad antrum and mastoid antrum. In noncholesteatomas ears, transcanal endoscopic creation of antrostomy hole/window abovethe tympanomastoid suture line was done to evaluate mastoid pathology.

Main outcomes measure: Evaluation of exposure after dissection just above the tympanomastoid suture line and presence of any damage tosurrounding vital structures.

Results: Tympanomastoid suture could be identified in both cadaveric temporal bone and living patient’s ears. Blind dissection above thesuture line, in every case successfully opened mastoid antrum without any damage to lateral semicircular canal and facial nerve.

Conclusion: Tympanomastoid suture in posterior wall of external auditory canal has been proved to be accepted as an important landmarkduring transcanal endoscopic approach to mastoid antrum.

October 26, 2015 TYMPANOMASTOID SUTURE. DR. SHEIKH SHAWKAT KAMAL

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Introduction:

Part of posterior bony wall of external auditory canal needs to bedissected during transcanal endoscopic approach to mastoidantrum for addressing its different pathology such ascholesteatoma or for evaluating the pathological shadow in itsradiogram. While approaching to mastoid antrum from attic,entry must be through aditus ad antrum. This aditus ad antrumalways remains hidden from direct endoscopic view. Moreoverthe lateral semicircular canal, tympanic segment or second genuof facial nerve and incus are situated at close proximity to aditusad antrum. So a cautious approach to aditus ad antrum isdemanded to avoid iatrogenic injury to these structures. On theother hand, direct transcanal endoscopic approach to mastoidantrum by opening a diagnostic hole/ window as mentioned bythe present author in his previous literature was highlydependable on landmarks1. Additional landmark would definitelyease this dissection.

Marchioni et al, in their early literature, depending upon thepresence of incus, described two landmarks during dissectiontowards mastoid antrum 2. They depicted that in presence ofincus, the tip of short process of incus pointed towards themastoid antrum. Whereas in absence of incus, according to them,position of mastoid antrum could be assumed by the meetingpoint of two imaginary lines of a triangle, one passed along thetegmen tympani and other passed along the lateral semicircularcanal. In a book’s chapter, written by Marchioni et al, thetranscanal endoscopic dissection towards antrum was advocatedafter keeping the tympanic part of facial nerve and lateralsemicircular canal under direct endoscopic view 3. The author ofthis study described a procedure in his previous article whereinitial bony dissection proceeded gradually from free margin ofscutum towards mastoid antrum with preservation of animaginary area of posterior wall that covered the presumed areaof mastoid segment of facial nerve and lateral semicircular canal4.However, the search of crucial landmarks for transcanalendoscopic dissection towards mastoid antrum is still continuingdue to the presence of some limitations in previous procedures.

Tympanomastoid suture line indicates the meeting point oftympanic bone and mastoid part of temporal bone at theposterior wall of external auditory canal. This study evaluated theworthiness of using tympanomastoid suture line as a landmark fortranscanal endoscopic dissection of the posterior bony wall to geta safe entry either from attic to antrum through aditus ad antrumor directly into antrum.

Cases and Methods:

This case series study was conducted on 5 (2 left and 3 right sided)dry adult cadaveric temporal bones and 11(4 left and 7 right) earsof patients aged from 12years to 45years in the Surgiscopehospital, a tertiary care private hospital, situated in Chittagong,Bangladesh from May’2014 to August’2015. Among the 11 ears ofthe patients, 8 ears had extensive attic cholesteatoma extendingto mastoid antrum and rest 3 noncholesteatomas ears had

suspected soft tissue shadow in CT radiograms of their mastoidantrum.

Dissection was first done on the dry cadaveric bones in author’spersonally arranged temporal bone lab before endeavor theprocedure on patients. All patients were informed about the risksand the consequences of the procedures before collecting theirwritten consent for the procedures.

Before starting the dissection, identification and appearance oftympanomastoid suture were observed first. For easyidentification, search of this suture line was started from thenotch of Rivinus. Two different dissection procedures wereperformed without keeping tympanic part of facial nerve andlateral semicircular canal under direct endoscopic view. In 3 (1 leftand 2 right) cadaveric temporal bones and in 8 (3 left and 5 right)cholesteatomas ears, the dissection was stared from the notch ofRivinus. After that, it was continued back towards posterior canalwall about 1mm above the tympanomastoid suture line untilexposing the mastoid antrum [fig.1]. Extension of cholesteatomain mastoid antrum decided the need of further extension of thisinitial dissection. On the other hand in 2(1left and 1 right)cadaveric temporal bones and in 3( 1 left and 2 right) ears ofnoncholesteatomas cases transcanal endoscopic antrostomyhole/window was performed above the tympanomastoid sutureline near the meeting point of the roof and posterior wall of theexternal auditory canal. Necessary tympanoplastic procedureswere performed after initial dissection in all diseased ears.

All the dissection was done by single surgeon-the author.

Figure 1: Endoscopic view of the posterior canal wall. Whitecolored letter ‘N’ indicated the notch of Rivinus, and blackcolored arrows indicated the faintly appeared tympanomastoidsuture line. Direction of dissection for the opening of mastoidantrum from attic was indicated by blue colored arrows.

Results:

Tympanomastoid suture could be readily identified in all cadaverictemporal bones. In 4 patient’s ears, it appeared as a vivid redcolored line in posterior canal wall [fig.2] where as in rest 7patient’s ears, it appeared as a faint line. It was also noticed inevery case that the height of medial half of the suture line was

October 26, 2015 TYMPANOMASTOID SUTURE. DR. SHEIKH SHAWKAT KAMAL

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higher than the lateral half of the suture line and this created adownward slope as the suture line ran from medial to lateral partof posterior bony wall. Dissection above the suture line, in everycase, successfully exposed the passage from attic to mastoidantrum through aditus ad antrum without any damage to lateralsemicircular canal and tympanic segment of facial nerve. Theexposed passage first went over the upper half of body of incus inattic, then passed medial to the lateral semicircular canal in aditusad antrum and finally ended into the mastoid antrum [fig.3].Following dissection just above the sloping part of the suture line,it was noticed that the slope of suture line resembled the slope offacial ridge. Dissection above the suture line also successfullycreated antrostomy hole/window without damaging facial orlateral semicircular canal.

Figure 2: Endoscopic view of vividly appeared tympanomastoidsuture line (black colored arrows).

Discussion:

Posterior bony wall of external auditory canal is formed with twobones- mastoid part of temporal bone from above and tympanicbone from below. The meeting point of these two bones formsthe tympanomastoid suture which provides passage fortransmission of Arnold’s nerve, branch of vagus nerve, whichinnervates part of skin of external auditory canal. Development oftympanic bone continues up to the age of 9th year5. This ongoingdevelopment of tympanic bone might cause variation in theposition of tympanomastoid suture in early age of life. Skin ofexternal auditory canal is adherent tightly with this suture thatnecessitates sharp dissection to separate skin from it duringelevation of tympanomeatal flap.

Direct endoscopic view of aditus ad antrum is not possible since itremains hidden behind the posterior canal wall and incus.Transcanal endoscopic dissection towards this concealed areashould be much preplanned in order to avoid injury to itssurrounding structure like tympanic segment of facial nerve andlateral semicircular canal.

While following the dissection procedure described by Marchioniet al the tip of endoscope is needed to bring near to the area ofdissection in order to keep tympanic segment of facial nerve andlateral semicircular canal under direct endoscopic view. This may

cause difficulty in instrumentation as both drill burr and tip ofendoscope are needed to pass through the isthmus of externalauditory canal (narrowest part of the canal). Moreover directvisualization of facial nerve and lateral semicircular canal at initialstage of dissection is barely possible in presence of intact incus. Itis also true that the tip of the short process of incus never pointstowards the antrum rather it points towards fossa incudis, a placeabout 1.2mm below the prominence of the lateral semicircularcanal. The position of the aditus ad antrum is medial to theprominence of lateral semicircular canal. On the other handimagination is very much subjective. So dissection around animaginary area could be difficult for the novice endoscopic earsurgeon.

This study observed that the dissections just above thetympanomastoid suture line, in all case, could safely open themastoid antrum without damaging its surrounding vitalstructures. These dissections were blind since they wereperformed without keeping the tympanic part of facial nerve andlateral semicircular canal under direct endoscopic view. As thissuture line was viewed even after keeping the tip of endoscope atthe lateral part of the external auditory canal, this ultimatelyfacilitated the easy instrumentation during dissection. Moreoverthis suture line is a solid landmark rather than an area ofimagination, so chances of subjective variations would be less.Dissection along a solid landmark with easy instrumentationfacility would definitely reduce the wariness of the surgeon aswell as reduce the time for operation. Although in most of thecases, tympanomastoid suture was appeared as faint line inposterior canal wall but careful search from the posterior borderof notch of Rivinus was found helpful to identify this faint sutureline. This was also observed that the creation of the diagnostictranscanal endoscopic antrostomy hole/window just above thetympanomastoid suture could be performed successfully andsafely.

Figure 3: Endoscopic view just after the initial opening of themastoid antrum. White colored star indicated the mastoidantrum and blue colored star indicated upper half of body ofincus. Tympanomastoid suture line was indicated by blackcolored arrows.

October 26, 2015 TYMPANOMASTOID SUTURE. DR. SHEIKH SHAWKAT KAMAL

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There is possibility of progressive change in the position oftympanomastoid suture line due to ongoing development oftympanic bone at early childhood period. This might bringuncertainty in the relation between the suture line and thesurrounding vital structures at that stage. So the safety ofdissection just above this suture line in early childhood periodcould be questionable. This study failed to address this issue as itonly included the adult ears.

Conclusion:

Dissection above the tympanomastoid suture line could safelyexpose the mastoid antrum in adult ear. This suture line hencecan be used as an additional worthy landmark for transcanalendoscopic dissection of posterior canal wall.

References:

1. Kamal SS. Transcanal Endoscopic Antrostomy Hole : Its Utility inRestoring Ventilation Pathway to Epitympanum and MastoidAntrum During Tympanoplasty. Researchgate. Sheikh ShawkatKamal. 9 May.2014. Web. 19 July. 2014.<https://www.researchgate.net/publication/264043810_Transcan

al_Endoscopic_Antrostomy_Hole__Its_Utility_in_Restoring_Ventilation_Pathway_to_Epitympanum_and_Mastoid_Antrum_During_Tympanoplasty>

2. Marchioni D, Molteni G, Presutti L. Endoscopic Anatomy of theMiddle Ear. Indian J Otolaryngol Head Neck Surg. 2011;63(2):101–113.

3. Marchioni D, Mattioli F, Villari D et al. Endoscopic Treatment ofCholesteatoma with Antral Extension. In: Presutti L, Marchioni D,editors. Endoscopic Ear Surgery- Principles, Indications, andTechniques. 1st ed. Thieme.2015.p.243-261.

4. Kamal SS. Transcanal endoscopic mastoid surgery withtympanoplasty for the management of cholesteatoma and itsrelated lesions of mastoid antrum. Researchgate. Sheikh ShawkatKamal. 22 April. 2011. Web. 23 May. 2014.<https://www.researchgate.net/publication/262559478_Transcanal_endoscopic_mastoid_surgery_with_tympanoplasty_for_the_management_of_cholesteatoma_and_its_related_lesions_of_mastoid_antrum >

5. Jones SM, Jones TA. Embryogenesis of the outer and middleear. In: Genetics, Embryology, and Development of Auditory andVestibular Systems. San Diego.The Plural Publishing.2011. p 131-152.