Within the Temporal Bone · Chapter 15 · Part IV · Inner ear and skull base
The Internal Auditory Canal and Translabyrinthine Exposure
Bony canal first, dura only for an agreed objective
Bench mode: one step per screen, large type, screen kept awake. Tap, use the arrow keys or say “next”.
- Specimen
- Well-preserved bone with posterior fossa dura; supervised
- Bench time
- A separate session after Chapter 14; your instructor sets the target
- Burrs
- Large to small diamond; blunt hook
- View
- Higher power, along the canal axis
- Before this
- Chapters 13–14
By the end you can
- Define dura, sinus, bulb and nerve before the labyrinth goes
- Remove the labyrinth without losing orientation
- Skeletonise the canal with its dura intact
- Name the four fundal nerves with the view stated
- Open dura only for an agreed objective, and record what was removed, opened and kept
The canal skeletonised from porus to fundus with both troughs and its dura intact, the crests and four fundal nerves named with the view stated, and a record of what was removed, opened and kept.
Opening the dura (Station 5) is done only if intradural anatomy is part of the session and the objective was agreed beforehand. Tumour removal and the transcochlear extension are not part of this chapter.
The landmark chain
Each landmark leads to the next. Tap any one to see every station where it appears.
15.1 Before you begin
The translabyrinthine route reaches the internal auditory canal through a deliberately opened labyrinth. In clinical use it sacrifices hearing, unlike approaches designed to keep labyrinthine function. It is an advanced exercise that needs a well-preserved specimen and specialist supervision; an isolated bone may not allow the intracranial relationships or closure to be shown.
Complete and record the hearing-preserving exercises first, so that you know what will be lost when the labyrinth is removed and how that loss changes your orientation. Review the mastoid, facial nerve, labyrinth, jugular region and posterior fossa relationships on the bone and its CT. The route is built from the surface inwards: an adequate mastoid exposure and defined dural and venous boundaries precede the labyrinthine stage.
Exposure of the bony canal and opening of its dura are separate stages: complete the first before you plan the second.
15.2 · The 5 stations
Define the boundaries
- Complete a wide mastoidectomy. Thin the middle fossa plate and the sigmoid sinus plate, and open the sinodural angle along the superior petrosal sinus.
- Remove the presigmoid bone to expose the posterior fossa dura from the sinus to the labyrinth; a thin island of bone may be left on the sinus.
- Skeletonise the jugular bulb from above and the mastoid segment of the facial nerve, keeping bone on the nerve.
- Confirm the labyrinth is outlined (Chapter 14) and photograph the field before any destructive step.
Middle fossa dura, sigmoid sinus, presigmoid dura, jugular bulb, facial nerve and labyrinth all seen and named.
The dural and venous boundaries are not yet defined. Do not start the labyrinthectomy.
Remove the labyrinth
- Follow the instructor's planned sequence: lateral canal, posterior canal to its ampulla and the common crus, superior canal to its ampulla (Chapter 14).
- Open the vestibule from the ampullae, keeping the tympanic and mastoid segments of the facial nerve under a shell of bone.
- As the canals disappear, keep orientation from the facial nerve, the vestibule and the dura, not from memory.
- Remember that the canal lies medial to the vestibule: its fundus lies against the vestibule's medial wall.
Vestibule open; facial nerve covered; the expected line of the canal marked between the middle fossa dura and the jugular bulb.
Recognisable labyrinth has gone and you are drilling towards an estimated canal position. Stop and reorient.
Skeletonise the canal
- Drill a superior trough between the canal and the middle fossa dura, and an inferior trough between the canal and the jugular bulb.
- Work from the porus, where the canal dura meets the posterior fossa dura, laterally towards the fundus.
- Expose the canal around most of its circumference, its dura intact. In the inferior trough the cochlear aqueduct marks the lower limit; the lower cranial nerves lie below it.
- Keep the burr moving along the canal axis, never across it.
Canal outlined from porus to fundus with both troughs, its dura intact.
Dura is torn or nerve is visible through it. Stop and record the opening.
The fundus
- At the lateral end, thin the last bone slowly: this is the most confined part of the canal, and uncontrolled removal damages structures before they are recognised.
- Identify the transverse (falciform) crest, which divides the fundus into upper and lower compartments.
- In the upper compartment find the vertical crest (Bill's bar): the superior vestibular nerve lies behind it; the facial nerve lies in front of it and deep in the translabyrinthine view, entering its labyrinthine segment.
- Below the crest, the inferior vestibular nerve lies posteriorly and the cochlear nerve anteriorly.
Both crests and all four nerves named, with the view and orientation stated.
You are about to separate neural tissue you cannot name with certainty. Reorient and obtain instructor confirmation.
Open the dura for an agreed objective
- Open the dura only if intradural anatomy is part of the session, and agree beforehand what will be inspected.
- Incise the presigmoid dura and reflect it; open the canal dura along its posterior aspect.
- Identify the facial and vestibulocochlear nerves from the brainstem to the porus, the loop of the anterior inferior cerebellar artery, the trigeminal nerve above and the glossopharyngeal and vagus nerves below.
- Preserve every nerve and vessel and avoid traction. A cadaveric nerve can be displaced without the warning that injury would give in a patient.
The angle and the canal contents named without traction or division.
The objective was not agreed before opening. Close the session at the bony stage.
15.3 What this exposure does not teach
The endpoint of this chapter is an anatomical one. Tumour removal, dissection of the facial nerve from a tumour, management of vascular adherence and the transcochlear and other extensions, each with its own cost, are not part of it; they need disease-specific technique, judgement and supervised clinical experience. The anatomical exposure is a foundation; it does not reproduce the pathology.
After an intradural exposure, cerebrospinal fluid can reach the middle ear and the wound through opened air cells, the antrum and the aditus; trace these routes on your specimen. Clinical closure depends on the approach and the operative findings. Where soft tissues are absent, describe the missing reconstructive components rather than claiming to have completed the operation.
At the end, record whether the dura was opened, which labyrinthine structures were removed, whether the cochlea or facial nerve was altered and which boundaries were kept: a precise record is worth more than “skull-base dissection completed”.
15.4 What the evidence says
Step-by-step anatomical dissections of the translabyrinthine and transcochlear routes set out the same sequence of mastoidectomy, labyrinthectomy and canal exposure, and the structures each extension sacrifices. They describe the specimen, not the operation in disease; the size of the field and the position of the jugular bulb vary between bones.
◉ See it on real specimens
Real dissection and operative photographs published openly by other surgeons and institutions. They open on the original site with full credit to the authors; they are not copied into this companion. Compare them with the drawings above.
15.5 Fault finding
| What you see | Probable cause | Correction |
|---|---|---|
| Field too narrow to reach the canal | Plates and sinus not decompressed widely enough | Widen the mastoid and thin the plates before going deep |
| Sinus torn while removing presigmoid bone | Burr working against the sinus plate | Thin with a diamond; lift the last flakes off |
| Facial nerve exposed at the fundus | Bill's bar not identified | Find the transverse crest first, then the bar |
| Jugular bulb opened in the inferior trough | High bulb not anticipated | Read the CT; skeletonise the bulb before the trough |
| Nerves named wrongly | Fundal map used without orientation | State the view and anterior before naming |
15.6 Viva at the bench
How do you find the facial nerve at the fundus from a translabyrinthine view?
Find the transverse crest first, then Bill's bar in the upper compartment. The superior vestibular nerve lies behind the bar; the facial nerve lies in front of it, deep in this view, entering its labyrinthine segment.
What is lost if a fundal diagram is memorised without its orientation?
The direction. Anterior lies on a different side of the page in the porus view and the middle fossa view, so a memorised picture without a stated view can put the facial nerve in the wrong quadrant.
By which routes can cerebrospinal fluid reach the middle ear after this exposure?
Through opened air cells, the antrum and the aditus into the middle ear, and through the wound. Trace them on the specimen.
One best answer
15.7 My dissection log
Saved on this device only. Print it or send the summary to your fellowship coordinator for review.
Score the dissection · review order
| Item | 0 · not achieved | 1 · partly | 2 · achieved |
|---|---|---|---|
| Dural and venous boundaries defined before the labyrinth | |||
| Labyrinth removed in a stated sequence, facial nerve covered | |||
| Canal skeletonised porus to fundus, dura intact | |||
| Crests and four nerves named with the view stated | |||
| Dura opened only for an agreed objective | |||
| Removed, opened and kept recorded |
The review order is a teaching method, not a validated scoring instrument. Structured end-product scales such as the Welling scale have been studied for rater reliability; they assess the dissection, not operative outcome, but they make feedback specific.
Before you assist at a translabyrinthine approach
Sources and credits
- Perry A, Carlstrom LP, Alexander AY, et al. Anatomical step-by-step dissection of complex skull base approaches for trainees: surgical anatomy of the translabyrinthine and transcochlear approaches. J Neurol Surg B Skull Base 2025;86:342–52. doi.org/10.1055/s-0044-1786736
- Hua X. Temporal bone anatomy: cadaveric dissection. Iowa Head and Neck Protocols. University of Iowa; 2018. iowaprotocols.medicine.uiowa.edu
Real dissection photographs with numbered drawings: Bassett Collection, Stanford University (CC BY-SA 4.0). Classic anatomical and operative plates: Gray’s Anatomy of the Human Body (1918, drawings by H. V. Carter) and Burghard’s A System of Operative Surgery (1909), both public domain, credited beside each plate. Remaining diagrams and charts: Dr Bhushan Kathuria & Team, not to scale. Operating-theatre frames and clips: our theatre, patient consent taken, identity not shown. Atlas photographs: Open Access Atlas of Otolaryngology (CC BY-NC 3.0), with their authors credited beside each image. This edition uses no AI-generated images. For professional education alongside supervised instruction and local protocols.
Dr Bhushan Kathuria & Team · Reg. No. HN-017323, Haryana Medical Council · A GeNext MedEd Foundation surgical education initiative.
‹ Chapter 14: The Labyrinth and Endolymphatic Sac RegionChapter 16: The Middle Fossa and Petrous Apex ›
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