Within the Temporal Bone · Chapter 16 · Part IV · Inner ear and skull base
The Middle Fossa and Petrous Apex
A corridor is defined by its limits
Bench mode: one step per screen, large type, screen kept awake. Tap, use the arrow keys or say “next”.
- Specimen
- Bone or block with the superior petrous surface and its dura preserved
- Bench time
- One session from above; inferior routes a separate session
- Burrs
- Medium and small diamond; blunt dura elevator
- View
- Higher power, from above
- Before this
- Chapters 4, 13 and 15
By the end you can
- Choose a corridor from the CT and the specimen, not from habit
- Name the landmarks of the middle fossa floor
- Find the internal auditory canal from more than one landmark
- Outline Kawase's rhomboid and its four boundaries
- Compare the superior and inferior routes to the apex
The middle fossa floor named, the internal auditory canal found from two landmarks with its dura intact, Kawase's rhomboid drilled to posterior fossa dura with carotid, cochlea and canal intact, and a record of each route attempted and its limits.
Inferior routes are a separate session. An apex lesion in a patient needs its own plan: drainage, biopsy, decompression or resection; this chapter describes anatomical access and its limits only.
The landmark chain
Each landmark leads to the next. Tap any one to see every station where it appears.
16.1 Before you elevate the dura
The middle fossa and the petrous apex show how several important structures constrain a small volume of bone. Study them on a specimen that preserves the superior petrous surface and its intracranial relationships. In a patient, an apex lesion needs assessment of its nature, extent, symptoms and relationship to hearing and neurovascular structures, and the aim may be drainage, biopsy, decompression or resection, each with a different plan.
Review the imaging before choosing a corridor: some routes are not available in a given bone. Read the CT for the arcuate eminence, the tegmen, the cochlea, the internal auditory canal, the carotid canal, the jugular bulb and any air-cell tract to the apex. Where dura or nerves were removed during harvesting, record that limitation before using the specimen to demonstrate a surgical plane.
A geometric triangle drawn on the surface does not fix the deep structures. Confirm every landmark against at least one other and the CT.
| Superior route (middle fossa) | Inferior routes |
|---|---|
| Petrous surface seen from above, temporal lobe retracted | Infracochlear: below the cochlea, through the hypotympanum |
| Canal found between nerve and eminence | Infralabyrinthine: below the labyrinth, through the mastoid |
| Apex reached through Kawase's rhomboid | Each limited by its own vessels and nerve |
| Limited by cochlea, carotid, trigeminal nerve and ridge | Feasibility decided by the individual anatomy |
16.2 · The 4 stations
Orient on the middle fossa floor
- Elevate the dura from posterior to anterior under the instructor's guidance, recognising its attachments rather than stripping indiscriminately.
- Identify the petrous ridge with the superior petrosal sinus, the arcuate eminence and the tegmen tympani laterally.
- Find the middle meningeal artery at the foramen spinosum and the mandibular nerve at the foramen ovale: the anterior limits of the elevation.
- Find the greater petrosal nerve leaving the facial hiatus and running anteromedially. Protect it and avoid traction transmitted towards the geniculate ganglion.
Ridge, eminence, tegmen, foramen spinosum, foramen ovale and greater petrosal nerve named.
The dura tears or the nerve is lifted with it. Stop and re-establish the plane further posteriorly.
Find the internal auditory canal
- Blue-line the superior canal under the arcuate eminence, or trace the greater petrosal nerve back to the geniculate ganglion, or both.
- The canal lies near the line bisecting the angle between the greater petrosal nerve and the superior canal. Begin medially, near the porus, where there is most room.
- Thin the meatal plane over the canal and follow it laterally. At the fundus the labyrinthine segment of the facial nerve runs forwards to the geniculate ganglion, with Bill's bar behind it.
- The cochlea lies in the angle between the labyrinthine segment and the greater petrosal nerve, close to the lateral end of the canal. Account for it before drilling there.
Canal roof thinned or removed from porus to fundus, dura intact; facial nerve and cochlea accounted for.
You are deepening a hole to find the canal. Return to the surface landmarks and the CT; do not use a small burr to make up for a large uncertainty.
Kawase's rhomboid
- Outline the rhomboid: greater petrosal nerve laterally, mandibular nerve anteriorly, petrous ridge medially, and the internal auditory canal and cochlea posteriorly.
- The horizontal petrous carotid lies beneath and parallel to the greater petrosal nerve: keep the lateral margin of the drilling medial to it.
- Remove the apex bone within the outline progressively, correlating each newly visible structure with the map, down to the posterior fossa dura.
- Preserve the cochlea in the posterolateral corner and the canal dura behind.
Rhomboid drilled to posterior fossa dura; carotid, cochlea and canal intact and named.
A deep boundary is uncertain. Return to the surface and the imaging before going deeper.
Compare the inferior routes
- Infracochlear: through the hypotympanum below the cochlea, between the carotid in front and the jugular bulb behind and below.
- Infralabyrinthine: through the mastoid below the posterior canal, between the facial nerve in front, the jugular bulb below and the sinus behind.
- Review each corridor on the CT and show each limiting structure on the specimen before extending it. A high jugular bulb can make either route unsuitable.
- Do not substitute one route's landmarks for another's.
Each attempted route named with its limiting structures demonstrated.
The corridor cannot be widened without unacceptable anatomical cost. Record that as the finding.
▶ Cholesterol Granuloma: Petrous apex, transnasal endoscopic
Opens on YouTube (needs internet). A different corridor to the same apex in a patient: compare what limits each route.16.3 Air-cell tracts and the comparison
Where aerated tracts run towards the apex, they help to illustrate its connection with the mastoid or the middle ear. They do not establish a safe direction on their own: interpret each within the surrounding cochlear, vascular and neural relationships. In dense bone, the absence of cells means relying more on verified anatomy, not deepening the field arbitrarily.
Record which route was attempted, which part of the apex was reached and what remained inaccessible, with the structures preserved and any deliberately sacrificed to extend the view. Finish by comparing the superior and inferior perspectives: the same cochlear, neural and vascular structures constrain different approaches, and the value of the exercise lies in recognising those limits, not in producing the largest petrosectomy.
16.4 What the evidence says
Stepwise cadaveric dissections of the middle fossa approaches and anterior petrosectomy describe the same landmarks and also their variation between specimens. Anatomical studies of the infracochlear route emphasise how the cochlea, carotid and jugular bulb vary in their spacing, so that the corridor is not available in every ear.
◉ 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.
16.5 Fault finding
| What you see | Probable cause | Correction |
|---|---|---|
| Middle meningeal artery torn | Elevation carried past the anterior limits | Stop the elevation at the foramen spinosum and foramen ovale |
| Superior canal opened at the eminence | Eminence taken as the canal | Blue-line the canal; confirm with the CT |
| Cochlea opened lateral to the canal | Drilling at the fundus too far forwards | Work medially first; map the cochlea's angle |
| Carotid exposed under the nerve | Rhomboid drilled too far laterally | Keep medial to the greater petrosal nerve |
| Air-cell tract followed into a vital structure | Tract taken as a safe direction | Interpret it within the surrounding anatomy |
16.6 Viva at the bench
Why can the arcuate eminence mislead you?
It is an orientation feature that does not correspond exactly to the superior canal in every specimen. Confirm with a blue-lined canal, the greater petrosal nerve and the CT.
What are the four boundaries of Kawase's rhomboid?
The greater petrosal nerve laterally, the mandibular nerve anteriorly, the petrous ridge medially, and the internal auditory canal and cochlea posteriorly. The petrous carotid lies beneath the greater petrosal nerve.
Does an aerated tract alone define a safe route to the apex?
No. It illustrates a connection but must be interpreted within the cochlear, vascular and neural relationships around it.
One best answer
16.7 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 |
|---|---|---|---|
| Corridor chosen from the CT, limits named | |||
| Dura elevated with the greater petrosal nerve undisturbed | |||
| Canal found from two landmarks, dura intact | |||
| Rhomboid outlined; carotid and cochlea intact | |||
| Inferior routes compared with limiting structures shown | |||
| Route, reach and what was inaccessible 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 middle fossa or apex surgery
Sources and credits
- Salgado-Lopez L, Perry A, Graffeo CS, et al. Anatomical step-by-step dissection of complex skull base approaches for trainees: surgical anatomy of the middle fossa approaches and anterior petrosectomy. J Neurol Surg B Skull Base 2022;83(Suppl 2):e232–43. doi.org/10.1055/s-0041-1725030
- Cömert E, Cömert A. Surgical anatomy of the transcanal infracochlear approach. Eur Arch Otorhinolaryngol 2022;279:159–68. doi.org/10.1007/s00405-021-06635-6
- Plontke SK, Kösling S, Schilde S, Wittlinger J, Kisser U. The infracochlear approach for diagnostic petrous apicotomy [in German]. HNO 2019;67:791–5. doi.org/10.1007/s00106-019-0687-6
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 15: The Internal Auditory Canal and Translabyrinthine ExposureChapter 17: Completion, Complications and Transfer to the Theatre ›
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