Within the Temporal Bone · Chapter 08 · Part II · Core dissections
The Facial Recess and Posterior Tympanotomy
Establish the corridor before you open it
Bench mode: one step per screen, large type, screen kept awake. Tap, use the arrow keys or say “next”.
- Specimen
- Adult temporal bone after a complete cortical mastoidectomy; canal wall intact; side recorded
- Bench time
- One session, mostly spent looking; your instructor sets the target
- Burrs
- Medium diamond to thin; small diamond in the recess; no cutting burr near the nerve
- View
- Higher power, the microscope tilted to look along the recess
- Before this
- Chapters 6–7: cortical mastoidectomy, antrum, incus and lateral canal
By the end you can
- Predict the facial nerve from the incus, lateral canal and digastric ridge
- Define the facial recess by its three boundaries before opening it
- Open the interval with a diamond burr, the nerve kept covered
- Name what the recess shows and relate it to the transcanal view
- Record the view and whether an instrument reaches its target under vision
Recess opened between the two nerves; facial nerve covered by bone throughout; chorda tympani intact; incus buttress and chain untouched.
Exposing the facial nerve (Chapter 13); removing the round-window overhang (Chapter 9); dividing the chorda to extend the opening, a separate planned step and never a rescue for an opening that has drifted.
The landmark chain
Each landmark leads to the next. Tap any one to see every station where it appears.
08.1 Before you open the recess
The posterior tympanotomy is a transmastoid route to the middle ear through the facial recess: the interval between the mastoid segment of the facial nerve medially, the chorda tympani laterally and the incus buttress superiorly. Their course and spacing decide how much corridor a given bone offers. A remembered triangle is only a starting model; the specimen defines the boundaries.
The objective is to open a defined interval while preserving its neural boundaries, not to find the middle ear by drilling an estimated hole.
Start from a complete mastoidectomy with the posterior canal wall intact and thinned. Inspect the incus region, the lateral canal and the posterior canal wall, and relate the expected descending facial nerve to all three. If the cavity is narrow or deep, widen it first: the recess is approached along its length.
Find both nerves before you open the space between them. Do not expect either to announce itself by colour.
A pale yellow line under thin bone suggests the facial nerve, but preservation, bone thickness and variation change what shows through, and dense bone or a cell can look similar. Identify the nerve by position and continuity: from below the lateral canal dome, down the line of the canal wall, to the anterior end of the digastric ridge.
08.2 · The 5 stations
Prepare the field
- Complete the mastoidectomy: open the antrum widely, thin the posterior canal wall evenly and lower the cavity floor towards the digastric ridge.
- Identify the short process of the incus in the fossa incudis; it points posteriorly towards the facial recess.
- Identify the dome of the lateral canal: the second genu of the facial nerve lies just inferior to it.
- Follow the digastric ridge forwards to its anterior end at the stylomastoid foramen. The line from the second genu to that point estimates the mastoid segment.
Incus, lateral canal, posterior canal wall and digastric ridge all seen and named.
The cavity is too narrow to look along the canal wall. Widen it before going on.
Define the boundaries
- Thin the bone over the expected facial nerve, strokes parallel to its course, until you can follow the canal as a continuous contour. Leave it covered.
- Find the chorda tympani where it leaves the facial nerve and follow it up towards the posterior canal wall and the drum.
- Note the incus buttress, the bone supporting the short process; it is the superior limit.
- Outline the interval before removing any bone from it. Its width and the point where the chorda arises vary from bone to bone.
Both nerves and the buttress identified, and the outline agreed with your instructor.
You are estimating the nerve from a diagram rather than seeing its canal. Thin and trace further first.
Open the recess
- Position the bone and microscope to look along the recess, not across it; burr and suction must not obscure each other’s contact points.
- Thin the bone within the interval in small increments, the burr moving parallel to the facial nerve, never towards it, a protective layer kept over the nerve.
- Watch for the change in space as the middle ear opens. Reassess the trajectory at once: a burr that was safe within bone becomes hazardous when a cavity opens beneath it.
- Enlarge only as far as the planned view requires, clear of the incus, the buttress and the drum. Inspect the corridor after each enlargement.
The middle ear entered between the two nerves; both intact, the facial nerve still covered.
The burr shaft rests on the canal wall or the incus, or you cannot see the contact point. Stop the drill and reposition.
Look through the opening
- Identify the long process of the incus descending to the incudostapedial joint, the stapes, and the stapedius tendon running forwards from the pyramidal eminence.
- Identify the promontory, the bulge of the basal turn, in the anterior part of the view.
- Look for the round-window niche at the posteroinferior edge of the promontory, low in the opening; the membrane itself is usually hidden by the niche.
- Compare with the transcanal view of Chapter 5: the same structures appear rotated and foreshortened. Tilt the bone or the microscope rather than assume the familiar arrangement.
Incus, stapes, pyramidal eminence, promontory and round-window niche named through the recess.
The target is not visible. Change the line of sight before you think of enlarging towards a nerve.
Inspect and record
- Inspect the facial nerve covering, the chorda tympani, the incus buttress and the ossicular chain.
- Record the field visible through the opening and whether an instrument can be introduced under continuous vision.
- If cochlear work follows, photograph this stage first: later drilling at the round window must not obscure whether the recess was opened without injury.
- Record any injury exactly. An opening with an injured chorda or an exposed nerve is a different result, not a larger version of the same one.
A defined opening, both nerves intact, the buttress untouched and the stage recorded.
You find an injury you did not see happen. Obtain instructor review before any further drilling.
08.3 Relate the recess to the transcanal view
Through the canal you look medially and the posterior mesotympanum is spread out, the chorda crossing above between incus and malleus. Through the recess you look forwards and medially down a narrow slot between two nerves: the incus and stapes above, the round-window niche below, the promontory beyond.
If a target cannot be seen, review the line of sight and the limits of the opening first. The obstacle is often the approach angle or a bony overhang beyond the recess, not the recess itself; discussing alternative views is part of learning the corridor.
A narrow recess, an unusual nerve course, or a high-arising or damaged chorda makes the exercise harder. Where the boundaries cannot be established with confidence, stop for instructor review, and describe an opening you could not make rather than conceal it by sacrificing a boundary.
08.4 Error lab · facial nerve exposed or chorda injured
On the bench: stop the drill, leave the field as it is and record the site, the instrument and the movement that caused it.
In the theatre: exposure of the nerve sheath, injury to the chorda and heat from a burr over the nerve each carry their own consequences. Nerve monitoring complements anatomical technique; it does not replace it.
The recess is defined by what you keep: two nerves and a buttress.
08.5 Fault finding
| What you see | Probable cause | Correction |
|---|---|---|
| No recess where expected | Nerve course not traced | Thin and trace the nerve before drilling the interval |
| Opening drifts forwards into the canal wall | Burr axis aimed at the drum | Re-aim along the nerve; keep the canal wall intact |
| Pale yellow line exposed behind the opening | Opening carried too far posteriorly | Stop; record the exposure; work anteriorly only if the chorda allows |
| Chorda frayed or divided | Burr worked laterally across it | Record it; do not extend the opening to hide it |
08.6 Viva at the bench
What are the three boundaries of the facial recess, and which varies most?
The mastoid segment of the facial nerve medially, the chorda tympani laterally and the incus buttress superiorly. The chorda varies most: the point where it arises from the nerve, and so the width of the recess, differs from bone to bone.
How do the lateral canal and the digastric ridge predict the mastoid segment of the facial nerve?
The second genu lies just inferior to the lateral canal dome, and the nerve leaves at the stylomastoid foramen at the anterior end of the digastric ridge. The line joining the two estimates the mastoid segment.
The target is not visible through the recess. Must the opening become wider?
No. First change the line of sight by tilting the bone or microscope, and look for a bony overhang beyond the recess. Widening towards a nerve is not the answer to a hidden target.
One best answer
08.7 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 |
|---|---|---|---|
| Facial nerve predicted and traced, still covered | |||
| Chorda tympani traced and intact | |||
| Incus buttress and chain untouched | |||
| Opening made along the nerve, not across it | |||
| Stapes, promontory and round window niche named | |||
| Stage photographed and recorded honestly |
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 open a facial recess in the theatre
Sources and credits
- Harris T, Linder T. Mastoidectomy and epitympanectomy. The Open Access Atlas of Otolaryngology, Head & Neck Operative Surgery (Fagan J, ed.). University of Cape Town. CC BY-NC 3.0. med.libretexts.org/
- Companion video: Posterior Tympanotomy, Temporal Bone Dissection Part 1 (GeNext MedEd Foundation workshop, Dr Raman Sharma). youtu.be/12c7D7gydDU
- Companion video: Cochlear Implant Surgery, Part 2: Posterior Tympanotomy. Dr Bhushan Kathuria & Team. youtu.be/EAH4sK8iEUE
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 7: The Antrum, Aditus and EpitympanumChapter 9: The Round Window and Cochlear Implant Corridor ›
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