THE TEMPORAL BONE

A Digital Study Guide
by
John E Buenting, MD

This is a local restricted-access copy, made because it had disappeared from the original site. It was made temporarily available to me today (2000 Jun 2) by Ahmed Aziz (aaziz@med.unc.edu).

The Temporal Bone Station:
The temporal bone drilling station is set up as shown in the photo. The microscope is covered with a plastic drape to protect from bone dust when the station is in use. The chair should be adjustable in height and suction and irrigation are essential. The drill and bits vary considerably and will not be covered in detail.

Initial Steps:
The specimen in this series is a cadaveric right ear; the approach is similar for fresh specimens. Soft tissue is removed using sharp dissection and the Lempert periosteal elevator. Once fairly clean, the bone is fixed into the holder; this is aided by tightening the set screws two at a time. Drilling is begun with the largest cutting burr, with the first cut along the temporal line. The second cut is made perpendicular to the first down toward the mastoid tip. These two lines intersect just posterior to the spine of Henle (shown at the arrow), and this region is the deepest part of the dissection initially. Drilling proceeds using continuous suction and irrigation and the largest burr possible; the burr size will decrease as depth into the mastoid bowl increases. The primary goal of the dissection initially is to define the landmarks that allow the surgeon to maintain orientation while drilling.

Identifying Early Landmarks:
The posterior bony canal wall (PCW), tegmen(T) and the sigmoid sinus(SS) are identified early as the developing mastoid bowl is progressively saucerized. As air cells are removed inferiorly, the digastric ridge(DR) is identified and preserved as a pointer to the facial nerve. As the tegmen is exposed superiorly, the sinodural angle(SA) is defined and all air cells removed in this area, an important part of mastoid surgery for cholesteatoma.


Completing the simple mastoidectomy:
Attention is again directed anteriorly and care taken to thin the bony posterior canal wall. This maneuver, combined with concentrated drilling medial to the spine of Henle, allows one to safely identify and enter the attic. Further thinning of the canal wall and definition of the tegmen will permit visualization of the head of the incus(I), first seen refracted through water in the depths of the cavity. The head of the incus in turn serves as a reliable landmark to identify the horizontal semicircular canal. The canal is embedded in the dense compact bone of the otic capsule, which also aids in its identification. Locating the facial nerve(VII) in its mastoid segment is made simpler by the above steps, as the nerve lies roughly on a line between the anterior tip of the digastric ridge and the lateral extent of the horizontal canal, shown by the dashed line in the image below. It lies directly inferior and medial to the fossa incudis as it finishes its tympanic segment. Visualizing the fossa incudis is also helpful in that the short process of the incus points directly to the second genu of the nerve overlying the lateral canal. Once the nerve is identified, the air cells near the nerve can be safely removed with a diamond burr, using long sweeping passes parallel to the course of the nerve. The bone overlying the tegmen and sigmoid are similarly thinned and the simple mastoidectomy is completed. The anatomy is well demonstrated in the image below, where the incudo-stapedial joint can be seen through the facial recess.

Drilling The Facial Recess:
Once a simple mastoidectomy has been finished, access to middle ear structures is limited. This can be addressed via the EAC with a vascular strip or tympanomeatal flap approach, or from the mastoid by a facial recess, extended facial recess or canal wall down approach. The trans-mastoid approaches are covered here, the most conservative of which is the facial recess with preservation of the chorda tympani. Drilling resumes at the base of the posterior canal wall just lateral to the facial nerve at its second genu. Using a small diamond burr, the typmanic cavity is entered. A narrow buttress supporting the short process of the incus is preserved between the lateral semicircular canal and the posterior canal wall and thus forms the superior margin of the dissection. The inferior margin is defined by the chorda tympani and the recess is bounded laterally by the tympanic ring. The main trunk of the facial nerve forms the medial wall of this small triangular space.

Extended Facial Recess:
The facial recess approach generally allows an adequate view of the round window and promontory and is thus commonly used for procedures requiring limited middle ear access, such as cochlear implantation. When slightly more exposure is required, as with a glomus tympanicum, the chorda tympani can be intentionally sacrificed and an extended facial recess performed. The inferior limit of the dissection is now limited primarily by the course of the main trunk of the nerve as it trends slightly laterally in the mastoid.

Canal Wall Down Mastoidectomy:
For operations requiring total access to the middle ear and mastoid, such as revision cholesteatoma surgery, the posterior canal wall can be removed. If a facial recess has been performed, a curette can be used to remove the bone of the posterior wall, aided by drill cuts made superiorly at the tegmen and inferio rly lateral to the facial nerve. The tympanic membrane, if present, should be reflected forward, as this maneuver clearly destroys the posterior half of the bony tympanic ring. It provides outstanding visualization of the entire middle ear, ossicular chain and tympanic and mastoid segments of the facial nerve, as well as the hypotympanum, eustachian tube orifice and oval window.