The posterior wall of middle ear cavity (Tympanum) is also known as retrotympanum. Important anatomic structures are lodged in this area. This area has assumed significance because of the difficulties encountered in clearing cholesteatoma from this area. This area is so narrow and has lot of crevises, it is very difficult to clear disease from this area.
This area is supposed to contain 4 important recesses. Each of these four recesses could hide cholesteatoma causing the surgeon to leave residual disease which could later recur. Precise knowledge of anatomy of this region is vital for the surgeon who wants to clear disease from this area. The recesses present in the retrotympanic area are:
Lateral tympanic sinus
Posterior tympanic sinus
Pyramidal eminence is the most prominent anatomical landmark of this area. This eminence hold the pyramidalis muscle. There are other prominences arising from this area projecting in various directions. They include:
External: Chordal ridge
Inferior: Pyramidal ridge
Superior: Suprapyramidal ridge
The 4 types of retrotympanic recesses are found under these eminences.
This is the most common and constant depression present in the retrotympanic area. Anatomically this sinus is located at the junction of the lateral and posterior walls of the tympanic cavity. Phylogentically this recess is considered to ba analogue of bulla tympanica seen in mammals. It lies between ponticulus superiorly and subiculum inferiorly. This recess is bounded by pyramidal ridge externally and promontory internally. Visualization of this area during middle ear surgery proves to be a challenge. During yester years small angled mirrors known as zinne mirrors were used. Now angled telescopes serves this function rather brilliantly. The sinus tympani is known to extend posteriorly up to the round window niche.
Types of sinus tympani:
Sinus tympani has been classified into three types depending on its depth. Note in type III it extends up to the level of lateral semicircular canal.
Figure showing types of sinus tympani
Figure showing anatomy of sinus tympani
Lateral tympanic sinus:
Proctor described this sinus in 1969. This sinus lies between three eminences of styloid prominence. These eminences include:
Posterior tympanic sinus:
Posterior sinus of middle ear cavity is one of the recently identified anatomical sinus inside the middle ear cavity.
Serial temporal bone dissections have shown that it is present in nearly 90% of dissected bones.
Position: It lies just posterior to the oval window.
Depth: 1mm or less
Width: 1.5 mm or less
In nearly 60% of dissected specimen a ridge of bone arising from the floor of middle ear cavity separates it from sinus tympani.
In 8% of dissected specimen, the sinus tympani and posterior sinus merged together to form one confluent sinus.
It has been demonstrated that cholesteatoma / granulation tissue may lie within this sinus making removal difficult leading on to residual disease.
Retraction pockets may also occur close to this area.
Figure showing posterior tympanic sinus
This recess lies between the promontory and tympanic annulus. It is bounded medially by the facial nerve and laterally by tympanic annulus. Running between these two structures at varying angulations is the chorda tympani nerve. Chorda tympani nerve always runs medial to the ear drum. Drilling in this area between the facial nerve, annulus and the angle formed by the chorda tympani nerve will lead into the middle ear cavity without causing a breach in the ear drum. This approach is used in cochlear implant surgery to place the electrode in the round window area. Hypotympanum can also be approached through this approach.
Figure showing middle ear anatomy
This is the posterior extension of promontory separating oval and round windows.
Rarely a spicule of bone arises from the promontory above the subiculum and runs to the pyramid on the posterior wall of the middle ear cavity. This spicule of bone is known as the ponticulus.