Posterior (Mastoid) Wall of the Middle Ear

Key Features (from superior to inferior)

1. Aditus ad Antrum

• A large irregular opening in the upper part of the posterior wall
• Connects the posterior epitympanic recess with the mastoid antrum
• Superiorly, the epitympanic recess is continuous with the mastoid antrum through this opening
• Clinically important: infections from the middle ear can spread through the aditus into the mastoid air cells, causing mastoiditis

2. Fossa Incudis

• A small depression just below the aditus
• Houses the short process (limb) of the incus and its suspensory ligament
• The short limb of the incus extends posteriorly and attaches here by a ligament

3. Pyramidal Eminence (Pyramid)

• A small hollow conical bony projection with its apex pointing anteriorly
• Located below the fossa incudis and medial to the chorda tympani opening
• Contains the stapedius muscle, whose tendon exits through the apex to insert into the posterior surface of the head (neck) of the stapes
• The canal within the pyramid curves downward and backward to join the descending portion of the facial nerve canal

4. Chorda Tympani Nerve Entry

• An opening on the posterior wall through which the chorda tympani nerve (branch of facial nerve [VII]) enters the tympanic cavity
• The chorda tympani enters via a small canaliculus (iter chordae posterius) and then crosses the middle ear anteromedially before exiting through the anterior wall
• It runs medial to the tympanic membrane and lateral to the long process of the malleus

5. Facial Recess

• A groove between the pyramid/facial nerve and the tympanic annulus
• Bounded medially by the facial nerve canal and laterally by the tympanic annulus
• The chorda tympani runs obliquely through this region
• Surgically important: a posterior tympanotomy can be cut here to access the middle ear from the mastoid without disturbing the tympanic membrane

6. Sinus Tympani

• A posterior extension of the mesotympanum into the posterior wall
• Lies deep to the facial nerve, pyramid, and stapedius muscle
• Variable in size; when extensive, it is one of the least accessible sites surgically (may require a 45° endoscope)

Diagrammatic Summary

Clinical Relevance