Relationship of the Facial Nerve to the Tympanic Annulus: A Direct Anatomic Examination

Objective: To examine the relation of the facial nerve to the only identifiable surgical landmark in the external auditory canal. Institution: Community-based teaching hospital. Study Design: Examination of formalin-fixed human temporal bones. Background: The transcanal approach is often used in tympanoplasty, canaloplasty, hypotympanotomy, and removal of tumors of the external auditory canal (EAC), such as exostoses and osteomas. Surgery of the EAC places the facial nerve at risk for injury as the nerve courses vertically in the posterior canal wall. Few articles have described the relation of the facial nerve's course to the tympanic annulus, the only identifiable landmark in the EAC. This study is the first to document the relationship of the course of the facial nerve with respect to the tympanic annulus by direct anatomic measurement of the temporal bone. Methods : Thirty-seven formalin-fixed cadaver temporal bones were studied after skeletonization of the facial nerve and tympanic annulus. Results: The facial nerve coursed lateral to the plane of the annulus in 70% of specimens, always in the posteroinferior quadrant. The nerve also coursed anterior to a plane through the most posterior point of the annulus in 73.1% of specimens, also exclusively in the posteroinferior quadrant. The course of the nerve was quite variable with respect to the annulus. Conclusions: The facial nerve is most vulnerable to injury in the posteroinferior quadrant in transcanal surgery. The annulus is not a reliable landmark for the facial nerve. Anecdotal evidence is cited and recommendations are offered.