Landmark structures to approach the internal auditory canal: a dimensional study related to the middle cranial fossa approach.

For the purpose of assisting the accurate topological identification of the internal auditory canal (IAC) during surgery through the middle cranial fossa, we conducted a dimensional study around the IAC using histologically prepared temporal bones. The anterior margin of the porus acoustics, the posterior margin of the porus acousticus, and the center of the fundus were located 19.7 mm, 16.8 mm, and 8.2 mm from the center of the malleus head, respectively. The angles formed by lines connecting those three points of the IAC and the ossicles were also measured. These values could be of help to identify the accurate localization of the IAC using the ossicles as surgical landmarks, when available. The common crus, i.e. a critical inner ear structure for hearing preservation, was also studied. The distance from the medial side of the common crus to the posterior wall of the IAC was relatively constant. However, the distance from the medial side of the common crus to the surface of the posterior cranial fossa was variable. Measurements in the vertical sections showed that the thickness of the superior bony wall of the IAC was similar to that of the cochlear otic capsule which should be kept unexposed for hearing preservation.