Using greater superficial petrosal nerve and geniculate ganglion as the only two landmarks for identifying internal auditory canal in middle fossa approach

The middle fossa approach is a surgical technique indicated for acoustic neuroma with advantages of complete tumor removal and hearing preservation. Various methods have been developed for identifying internal auditory canal (IAC). Here, we describe a new method, which uses greater superficial petrosal nerve (GSPN) and geniculate ganglion (GG) as the only two landmarks to identify IAC. In this study, the surgical anatomy and relations between GSPN, GG, petrous ridge, and IAC were measured on 20 temporal bone specimens and 40 HRCT scans of normal inner ear structures. The temporal bone study showed that the GSPN is nearly parallel to the petrous ridge, with an average angle of about 6 degrees. The line perpendicular to the GSPN from the tip of the GG to the petrous ridge reaches at the porus acousticus, with an average distance of 12.95 mm. The temporal bone HRCT scan study revealed that the length from the tip of the GG to the midpoint of the IAC portion on the line perpendicular to the petrous ridge is about 9.9 mm. We applied this method on 20 temporal bones by drilling the point away from the tip of the GG about 9.9 mm on a line angled with the GSPN about 96 degrees. All IACs were safely identified without damaging the cochlea, the labyrinthine portion of the facial nerve, or the vestibule. In conclusion, the GSPN and GG can be the only two landmarks for safely identifying the IAC without injury to the inner ear structures in the middle fossa approach.