Topographical relationships among the facial nerve, chorda tympani nerve and round window with special reference to the approach route for cochlear implant surgery.
The topographical relationships among the facial nerve (FN), chorda tympani nerve (CT), and round window (RW) in 22 temporal bone specimens were analyzed morphometrically in order to examine which route is widest through the facial recess between the FN and CT during cochlear implant surgery and in order to establish some criteria to assist in the evaluation of the best surgical approach. Two lines, i.e., the FN-RW line and CT-RW line, were speculated as limitations of a visual field for this surgery. According to the relative position of these structures, including the posterior wall of the external auditory canal (EAC) and an inserted pin-gage that indicates the hypothetical widest approach route, the relationships were classified into five types. Most frequently, the widest approach route through the facial recess did not point directly at the RW, but at the basal turn at the promontory. Moreover, this approach route crossed the FN-RW line in a posterior to anterior direction and the CT-RW line frequently crossed the posterior wall of the EAC. The latter seemed to provide a critical landmark for avoiding damage to the CT during cochlear implant surgery. Therefore, we recommend inserting the electrode into the basal turn.