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 analy zed 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 su rgical approach. Two lines, i.e., the FN-RW line and CT-RW line, were specu lated as limitations of a visual field for this surgery. According to the r elative position of these structures, including the posterior wall of the e xternal auditory canal (EAC) and an inserted pin-gage that indicates the hy pothetical widest approach route, the relationships were classified into fi ve types. Most frequently, the widest approach route through the facial rec ess did not point directly at the RW, but. at the basal turn at the promont ory. Moreover, this approach route crossed the FN-RW line in a posterior to anterior direction and the CT-RW line frequently crossed the posterior wal l 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. (C) 2000 Wiley-Liss, Inc.