[Surgical anatomy of the facial nerve and chorda tympani with special reference to cochlear implantation].

The surgical anatomy of the facial nerve (FN), chorda tympani nerve (CT) and round window (RW) in 26 Japanese temporal bone specimens was analyzed. The relationships were classified according to the relative position of these structures and the posterior wall of the external auditory canal (EAC). Most frequently, the widest approach route through the facial recess did not point directly at the RW, but rather at the basal turn. Moreover, the approach route crossed the FN-RW line in a posterior to anterior direction (76.9%) and the CT-RW line frequently crossed the posterior wall of the EAC (92.3%). 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 at the promontory. Several other anatomical indices were measured and we extracted the index which was related to the maximum approach amplitude and direction. Preoperative diagnosis may be possible using these indices.