The surgical anatomy for multiple-electrode extracochlear implant operations.

Direct access to the whole length of the cochlear turns via endaural middle ear approach for the placement of extracochlear electrodes is severely restricted. Approximately 10 mm. of the cochlear turns are accessible, being less than a third of their length. The middle cranial fossa, the facial nerve, the internal carotid artery and the temporomandibular joint restrict the access. A further restriction is caused by the position of the cochlea and the direction of its axis. The anterior part of the cochlea lies anterior to the tympanic membrane and medial to the temporomandibular joint, thus limiting an endaural approach to a postero-lateral direction. Despite this limitation small sections of the basal, middle and apical turns of the cochlea can be reached.