A silastic positioner for a modiolus-hugging position of intracochlear electrodes: Electrophysiologic effects

Hypothesis: It was postulated that an electrode array that achieved a close modiolar proximity would result in reduced threshold levels and amplitude slopes, as measured with electrically evoked auditory brainstem responses ( EABRs). Background: Quality and quantity of auditory information transmitted by a c ochlear implant to patients with sensorineural hearing loss depend on spati al and temporal resolution achieved by the electrical intracochlear stimula tion. Methods: To improve spatial resolution, a new electrode system was develope d by Advanced Bionics Corp., with the intention of obtaining greater modiol ar proximity. The implant version specified for animal experiments consists of a straight electrode array of seven embedded platinum discs and a so-ca lled Silastic-positioner. The Silastic positioner is shaped to Follow the d imensions of the scala tympani with a concave (triangular) inner side, whic h fits the form of the electrode array. The aim of the study was to evaluat e the influence of a modiolus-hugging electrode position in contrast to a c onventional electrode position on EABR in short-term animal experiments. Sh ort-term electrophysiologic studies were performed on six adult cats. After local intracochlear application of neomycin solution (50 mg/mL), electrode s were inserted into the scala tympani. Electrically evoked auditory brains tem response threshold levels and EABR amplitude slopes were systematically investigated with and without the positioner. Results: Electrically evoked auditory brainstem response measurements revea led a distinct apicobasal threshold shift, with increasing thresholds towar d the basal end of the electrode. After insertion of the positioner, this s hift diminished or was inverted and EABR thresholds and amplitude slopes we re reduced significantly. Conclusions: Threshold and amplitude slops data emphasize the functional be nefit of the positioner system, especially for the stimulation of electrode s in the more basal channels.