Auditory brain stem implant: effect of tumor size and preoperative hearing level on function. The auditory brain stem implant is an investigational device designed to provide hearing sensations to patients without functioning auditory nerves. We analyzed results from 17 implants in 15 patients to determine if tumor size or preoperative hearing level might be related to proper device function. We found no significant correlation between preoperative hearing level or tumor size and device function. We also found no significant correlation between preoperative hearing level and tumor size in these 15 patients. 