Impact of Floating Mass Transducer Coupling and Positioning in Round Window Vibroplasty

Objective: The round window application of the Vibrant Sound bridge, the so-called round window vibroplasty, is gaining increasing popularity for hearing rehabilitation of patients with mixed hearing loss or conductive hearing loss. In these patients, conventional hearing amplification and/or surgical restoration is either not possible or has failed because of chronic ear disease, extensive otosclerosis, or malformations. The exact mechanisms of direct cochlear stimulation via the round window membrane are not yet completely understood. It is unclear what kind and what degree of contact is required between the floating mass transducer (FMT) and the round window membrane (RWM) to elicit a functional hearing perception with the implant. We investigated the coupling efficiency between the FMT and the RWM and how the efficiency is altered by the FMT position, the degree of FMT-RWM contact, and the use of a soft tissue coupler. Study Design: Prospective cohort study. Setting: Tertiary referral center in Western Australia. Patients: Patients undergoing round window vibroplasty for a mixed or conductive hearing loss otherwise not aidable. Intervention: Patients underwent round window vibroplasty and received audiological and coupling analysis in the follow-up. These data were then correlated with FMT positioning and the extent of FMT-RWM interface as determined by postoperative high-resolution temporal bone computed tomography. Main Outcome Measures: Coupling and hearing levels in relation to FMT positioning and degree of FMT-RWM contact. Results: Of 10 patients, 8 were available for vibroplasty behavioral threshold testing. In 2 patients, testing could not be done because of wound breakdown requiring device explantation in 1 case, and in the other case, the bone conduction thresholds dropped 2 months after implantation, thus falling out of the performance range of the device. Postoperative FMT migration occurred in 50% of the patients (3/6) with recurrent chronic ear disease and status after multiple previous ear operations. All patients, including the 3 patients requiring surgical repositioning of the FMT, attained significantly improved speech in quiet and speech in noise when compared with the preoperatively best aided performance. All patients showed significantly improved average Abbreviated Profile of Hearing Benefit scores with the use of the FMT. Direct (partial or complete) contact with the RWM resulted in good coupling efficiency; soft tissue coupling resulted in a reduced coupling efficiency.