Analysis of vibrant soundbridge placement against the round window membrane in a human cadaveric temporal bone model

Objective: To evaluate optimal placement of the Floating Mass Transducer of the Vibrant Soundbridge (Med-El, Innsbruck, Austria) against the round window membrane, particularly the impact of interposed coupling fascia and of covering materials. Methods: Six fresh human cadaveric temporal bones were used. After mastoidectomy, posterior tympanotomy and removal of the round window niche, the Floating Mass Transducer (FMT) of the Vibrant Soundbridge (VSB) was placed against the round window membrane (RWM) in the following ways: in direct contact, or with interposed fascia. Both conditions were combined with a second parameter: no cover over the FMT or covered with fascia, fat or cartilage. The inner ear was stimulated through the VSB with a frequency sweep from 0.1 to 8 kHz at 1 V RMS. Stapedial footplate vibrations were recorded with a PSV-400 Scanning Laser Doppler Vibrometer (Polytec, Waldbronn, Germany). Results: A learning curve exists for optimal placement of the VSB. Sufficient removal of bone around the round window is essential. Without covering materials, there is increased transmission of vibrations if fascia is interposed between the RWM and the FMT. If there is no interposed fascia, vibration transmission is increased with a fascia or fat (but not cartilage) cover. There is no added advantage of cover and interposed fascia, either is as good as the other. Conclusion: Optimal placement of the VSB against the round window relies heavily on surgical precision in placement. There is improved transmission of vibrations with either interposed fascia, or with a covering material.