Experimental comparison between forward and reverse Vibrant Soundbridge cochlear stimulation

Various couplers exist for the Vibrant Soundbridge implant to connect the actuator to a middle ear structure. While the surgical indication mainly distinguishes between purely sensorineural hearing loss and mixed hearing loss, and audiological indication ranges are designed accordingly, we believe that the biomechanical distinction between forward and reverse cochlear stimulation is more meaningful. To evaluate this, we apply all common couplers in temporal bone experiments and correct the measurements for adequate comparability.

The study was conducted at two research labs, each with n = 10 cadaveric temporal bones. Laser Doppler velocity measurements on the stapes were used for direct comparison between the coupling methods. Therefore, reverse cochlear stimulation requires a correction factor due to pressure loss through a third window. This was calculated by measuring intracochlear pressure differences in the same specimens.

Compared to the stapes LDV measurement, reverse stimulations require a correction factor that continuously decreases in the high frequency range: approximately +22.4 dB between 125-250 Hz, +7.9 dB between 300-800 Hz, and +4.8 dB above 1000 Hz. Taking this into account, there is still a considerable difference between the forward and reverse stimulation methods of 15-22 dB in the frequency range <2000 Hz and approximately 7 dB >2000 Hz to the favor of forward stimulation.

Forward couplings, including direct stapes coupling, are mechanically superior to reverse stimulation. The stapes head coupler performs 7-25 dB better than the round window coupler and similar or even superior to incus couplers.