gms | German Medical Science

Introduction: In recent years patients with moderate to severe hearing loss can be treated by transcutaneous bone conduction and active middle ear implants. Some of these implants use analogue technique such as the Bonebridge (MED-EL, Innsbruck, Austria). The transmission loss in analogue systems due to unknown and often larger distances > 10 mm between the audio processor and implant, as encountered in intra-operative applications, causes inter-individual variability. We developed an inductive link to transmit a calibrated stimulus signal to the Bonebridge implant.

Method: An artificial mastoid (type 4930, B&K, Denmark) was used to measure and compare the output acceleration of the BC-FMT driven with the AP304 (MED-EL, Innsbruck, Austria) and the developed transmission system. The stimuli were 13 single sinusoids with frequencies ranging from 100 Hz to 10 kHz. The change of voltage in the primary loop due to coupling coefficient changes was used to estimate the distance between the transmitter and receiver coils. The transmission loss was compensated using the estimated distance.

Results: The acceleration output amplitude of the Bonebridge driven with the AP304 showed 10 to 20 dB drop between 0.1 and 10 kHz when the distance between the AP304 and receiver coil increased from 2 to 17 mm. However, the amplitude drop from 2 to 11mm lay between 3 to 10 dB. Acceleration amplitude of the Bonebridge driven with the developed transmission system showed up to 17 dB drop at the same frequency range before compensation. Compensated acceleration amplitudes of BC-FMT driven with the developed system showed <3 dB variations.

Conclusion: The unwanted variability caused by the unknown distance between the transmitter and implant were estimated and compensated. The intraoperative stimulation can be used to provide an accurate signal to determine the patients" audiological threshold level.