Investigation of bone conduction stimulation efficiency on the skull surface and below assessed by cochlear promontory vibration.

OBJECTIVE

Stimulation sites closer to the cochlea result in higher output for bone conduction devices (BCD), increasing maximum output and reducing energy consumption. In our study, alternative coupling sites closer to the ear canal (EC), in cortical and spongious bone, were investigated and the influence on ipsi- and contralateral bone conduction output was quantified.

METHODS

Cochlear promontory (CP) vibrations were measured by 1D laser Doppler vibrometry, using five freshly frozen (10 ears) whole human cadaver heads. A percutaneous actuator was implanted at different positions: five on the surface 25 to 55 mm posterior to the EC and two recessed positions 20 mm posterior to the EC at 5 to 8 mm depth. Stimulation was performed using a stepped sine, consisting of 78 frequencies from 0.1 to 10 kHz.

RESULTS

In comparison to positions 50-55 mm to the EC, average CP vibrations showed significantly increased CP vibrations, ranging from 6.5 dB to 13.9 dB between 0.5 and 4 kHz at closer and recessed positions. In contrast, contralateral results showed smaller output amplitudes (0.5 kHz < f < 4 kHz). For positions closer to the EC, transcranial attenuation was considerably increased up to 25.2 dB at 4 kHz.

CONCLUSION

Our results show a significant increase of CP vibrations in stimulation direction, for positions closer to the cochlea. Moreover, the feasibility of bone anchors in spongious bone was demonstrated, leading to similar output when compared to positions on the skull surface.