In vitro model for intraoperative adjustments in an implantable hearing aid (MET)

Assessment of the optimal static preloading of Otologics Middle Ear Transducer (MET) Ossicular Stimulator, when coupled to the incus.

The MET Ossicular Stimulator is a partially implantable electromagnetic middle ear hearing device that transmits vibrations to the ossicular chain. The vibration patterns were measured with laser-Doppler vibrometry.

Experimental.

We used three human cadaveric temporal bones (TB) and one MET ossicular stimulator.

Laser-Doppler vibrometry was used for the selection of TBs. The cochlea was subsequently extirpated from the posterior side to measure the vibrational patterns (VP) of the footplate. Three TBs with different VP were selected based on data obtained from volunteers with normal hearing (n = 110): one TB with a VP larger than +1 SD, one TB with a VP in the range of +/-1 SD, and 1 TB with a VP smaller than -1 SD. Transfer functions were calculated between VP of the measurement points at the coupling rod, umbo, incus, and footplate. The TBs were subsequently defrosted. The MET was implanted and coupled to the ossicular chain. Different coupling loads were measured at the incus, the umbo, and the footplate.

Optimal transfer function between the MET transducer and the oval window was achieved during contact when the coupling rod advanced 0.0625 mm (90 degrees rotation). Additional advances of 0.0625 mm (180 degrees turn = 0.125 mm) resulted in a decreased vibrational amplitude, ranging between 20 and 40 dB below 3 kHz. The lowest linear distortion occurred up to 10 kHz during direct contact without advancing the coupling rod.