Stapes Vibration Produced by the Output Transducer of an Implantable Hearing Aid: Experimental Study

Abstract

\s=b\The function of the output transducer of an implantable hearing aid was assessed by applying it to the stapes head in seven fresh human cadaver temporal bones while observing vibration of the stapes under a microscope with the use of a stroboscope and a video measuring system. The transducer was a 5 \m=x\1.2 \m=x\0.6-mm piezoelectric ceramic bimorph with attached metal holder. Transmission changed with the amount of force holding the transducer tip on the head of the stapes. An optimal connection between the vibrator and stapes produced better transmission than a tight or loose connection. Gluing the connection with cyanoacrylate cement decreased transmission in the optimal connection, produced no change in the tight connection, and improved transmission in the loose connection. Comparison of stapes displacement produced by the vibrator at 1 kHz with that produced by normal middle-ear sound transmission revealed that the vibrator-induced stapes displacement for a 1 V peak-to-peak input was equivalent to that produced by a sound stimulus of 90-dB sound pressure level at the tympanic membrane.The aim of an implantable hearing aid (IHA) is to decrease some of the disadvantages of conventional hearing aids, such as sound distortion, acoustic feedback, and cosmetic appearance.1"4 Implantable hearing aids can be partial or total. In a partial IHA only the output transduc¬ er is implanted; the remainder of the hearing aid components (microphone, amplifier, and battery) remain in their usual location behind or in the pinna. Sound reaching the micro¬ phone is transformed into an electric signal and then amplified and condi¬ tioned just as in a conventional hear¬ ing aid. In the partial IHA developed in Japan, the amplifier output passes to a primary induction coil that lies on the skin over the mastoid. A second¬ ary implant coil lies under the skin adjacent to the primary coil; its out¬ put goes to a piezoelectric vibrator attached to the stapes head." Energy is transferred from the primary to secondary coil across the intact skin by electromagnetic induction and then to the vibrator. By eliminating the receiver and connecting the output stage of the aid directly to the stapes, there appears to be improved sound fidelity with low consumption of elec¬ trical energy.7 In a total IHA, all of the components are implanted in the temporal bone.In a partial IHA, the ossicular vibrator is the most important compo¬ nent. Though engineering tests,5 ani¬ mal experiments,6 and clinical tests7 have shown that the physical proper¬ ties of the vibrator are adequate for implantation, the nature of the opti¬ mal vibrator-stapes articulation is still unclear. The purpose of this arti¬ cle is to report studies on the follow¬ ing: (1) the sensitivity of the vibrator when applied to the stapes; (2) the effect of tight and loose connections on the transmission of vibration; and(3) the effect of gluing the articula¬ tion. MATERIALS AND METHODSSeven fresh normal human temporal bones were used. The temporal bone speci¬ mens were obtained wi...