[Principle requirements of an electromechanical transducer for implantable hearing aids in inner hearing hearing loss. II: Clinical aspects]

Abstract

In cases of sensorineural hearing loss, an electromechanical transducer of implantable hearing aids should partially or completely compensate for the cochlea amplifier (CA) failure that occurs in most of these patients. CA failure can be clinically identified by a positive recruitment and amplitude loss in otoacoustic emissions (OAE). In a healthy ear, the basis of the CA is the motility of the outer hair cells. A transducer that replaces functions of the CA can be termed CAI (cochlea amplifier implant). A CAI should fulfill five requirements: (1) production of a vibratory output signal, (2) reinforcement of the travelling wave in the inner ear, (3) imitation of the physiological, nonlinear behavior of the CA, (4) reinforcement without distortion, and (5) steady amplitude at rising frequencies. Compensation of frequency selectivity of the CA would also be desirable; however, this can hardly be expected from an implant. If the requirements are fulfilled, one can expect superior sound and hearing quality compared to a conventional hearing aid. Frequency should range from 50 to 10,000 Hz and have an increasing transfer function beyond 1000 Hz. Equivalent levels in the high frequency range should clearly exceed 100 dB SPL. Total distortion should at best amount to 1%. The CAI should function as a totally implantable hearing aid requiring minimum energy. The outer ear canal has to remain open so that occlusion cannot occur; the ossicular chain should be kept intact or repairable, and the implantation technique should allow explantation. The quality of the implant should make possible a CE approval in Europe and an FDA approval in the USA. Furthermore, there should be no risks involved if computer tomography, MRI, or PET examination are undertaken. Underwater diving to a depth of 2-3 m should also be permissible. Minimization of risks that occur near strong magnetic fields or high frequency fields is of great practical importance.