The bone-anchored hearing aid. Principal design and a psychoacoustical evaluation.

Hearing aids can be divided into two groups, distinguished by the principle of how sound is transmitted to the cochlea. The greatest and most important group is that consisting of air conduction (AC) hearing aids, whereas the other group consisting of bone conduction (BC) hearing aids is relatively small but nevertheless important for patients with particular types of hearing disorder. Conventional bone conduction hearing aids are not in common use, mainly because of several drawbacks. The implantation of a titanium screw into the mastoid portion of the temporal bone has made it possible to obtain a permanent and reaction-free rigid penetration of the skin. A newly developed bone conduction hearing aid was connected to such a screw. This new hearing aid is named 'the bone-anchored hearing aid' (BAHA) and it is characterized by a single housing construction. The transducer piston of the BAHA is directly connected to the titanium screw by a low-profile coupling arrangement. Ten patients were equipped with this new hearing system. In a comprehensive questionnaire, all patients reported that they preferred the BAHA to their old hearing aid. The major reasons were improvements in practical arrangements, wearing comfort, aesthetic appearance, and sound quality. Improvements in speech reception were not verified by the discrimination tests, but 6 out of 10 patients subjectively reported clearly improved sound quality. The group of patients who may benefit from the BAHA of present standard includes patients with a sensorineural hearing loss of at most 50-60 dB HL.