[Implantable hearing aids].

Abstract

The purpose of these hearing aids is to improve the quantity and quality of amplification of the sound wave. Their operating principle is based on the existence of a transducer which transforms the electric signal captured by the microphone into a mechanical, vibrating wave. This transducer can act either directly on the temporal bone by short-circuiting the middle ear (and is then called a bone conduction implant) or directly on the ossicular chain (in which case it is called an ossicular transduction implant). A) There are 2 types of bone conduction implant: the "Audiant Bone Conduction" system of Hough-Vernon (Xomed), where an outside electromagnetic coil sends signals transcutaneously to a magnetic screw embedded on the temporal bone. Bone conduction must be normal. The "Bone Anchored Hearing Aid" (Nobelpharma) where the outside electromagnetic coil sends signals directly to the magnetic screw that is percutaneous. The mean thresholds in bone conduction can reach up to 45 dB. The drawbacks of this system lie in its inaesthetic appearance and the risk of infection. B) With ossicular transduction implants, the transducer may be: either an electromagnetic coil influencing a magnet fixed to the tympanum, on the ossicular chain, or replacing an element of the ossicular chain (these hearing aids are marketed by Richards); or a piezo-electric crystal which produces vibrations when it is subjected to an AC current, and which can be positioned on the stapes (Yanagihara). This system consumes less energy, gives excellent results for the high frequencies, but is more cumbersome.