Middle Ear Implants

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Answer

Middle ear implants (MEIs) have three potential benefits. The first, and most important, is the possibility of generating more high-frequency amplification than can be obtained through conventional amplification. Since the middle ear ossicles are driven directly, there is no feedback path (such as with an air conduction receiver feeding back to the microphone). Because of the lack of such a pathway, the amplified sound is more stable with no chance of either audible or ''subaudible'' feedback. ''Subaudible feedback'' is a term that I first read about in Robyn Cox's 1979 monograph on earmold acoustics. It refers to the constructive and destructive interaction of sound that results in a peakier frequency response, despite not being audible- that is, it is at a point just below audible feedback. Clinical reports indeed support that MEIs can generate more high-frequency amplification than can conventional hearing aids. The second benefit is that (depending on the MEI), nothing needs to be placed in the ear canal. The lack of an occlusion effect (and the lack of insertion loss) can go a long way towards a more satisfied consumer. There are some MEIs that are currently undergoing regulatory approval, and these may have the external coil located in the ear canal, and of course, this second benefit would not apply to this particular MEI. The third benefit is related to cosmetics. Two out of the three electro-magnetic MEIs that have either received regulatory approval (Symphonix [FDA approved], Otologics [still in FDA process]) have the external coil hidden in the hair behind the ear. The third currently requires the presence of a coil in the ear canal and as such may be visible (Soundtec [still in FDA process]). The two piezoelectric MEIs that are either currently available (IMPLEX [approved by CE] and St. Croix Medical [still in FDA process]) are completely implantable, and as such are cosmetically invisible.

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Marshall Chasin, M.Sc., is adjunct professor in Experimental Acoustic Phonetics at the University of Toronto and in Audiology at the University of Western Ontario. He has published extensively and has spoken internationally about middle ear implants, bone anchored hearing aids, and prevention of hearing loss for musicians. Publications include three books (Musicians and the Prevention of Hearing Loss (1996), CIC Handboo (edited, 1997), and Noise Handbook Primer (1999)), and the 1997 monograph,
''Implantable Hearing Aids'' (Trends in Amplification, September 1997).