Bone-Anchored Hearing Aids
This is taken from a great post by Dr. Mark Ross:
“A bone-anchored bearing aid is basically a modern development of a bone-conduction (b/c) hearing aid, something that has been with us for over 60 years. In a traditional b/c hearing aid, the user places a tight, elastic, metal band over his or her head. A bone-conduction vibrator (oscillator) is fixed to one end of the band and placed behind the ear and connected to a body-worn hearing aid via a wire cord. These oscillators are essentially the same devices still being used during routine audiometric testing. People wearing such b/c hearing aids would frequently complain of discomfort and headaches because of the constant pressure of the vibrator behind their ears.
The people who were candidates for b/c hearing aids were those whose hearing losses were primarily conductive, that is the sound was not being “conducted” through either the ear canal or middle ear because of some impediment to the transmission of the sound vibrations. This impediment could be caused by ear-canal infections (external otitis), chronic drainage from the middle eat or a congenital malformation of the outer or middie ears – anything that precluded the insertion of an earmold in the ear canal.
In these instances, it would be appropriate to fit the person with a b/c hearing aid in order to by-pass the outer and middle ears and directly vibrate the cochlea through the skull. In other words, instead of amplified sound being transmitted through the ear canal to the inner ear via a traditional hearing aid, tiny skull vibrations would deliver the same sound signals to the inner ear (the cochlea) via bone conduction. Once sound signals from either source arrive at the inner ear, they are analyzed by the auditory system in exactly the same way.
The BAHA® requires that a titanium screw be inserted into the temporal bone behind the ear. This sounds rather fearsome, but I’m informed that it is really a minor surgical procedure. After about three months, the process of “osseointegration” and healing is completed. Osseointegration refers to a process in which an organic connection occurs between implanted material (the titanium) and living tissue. This type of connection provides for a much more efficient way of transferring energy from an external vibrator through the skull to the cochlea than traditional b/c oscillators. The titanium screw terminates in an abutment into which the hearing aid can easily be attached and detached.
The hearing aid itself (there are several models, ranging from ear level to body-worn types) contains the usual hearing aid circuitry and can be modified and upgraded independent of the inserted portion which is permanent. The vibrations produced by the BAHA® are transmitted to the embedded titanium screw and passed along to the cochlea via the bone conduction pathway.
Unlike previous generations of b/c hearing aids, because of its vibratory efficiency, the BAHA® is also appropriate for people with mixed hearing losses; i.e. part conductive and part sensorineural (up to about a sensorineural component of about 45 dB). The ear-level instrument contains a direct audio input jack into which assistive listening systems or an external telephone coil can be plugged. Further information about the BAHA® can be obtained from www.entific.com.”