May 6, 2008 -- A device screwed into the skull that transmits sound via bone can improve hearing in patients with certain kinds of hearing loss by as much as 50 percent, according to the largest study of devices, known as bone-assisted (or bone-anchored) hearing aids, or BAHAs.
"This is a very important technology for a number of patients who have difficulty hearing," said Daniel Lee, a doctor at the Massachusetts Ear and Eye Infirmary in Boston, Mass., who was not involved in the study.
"Patients are getting better choices for improving their hearing," said V. Suzanne Jeter, a researcher at Loyola University who presented the study at the 10th International Conference on Cochlear Implants and Other Implantable Auditory Technologies in San Diego.
BAHAs have been used for years to restore hearing to patients who suffer from outer and middle ear problems like tumors, chronic infection, congenital disorders and other problems that stop sound from entering the inner ear.
Unlike cochlear implants, the device cannot help deaf patients whose inner ear, which contains three tiny bones that contribute to hearing, has been damaged or destroyed.
Deaf patients sometimes use cochlear implants, which electrically stimulate the nerve leading from the ear to the brain, to regain hearing. Instead, BAHAs mechanically vibrate the bone that then stimulates the intact inner ear, triggering a nerve impulse that then travels from the ear and into the brain.
Simple surgery is required to implant the device.
After a patient is anesthetized, a small U-shaped incision is made, and a 2- to 3-mm hole is drilled into the mastoid bone behind the ear. A small titanium screw is then inserted into the hole, and the incision is sealed.
Over the next three to six months the bone grows into the screw, locking it into place.
Then all the patient has to do is clip a microphone and sound processor the size of a thumb knuckle to the screw.
When the microphone picks up sound, the sound processor creates a series of mechanical vibrations that travel through the screw, into the mastoid bone and then into the inner ear.
Using bone to hear may sound like a strange idea but, as the recent study shows, it is effective. It's easy to hear a tuning fork when you hold it next to your ear, but place the base of that humming metal on the bone behind your ear, the same bone the BAHA uses, and suddenly the sound is amplified.
It has been proposed that snakes use their jaw bones in a similar way to detect approaching prey.
The device can be unclipped for sleeping or showering. While patients hear sounds, there is no detectable physical pressure. As with any surgical implant, patients must clean the skin around the screw regularly to prevent infection.
Depending on how hearing is measured, the researchers reported patients improved their hearing by anywhere from 25-50 percent, said Jeter.
The bigger challenge is getting the devices into patients who need it, said Lee. Each operation costs between $10,000 to $15,000, which may or may not be covered by insurance or states.
"The technology is certainly important," said Lee. "But many states aren't paying for the surgeries that would give people better hearing."
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