A bone-anchored hearing aid (BAHA) or bone-anchored hearing device,is a type of hearing aid based on bone conduction. It is primarily suited for people who have conductive hearing losses, unilateral hearing loss, single-sided deafness and people with mixed hearing losses who cannot otherwise wear ‘in the ear’ or ‘behind the ear’ hearing aids. They are more expensive than conventional hearing aids, and their placement involves invasive surgery which carries a risk of complications, although when complications do occur, they are usually minor.
Two of the causes of hearing loss are lack of function in the inner ear(cochlea) and when the sound has problems in reaching the nerve cells of the inner ear. Example of the first include age-related hearing loss and hearing loss due to noise exposure. A patient born without external ear canals is an example of the latter for which a conventional hearing aid with a mould in the ear canal opening would not be effective. Some with this condition have normal inner ear function, as the external ear canal and the inner ear are developed at different stages during pregnancy. With normal inner anatomy, sound conducted by the skull bone improves hearing.
A vibrator with a steel spring over the head or in heavy frames of eyeglasses pressed towards the bone behind the ear has been used to bring sound to the inner ear. This has, however, several disadvantages, such as discomfort and pain due to the pressure needed. The sound quality is also impaired as much of the sound energy is lost in the soft tissue over the skull bone,particularly for the higher sound frequencies important for speech understanding in noise.
The bone behind the ear is exposed through a U-shaped or straight incision or with the help of a specially designed BAHA dermatome. A hole, 3 or 4 mm deep depending on the thickness of the bone, is drilled. The hole is widened and the implant with the mounted coupling is inserted under generous cooling to minimize surgical trauma to the bone.
Some surgeons perform a reduction of the subcutaneous soft tissue. The rationale for this is to reduce the mobility between implant and skin to avoid inflammation at the penetration site. This reduction of the soft tissue has been questioned and some surgeons do not perform any or a minimum of it. The rationale for this is that any surgery will result in some scar tissue that could be the focus of infection. The infections seen early during the development of the surgical procedure could perhaps be explained by the lack of seal between implant and abutment allowing bacteria to enter the space. A new helium tight seal may be advantageous and prevent biofilm formation. This will also allow the surgeon to use longer abutments should a need exist. Three to six weeks later or even earlier, the audiologist will fit and adjust the hearing processor according to the patient’s hearing level. The fitting will be made using a special program in a computer.