Bone anchored hearing aid
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A bone-anchored hearing aid (BAHA) is a type of
hearing aid A hearing aid is a device designed to improve hearing by making sound audible to a person with hearing loss. Hearing aids are classified as medical devices in most countries, and regulated by the respective regulations. Small audio amplifiers su ...
based on
bone conduction Bone conduction is the conduction of sound to the inner ear primarily through the bones of the skull, allowing the hearer to perceive audio content without blocking the ear canal. Bone conduction transmission occurs constantly as sound waves vibr ...
. It is primarily suited for people who have conductive hearing losses,
unilateral hearing loss Unilateral hearing loss (UHL) is a type of hearing impairment where there is normal hearing in one ear and impaired hearing in the other ear. Signs and symptoms Patients with unilateral hearing loss have difficulty: * Hearing conversation on their ...
, 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 Hearing loss is a partial or total inability to Hearing, hear. Hearing loss may be present at birth or acquired at any time afterwards. Hearing loss may occur in one or both ears. In children, hearing problems can affect the ability to Language ...
are lack of function in the
inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...
(
cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory org ...
) and when the sound has problems in reaching the nerve cells of the inner ear. Examples 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.


Medical use

Bone-anchored hearing aids use a surgically implanted
abutment An abutment is the substructure at the ends of a bridge span or dam supporting its superstructure. Single-span bridges have abutments at each end which provide vertical and lateral support for the span, as well as acting as retaining walls ...
to transmit sound by direct conduction through bone to the inner ear, bypassing the external auditory canal and middle ear. A titanium prosthesis is surgically embedded into the skull with a small abutment exposed outside the skin. A sound processor sits on this abutment and transmits sound vibrations to the titanium implant. The implant vibrates the skull and inner ear, which stimulate the nerve fibers of the inner ear, allowing hearing. The surgery is often performed under local anesthesia and as an outpatient procedure. An important piece of information for patients is that if they for whatever reason are not satisfied with the BAHA solution, removing the implant is easy. No other ear surgical procedure is reversible like this. By bypassing the outer or middle ear, BAHA can increase hearing in noisy situations and help localise sounds. In addition to improved speech understanding, it results in a natural sound with less distortion and feedback compared with conventional hearing aids. The ear canal is left open for comfort, and helps to reduce any problems caused by chronic ear infections or allergies. In patients with single-sided sensorineural deafness, BAHA sends the sound by the skull bone from the deaf side to the inner ear of the hearing side. This transfer of sound gives a 360° sound awareness. BAHAs may facilitate normal speech development.


Chronic ear disease

This fairly common condition is often associated with continuous or intermittent drainage from the ear canal. These patients may also have a hearing loss and need amplification. A conventional air conduction aid with a mold placed in the ear canal opening may not be appropriate due to the drainage, and may even provoke drainage. If the hearing loss is significant an air conduction aid may have difficulty overcoming the dysfunction of the eardrum and middle ear bones. Bone conduction hearing device bypassing the middle ear may be a more appropriate treatment for these patients. Good transmission of sound in the bone, with reduced attenuation and distortion may be possible.


Single-sided deafness

A person with
unilateral hearing loss Unilateral hearing loss (UHL) is a type of hearing impairment where there is normal hearing in one ear and impaired hearing in the other ear. Signs and symptoms Patients with unilateral hearing loss have difficulty: * Hearing conversation on their ...
may have functional difficulty hearing even when the other ear is normal, particularly in demanding situations such as noisy environments and when several people are speaking the same time. A complication in single-sided deafness is hearing impairment in the hearing ear. Conventional ear surgery involves a risk of hearing loss due to the surgical procedure. Most ear surgeons are thus reluctant to perform surgery on an only hearing ear. The BAHA surgery avoids this risk and may be an appropriate treatment. An extended trial of a BAHA system with a headband prior to surgery led to more realistic expectations. In the trial, 50% of the candidates wished to proceed to surgery.


External ear canal problems

Irritation in the external ear canal due to inflammation or eczema may be a condition for which a conventional air conduction aid is not an appropriate treatment. Direct bone conduction may be an option.


Malformations

Patients with malformations are not always suitable for reconstructive surgery.
Treacher Collins syndrome Treacher Collins syndrome (TCS) is a genetic disorder characterized by deformities of the ears, eyes, cheekbones, and chin. The degree to which a person is affected, however, may vary from mild to severe. Complications may include breathing prob ...
patients may have significant malformations with ossicular defects and an abnormal route of the facial nerve. These structures, as well as the inner ear, could be in danger at surgery. Patients with
Down syndrome Down syndrome or Down's syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. It is usually associated with physical growth delays, mild to moderate intellectual dis ...
may have a narrow ear canal and middle ear malformation leading to impaired hearing. Some part of the cognitive delay seen in these children may be partly due to their poor hearing. The surgery can only take place once the skull is at least 2.5 mm thick. Children with certain syndromes may have a slighter build, thinner bone, or unusual anatomy. Other children may have a thicker skull at a younger age, so it is difficult to give a specific age for surgery. In the U.S., the
Food and Drug Administration The United States Food and Drug Administration (FDA or US FDA) is a List of United States federal agencies, federal agency of the United States Department of Health and Human Services, Department of Health and Human Services. The FDA is respon ...
only approves BAHA implantation of children aged five years or older. For infants and young children prior to surgery, the sound processor can be worn on a head band or soft band which the infant wears to hold it against the skull.


Side effects

Complications of BAHA systems can be considered as either related to the bone (hard tissue) or the
soft tissue Soft tissue is all the tissue in the body that is not hardened by the processes of ossification or calcification such as bones and teeth. Soft tissue connects, surrounds or supports internal organs and bones, and includes muscle, tendons, ligam ...
. * Bone ** Failure of osseointegration ** Chronic infection ** Trauma * Soft tissue ** Irritation of the skin surrounding the implant ** Skin flap
necrosis Necrosis () is a form of cell injury which results in the premature death of cells in living tissue by autolysis. Necrosis is caused by factors external to the cell or tissue, such as infection, or trauma which result in the unregulated dige ...
(death of the skin flap, e.g. due to interrupted blood supply) ** Overgrowth of skin over the device ** Wound dehiscence (splitting apart of the wound) ** Bleeding or
hematoma A hematoma, also spelled haematoma, or blood suffusion is a localized bleeding outside of blood vessels, due to either disease or trauma including injury or surgery and may involve blood continuing to seep from broken capillary, capillaries. A he ...
formation ** Persistent pain Soft-tissue complications are much more common, and most are managed with topical treatments. Children are more likely to develop both kinds of complications than adults. Sometimes, a second surgical procedure is required. Complications are less likely with good wound hygiene. Other drawbacks of BAHA include accidental or spontaneous loss of the bone implant, and patient refusal for treatment due to stigma.


Surgical procedure

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. The original surgical procedure has been described in detail by Tjellström et al. 2001.


Handling

An area where skin is penetrated requires care and cleaning because of the risk of inflammation around the abutment. Daily cleaning is required.


Small children

Hearing is of importance for a normal speech development. The skull bone in children is often very thin and softer than in the adult. Surgery is thus often delayed until the age of four to five years. In the meantime, the child with bilateral atresia can be fitted with a band around the head with a coupling for a BAHA. This may be done at the age of one month. Infants at this age may tolerate this well.


History

Patients with chronic ear infection where the drum and/or the small bones in the middle ear are damaged often have hearing loss, but difficulties in using a hearing aid fitted in the ear canal. Direct bone conduction through a vibrator attached to a skin-penetrating implant addresses these disadvantages. In 1977, the first three patients were implanted with a bone-conduction hearing solution by Anders Tjellström at the Ear, Nose, and Throat Department at
Sahlgrenska University Hospital The Sahlgrenska University Hospital ( Swedish: ''Sahlgrenska Universitetssjukhuset'') is a hospital network associated with the Sahlgrenska Academy at the University of Gothenburg in Gothenburg, Sweden. With 17,000 employees the hospital is the lar ...
in Gothenburg, Sweden. A 4-mm-long titanium screw with a diameter of 3.75 mm was inserted in the bone behind the ear, and a bone conduction hearing aid was attached. The term osseointegration was coined by Professor Brånemark. During animal studies, he found the bone tissue attached to the titanium implant without any soft tissue in between. He also showed an such an implant could take a heavy load. His definition of osseointegration was "direct contact between living bone and an implant that can take a load". The first clinical application of titanium was in oral surgery, where implants were used for retention of dentures. Brånemark sought an acoustic way to evaluate osseointegration. A patient with implants in the jaws was fitted with a bone vibrator on one of his implants. When tested, the patient experienced very loud sound even at low stimulation levels, indicating sound could propagate very well in the bone. It has later been shown by Håkansson that the sound transmission in bone is linear, indicating low distortion of the sound. The implant in the bone is made of titanium and will osseointegrate. The hearing instrument is impedance-matched.
Osseointegration Osseointegration (from Latin ''osseus'' " bony" and ''integrare'' "to make whole") is the direct structural and functional connection between living bone and the surface of a load-bearing artificial implant ("load-bearing" as defined by Albrekt ...
has been defined as the direct contact between living bone and an implant that can take a load, with no soft tissue at the interface.


Models

There are several models of bone-anchored hearing aids available. Most common are the BAHA from Australian company Cochlear, and the Ponto from Danish manufacturers Oticon Medical. There are also other types of bone conduction hearing aids, such as implants and non-surgical devices.


Costs

In the US, the cost of the Baha device is about $4,000. In the Netherlands, the cost of the device is around €3000 (in 2008). The cost of the titanium implant, surgery, and aftercare from surgeon and audiologist must also be considered.


References

{{reflist Hearing aids Otorhinolaryngology