Bone Conduction Hearing Aids/BAHA� (Bone Anchored Hearing Aids) Home

Bone Conduction Hearing Aids

A bone conduction hearing aid is different from a conventional air conduction hearing aid. A conventional hearing aid amplifies sound and passes it through the earmold, into the ear canal, through the middle ear and to the inner ear. This type of hearing aid is not appropriate for a child with Atresia, as the ear canal is blocked and the sound cannot get through. The sound must be transmitted in a different manner, through "bone conduction."

A bone conduction hearing aid consists of a typical hearing aid that has been modified. Instead of sending the sound out through the earmold, the signal travels through a wire leading to an oscillator, which vibrates. When this oscillator is placed against the skull (usually behind the ear on the mastoid bone), the sound vibration is carried through the skull bone to both inner ears.


As with any other hearing aid, the quality of sound from a bone conduction hearing aid is not perfect. Both the dynamic range and the frequency range are limited, resulting in sound similar to listening through a telephone. Another disadvantage, which is specific to hearing by "bone conduction," is that the listener hears in "mono." When the bone is vibrated in any location, the sound travels to both inner ears at almost the same time. This causes significant difficulty in determining the location of the source of a sound.

Our Son's Bone Conduction Hearing Aid

Our son has worn his hearing aid three different ways. When we first got it, it came with a rigid metal headband, with the hearing aid on one side and the oscillator on the other, connected by a wire.

Since our son was so young when he first got his hearing aid, it was suggested that we remove it from the metal headband and use a cloth headband instead. The audiologist thought that the metal headband might be too tight and uncomfortable for a baby. She also showed us an article that suggested that the metal headband could put too much pressure on a baby's skull, which was still not completely formed. The article postulated that this could create a "dent" in the skull or an area of increased bone density, which would make the hearing aid less effective. We have not seen any additional literature supporting or refuting this.

So, we decided to sew a cloth band, with a loop for the hearing aid to rest in (assisted by velcro). The oscillator was attached to our son's skin with a small piece of double sided wig tape, and to the headband with a small loop of scotch tape (the band would slip off of the smooth oscillator without it). Check out the pictures below.

We should also mention that we had our son's hearing aid modified slightly. We were told that with a bone conduction hearing aid, the wire is quite vulnerable and likely to become broken. The wire was connected directly to the hearing aid, though, so whenever the wire stopped working properly (resulting in an intermittent or crackly sound) we had to send away the whole hearing aid for repair. We had the aid modified so there was a detachable connector between the wire and the hearing aid, similar to that which was already connecting the wire to the oscillator. This way, we are able to have spare wires on hand, and replace them ourselves (see the pictures below). The extra wires we ordered are shorter than the typical wires, as they only have to be long enough to go behind our son's head, rather than along the length of the metal band. The longer wires we had originally used to dangle, tickling the back of his neck and getting caught on things, pulling his hearing aid off.

This hearing aid set-up may sound complicated, but really it worked quite well and we used it for three years. Minor drawbacks included the tendency for the headband to get rubbed off, pulling off the whole hearing aid, and the fact that many people thought our son was a girl.

We changed our son's set-up when he was three years old. An audiologist had suggested that his one ear might be just big enough to hold an earmold to hang the hearing aid from. So we had an earmold made, similar to what is used for a typical air conduction hearing aid. It was not used to conduct sound into the ear canal, just to hold the hearing aid in place. It stays in our son's ear quite well, and the oscillator is held in place with wig tape alone on the opposite side: no headband. This arrangement seems to be more comfortable, it doesn't fall off as often, and looks nicer for a boy. (See pictures below)

This setup does not put any pressure on the oscillator, but also has nothing touching the oscillator, so little of the energy is lost. Hearing tests comparing this to the traditional headband show slightly poorer hearing thresholds at two of the lower frequencies tested (by 5 to 10 dB) but slightly better thresholds at one of the higher frequencies (by 5 to 10 dB). Given the fact that this set-up is much more comfortable than the metal headband, and that our son uses an FM system at school, the audiologist suggested continuing with this arrangement for now. Our next step will be to pursue a BAHA� for him.

BAHA� (Bone Anchored Hearing Aids)

The BAHA, Bone Anchored Hearing Aid, was developed in Sweden, and has apparently been used for many years with great success. It has been offered in North America for many years for adults, but it is fairly recent that it has been accepted for children. Simply put, the BAHA� makes use of a titanium "abutment" that has been surgically implanted behind the patients ear. The hearing aid then clips onto this abutment, and transmits the sound by bone conduction, just like the normal bone conduction aid. The advantage is that the vibration is transmitted directly to the bone, instead of through the skin. This direct contact translates into several advantages: a lower power hearing aid can be used, the direct connection reduces distortion in the sound, and a wider frequency range can be produced. If we say the bone conduction aid sounds like a telephone, then the BAHA� would sound more like a good quality FM radio (tuned into a clear station of course).

Pictures from the Entific Web Site

The youngest recommended age for a child to receive a BAHA� seems to vary depending on the Country and the Dr. performing the surgery. We have hear of a child as young as 3 years old getting a BAHA. Our doctor (Dr. DiToppa, Edmonton, Alberta, Canada) suggests a minimum age of 6 years old. The main reason to wait is so there is sufficient mastoid bone thickness to hold the abutment. If the bone is a little thin, the surgeon can add some Gortex gauze to stimulate the bone to grow thicker after the surgery. Our son got the first stage of the BAHA procedure done just after he turned 7. The bone was a little thin, and therefore he added some of the gauze.

Another point about the BAHA that seems to vary is whether a two-stage procedure or a one-stage procedure is performed. Adults almost always get the one-stage procedure. In Canada, it seems that children always get the two-stage procedure. We have heard of children getting the one-stage procedure, but these are all in the US. In the first stage, a titanium screw is implanted, and then covered by the skin. This is left to osseointegrate with the bone for up to 6 months. The second stage attaches the abutment to the integrated screw. This abutment sticks through the skin, and the BAHA is attached to it.

In our sons case, the cost of surgery and the BAHA were both covered by the medical system. However, when this aid wears out, we will be on the hook for the replacement. Currently, the BAHA Classic (See it here) is going for $3100CAD. The BAHA Compact (See it here) is the newest version of the BAHA, and costs ~20% more.

The latest development in BAHA�s, is getting them bilaterally. The FCC has approved it, and we have talked to a few adults with bilaterally fitted BAHA�s. There have been studies recently released showing how bilateral fittings help in noisy situations, and enable the wearer to localize sounds, even when they have used a monaural bone conduction aid all their life.

We found some useful information at:

The Patients BAHA web site http://www.patients-baha.com

A Family from Belgium where two of their three boys have BAHA's, http://users.pandora.be/microtia/

Entific: A company that manufactures the BAHA http://www.entific.com

 

Bone Conduction Phones:

http://www.globe-mart.com/comm/phones/specialneeds/MIRAFONE.htm

http://www.temco-j.co.jp/html/English/HG16.html

 

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