Bone Conduction Aids

Hearing through bone conduction is normally only used where there is a significant conductive hearing loss or where it is not possible to use air conduction aids.

Bone conduction hearing aids are therefore used by much less than 1% of hearing aid users. Indeed bone conduction aids are very often used because there is no alternative and not because they are necessarily capable of giving the best performance. It should be remembered that bone conduction vibrators will excite both ears about equally no matter which side they are placed. In essence the bone conduction hearing aid is identical to an air conduction aid apart from its output where a bone vibrator is used in place of an earphone. However, the use of a bone vibrator places a number of requirements on the design of the hearing aid and limits the type of aid that can be worn; spectacle, headband or bone anchored aid.

The Bone Vibrator

The bone vibrator is mounted on a headband which must exert pressure on the head in order to maintain a good contact. A reduction in pressure is likely to produce a considerable falling off in the effectiveness of the device. The major problem with bone conduction is keeping the vibrator applied without causing discomfort.

It is normally worn behind the ear on the hairless part of the skull known as the 'mastoid process.' The effect may alter with quite small changes in position. They can be placed elsewhere on the head but there can be significant reduction in the level of sound perceived. This may not be a problem if the conductive hearing loss is such that there is sufficient power in the hearing aid to counteract this loss without undue distortion.

Bone vibrators, like earphones, are designed with electrical characteristics to match the outputs of specific hearing aids, therefore care has to be taken as to which particular bone vibrator is used. Manufacturers and suppliers will be able to recommend the appropriate vibrator.

Body Worn Aids

A body worn aid for use with a bone vibrator has to be of the high powered type due to the much higher power requirements compared with the air conduction earphone. Most high powered body worn aids will work satisfactorily with an appropriate bone vibrator.

Post Aural and In-the-Ear Aids

The normal manufacturers range of post aural and in-the-ear aids are designed solely for air conduction and cannot be purchased as bone conduction aids.

Spectacle Hearing Aids

Spectacles provide an ideal way for the bone vibrator to be held in place in an inconspicuous manner with the minimum discomfort.

The most widely available type is where the vibrator is housed in the back of the curved part of the arm of the spectacles and the microphone, battery and amplifier in the rest of the side arm. In some cases an aid can then be worn on both sides and in others, particularly with the more powerful aids, the amplifier is housed in the second arm. Spectacle aids of this type are expensive and require the cooperation of an optician and the hearing aid supplier for fitting.

Headband Hearing Aids

As the name implies headband hearing aids have the whole aid mounted on a band over the head. Often the microphone, battery and amplifier are housed on one side while the bone vibrator is on the other. These aids have the advantage of being worn in an inconspicuous manner but lack the higher output available from body worn aids.

All ages can wear headband bone conduction hearing aids as the headbands can be made to fit any size. One of the major problems is keeping the headband on the head. This can be helped by covering the band with textured fabric which will not slip on the hair, or tying tape around the back of the neck onto each side of the band.

Bone Anchored Hearing Aids

A method of fitting a titanium screw into the mastoid process making it possible to mount a vibrator on to it has been developed in Sweden. This makes a good system for transmitting sound into the head involving no pressure.

The Bone Anchored Hearing Aid

Traditional Bone Conduction Hearing Aids have a number of drawbacks. They can be uncomfortable and rather cumbersome. The bone conductor is kept in place either by a steel headband or by the spring loaded arm of a spectacle aid.

Headaches and soreness of the skin caused by the pressure from the vibrator against the skull are common problems along with keeping the headband on the head and in the correct position.

The Bone Anchored Hearing Aid uses the principle of osseointegration (bonding with the bone) to overcome these problems by attaching the aid to a small titanium screw which is implanted behind the ear.

Surgical Procedure and Treatment

The surgery is normally carried out in two stages and can be done under either local or general anaesthetic. Both operations are minor and take between 45 minutes and 1 hour to perform.

Stage One

During this operation a small titanium screw 3-4 mm long is implanted into the bone directly behind the ear. This screw is called an implant or fixture. The side chosen for the implant is usually the side with the best bone conduction but this is discussed with the doctor before the operation as there may be other considerations to be taken into account.

Stage Two

After 3-4 months the implant will have securely bonded with the bone. The second operation will now be carried out during which a titanium sleeve or abutment is connected to the implant through the skin. It is this which provides the direct connection between the hearing aid and the implant.
(Note: for adults in some cases the two stages may be combined into one procedure.)

What Happens Next?

The site of the surgery has to be dressed and cleaned in the outpatients clinic on a regular basis for at least two weeks after the second operation.

Approximately one month later the hearing aid can be fitted.
(In the case of a one-stage procedure there will be a slightly longer time between the surgery and the fitting of the hearing aid.)

The hearing aid is fitted by the Audiologist after an initial check-up by the doctor. At the first fitting the Audiologist will ensure that the patient is familiar with the volume and tone controls and will show how to connect and disconnect the hearing aid from the abutment.

Routine audiological tests follow after a few weeks and then regular check-ups take place initially after 4-6 months and then every 6-12 months.