Bone conductive hearing aids

Bone Conductive Glasses

Function

The microphone, processor and vibrator are situated along the arms of a pair of glasses. At the front, the microphone captures the sound and transmits it to the processor in the form of an electric signal. Then the processed and amplified sound activates the vibrator situated against the mastoid bone. The vibrator can be assimilated with a speaker that has had the membrane replaced by a plate that vibrates with the variations in electric current.

Indications

Although mainly designed to treat conductive hearing loss, this device can also be used for perceptual and mild or moderate mixed hearing losses where conventional air conduction hearing aids cannot be used (aplasia of the pinna and/or ear canal, chronic otorrhea, advances otospongiosis, otosclerosis or cholesteatoma)

Advantages and Disadvantages

The main advantage of this system is that it is completely non-invasive and easy to use. However, signal processing, the bandwidth and the power of the vibrator are all limited. Furthermore, the efficacy of these devices depends highly upon having good contact with the bone, the thickness of the overlying skin, and the pressure that the device can exert on the bone.

Bone Anchored Hearing Aids (BAHA)

Function

A titanium screw of a few millimeters in length is placed into the mastoid bone. BAHAs are composed of two components: the fixture anchored to the bone, onto which the abutment, which sticks out of the skin, is affixed. The microphone and microprocessor are connected to the abutment. In this way, the sound, transformed to processed vibrations, is transmitted directly to the inner ear via the abutment and the screw.

Indications

Like bone conduction glasses, BAHAs are only used when there is no benefit to using conventional air conduction hearing aids. However, some cases of conductive, perceptual or mild or moderate mixed hearing losses can be well treated with this device. It can be used in adults, but also in children from five years of age, who use a headband to maintain good contact between the vibrator and the bone.

BAHAs can also be used when surgery can either not be carried out in the middle ear, or would not provide any benefit to the patient. Finally, in the case of severe or profound unilateral hearing loss, the Contralateral Routing of Signals (CROS) system can be used to provide complementary information to the healthy contralateral ear.

Advantages and Disadvantages

The use of a titanium abutment reduces the risk of skin infection, and the fact that it is anchored into the bone limits the constraints linked to the thickness of the skin and the quality of the contact between the device and the bone. The use of a BAHA therefore confers improved sound quality and loudness over bone conductive glasses.

The main disadvantage is the need for surgery, although it is minor, and the need to take continual precautions to avoid infection. Furthermore, although the bandwidth and processing abilities are greater than in the non-invasive device described above, they remain limited.

Middle Ear Implants

Function

Like the two devices described above, sound is capture by an external microphone and treated by a sound processor. The difference is that the electric signal is relayed to a vibrator that can be one of two types:

	Electromagnetic: When a coil surrounding a magnet is placed under electric tension, the induced magnetic forces cause the coil to move. In fact, when the current passes in one direction, the coil moves to the outside, and when the current changes direction the coil travels in the other direction. This is the same principle that is used in electrodynamic loudspeakers.
	Piezoelectric: A piezoelectric element with the intrinsic property of deforming when placed under electric tension is used. By changing the current, the deformation (and therefore vibration) can be controlled.

These vibrators can be affixed at different locations along the ossicular chain: malleus, incus or to a piston if the stapes has been removed. In this way, the induced vibration amplifies the natural movements of the ossicular chain.

Indications

When conventional hearing aids cannot be used or will not show any benefit, the middle ear implant can be used as an alternative. It can provide relief to hearing impaired adults that have bilateral conductive, perceptual, or middle or severe mixed hearing losses that still have an intact ossicular chain.

Advantages and Disadvantages

The main advantage of the middle ear implant is that it amplifies the natural movement of the ossicular chain. As a result, the quality of the re-established hearing is improved over bone conduction devices.

However, the main disadvantage is the need for surgical intervention to affix the vibrator onto the ossicular chain, and the need for the latter to be intact for the device to work.