| The
Bone Anchored Hearing Aid is based upon a discovery made by
Dr. Per-Ingemar Branemark almost 30 years ago. While he was
working on orthopaedic prostheses he discovered that titanium
metal fused directly with bone. There is minimal skin reaction
and the metal can protrude through the skin. He has spent the
remainder of his life promoting osseointegration of titanium
to bone in relation to prosthetic use. In patients with false
teeth it is particularly effective for holding the teeth to
the jaw. It allows reconstructive materials to be attached directly
to the bone and, for example, allows false noses and eyes to
be held in good position in patients who have had large resections
of face and head for cancer.
As a separate
development, osseointegration of titanium has been used to
allow a titanium peg to fuse to the skull near the ear and
an abutment from this protrudes through the skin. A hearing
device similar to a hearing aid is attached to this abutment.
This allows direct vibration from the hearing device to the
bone of the skull. This bypasses the conductive mechanism
of eardrum and ear bones and stimulates the cochlea of the
inner ear directly. The direct coupling of the device to bone
is much more efficient from an acoustic and power requirement
standpoint than traditional bone conducting aids. More sound
energy is transferred at the higher frequencies. Thus it is
possible to gain significant amplification with poorer cochlear
function (nerve hearing). Good amplification is possible with
bone conduction hearing reserves at 2 and 4K of 60 decibels.
The system
is useful in hearing impaired patients with bilateral discharging
mastoid cavities, bilateral chronic otitis externa which weeps
and prevents use of a standard air conduction hearing aid,
and for patients who have congenital absence of the ear canal.
It can be used on children as young as five years of age.
The specific
disadvantage of the system is that there is a protruding metal
abutment behind the ear. The hearing device is attached directly
to it by a plastic button, which detaches if there is any
trauma, so that the abutment passing through to the skull
is not avulsed or injured. Long term, 95% of the patients
have no significant skin reaction around the implant.
The operation
is a single stage procedure in which a titanium screw is inserted
behind the ear under local anaesthetic. A split thickness
skin graft from behind the ear is used to cover the internal
screw and the abutment is attached. After three-months of
healing the hearing device can be worn. There is a simple
insert and twist connection so removal for sleep is easy.
The hearing
aid is a square box approximately 0.75 x 2 x 3 cms. Colour
varies from either beige to black and the components allow
certain alterations of amplification depending on the
patient’s
specific hearing deficit. As previously noted, use of this
device is limited to patients where a conventional air conduction
hearing aid cannot be used. Cost is also a limiting factor.
Unfortunately the system is not fully covered under BC Medicare.
The internal screw is covered but the abutment and hearing
device are not. Over 800 have been done in Sweden. For more
information on the BAHA, please refer to www.entific.com.
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What
Happens in a Bone Anchored Hearing Aid Operation
|
Bone Anchored Hearing Aid |
- 1.
skull bone
- 2.
soft tissue
- 3.
titanium screw
- 4.
titanium abutment
- 5.
coupling
- 6.
transducer
- 7.
housing
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| Indications
for Bone Anchored Hearing Aid |
|
1.
Congenital or acquired ear canal/middle ear anomalies.
2.
Chronic Otitis Media where conventional aids increase
infection rate.
3.
Otosclerosis where conventional aids or surgery are
not options.
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