ELECTRONIC
IMPLANTABLE HEARING DEVICES
BAHA
With bone conduction devices
the sound is transmitted to the cochlea basically independently of the external
and middle ear.
With the introduction of the
BAHA, a new type of bone conduction excitation was introduced, namely, direct
bone conduction.
Hearing through direct bone
conduction is defined as "sound transmission via bone conduction without the
skin and soft tissue being part of the vibration transmission path between the
transducer and the skull bone.
Direct bone conduction
provides a more sensitive input for vibrations to the skull and also an improved
comfort.
A number of different
biomechanical investigations have described the improvements in hearing obtained
by direct bone conduction.
It has been pointed out that
hearing by bone conduction is a natural way of hearing. When listening to one's
own voice, one is listening to both airborne and bone-conducted sound. The
portion of one's own voice transmitted by bone conduction has been estimated to
be of the same order of magnitude as the airborne component.
Most people do not recognize
their own voices on a tape recorder, because the tape recorder records only the
airborne component of the voice.
This observation indicates
that the quality of bone-conducted sound is quite good, because most people do
not hear their own voice as distorted.
The air-bone cancellation
experiment, first made by von Bekesy in 1960, further indicates that
bone-conducted-sound is normal sound.
This experiment showed that a
pure tone transmitted by bone conduction could be cancelled by simultaneously
presenting an airborne tone of the same frequency and loudness level;
cancellation occurred at a certain phase relation between the two tones. The
conclusion is that, at the basilar membrane level and for the frequencies
tested, it makes no difference whether the excitation originates from the
airborne or the bone conducted sound.
In 1966 Tonndorf demonstrated
that bone-conducted sound is transmitted
to
the inner ear by three basic modes of excitation:
-
inertial movement of middle
ear ossicles and inner ear fluids,
-
compression of the cochlear
shell, and
-
sound radiation from the
vibrations in the skull to the external and middle ear spaces.
Dynamic
range:
The difference between the direct bone conduction threshold and the maximum
output of the device. For a particular patient, the dynamic range is reduced by
the same amount as the degree of sensorineural hearing loss.
Functional gain:
In
a bone conduction device, the functional gain is determined not only by the
device performance but also by the degree of conduction hearing loss (ABG). FG
is minimal in a patient with ABG of 0, & maximal in a patient with an ABG of
60dB.
PRINCIPAL DESIGN OF THE B A H A
Over the years, the design presented in the picture has shown to be valid.
It
consists of :
-
a conventional microphone
and amplifier,
-
a specially deigned
transducer and
-
a coupling arrangement to
attach the device to the skin-penetrating and bone-anchored implant.
The Basic Ear-Level
Device-BAHA Classic 300
The BAHA Classic 300 has a
design characterized by an anatomic positioning of the volume control, as
illustrated in Figure .
It has a gain control and a
continuous tone control for low frequencies and a three-position switch (N =
normal, L = low, and E = electrical) for the selection of two different
high-frequency responses.
With the switch in the E
position, the internal microphone is disconnected, and only the electrical input
is active. Through the electrical input, external sound sources such as a
tele-coil unit or a battery-operated tape recorder can be connected.
The lifetime for using
zinc-air batteries (type 675) is approximately 1 to 3 weeks, depending on the
volume setting and the surrounding sound level.
The outer dimensions are
approximately 33 x 22 x 11 mm, and the weight (including battery) is 16 g.
The maximal recommended bone
conduction threshold is 45 dB hearing level (HL), taken as an average of 0.5,
1,2, and 3 kHz.
The Body-worn Device-BAHA
Cordelle
The need for a more powerful
device soon became evident, and the body-worn device, the BAHA Cordelle, was
designed.
The output of this device
exceeds that of the Classic 300 by 10 to 15 dB at lower frequencies and by 5 to
7 dB at higher frequencies, as shown in Figure.
Also, the resonance frequency
is shifted from approximately 1000 Hz for the BAHA Classic to approximately 750
Hz for the BAHA Cordelle.
This lower resonance frequency
was chosen because patients with severe sensorineural hearing losses usually
have best residual hearing capacity in the low-frequency region. The processor
uses a K-amp preamplifier, a specially designed power amplifier, and a 9V
rechargeable battery as power source.
A Miniaturized Devices
are now available.
The Test Rod
The
test rod is a plastic bar to which a bayonet or snap coupling is attached at one
end. It is intended for (1) preoperative assessment of a candidate, (2)
education and demonstrations, and (3) quality control of any BAHA device. The
flat end of the bar can be pressed against the skin over the temporal bone
behind the outer ear or positioned between the teeth (with the patient biting on
it) so that the patient can hear with the BAHA device.
If a possible candidate
experiences acceptable sound when biting or pressing the test rod on the skin
over the mastoid, this patient should be able to wear a BAHA successfully. It is
sometimes difficult to judge from an audiogram which side is preferable for
placement of the implant. By simply pressing the test rod first on the left side
and then on the right side of the skull while listening to some samples of
sound, one can determine the preferred side.
CONTRAINDICATIONS AND
SELECTION OF PATIENTS
The most important issue in
achieving a good result is selecting the patients carefully and being aware of
the limitations of the devices and the contraindications for their use.
Clinical Indications
Chronic Ear Disease
The authors have found that
patients with a draining ear in need of amplification compose the largest group
for whom these devices are indicated.
Patients with a chronic ear
condition which starts to drain when an ear mold is placed in the external ear
opening are also suitable candidates.
Patients who have had radical
surgery with meatoplasty often experience acoustic feedback.
It is difficult to make a mold
which will seal the meatus completely, especially because these patients often
need high amplification.
Ear Canal Atresia
Atresia surgery is one of the
most challenging otologic procedures. This surgery involves a significant risk
in the hands of the less experienced surgeon. In spite of initial good results,
there is also a tendency for re-stenosis. In bilateral cases the BAHA is an
excellent alternative and there are reports that fitting could be of advantage
in unilateral cases. The ability to listen in noisy surroundings and to localize
sound may be improved.
Maximal Conductive Loss
in the Only Hearing Ear
Ear surgery always involves a
risk of cochlear damage. A dead ear after ear surgery could indicate that the
patient has less robust inner ear function. Surgery on the other ear could
involve a greater risk. In patients with a maximal conductive loss (e.g., in
patients with otosclerosis), it is difficult to overcome the air-bone gap using
an air conduction hearing aid. Because the air-bone gap is of no importance with
the BAHA, the results in these patients are quite good.
External Ear Canal
Problems
Some patients who have air
conduction hearing aids have problems with ear canal irritation. Although they
may have tried molds made of many different materials and ventilated ear molds,
these patients experience problems with itching and moisture and can use the
hearing aid only during short periods of the day.
Discomfort Caused by the
Occlusion Effect
Some patients wearing an
ordinary air conduction hearing aid feel that one of the most disturbing
problems is caused by the occlusion of the external ear canal. This problem is
difficult to address with a conventional air conduction hearing aid and is, of
course, solved with a BAHA.
Audiologic indications:
It has been shown that if the
bone conduction threshold is 45dB or better, more than 80% of the patients are
satisfied using this device.
The better the cochlear
reserve, the better are the chances for a satisfied patient.
The propagation of the sound
through the skull is quite complex , and the attenuation of the sound varies
with frequency and shape of the skull.
CONTRAINDICATION
Psychiatric disease,
Immature personality,
Drug and alcohol abuse,
Inability to follow given
instructions,
Inability to maintain adequate
hygiene (relativeCI), because of adverse skin reaction.
Diabetes, psoriasis, … not
included.
SURGICAL PROCEDURE
Initially a two-stage
procedure was recommended.
The insertion of the implant
in the bone was followed by a 3-month healing period when osseointegration was
allowed to take place.
The rate of implant losses in
the mastoid process was quite low, and the load of the implant caused by the
hearing instrument was also low.
These low rates led to a
5-year study comparing the traditional two-stage procedure and a one-stage
procedure.
Studies showed no
statistically significant difference between the two, and today a one-stage
procedure is recommended in adults.
In children a two-stage
procedure is used;
The bony implant, the fixture,
is the same as when the technique was introduced in 1977, but the coupling is
new.
The current surgical technique
used by Tjellstrom is described here in some detail.
The implantation is much the
same among the experienced surgeons working with the system, but the extremely
important soft tissue work varies.
The end result should always
be a very thin, hairless skin at the penetration site.
Marking and Draping
In adults the surgery is
generally performed under local anesthesia as an out- patient procedure. Before
the patient is draped, the skin is marked to indicate the intended implant site
with the help of a dummy BAHA .
The implant should not
be positioned so that the hearing instrument will touch the pinna. This contact
could cause acoustic feedback. When the mark has been made, the external ear is
folded anteriorly, and the implant site is covered with a plastic sheet
Incision
A
U-shaped incision, 2.5 cm in diameter, is made with the attachment anteriorly.
The periosteum is exposed. A hole is made in the periosteum approximately 6 mm
in diameter to accommodate the 5.5-mm diameter flange of the fixture.
Drilling
The drilling starts with a
guide drill with a safety guard allowing only 3 mm of penetration.
The drill speed is 1500 rpm.
Generous cooling is always applied during all drilling, tapping, and implant
insertion to reduce heat trauma that could jeopardize osseointegration and
result in a fibrous encapsulation of the implant.
Such an attachment involves a
great risk of implant failure when loaded. The hole is gradually made larger to
provide good visibility at the bottom of the implant seat.
Tapping
The tapping is made with the
titanium tap which is sterile-packed in a glass container.
Fixture Insertion
The implant also is
sterile-packed in a glass container. The fixture is placed in one of the small
tubes on the titanium organizer.
Subcutaneous Tissue
Reduction
For a long-lasting,
reaction-free skin penetration, the subcutaneous tissue reduction is of great
importance.
The skin flap is thinned; the
result should be a flap without any hair follicles and with the thickness of a
split-thickness skin graft.
It is better that the flap be
too thin than too thick. The soft tissue around the implant site is removed to
allow the skin to slope nicely down to the implant area.
Attaching the Coupling
When the flap is in place a
4-mm disposable dermatologic punch is used to make a hole exactly over the
fixture.
Healing Cap and Draping
A plastic healing cap is
placed on the coupling, and ointment-soaked gauze is placed under the healing
cap to avoid postoperative hematoma or swelling. It is
important that the gauze not
be too tight.
Postoperative Handling
6 weeks after surgery, the
hearing instrument (the BAHA) can be attached to the coupling and adjusted
according to the patient's hearing loss. The patient is instructed to clean the
area daily with soap and water and to use some mild ointment during the first
months after surgery. After that, most patients use ointment only occasionally
or if some skin irritation should occur.
CLINICAL AUDIOLOGY
Warble tone thresholds have
been measured with the BAHA and compared with the unaided situation. The
difference was highly significant (P < 0.001), and the mean difference was 30
dB.
When the bone-anchored device
is compared with a conventional bone transducer, however, the BAHA was better in
all frequencies except 500 Hz.
When the threshold levels are
measured in a quiet environment, significant differences between the devices
would not be expected, because the patient adjusts the hearing aid at the most
comfortable level.
Word discrimination in a noisy
environment was better with the BAHA than with the conventional bone transducer
in this study. The difference of 6.2 dB was significant. Cremers and co-workers
and van der Pouw et al have verified these results.
