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As printed in Hearing Health, volume 19:2,
Summer 2003
By Vasant Kolpe, Ph.D.
Earmolds are changing. Long an integral part of the
management of hearing loss, they couple behind-the-ear
(BTE) and in-the-ear (ITE) hearing aids with the ear
and seal the ear canal, delivering amplified sound and
preventing it from escaping. A well-fit earmold accomplishes
its task without feedback, a squealing sound that is
uncomfortable if not painful for the person wearing
the aid as well as others nearby. The challenge of the
clinician is to provide the user with a secure fit to
avoid this occurrence without compromising wearing comfort.
Innovations over the last decade include the creation
of new products for making ear impressions and processes
for the production of customized earmolds. In addition,
the advent of "compliant" materials provides
an alternate method of sound delivery through a new
category of products - acoustic couplers.
Making an Impression
Recent advances in earmolds are welcome but there are
still no automatic solutions for achieving both effective
sound delivery and an acceptable comfort level for each
hearing aid user. This is partly due to the uniqueness
of ear canals. They are, in fact, as individual as fingerprints;
they vary in size, shape and some even have unusual
bends.
Poor fitting earmolds have frequently been the source
of discomfort and frustration since their introduction
over a century ago when plaster of Paris was used to
make "casts" or impressions of the ear canal.
The dawn of polymer science in the 1920s encouraged
the use of a solution of acrylic in a "powder and
liquid" impression material. Now, we have state
of the art silicone compounds that are similar to those
used in the dental office for making tooth and jaw impressions.
The first step to having a custom-made earmold is when
your clinician inserts a small plug of cotton into your
ear to protect your eardrum and uses a syringe to fill
your ear canal and outer ear with a soft plastic. After
several minutes, the plastic hardens and is removed.
The impression is then sent to a lab to provide the
model for your earmold that will be manufactured in
a material indicated by your hearing healthcare provider.
Impressions are a static measurement or "still
snapshot" of your ear canal. It is critical to
note, however, that the ear canal changes in response
to normal jaw motion as well as head position and posture,
time of day when the impression is made and whether
the person is wearing a hearing aid immediately before
the measurements are made. Changes in the position of
the jaw can cause a variance in canal diameter of more
than 30 percent in some people. All of these variables
can contribute to problems of fit in the earmold based
on the ear impression, no matter how much care is taken
in the impression-making process.
Another possible contributor to poor fit is poor laboratory
replication of impressions. Laser sintering, a technology
developed and released in the last few years to address
this problem, uses lasers to scan the ear impression.
The data from the scanning is then processed to produce
an earmold by using thermal heat generated by laser
irradiation to "melt grow" a polymer coating
layer by layer.
Soon this technology will advance to taking an impression
by directly scanning the ear canal and electronically
transmitting the resulting measurements and data to
the laser sintering system at the manufacturer's location.
Although this method will speed up the delivery of hearing
aids, again it provides a still snapshot rather than
a video-like picture of the ear canal.
Sealing the Leaks
Even with the elimination of shrinkage in impression-making
through the use of modern materials and the assistance
of laser-based systems in the production process, the
incidence of poorly fitting earmolds and hearing aid
shells has essentially stayed at the same rate, approximately
20 percent, throughout the last 10 years. Findings of
a 2002 survey of hearing aid users highlight the effect
of this continued problem by revealing that feedback
due to a poor physical fit is the third most common
dissatisfaction with hearing aids. This points to the
conclusion that hearing science and industry must focus
on the prevention of sound leakage from the ear canal,
a difficult task when it is constantly changing in shape
and volume with jaw motion.
With traditional custom earmolds, the solution possibilities
are to use more viscous impression materials, take open
jaw impressions and/or request that the earmold laboratory
coat the ear impressions with extra wax layers. An alternative
comes from a newcomer in the field: compliant acoustic
couplers utilizing ready-made sound sealing material
- viscoelastic polyurethane foam - that expands and
contracts as the canal size increases and decreases,
maintaining a seal by keeping up with the activity in
the ear canal related to jaw movement. It is also very
comfortable as it is as much as 30 times softer than
a silicone earmold at body temperature. The use of compliant
couplers with both BTE and ITE hearing aids is growing
quickly in Japan and in the U.K. and their recent entry
into the U.S. marketplace is garnering attention.
Compliant couplers are currently available in two forms,
both developed by Hearing Components, Inc. Comply Snap
Tips are made of high-tech slow recovery foam that requires
no impressions, no re-makes and no wait time. They attach
to the end of the sound tubing between a BTE and the
ear canal in place of a custom earmold or fit around
the end of an ITE shell. A built-in wax guard helps
keep the sound tube clear and the hearing aid user changes
the tip about every two weeks to give optimum performance
and hygiene. Overall cost is about the same as traditional
earmolds.
While offering a fitting solution for some hearing
aid wearers, compliant acoustic couplers are not for
everyone. Some patients with limited dexterity and vision
may find them difficult to use and the tip attachments
are not recommended for children under five years of
age.
Another form of compliant coupler does work well with
small children. Comply Soft Wraps are thin foam strips
with adhesive backing that can be wrapped around standard
pre-bent tubing connected to the hearing aid. The wrap
is squeezed tightly so that after the tubing is gently
placed in the ear canal and as the compliant material
recovers, it creates an active seal.
Infants from birth to approximately 9-months of age
experience rapid ear canal growth and custom earmolds
require frequent replacement. With the compliant foam
alternative, a thicker wrapping controls feedback as
the ear grows.
Although the foam wraps do not work for all infants,
the parents of those who can wear them can easily and
safely manage this acoustic coupler and clinicians can
immediately fit infants with a hearing aid without the
need for impressions and earmold production.
Potential for managing hearing loss is greater than
ever with today's hearing aids. It remains true, however,
that hearing aid users' success and satisfaction do
not hinge solely on the sound processing technology
in their instruments. Earmolds and other methods of
sound delivery play a key role.
The evolution of earmolds continues and the rewards
are clear each time a comfortable fit with quality sound
transmission leads a hearing aid consumer to become
a consistent and satisfied user and re-enter the hearing
world.
Dr. Vasant Kolpe has a Ph.D. in polymer science from
the Univ. of Akron and is currently
company scientist at Hearing Components, Inc. He worked
as a materials researcher at 3M Corporation for 37 years,
holds numerous patents and can be contacted by writing
vkolpe@hearingcomponents.com.
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Earmolds 101
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