Forum Proceedings

Summary

Manufacturers, researchers and clinicians have identified computerized ear canal measurement (e.g. 3D laser scanning), automated earmold production (e.g. one-day turnaround, computer automated), earmold materials (e.g. reverse thermal gels) and designs (multi-material, pneumatic, etc.) as addressing high-priority needs for persons with hearing impairments, clinicians and production technicians. Technology solutions represent good business opportunities for manufacturers.

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Market

It is estimated that more than 20 million people in the United States experience some form of hearing loss.  However, according to the 1990-91 National Health Survey, only 18% of those who identified themselves as having hearing problems use hearing aids (over the age of three and non-institutionalized). The reasons people who experience hearing loss but chose not to use the available technologies include: "hearing aids do not perform in noisy situations" (7.1 million), "provide too much whistle or feedback" (6.4 million), "do not work well" (4.8 million) or "work only in limited situations" (4.3 million), "have poor sound quality" (3.9 million), "break down too much" (3.4 million), "can not be used on the telephone" (3.1 million), and "negative experiences of friends" (3.9 million) (Kochkin, 1997).

The earmold provides several basic functions. First, it couples the hearing aid with the user's ear. It channels the sound from the hearing aid, through the ear canal, to the eardrum. The earmold also helps to secure the hearing aid in place. The challenge is to provide the user with a secure fit. Yet the tighter the fit, the more uncomfortable the device is to wear. A well-fitted earmold directs sound from the hearing aid to the ear without feedback, thus allowing the user to hear comfortably (Lachapelle, 1999). Earmolds are required for all hearing aids, and since the anatomical structure of the ear varies from person to person, the majority (80%) of all earmolds are custom-made.

Feedback is experienced by 6.4 million hearing aid users. There are two types of acoustic feedback: (1) produced internally from the hearing aid, indicating a need for repair; and (2) the more common cause, externally produced feedback due to leakage of amplified sound, that radiates from the speaker and then is picked up by the microphone and re-amplified.  In many cases, the feedback can be addressed by either repositioning the hearing aid or by reshaping the earmold so that its fit conforms more closely to the shape of the ear canal (Smedley & Schow, 1998; Sweetow, 1998).

Feedback occurs when the hearing aid does not fit properly and the output signal leaks around the earmold, is received by the hearing aid microphone, and is amplified. Other causes of feedback include the vents that are drilled into them. Vents are used to reduce the "plugged up" feeling experienced when the user speaks. However, the vent also provides an opening for the sound to create feedback within the hearing aid. At high amplification the output signal can again be picked up by the hearing aid microphone and be amplified. Users who experience significant feedback will adjust the hearing instrument's gain, or will turn it off completely. In the worst-case situation, the hearing aid user will stop wearing the device all together.

There is a need to improve the comfort of earmolds while maintaining the secure fit necessary for proper hearing aid function, including the reduction of acoustic feedback. Chewing, yawning, and other facial movements change the geometry of the ear canal structure. As the anatomic structure changes, the fit of the earmold is affected causing an increase in acoustic feedback. The hearing instrument may dislodge from the ear if the ear canal's shape is changed.

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Current Technology

Health care professionals must evaluate each person individually as to the material and style selection of earmolds to best meet their needs and to ensure the highest success rate with the hearing aid. Issues to consider include:

• whether the user is active or sedentary in their lifestyle,
• user dexterity (for example, the persons ability to handle hearing aid insertion, daily care, and cleaning of the earmold),
• the anatomy of the individual's ear and the affect it has on the choice of material or style of hearing aids that are to be used (anatomic considerations when choosing a hearing aid include: a deformed outer ear, the depth of the concha, whether ear canal is of sufficient diameter and whether there is a sharp enough bend to hold the hearing aid),
• growth changes, (in particular children),
• changes in morphology of the ear canal as it slowly adapts to the continuous pressure of the device (continuous pressure may cause the area to expand slightly),
• amplification objectives of the fitting,
• toxicity or allergies to plastics,
• appearance -- color selection, hearing aid style and earmold design options, and
• the number of modifications that may be required after delivery of the device (Microsonic, 2000b).

Custom modification of the earmold aids in the overall fit (comfort & security) and minimizes acoustic feedback.  These custom designs are handmade which is time consuming and costly. Yet, the time taken to properly fit the ear canal in the beginning may reduce the need for modification to the earmold shell later.

Technology for the hearing industry has been transferred from the dental industry, however material requirements for dental and hearing applications differ. Earmold material (1) must be flexible and accommodate changes in the ear canal throughout the day. For example, after being worn a few hours, the earmold will cause the ear canal to stretch. (2) The ear canal is flexible, warm and moist and has many sharp angles and changes of direction. This same flexible material must provide a snug secure fit without causing irritation. (3) Creating the earmold shape depends upon measurements where the clinician cannot see the actual anatomy where the product will be used.

Impressions are a static measurement (snapshot) while the ear canal constantly changes in response to such factors as head position, jaw movement, time of day when the impression is made, posture and whether the person was wearing a hearing aid just prior to the measurements being taken. Changing the position of the jaw can cause a variance in canal diameter of more than 30% depending on whether the mouth is opened or closed.  It is best to make more than one impression.

Casting materials and impression making techniques differ somewhat between clinicians and from client to client. The technician's techniques for making earmolds from these impressions depend upon their training. "This process [from making the impression to a finished earmold] is considered by industry insiders as an "art form" rather than a science which cannot be repeated consistently. [For example,] ten impressions [for the same client] made by ten clinicians would yield ten very different earmolds" (Stakeholder Forum Participant, 2000).

Casting material shrinks as it hardens.  The length of time between when the impression is made by the clinician versus when the earmold is produced by the technician will vary greatly by days.  The amount of shrinkage depends upon the time difference between measurement and production. For this reason, it is unreasonable for the manufacturers to keep impressions for an extended period of time and new impressions must be taken each time the earmold is reproduced.

There are a number of options in the type of materials used to create the earmold, which differ between manufacturers, with client needs, and with the type of the hearing loss. The clinician may also prefer one material or another based upon their ability to customize the earmold's shape after it has been cast.

Proper selection of the earmold material is critical to improve the overall fit and comfort to the user. Silicone, acrylics and polymers are examples of earmold materials used today. Some materials may cause allergic reactions for the user, some provide options in colors, while others are simply more comfortable for the user (individual perception). Material characteristics or properties may change over time and become hard, or may experience shrinkage causing poor fit or discomfort to the user.

Acoustic modifications of the earmold can significantly enhance the sound characteristics of a hearing aid.  Three of the most common options include; venting, dampers and horn effects. Each will affect different portions of the hearing aid response curve.

Venting is an opening that is drilled into the earmold to release low frequency sound. This reduces the "plugged feeling" experienced by the hearing aid user while speaking, described as "talking inside a barrel." This sensation is called the occlusion effect.

Dampers are materials that are used to alter the frequency and decrease unwanted peaks of sound waves. Common materials used include wool, plastic and metal, which fit inside the earmold tubing.

The horn effect is provided when the bore of the earmold increases as it goes deeper into the ear canal. It increases and extends the high frequency sound waves. A reverse horn effect is achieved by adapting the earmold to gradually narrow towards the inner portion of the ear canal (Microsonic, 2000a).

The body considers the earmold as a foreign body within the ear canal and tries to remove it by increasing the production of oils or wax. In addition, skin sensitivity is heightened with a foreign body in place. Improper measurements (or earmolds that change physical characteristics, such as hardening, over time) may cause irritation and soreness that may result in the hearing aid not being used.

Earmolds have an average lifespan of about two years but lifespan is dependent upon changes to the ear canal (e.g. canal growth in a young child); by the materials used (e.g. polymers have a relatively short lifespan and quickly discolor from skin oils and earwax); by how well the earmold is maintained (cleaning), and by changes to client's audiogram (requiring a different earmold).

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Technology Requirements

Manufacturers, researchers and clinicians have identified computerized ear canal measurement (e.g. 3D laser scanning) as an improved method by which to perform accurate, consistent and repeatable ear canal measurements without the use of casting materials. There is a current need for improved casting materials that are quick setting, comfortable to the client and do not shrink. Consistent and rapid earmold production is highly desirable. Earmolds materials and designs must be biocompatible, fit accurately and comfortably, adapt to ear canal changes throughout the day and remain stable over long periods of time.  Low cost, disposable earmolds are desirable. Earmolds must be aesthetically acceptable (e.g. smaller, skin colored, etc.). Technology solutions address important user needs and represent significant business opportunities for manufacturers.

Computerized Ear Canal Measurement - Create 3-D images and measurements (e.g. by using lasers, MRI or CAT scan etc.) that can be transformed into a mathematical representation of the ear canal and used to produce the earmold. This will ensure consistency in measurements, eliminate errors caused by shrinkage, and provide a permanent record of the impression to be kept on file. Issues to be addressed include transient changes to ear canal geometry (due to jaw position, posture, swelling, etc.); hair, wax and other debris in the ear canal; and variations in surface conformity (e.g. skin over bone versus skin over soft tissue); points of increased pressure within the ear canal, or areas that cause skin irritation.

Improved Casting Material - Although Computerized Ear Canal Measurement is the ideal, there remains a current need for an improved casting material that is comfortable to the client, non-irritating, and non-allergenic; remains fluid until it comes in contact with the ear canal; sets quickly once in contact with the ear canal and does not shrink thereafter.

Automated Earmold Production - Eliminate variability in methods and materials starting with the fitting process through to the finished product. Eliminate the "art" of creating an earmold.  A one-day turn around from measurement to use would be ideal.

Earmold Materials and Design:

• Should adapt to changes within the ear (e.g. due to head position, jaw movement, swelling, etc.) while maintaining a comfortable, secure fit.
• Should be easy to clean (quick; safe and easy to use cleaning solutions etc.).
• Should be aesthetically attractive in color and appearance.
• Should use tubing materials that are compatible with the earmold and will not change color, shrink or deteriorate.
• Should use ear-hook dampers that do not clog, or are self-cleaning
• Should be easy to insert into the ear canal.
• Should be non-irritating and non-allergenic.
• Should NOT harden, become less flexible, or generally change its physical characteristics over time.
• Should NOT shrink over time.
• Should NOT foster the buildup of bacteria; "self-disinfecting" when in contact with open sores.
• Should NOT absorb or be damaged by ear canal acidity, water, salt, oil, wax, etc.
• Should NOT induce increased oil and wax production.

Suggested Designs:

• Develop a compound earmold with a gel-like material surrounding the earmold core that will flex and conform to changes in the ear canal (material might be contained within sleeves or compartments).
• Reverse thermal gel that conforms to shape and hardens within the ear canal at body temperature (while remaining pliable) and softens at room temperature (for easy insertion).
• "Pneumatic earmold" that can be "pumped up" to provide a secure fit and deflated for insertion or removal.
• Pre-fabricated earmolds in standard sizes and shapes. Each earmold fits a certain range of (ear canal) sizes and shapes. This technology will enhance the development of disposable earmolds that are less expensive to purchase and reduce the need for fitting adjustments after initial measurements have been made.
• Earmold with properties/characteristics of eye contact lens.
• "Chameleon-like" material that adapts to match the wearer's skin color -- similar to material used with photo-gray eye lenses.
• An open earmold (with digital signal processing).

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References

Kochkin, S. (1997). Customer Satisfaction & Subjective Benefit with High Performance Hearing Aids. Knowles Electronics. Available: www.knowlesinc.com [April 19, 2000].

Lachapelle, R. (1999). The Earmold. Available: www.rayshearing.com/earmold.htm [April 10, 2000].

Microsonic, I. (2000a). Custom Earmolds -- Acoustic Options. Available: www.earmolds.com/acoustic_opt.htm [April 10, 2000].

Microsonic, I. (2000b). Patient Evaluation for Earmold Selection. Available: www.earmolds.com/patient_eval.htm [April 10, 2000].

Smedley, T, & Schow, R. (1998). Problem-Solving and Extending the Life of Your Hearing Aids. In R Carmen (Ed.), The Consumer Handbook on Hearing Loss and Hearing Aids - A Bridge to Healing (pp. 130-158). Sedona: Auricle Ink Publishers.

Stakeholder Forum Participant. (2000). New York City.

Sweetow, R. (1998). Hearing Aid Technology. In R Carmen (Ed.), A Consumer Handbook on Hearing Loss & Hearing Aids - A Bridge to Healing (pp. 111-129). Sedona: Auricle Ink Publishers.

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