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Originally Published in Hearing Review, May, 2007

Coronary Stents Enable Same Day Fitting Scenario.
By: Ed Desporte, MS., Roger Juneau, BSME
Quest for the non-custom custom device
There is a hot renewal of interest in the successful same-day fitting scenario. This is hardly a new concept: Ironically, the first stock earmolds were introduced in 1922 (2). The quest for the same-day fitting springs from the inconvenience of having to take a custom impression of the ear, the delay between the decision to pursue amplification and the actual fitting (caused by the customized fabrication of an earmold or an in-the ear instrument), the unpleasant sensation experienced by the potential hearing instrument wearer while experiencing the ear impression, and the uncertainty of how successful the custom earmold or hearing instrument will fit.

One of the fundamentals of successful hearing instrument fittings has been (surprise!) the fit. More to the point, it has been the coupling of the hearing instrument to the wearer’s ear so that the amplified/processed signal can be efficiently delivered to that wearer’s hearing system. That efficient delivery demands that the coupling is true to the contours and configurations of the individual’s outer ear and ear canal architecture, provides a comfortable fit in all daily situations, and provides a secure acoustic seal to eliminate - or at least minimize – uncontrolled leakage of signal from around the hearing instrument or earmold and out of the ear.
One Shape For A Dynamic Ear?
The primary method of achieving a custom fit has been through the use of ear impressions. In fact, the first U. S. patent for a custom earmold was granted in 1926, and assigned to The Western Electric (1). From the beginning, these custom earmolds were produced by dental laboratories; small wonder, since similar materials were made for ear and dental impressions, and similar materials were used for both denture subassemblies and earmolds.

Aside from the discomfort and delays, the custom ear impression has one overwhelming flaw: It is a static “snap shot” of the ear canal. At its very best, it represents that ear canal in one condition only. This flaw is shared regardless of whether that condition is “mouth open” or “mouth closed”.

In truth, the human ear canal is dynamic in nature, in large part due to the motion of the temporomandibular joint (TMJ). That dynamic action along the anterior-posterior plane of the ear canal varies by three to five millimeters with talking and chewing. The degree of TMJ action not only varies from individual to individual, research has shown that it varies significantly from one individual’s ear to the other (4). Complicating the attempt to fit a statically designed instrument into a dynamic environment was the fact that almost all custom fit hearing instruments have been constructed of rigid acrylic.
Evolution of Same Day Fitting Devices
Hearing instruments constructed of soft materials had been attempted in the past, and had been perceived by many as preferable to acrylic shells on the basis of comfort, adaptability to changes in the ear canal, and better sound quality. Most of those attempts were not successful: Most used thermoplastic elastomers that were prone to shrinkage, hardening, and cracking over time. Most mimicked traditional shell technology in that they were hollow, tending to bend upon insertion. In the late 1990s, a method of bonding silicone to an acrylic faceplate was patented (5). Uniquely, the method employed very low durometer silicone, and the electronic components were completely embedded in the silicone with no interior spaces. Utilization of that technology did overcome many of the short-comings. Nevertheless, when utilized in the custom fit format, these soft instruments were subjected to the same shortcomings of custom ear impressions as their acrylic counterparts

While there have been a number of approaches to achieve same-day fittings, most have been variations of the standard model of earmold/hearing instrument manufacturing.

For behind-the-ear hearing aids, “instant earmolds” utilizing acrylic technology date back to 1957 (3). Produced at the dispenser’s office, the primary purpose was to eliminate the delay, although high-fidelity representation of the ear canal was also a goal.
Same Day Fitting Scenarios
In-the ear hearing aids have generally followed one of two different paths.
1. Faceplates provided by a manufacturer with an earmold/hearing aid lab on site.
2. Manufactured non-custom, acrylic instruments designed to fit the mythical “standard” ear.
Very few of these approaches have been successful.

In the former case, the focus was to duplicate – on a smaller scale – traditional custom hearing instrument manufacturing. This required significant investment: special licensing, equipment, dedicated lab space, and talented well-trained personnel.

The latter case – besides being dependent upon the concept of an “average” ear canal – utilized instruments manufactured with acrylic shells. In most cases, these instruments failed because they did not provide the acoustic seal necessary for acceptable and useful gain before feedback or proved uncomfortable because of their hard, unyielding construction. Because of the acrylic nature of the shells, attempts to address these acoustic seal failures and comfort issues were limited to exterior modifications:
  • Acrylic add-on material could be applied to the shell surface
  • These coatings often appeared rough and unfinished.
  • More importantly, the application of these materials was almost always imprecise, often resulting in discomfort or undesirable acoustic side-effects.
  • Foam could be applied to the receiver tip or at the aperture area
  • Many multiple sizes needed to achieve a fit.
  • Hard to insert.
  • Uncomfortable and stuffyCannot not be reused and must often be replaced as cellular foam quickly becomes soiled
Soft vs Acrylic
An obvious alternative to acrylic instruments in this same-day scenario is the use of instruments constructed of soft material. These have been very well accepted from a comfort stance, but share the shortcoming of being incapable of adapting to varying ear topography. As a result, many fittings were compromised by the implementation of dated, low power analog technology. Photo below shows cross-sectional view of early soft silicone device.
Unlike rigid acrylic instruments, however, the soft format provides the possibility of alternative methods to adapt a non-custom fit to accommodate an individual’s ear canal. Instead of relying on exterior “add-ons”, the soft format made interior adjustments a possibility. A number of solutions have been attempted.


Fixed-Force Spacers (springs, plastic rings)
- Different sizes required
- Placement of the spacer inexact
- Fixed force applied by the spacer cannot be changed

Bladder technology
- Complicated clinical procedure required to interface pressurizing medium to the bladder(s).
- Pressurizing the bladder(s) with the instrument in ear may lack control to deliver desired comfort.
- Once sealed, the pressure inside the bladder(s) may not be variable, and may not provide consistent comfort throughout the day.
- Not consistently manufacturable
A solution that could overcome the inexactness and the unvarying pressures of the previously mentioned strategies was found outside the hearing industry: “Memory metal”, specifically Nitinol, is most commonly associated with its use in the manufacture of coronary stents.
Of the many mechanical properties unique to Nitinol, the most important is superelasticity: the transformation from rigidity (the Austenitic phase) to malleability (the Martensitic phase) upon cooling, and the reverse transformation from Martensitic to Austenitic upon heating. Two critical characteristics exhibited in the Austenitic phase are the loading plateau and the unloading plateau, usually diagrammed on a stress strain curve. The loading plateau stress is the stress level at which material at a specific temperature above the Austenite finish temperature, Aƒ, will force the Austenite phase into Martensite.
This produces an almost constant stress level over a relatively large range of strain (up to about 8%). This means that the stent moves until it meets a specific resistance, not a fixed distance. Therefore, a five millimeter expansion range delivers the same wall pressure to a small ear canal as it does in its larger counterpart. Fig. 1 shows a ReadyWear device with embedded stent in a room temperature state. Fig 2. shows stent reacting to body temperature and expanding to conform to the ear canal wall.


Fig.1 and Fig.2 illustrates the SmartWear device expanding to fit ear canal.

Stented technology
  • Can be calibrated to activate in response to a specific temperature (e.g. ear canal temperature)
  • Can be calibrated and polished to provide precise (to nearest 100th of a Newton) resulting in precise interfacial pressure between the ear canal and the soft hearing instrument body
  • Excellent biocompatibility. Stents emerged from coronary implant surgery
  • The constant stress level provides for dynamic reaction to changes in the ear canal.
  • Enables the use of state of the art signal processing in same-day fitting scenarios.
Summary
Stented technology offers more than simply improving the fit of same-day fit scenarios. It offers a method to overcome the shortcomings of the hearing instrument or the earmold manufactured on the basis of a static (though custom) ear impression. It brings to reality the scenario of taking a non-custom instrument off the shelf, placing it in the wearer’s ear, having the embedded stent sense body temperature, and having the stent expand to seek out the unique configuration of the wearer’s ear canal.

With the vagaries of fit issues removed, the clinician can then focus on meeting the electroacoustical needs of the patient.

In fact, the technology leads us past the narrow realm of hearing instrument fittings and toward the concept of the self-forming listening system. The road is then opened to recreational, industrial, and personal communication applications.
About the Authors:
Roger Juneau, is President of General Hearing Instruments, Inc. Respected industry lecturer and has published numerous technical articles on hearing industry technology. Author of several US and international patents. Honored as 2003 Inventor of the Year by New Orleans CityBusiness publication



Ed Desporte, MS, is Director of Product Development and Quality Assurance at General Hearing Instruments, New Orleans, LA. Author of technical hearing articles, lectures on tinnitus treatment therapy, and co-inventor of several hearing industry related patents.
Correpondence can be addressed to ed@generalhearing.com



Bibliography:

1. Berger, K: The Hearing Aid: Its Operation and Development, Third Edition. Michigan: The National Hearing Aid Society, 163.
2. Berger, K: The Hearing Aid: Its Operation and Development, Third Edition. Michigan: The National Hearing Aid Society, 163
3. Berger, K: The Hearing Aid: Its Operation and Development, Third Edition. Michigan: The National Hearing Aid Society, 176.
4. Oliveira, R: The Dynamic Ear Canal. In: Balanchanda, BB (ed). The Human Ear Canal. San Diego: Singular Publishing Group, Inc. pp 86-91, 1995.
5. Creel, LP, Desporte, EJ, & Juneau, RP. Soft-Solid Instruments: A Positive Solution to the Dynamic Ear Canal. The Hearing Review, 1999, Volume 3, pp 40-43.
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