Eliminating feedback while maintaining sufficient use gain is a problem for many hearing instrument dispensers who fit CIC instruments. Patients and dispensers are often frustrated when trying to solve the feedback problem. In fact, surveys show that as many as 75% of dispensing professionals identify feedback as a “common problem area” when fitting CICs.1 Other causes of patient dissatisfaction with CICs are irritation and/or discomfort in the bony portion of the ear canal and changes in comfort or feedback with jaw movement because of the anatomy of the external ear canal.2 The hypothesis put forth in the following article is that the “best solution” to these problems is a shell modification that would address feedback while compensating for jaw movement and providing a comfortable fit.

Some shell build-up materials are reported to have several drawbacks. Acrylic materials will harden and may contribute to patient discomfort. There is no defined place on the shell to place the build-up layer, and optimum thickness is not specified. Additionally, this hit-or-miss approach is not reliable for all patients. Foam “doughnuts,” applied by means of an adhesive strip, have the advantage of being compliant and “giving” with jaw movement, but they need to be reapplied often, and can easily become soiled or torn. Manufacturing a flexible soft plastic tip at the end of the shell (i.e., canal flex-tip) may help preserve the seal and prevent feedback. However, these soft materials can harden over time and lose elasticity; furthermore, they may discolor and/or separate from the rest of the shell.

The goal of the following study was to devise a shell modification that addresses the above issues and provides a reliable, permanent method of controlling feedback without sacrificing comfort, durability or cleanliness. It was theorized that a ring of silicone material (Fig. 1), strategically placed, would provide greater retention and solve the complaints of some patients who say instruments “walk out of the ear,” possibly contributing to feedback. 

The following study examines comfort, usable gain and retention for custom instruments manufactured with the addition of a compliant ring of silicone. This ring was placed on the shell in an indented groove to allow for the expansion and contraction of the material with jaw movement, and sealed in place with a heat-curing process.

Three experiments were conducted to answer the following questions: 

1. Would a compliant ring result in a reduction of feedback? 
2. If so, is there an optimum position on the shell to place the ring?
3. Do compliant rings adversely affect comfort or ease of insertion? 
4. Do aids with the compliant ring have greater retention? 
5. Is this approach applicable to all shell styles? 

Experiment 1: Usable Gain Before Feedback 

Procedures: The first experiment was designed to determine whether hearing aids with compliant rings would result in more usable gain before feedback over aids without rings. Different positions of the ring were evaluated to determine if a particular position would yield better results. Three different styles of aids were evaluated, although the focus of the study was on CIC instruments. Subjective evaluations of comfort and ease of insertion/removal were used to assess ring size and shape. 

Fifteen hearing-impaired subjects were fit with two-channel, programmable WDRC custom instruments. All subjects were fit binaurally, except one subject, for a total of 29 ears. Subjects wore the set of test instruments that provided the most usable gain and comfort. Follow-up visits assessed long-term comfort and potential problems with the compliant rings.

Four instruments were built for each ear: one without a compliant ring, and three with rings placed in different positions on the shell. The three positions were labeled: 

• Tip: Between the medial end of the instrument shell and the second bend; 
• Mid: At the second bend of the ear canal; and 
• Faceplate: Between the first and second bend of the ear canal. 

Ten subjects were fit with CICs, two with ITCs and three with ITEs. For the ITE instruments, only “mid” and “faceplate” positions of the compliant rings were evaluated due to the shorter canal length of this style. 

Usable gain before feedback was measured with the aid in the subject’s ear. Any problems with discomfort or difficulty with insertion were noted. The gain of each instrument was increased in 1 dB increments until the maximum gain without feedback was reached. This was measured in a quiet room and with vents closed to ensure maximum gain from the instrument. Measures were made in two conditions: 1)With mouth closed, and 2)While making exaggerated mouth movements simulating the articulation of the /i/ and /a/ sounds (without vocalizing). Measures for each test aid were repeated twice and averaged.

• Results: Eleven of the 15 subjects, or 21 out of 29 ears (73%), demonstrated more usable gain without feedback when wearing instruments with the compliant rings for the CIC aids. The “mid” position (at the second bend) provided the greatest increase in gain with jaw movement (3.69 dB for “mid” position, 2.54 dB for “faceplate” position, and 0.39 dB for “tip” position), as shown in Fig. 2. The closed-jaw position did not demonstrate the same differences, suggesting that the compliant ring is most beneficial for dealing with the dynamics of the ear canal. Overall, the CIC instruments showed greater increases in gain with the compliant rings than either the ITCs or ITEs. This could be a result of the lack of a second bend on the canals of the ITCs and ITEs.
• If insertion was found to be difficult, or if discomfort was noted, then ring size or shape was modified. A ring dimension was determined that did not compromise comfort yet provided an improved acoustic seal.

Fig. 2. Average gain increase relative to control for three different ring positions for the CIC aids.

Experiment 2: Confirmation of Preliminary Testing

Procedures: The purpose of the second experiment was to confirm the results of the first experiment while holding constant the variables of ring size and position. The optimum ring dimension was used and the ring was placed at the “mid” position. This was the position that was determined to yield the best results in Experiment 1. Maximum gain before feedback was determined for aids with and without the compliant ring.

• Ten hearing-impaired subjects with sensorineural hearing losses were fit binaurally with two-channel, programmable WDRC completely-in-the-canal (CIC) instruments. Two pair of instruments were made for each subject—one pair of instruments without compliant rings and one pair with compliant rings. Subjects wore each set of instruments for five weeks.
• Maximum usable gain was subjectively determined in a procedure identical to that used in the first experiment, (i.e., setting each aid to maximum gain before feedback in both a closed-mouth position and with jaw movements). Measures were repeated three times during three separate test sessions for a total of nine measures. Peak and high frequency average (HFA) gain were measured in a 2cc coupler for each aid, and gain differences were calculated relative to these measures. 
• The APHAB3 (Abbreviated Profile of Hearing Aid Benefit) was administered to this group of subjects to verify perceived benefits of amplification. Subjects were also informally surveyed regarding occlusion and comfort.

Results: Results for the 20 ears were divided into three categories based on the gain realized with the compliant ring (compared to instruments with no ring): 1) Gain increase of more than 2 dB; 2) No gain increase (-2 dB to + 2 dB), and 3) Gain decrease of more than 2 dB. Fig. 3 displays the number of ears in each category for the two conditions of closed mouth and moving jaw. In both conditions, over 50% of the ears achieved greater than a 2 dB increase in gain with the compliant ring relative to no ring. Fig. 4 displays the average gain values for the three categories in each condition. Results showed that 60% of ears fit with CICs incorporating the compliant rings had an average increase in usable gain of 6.1 dB when the mouth was closed; 55% had an average gain increase of 5.6 dB when the jaw was moving. Less usable gain was experienced in 15% of the ears when fitted with the compliant rings, and 25% showed no difference between instruments with the ring and instruments without the ring.

Fig. 3. Distribution of ears categorized by increased gain obtained with compliant ring.

Fig. 4. Difference in HFA gain for compliant ring versus hearing instrument with no ring.

• Subjective hearing aid benefit, as measured by the APHAB, was the same for aids with compliant rings compared to those without rings. APHAB results for all patients revealed benefit with amplification (comparing aided versus unaided conditions) for the EC, RV and BN subscales. The presence or absence of a compliant ring did not affect complaints of occlusion or discomfort.

Experiment 3: Evaluation of Retention Properties

• The purpose of this experiment was to determine if the compliant ring improves retention of the aid in the ear. This was measured by determining the amount of force required to remove the aid from the ear.
• Nine subjects with clean and clear ear canals were used for a total of 18 ears. Measures were compared for two pairs of custom-built “dummy” CIC instruments each (i.e., complete shells and faceplates without any electronic components). One pair was built without compliant rings, and the other pair had compliant rings at the “mid” position. Standard CIC retrieval lines were attached and were matched for position (length and angle) for each comparison set.
• A 1-lb.-force gauge was used to measure the extraction force necessary to remove the instrument from the ear using the retrieval line. The clip of the gauge was attached to the ball of the retrieval line. The measurement was repeated five times for each instrument.

Results: A higher extraction force suggests better instrument retention. The extraction force increased from an average of 0.13 lbs. for instruments without compliant rings to an average of 0.25 lbs. for instruments with compliant rings, an increase of 92%. This result suggests that, on average, CIC instruments with compliant rings have better retention in the ear than those without compliant rings. 

Discussion

The use of a compliant silicone ring placed at the second bend of the ear canal was found to provide improvement in the fitting of CIC instruments in terms of reduced feedback, increased usable gain and improved retention. The placement of the ring at the second bend is crucial in delivering the most benefit from this modification; therefore, its use may not be as beneficial for shell styles which do not capture the second bend of the ear canal, such as in-the-canal (ITC) or in-the-ear (ITE) styles. The use of silicone material results in a ring that is flexible and easy to keep clean. Applying the ring in a drilled groove enhances its durability and comfort by providing for the contraction and expansion of the material with jaw movement. Thus, the patient has a more comfortable instrument that is less prone to feedback with jaw movement. In addition, the instrument can deliver more gain without feedback, and is less likely to “walk” out of the ear than comparable CIC instruments without this shell modification.

References

1. Staab W: Deep canal hearing aids. In M Chasin’s (ed.) CIC Handbook. San Diego: Singular Publishing Group, Inc., 1997: 12.
2. Oliveria R, Hammer B, Stillman A, Holm, J, Jons C & Margolis R: A look at ear canal changes with jaw motion. Ear Hear 1992; 13 (6): 464-466.
3. Cox R & Alexander G: The Abbreviated Profile of Hearing Aid Benefit. Ear Hear 1995; 16 (2): 176-186.

This article was submitted to HR by Jennifer L. Robinson, MS, research audiologist; Mary E. Meskan, PhD, group leader of research audiology; and Bonnie Siu, PhD, research audiologist, at Beltone Electronics Corp., Chicago; and Ankur Chhadia, a design engineer currently working on his MD degree at the Univ. of Illinois in Chicago. Correspondence can be addressed to HR or Mary E. Meskan, PhD, Beltone Electronics Corp., 4201 W. Victoria St., Chicago, IL 60646; email: MeskanM@beltone.com.