Ear canal dynamics

Abstract

The dynamics of the ear canal, and the wide diversity of dynamics in the patient population, often pose challenges in fitting custom earmolds and hearing aids. These problems commonly include acoustic feedback, discomfort, and retention problems. Garcia1 and Pirzanski and Berge2,3 have reported that the softness of the ear canal cartilage varies among individuals. While most subjects had firm canals, others had remarkably soft canals. Oliveira4,5 and Pirzanski and Berge2,6 have pointed out that mandibular movements result in canal cartilage compression and expansion, which lead to changes in the volume of the ear canal. The magnitude of these changes varies significantly among individuals. These reports were limited because they covered the general population and investigated canal cartilage softness and mandibular movements independent of each other, not as an interactive complex. Further, they failed to address the issue of whether or not ear canal dynamics could be satisfactorily assessed prior to impression taking, thereby allowing the clinician to choose the most appropriate impression technique and material for a given patient. RESEARCH OBJECTIVES The objectives of this study were to determine if each of the following hypotheses was true: The impression-taking technique and material do not matter as long as the impression is complete. Children's ear canals are softer than adults' ear canals and less affected by mandibular movements. Female and male ear canals exhibit similar dynamics. The right and left ear canals of a subject are similarly soft/firm and affected to the same extent by mandibular movements. The softness of the ear canal and its widening resulting from mandibular movement can be correctly quantified during patient examination. METHOD The data for this study were gathered from research assignments submitted by audiologists enrolled in the distance-education AuD degree program at the Pennsylvania College of Optometry (PCO) School of Audiology.7 The research method was to investigate ear canal dynamics by taking ear impressions using various techniques and materials and then measuring and comparing those impressions. Impressions taken with a higher-viscosity (firmer) silicone are known to expand the ear canal more than impressions taken with a low-viscosity (light) silicone.1–3 Similarly, impressions taken with the subject's jaw open are commonly bigger at the canal area than impressions taken with the jaw closed.2,4–6 Prior to impression taking, the audiologists evaluated each ear and classified the canal cartilage as soft, medium soft, or firm. They classified mandibular activity in the ear canal as none, moderate, or severe. These perceptions were later compared with impression measurements. Four ear impressions were taken from the right ear and four impressions from the left ear of each of more than 800 subjects. The subjects had not been included in previous studies done by these authors. We analyzed more than 6000 impressions. The first set of impressions was made with a low-viscosity silicone and then a higher-viscosity silicone, with the subject's jaw being closed in each case. The other two impressions were both taken with a higher-viscosity silicone, one with the subject's mouth closed and one with it open. The diameter of the canal area was measured with a caliper in the anterior-posterior plane at three areas on each impression. The areas were: the aperture, mid-section, and second bend (see the insert in Figure 2).Figure 2: Ear canal dynamics at the canal aperture, the acoustic seal area, and at the second (medial) bend.For the purpose of this study, the measure of the canal softness was the difference in the measurements between the impressions taken with a higher-viscosity (HV) silicone and a low-viscosity (LV) silicone, with the subject's mouth closed. The measure of the magnitude of canal widening with mandibular movements was the difference between the open-jaw (OJ) impression and the closed-jaw (CJ) impression, both taken with an LV silicone. Finally, the measure of the ear canal maximum expansion was the difference between the OJ HV impression and the CJ LV impression. The HV silicone impression materials used in this study came from established earmold labs and the silicones are commonly used by hearing professionals around the world. Higher-viscosity silicones are safe and known to significantly reduce fitting problems in hearing instruments.6,8,9 DATA VERIFICATION The first challenge we encountered in analyzing the data was to assess the accuracy and reliability of the measurements. We recognized that some of them could be inaccurate, and we rejected data that appeared to be obviously flawed. The remaining data, derived from 744 subjects (1488 ears, 5952 impressions), were compared with data recently published by Oliveira.10 The black field in Figure 1 comes from Oliveira's research and shows changes in the ear canal volume due to mouth opening. The red curve is the linear increase in the ear canal diameter with mouth opening based on the measurements taken by the audiologists. While there are some differences in the compression area, in the expansion area the similarity of both graphs is striking. Based on this we concluded that the measurement data were accurate for further analysis. The results are discussed below.Figure 1: Comparison of research data. Data regarding ear canal TMJ activity obtained by these authors were consistent with Oliveira's research.RESULTS The purple line on the X-axis in Figure 2 is the reference line that represents ear impressions taken with subjects' mouths closed and light silicone used (CJ LV impressions). It is understood that such impressions do not stretch the ear canals. The brown, red, and green curves illustrate the canal expansion/compression obtained with the “tighter” OJ HV impressions. The data for each curve were sorted from the smallest to the largest value. As provided, the majority of the canals exhibited similar dynamics at the canal aperture and second bend. The area between the canal's two bends, which is the acoustic seal area for hearing instruments, was stretched the most. This is an important finding. It suggests that hearing instruments made from impressions that do not sufficiently expand the ear canal at the seal area may be more susceptible to acoustic feedback. The rest of this article discusses ear canal dynamics affecting the acoustic seal area in the ear canal. Figure 3 illustrates ear canal widening measurements for the right and left ears for three different impression techniques. The dark-green and light-green curves show the expansion in the right and left ear canals respectively, resulting from HV impressions. The brown and black lines show the widening in the right and left canals related to taking OJ impressions. Finally, the red and blue curves show maximum expansion in the right and left ears resulting from OJ HV impressions. The light-green, dark-green, brown, and black curves are so close to each other that making a distinction in most areas is almost impossible.Figure 3: Ear canal expansion at the acoustic seal area as it pertains to ear canal tissue softness, mandibular movements, and maximum canal expansion.An interesting observation can be made here. It appears that the right and left ear canals were equally soft/firm and equally affected by mandibular movements. Because the data for each curve were sorted from the smallest to the largest value, it would be inappropriate to reason that the right and left canals of each subject exhibited similar dynamics and were stretched to the same extent. In fact, as shown in Figure 4, the left ear canals (blue line) and the corresponding right canals (orange line) were commonly (66%) asymmetrical. A pair of canals was considered symmetrical if the difference in canal widening between the right and left ear was less than 0.2 mm. This finding was consistent with previously reported data.9,10Figure 4: Asymmetry in ear canal dynamics. While all human left and right ears exhibited similar dynamics, ears belonging to a given individual were in most cases asymmetrical.For further analysis, we divided the measurement data into five age groups: (1) children 12 years and under, (2) teenagers 13–19, (3) young adults 20–35, (4) adults 36–60, and (5) seniors over age 60. Our purpose was to investigate the effect of aging on ear canal dynamics. The five curves in Figure 5 show the ear canal widening with OJ HV impressions for these groups. These curves were smoothed to make it easier to identify trends. The light-green field that covers ear canal expansion from 0.0 to 0.5 mm is considered a low-risk area for loose fit and feedback problems. Earmold labs typically coat ear impressions with wax to create a slightly tighter fit for the resulting shell/mold.12,13Figure 5: Age-related magnitude of ear canal dynamics.The curve representing the children's ear canal dynamics leaves the green area last. It means that pediatric canals are the firmest and least affected by jaw actions. Their firmness was surprising! In contrast, seniors' canals were the most active and most susceptible to expansion with OJ HV impressions. Still, regardless of subjects' age, a certain number of canals in each group were very dynamic; see the yellow oval area. It appears that male and female ears exhibit similar behavior in compression-expansion with OJ HV impressions (Figure 6).Figure 6: Young adult female and male ear canals exhibited similar expansion properties.One hundred and eleven pairs of ears were perceived by the audiologists as firm and unaffected by TMJ movements. It was expected that those ear canals would show minimal expansion with OJ HV impressions, but, as Figure 7 indicates, that assumption was incorrect. OJ HV impressions expanded almost half of the ears by more than 8%. An 8% expansion is the equivalent of a 0.5-mm stretching for an average size canal.Figure 7: Ears classified as firm and not affected by jaw movements were in most cases significantly expanded by OJ HV impressions. The right and left ear of a subject commonly exhibited asymmetrical expansion.This does not appear to be much expansion until one takes into consideration that the vent equivalent increases by 1 mm for each 0.5-mm of the canal widening (Figure 8). Figure 8 is based on images of ear impressions scanned digitally. Pay particular attention to the green circles representing the venting. The doubling of the diameter makes the vent area four times as large! This suggests that even after waxing that makes the shell about 0.5 mm tighter, significant venting problems may exist if CJ LV impressions are taken.Figure 8: Equivalent of venting with ear canal widening.One of the issues reported is intermittency (sound going on and off) with mouth movements. This is because an additional uncontrolled 3-mm or greater venting may lower sound pressure at the eardrum to a level that makes the user perceive the aid as off.14 Figure 9 shows the canal firmness/softness data. The perceived ear canal softness is compared with the measured average canal widening with HV impressions for each age group. After ear examination, the audiologists were confident that pediatric ear canals were the softest. However, the measurement data analysis did not support their observations. The data showed that pediatric canals were the firmest and the seniors' ear canals the softest.Figure 9: Perceived softness of human ear canals. Pediatric and geriatric ears were initially perceived as the softest. However, this perception was disproved by the actual measured canal expansion.Figure 10 shows a comparison of TMJ movements with actual measurements taken from CJ and OJ impressions. In this case, the audiologists' perception that mandibular movements affected pediatric canals the least and seniors' canals the most was correct.Figure 10: Perceived ear canal activity. Pediatric ears were perceived as the least active and geriatric ears as the most active with TMJ movements. This perception proved consistent with the actual measured widening.Regrettably, these findings cannot help the clinician take better impressions. Look at the range of changes in the ear canal diameter across age groups shown in Figure 9 and 10. The seniors' ear canals were stretched on average by only 0.1 mm more than pediatric canals. The average difference in the canal expan-sion due to TMJ was 0.2 mm. If you consider that the thickness of a sheet of copy paper is 0.1 mm, you'll realize that predicting such minor changes in the canal diameter would be a most challenging task. The difficulty of predicting how much an ear canal would expand became even more evident when the perceptions were compared with the number of accurate observations. Figure 11 shows that for canals classified as soft, medium-soft, and firm the rate of accurate evaluations was 8%, 16%, and 83%, respectively. Only 34% of all evaluations were accurate. Note that the majority of canals in each group were firm (70%, 80%, and 83%).Figure 11: The softness of the canal in the patients' ears could not be accurately predicted. Only 34% of such guesses were accurate.For the magnitude of TMJ movements, see Figure 12. Eighty-one percent of the observations for inactive ear canals were correct, 17% for moderate movement were correct, and only 10% were correct for severe movement. The total of accurate observations was 48%. Note that most canals in each group did not stretch more than 0.5 mm with TMJ movements (81%, 75%, and 70%).Figure 12: TMJ-related canal widening could not be properly quantified. Changes in only 48% of ears were satisfactorily assessed.MAJOR FINDINGS The findings in this study were consistent with previous research.1–11 We found a wide diversity among the population. While most subjects had firm and inactive canals, some subjects had remarkably dynamic ear canals. Furthermore, this study found: The impression-taking technique and material affected the size of the impression in the canal area. Most canals were firm and were not significantly affected by mandibular movement. In most cases, impressions taken with a higher-viscosity (firmer) silicone and with the patient's mouth open expanded ear canals the most. The acoustic seal area was the most dynamic area in the ear canal. The perception that children's canals are the softest is incorrect. In fact, children's canals are firmer than those of all other age groups. The false perception can be linked to the fact that clinicians are unaware that the cartilage of the pinna is not physically contiguous with the cartilage of canal. Since the pinna on a child's ear is unusually soft, the audiologists incorrectly assumed that the canals were soft as well. Ear canals were least affected by mandibular movements in children and most in seniors. Ear canals of male and female subjects exhibited similar dynamics. The two ears of a subject commonly exhibited asymmetrical dynamics. This explains why in some binaural fittings only one hearing aid requires a modification or remake.13 The audiologists could not satisfactorily quantify the dynamics of ear canals through ear examination in any age group. CONCLUSIONS It came as a surprise that so many common perceptions and observations related to ear canal dynamics were incorrect. This suggests that clinicians should be cautious about advising manufacturers how soft/firm or active the ears of their patients are. Far more helpful is to take ear impressions that carry such information “imprinted” in them. It appears that ear impressions taken with a viscous (firm) silicone and the patient's mouth wide open provide the most anatomically accurate ear imprints. As indicated before, the open-jaw, high-viscosity impression technique is safe and reliable and has the potential to reduce significantly hearing aid remakes and to increase patient satisfaction with hearing instruments.2,3,8,9,14 It is worth pointing out that if the ear canal is firm and/or not affected by mandibular movements, the OJ HV impression will not differ from an impression taken with any other technique. However, if the ear canal is soft and/or active, the OJ HV impression will most precisely capture the canal dynamics. This will offer the greatest probability that the otoplastic will have a proper seal and fit securely and comfortably.