Custom earmolds and hearing aids shells are derived from ear impressions taken individually from each patient's ear. The procedure for impression taking is so simple and straightforward that one might think nothing could go wrong. Such a presumption would be incorrect.
Earmold labs frequently report the quality of ear impressions is low. Indeed, they estimate 50% of hearing aid remakes result from poor impressions.1 In return, hearing professionals claim they always follow the same procedure, so all hearing aids should fit, but in reality -- they do not. In binaural fittings, for example, one hearing aid might fit satisfactorily whereas the other might require a remake. The clinicians question the reliability of the manufacturing process, and the earmold labs question the integrity of the ear mold impression technique.
Still, both parties emphasize impression taking is an act of art that requires a lot of experience. To assist the professional, earmold labs offer a wide variety of syringes, pistol injectors and impression materials, so that the professional could choose whatever suits his or her personal needs and master the art of impression taking.
The difference between art and science is that while discussing art, we express our feelings and beliefs regarding the subject, but we neither have to agree or draw conclusions. Everybody's opinion can be equally valuable. In science, we discuss the differences in observations and our conclusions are based on facts and data. If we disagree, one party must be wrong. In impression taking, we disagree on many issues.
The information in this article has been gathered by fifty-six international students participating in a doctoral level distant learning course on earmold technology. 2 In a series of telephone interviews with earmold labs and hearing aid manufacturers the doctoral students sought answers to the following questions:
- Which impression materials they recommended; low, medium, or higher viscosity? Any particular material? Why?
- Did they recommend a silicone or traditional liquid-and-powder impression material? And why?
- Did they recommend the patient move their jaw while the impression material cures in the ear - or not? And why?
- Did they recommend the use of an impression syringe or an impression injector? What kind?
- Did they recommend the same impression technique and material for all hearing instruments, or did their recommendations depend, for example, on the instrument style or the patient's degree of hearing loss?
In total, members of the technical staff from 34 companies worldwide were interviewed.
At first, the AuD students were surprised that most companies did not have any official policy regarding impression taking techniques. One noted that staff audiologists spoke from their experience, whereas customer service representatives tried to figure out what the company policy could be. Different opinions were not only obtained from different companies, but often the staff at the same company disagreed on the same issue. It happened on several occasions that after the initial interview, another person from the company called the audiologist with different recommendations.
Ear impressions seemed to be a hot potato.
A number of students remarked that their phone calls were transferred from customer service to production to a staff audiologist. Still, some reps were uncomfortable with discussing options in impression taking. Many stated up front that the audiologist could use any impression material and technique they were comfortable with. The labs would accept all impressions made from any material as long as the impression was complete. They also claimed that their expert technicians could fix even poor impressions. If more detailed information was offered, there were significant differences in the understanding of the factors that constitute good ear impressions.
Most labs recommended silicones over the traditional liquid-and-powder impression material citing silicone's better shape stability and less shrinkage. One lab stood firm for liquid/power saying that they had good experience with it and impression damage was less frequent than commonly claimed, provided the material was mixed properly.
Recommendations for impression material viscosity varied from low to medium to higher. If a low viscosity material was recommended, it was in order to avoid the ear tissue stretching and provide comfort for the resulting instrument. It was often pointed out that cartridge silicones supposedly flow more freely into the ear canal and fill all crevices and undercuts. Medium and higher viscosity silicones were recommended to prevent acoustic feedback and make the instrument fitting more secure.
Companies commonly recommended their own brand of material. If the company sold only one type of silicone the choice was obvious; it was their silicone. Some reps stated that they offered one silicone to make the one-stop shopping more convenient for the professional and did not really know the properties of the material. They thought it was good though. If the company sold several silicones, often possessing different viscosity, offering a recommendation was a challenge. Since pistol injectors are marketed more aggressively, most reps guessed that silicones in cartridges were preferable. In insolated cases, the reps would admit that they did not recommend the material they offered but sold it only as a result of customer demand.
Some of the staff interviewed was not aware that viscosity could vary and often concluded that it must be irrelevant if it was not printed on the package. If a second party's well known brand silicone was recommended, the reps often disagreed with each other whether it was a medium or higher viscosity. Some thought that all silicones had the same viscosity. Others were confused with viscosity and the shore value.
Trying to asses what viscosity is preferred by the majority of hearing professionals, one of our Au.D. students interviewed a major US distributor and learned that low viscosity cartridge silicones were being abandoned and more viscous hand mixed one-to-one silicones had recently become the most popular.
Impression taking techniques
Most respondents did not see any need for using different impression taking techniques for different products. If the impression shows the ear structures 2 mm past the canal second bend, it was considered satisfactory to build any product. If a high gain instrument was built, the impression would receive a thicker wax or plastic coating, they said.
If more specific advice was offered, most labs preferred closed jaw impressions, a minority preferred "chewing" impressions. The idea of having the patient chew before the material sets in the ear was supported with some interesting thoughts. It was felt that chewing moves the silicone further into the canal and helps to get better seal at the bony portion. Another belief was that chewing moves the soft ear tissue in the area it should be. Open jaw impressions were recommended mainly for hearing aid remakes due to feedback with jaw or head movements or an insecure or loose fit. At times, taking an open jaw impression was advised for new orders on the condition that excessive jaw movement had been observed during the patient's otoscopic examination. Some labs still believed that a chewing impression was the best remedy for such problems. Other labs suggested sending two impressions, an open jaw and a closed jaw impression, so the technician could to assess the situation. However, the reps were quick to add that if two impressions were received, the open jaw would most often be used.
Some reps stated that their expert technicians could identify the viscosity of the silicone and the impression technique employed and adjust the manufacturing process accordingly. Other believed that such methods did not exist or admitted to not having those skills.
During the interviews the Au.D. students discovered a certain pattern in the opinions. While the production staff frequently recommended higher viscosity silicones and the open jaw technique, staff audiologists liked lighter silicones and jaw movements. In many cases, the advice coming from the first party was supported with in-house research data that proved higher viscosity silicones and open jaw impressions superior. This diversity in opinions came as a surprise to the common belief that the clinician taking the impression knows best what impression material/technique is to be used for a given patient because he or she can judge the flexibility of the cartilage and the impact of jaw movements on the tissue displacement first hand.
In light of these interviews, the audiologists frequently expressed their concerns regarding the apparent disaccord in recommendations for best ear impressions and wondered if they received the best possible product for their patients. Several hands-on studies were undertaken to assess the situation. One of the most interesting was sending three sets of ear impressions to three different earmold labs. Different impression techniques and materials were employed in taking those impressions. The labs were all asked to make molds using the same manufacturing technique. Upon fitting the molds, the subjects were advised to wear each mold for some time and report their findings. For the first lab, all subjects felt that the higher-viscosity open-jaw molds were the most comfortable. In addition, seven of the eight subjects reported that the open jaw molds caused less occlusion, with one reporting no difference. For the second lab, three patients preferred molds made from higher-viscosity open-jaw impressions, two chose closed jaw molds, and one liked the chewing mold. For the last lab, more than half of the subjects reported that molds made from the higher-viscosity open-jaw impressions hurt deep inside the ear.
This study made an interesting point; satisfaction of an earmold fitting depended not only on the impression taking technique but also on the manufacturing process. Not all labs were equal in their skills. Several students concluded that such variances would be greatly reduced with the development of a laser impression scanning device.
To learn what fellow audiologists thought of taking tighter impressions, one student conducted an informal survey at a convention and found that 43% of the attendees were taking open jaw impressions. This is much higher than the earmold labs believed they receive. Two important conclusions can be drawn from this. The first is that despite being discouraged from taking open jaw impressions, hearing professionals see the benefit of this new technique and use it frequently. The other is that despite speaking against such impressions most labs are capable of processing them into molds and hearing aids with a satisfactory fit.
Although new research may be helpful in developing more efficient techniques for hearing aid fittings, there is currently enough reliable information to offer the professional a more science-based approach to impression taking.
Viscosity of an impression material is defined as a measure of the material consistency before polymerization. A low viscosity silicone has soft consistency, a higher viscosity silicone is firmer. Figure 1 shows viscosity of commonly used audilogic silicones. The viscosity of these silicones was determined through an extrusion test.
Figure 1. Viscosity of selected silicone ear impression materials.
As provided, audilogic silicones exhibit a wide range of consistency: low viscosity silicones are up to eight times lighter than higher viscosity silicones.
Silicones provided in a rectangular-flange 48-ml cartridge are the lightest. There are no medium or higher viscosity silicones in this type of cartridge. Silicones in the new rounded-flange S50 50-ml cartridge are either low or medium viscosity. There are no higher viscosity silicones in S50 cartridges, as it is commonly claimed. One-to-one hand-mixed silicones are either medium or higher viscosity, so are the tub-and-accelerator type silicones. All silicones, including those in cartridges are thixotropic, which means they do not flow freely.
The liquid/powder material was not included in the chart because it did not exhibit any specific viscosity. From the moment of combining, the viscosity of liquid/powder steadily increased from medium to higher. In contrast, the viscosity of a silicone remained almost unchanged for about one to two minutes.
The shore value refers to the silicone after-cure hardness. The lower the value the softer the finished impression will be. The shore value does not relate to flow characteristics of the silicone before hardening, as provided in Figure 2. Therefore, it should not be used to estimate the material after-mix consistency.
Essentials in impression taking
During an impression, the injected silicone flows up against the oto-block, spreads, and stretches the cartilaginous canal tissue out toward the ear aperture. The canal opens up and its increased diameter is captured in the impression. The higher the viscosity of the silicone, the more stretching may occur. The stretching is a desirable effect, as it ensures a proper hearing aid seal, lowers the risk of feedback and provides a more secure fit for the instrument.
The use of a light silicone does not prevent discomfort or ease hearing aid insertion. In-field hearing aid fitting results proved these ideas false. 3 In fact, a loose or insecure fit hearing aid may require frequent pushing back into the ear and this may lead to tissue soreness and discomfort that is often misunderstood as a tight fit. Soft silicones are rarely recognized as the culprit of unsuccessful fittings. It appears that higher viscosity silicones are more suitable for taking ear impressions from all ears regardless of how soft or firm the ear is, provided acoustic seal and a precise fit are the focus.
Many professionals consider acoustic feedback caused by jaw movements to be the most challenging fitting problem. The reason for such feedback is that the action of jaw opening stretches the ear cartilage and widens the canal. 4 Although this condition affects most adults, not all of them will experience feedback while wearing their hearing aids. This is because most hearing aids operate at a gain level too low to trigger acoustic feedback.
Another frequent fitting problem is that while jaw movements break acoustic seal, the loudness of the instrument is temporarily reduced causing the on/off effect in the instrument's performance. In addition, the widening of the canal often loosens the fitting of the instrument and makes it insecure. For patients experiencing such problems, the best solution is to take an open jaw impression that accurately shows the magnitude of the canal widening.
Figure 3 shows the effects of both the material viscosity and mandibular movements on the canal tissue displacement. The Au.D. students evaluated one hundred and twenty impressions taken from thirty ears through measurement of each impression at the canal aperture, midway between the first and second bend, and at the second bend. The data showed significant differences in the ability of the canal tissue to stretch. The cartilage at the canal aperture in the majority of the ears appeared to be firm and was not stretched even with a more viscous silicone. In contrast, the tissue between the two canal bends was more elastic and mandibular movements dominated its stretching. Similarly, the ear tissue at the canal second bend proved to be significantly expansive, with jaw movements leading the stretching.
This data offers several interesting insights into ear anatomy and impression taking. If a low-gain short-canal hearing aid is ordered, the viscosity of the silicone and the impression taking technique will not matter, at least in most cases. On the other hand, an impression for a higher gain instrument should be taken with a more viscous silicone and the patient's mouth wide open. This will maximize the instrument's acoustic seal and lower the risk of acoustic feedback. To ensure the instrument's comfortable fit deep in the ear, the canal tip on the impression must be skillfully trimmed. The ear tissue at and past the canal second bend is more sensitive to touch than the cartilage between the bends and will not tolerate pressure from the instrument.
The Au.D. students who focused on hearing aid remakes secondary to; acoustic feedback, insecure fit, discomfort, or poor sound quality, found that for 114 out of 136 (84%) patients, taking open jaw impressions with more viscous silicone remedied or diminished the fitting problems. Of course, had such impressions been taken in the first place, the majority of the remakes could have been avoided.
Unfortunately, some earmold labs are against open jaw impressions and more viscous silicones because they lack confidence in their skills in processing such impressions into comfortable hearing aids.
Several of the students conducting this study were staff audiologists for various hearing aid companies. It was encouraging to learn that they initiated a review of the company recommendations for impression taking and undertook in-house research to optimize their manufacturing process.
There is still a lot that can be done to transform earmold impression taking from art to science. Dispensing professionals and manufacturing labs should review their earmold impression recommendations and procedures and employ only those that have been verified through research. This would greatly increase patients' satisfaction with their custom hearing instruments.
More detailed information regarding impression taking and hearing instrument troubleshooting can be found in articles listed under Recommended Readings.
The authors wish to express their gratitude to all Au.D. students of the PCO ACD 602 course for their efforts and dedication to change earmold impressions from art to science. They also acknowledge support in this project from Dreve, Earmold Design, Egger, Frisch, Magnatone, Microsonic, Mid States Lab, Oak Tree Products, Oticon, Phonak, Precision Labs, Siemens, Starkey, and Westone.
1. Impression Material Shopper, The Hearing Review, 1995, 2(4):12-15
2. Pirzanski C, Berge B. Course ACD 602 Earmold technology, School of Audiology PCO, Summer 2000 through Fall 2001.
3. Pirzanski C, Impression Procedures for CIC Hearing Aids, The Theory and the Practice, Starkey Canada internal study, 2001
4. Oliveira R: The dynamic ear canal. In Ballachanda BB, ed. The Human Ear Canal. San Diego: Singular Publishing Group, 1997: 83-111.
On ear impressions:
On earmold technology:
On hearing instrument troubleshooting:
TIPS FOR BETTER IMPRESSIONS
Use a more viscous silicone
When using a pistol injector, use a medium viscosity silicone in an S50 cartridge. If you intend to use a light silicone in the 48 mm cartridge, carefully check the softness of the ear tissue. If it is firm, go ahead with the impression. However, consider that most ear canals are firmer at the aperture and softer past it. If you want an enhanced seal for the instrument, you may be better off using a more viscous syringed silicone. If a remake is warranted, always take the new impression with a more viscous silicone, regardless of the fitting problem. You may indicate the silicone's viscosity in the order form. Not everybody is equally skilled in evaluating the finished impression.
Take an open jaw impression
Hearing instruments made from open jaw impressions have fewer feedback problems, fit more securely and comfortably, and are reported to diminish the occlusion effect. To make the procedure more comfortable for the patient, insert a mouth prop at the corner of his or her mouth on the side the impression is taken prior to syringing, and offer them Kleenex. After injection, let the silicone cure undisturbed. Do not stretch the ear and do not push on the material at the concha. Remove the mouth prop when the silicone has set. Mark the open jaw impression. Just by looking at an impression nobody would be able to tell how you took it. For hearing aid remakes, it is important to state the technique to assist the technician in proper troubleshooting.
Evaluate the impression
Check that the impression extends past the canal second bend and that it is free from voids particularly between the canal first and second bend as this will be the seal area for the instrument. Detect and mark the imprints of any natural growths and ridges in the ear, and request that the impression is modeled in such a manner that a wax or plastic coating will not gather in them or around them creating tight spots.
Chester Pirzanski, BSc and Brenda Berge, AuD are Adjunct Faculty at the School of Audiology PCO. Chester is available to teach classes on ear impressions, email: email@example.com
The authors wish to express their gratitude to all Au.D. students of the PCO ACD 602 course for their efforts and dedication to change earmold impressions from art to science. They also acknowledge support in this project from Dreve, Earmold Design, Egger, Frisch, Great Lakes Lab, Magnatone, Microsonic, Mid States Lab, Oak Tree Products, Oticon, Phonak, Precision Labs, Siemens, Starkey, and Westone.
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