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Earmolds: Are soft materials superior?

Pirzanski, Chester

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doi: 10.1097/01.HJ.0000294111.22128.00
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The variety of materials available to manufacture custom earmolds is constantly growing. Unfortunately, this does not mean that choosing the right material for the patient has become easier. In fact, the wider the selection, the more confusing the issue.

One purpose of this study is to review research results that pertain to the issue of whether or not soft materials are superior to hard materials in manufacturing long-lasting hearing aid earmolds derived from ear impressions. The conclusions here do not pertain to the potential benefits and drawbacks of using other soft materials such as foam, soft wraps, or soft coats.

The article also discusses other issues related to the selection of earmold material.

EARMOLD MATERIALS

Most earmold labs offer, often under different names and for a ranges of prices, the following earmold materials:

  • hard acrylic (Lucite) and hard ultra-violet (UV) resins
  • soft acrylic and soft UV resins
  • medical-grade silicone
  • polyvinyl chloride (also known as vinyl or PVC)
  • polyethylene

Earmold materials vary in their physical properties, particularly in the degree of softness, extent of shrinkage, and finishing techniques.

Shore value

The softness of a given material is described as the shore value. The lower the shore value, the more flexible the material. Earmolds made from a soft acrylic and soft ultra-violet material have 40 to 50 shore hardness, vinyl earmolds 30 to 50, and silicone earmolds 25 to 55, depending on the material and manufacturer. Rigid materials have 90-shore end hardness.1

Contraction ratio

The earmold manufacturing process, for most molding materials, is based on polymerization. In its non-polymerized condition, the liquid plastic consists of monomer molecules that compound during reaction to long chains of polymers. Single-monomer molecules are farther apart in the liquid condition than are the polymers in the cured condition. As a result, the transition from the non-cured to the cured condition causes contraction in the earmold body. The linear contraction is approximately 2% for hard acrylic, 1.7% for ultraviolet resin, and 0.4% for silicone. Soft acrylics shrink 2% or more, during and after casting. Earmolds molded by injection of a liquid or melted vinyl contract up to 2.5% within 24 hours after molding.1

Earmold finishing

Hard-body earmolds require sanding and polishing to make them cosmetically appealing. The surface of soft earmolds is commonly treated chemically for the same reason. This process slightly reduces the dimensions of the earmold.

In order to offset the negative impact of earmold material contraction/shrinkage and earmold surface treatment, ear impressions are built up by the earmold lab technician prior to manufacturing the mold. The thickness of the build-up, which is commonly made of wax, depends on the physical properties of the material being processed.2

CONSIDERATIONS FOR EARMOLD MATERIAL SELECTION

In selecting the earmold material, the acoustic seal of the earmold and comfort are usually the key factors.

Acoustic seal

Proper earmold seal is the most important issue in hearing instrument fittings. If the earmold leaks acoustically, it becomes susceptible to acoustic feedback. It has been frequently stated that soft earmolds, particularly those made from silicone, provide a superior seal. However, the research data do not support this opinion.

Study one

In a study conducted on 16 subjects, Macrae used one ear impression to make four earmolds for each ear.3 The impression was coated with wax and then earmolds of vinyl, silicone, polyethylene, and hard acrylic were cast. In total, 64 earmolds were manufactured.

The accuracy of the earmold seal was then tested as follows: A tympanometer was used to apply air pressure to the ear canal through a tube running through the mold.4,5 When sustained pressure was measured for 5 seconds, the seal was considered satisfactory. If the pressure decreased, the seal was inadequate. The study did not find that soft earmolds seal better. In fact, all soft and hard earmolds leaked and failed the test.

Apparently, three factors contributed to this negative result.

First, the impression material was likely low-viscosity (light) and therefore it did not stretch the ear tissue in the subjects properly, allowing for earmold leakage.

Secondly, the wax coating applied to the impressions was probably too thin to ensure an effective seal for the finished molds.

Lastly, the pressure seal test was very demanding. A verification of the method conducted by this author determined that even a slight around-earmold leakage, equal to the leakage that would occur through a 0.5-mm vent, was able to reduce the pressure in the ear almost to zero, even with the air pump continuously running.

Study two

To investigate further the effects of impression waxing and ear tissue stretching, Macrae conducted another test.3 The research concept and results are provided in Figure 1.

Figure 1
Figure 1:
Comparison of seal provided by silicone earmolds manufactured under five different conditions. Earmolds ML-O made from non-waxed multilayer impressions provided the most effective seal.

He made two impressions from one ear of each subject. The first was taken with a soft impression material, the other using a multilayer impression with the primary impression made of a heavy-bodied silicone. Four wax coatings of varying thickness were used to make four earmolds from the first impression, one earmold for each coating condition. Earmolds EM-A were made with the thinnest impression coating (A), whereas earmolds EM-D were made with the thickest coating (D). Multilayer impressions were not coated prior to making earmolds ML-O. All earmolds were made from the same medical-grade silicone and had a vinyl tubing for the air pressure test.

Macrae found that making an earmold from a silicone would not guarantee a proper seal. The efficiency of sealing varied and strongly depended on the thickness of the wax applied to the impression. Silicone earmolds made from coating A sealed in 15%, whereas silicone molds made from coating D sealed in 65%. Generally, the thicker the coating the more effective the seal.

Interestingly, the study found that silicone ML-O earmolds made from non-waxed impressions sealed most efficiently, reaching 88%. This was achieved even though the ML-O earmolds had smaller canal diameters than the extra-tightly fitting EM-D earmolds. It is also worth noting that earmolds EM-C, which had on average the same canal diameter as earmolds ML-O, provided a much less effective seal, just 50%.

This surprising fact can be explained as follows: Ear canal tissue is soft, but its softness is not a rubber-like flexibility. Certain areas of the cartilage appear to be more forgiving than others. Earmolds EM-D made from heavily coated impressions taken with a light material did not correspond properly with the anatomical structures of subjects' ears. Tightly fitting molds overstretched the ear tissue in some areas but did not stretch it enough in other areas. This resulted in poor seal and leakage.

The most efficient seal in earmolds ML-O was achieved by the employment of the multilayer technique. Although the multilayer impressions stretched the ear tissue significantly more than the low-viscosity impressions, the stretching reflected the forgiveness of the natural ear tissue. Therefore the fit of the resulting ML-O earmolds was anatomically most accurate.

Problems with loosely fitting earmolds

The use of light impression materials is a common cause of loosely fitting earmolds, both soft and hard. Figure 2 shows a soft earmold made from an impression taken with a light material inserted into a bisected control ear cast from another impression taken with a standard-viscosity silicone. The earmold is obviously loose, mainly in the ear canal area. This loose fit increases the risk of acoustic feedback.

Figure 2
Figure 2:
A soft earmold may be unable to effectively seal the ear canal if the mold fits loosely.

It is doubtful that soft molds can prevent acoustic feedback through adjusting to the shape of the ear. A loosely fitting earmold is smaller than the actual patient's ear and allows for sound leakage around the mold. If such a mold is forced deeper into the ear, the leakage may be reduced and feedback eliminated. This, however, will distort the mold shape and, in most cases, will not have a lasting effect. The mold will eventually recover from stress and become loose again.

It should be mentioned that there are some thermoplastic materials that soften under body temperature. However, for the reasons discussed above, it should not be assumed that an increase in earmold softness would improve the efficiency of its seal. A loosely fitting earmold will not become tighter just because the material becomes softer.

Some experiments have been conducted in which earmolds were manufactured using thermoplastics that were able to expand in the patient's ear under body temperature and provide an air-tight seal within minutes after insertion.1 The theory was that the ear cartilage could accommodate and tolerate virtually everything placed in it within reason. While some patients benefited from the new self-sealing earmolds, the project was soon terminated because too many earmolds were reported to be excessively tight. As for the general patient population, the supposition that the ear tissue would stretch and conform to the tighter mold and that the softness of the mold would prevent discomfort proved erroneous.

Other findings

Soft earmolds do not prevent acoustic feedback related to jaw movements.6 The opening of the patient's mouth pulls the ear tissue at the front canal wall forward and commonly widens the canal 0.5 mm, or more.7 This may allow for enough sound leakage from the ear canal to cause acoustic feedback.

Figure 3 shows a soft earmold made from a closed-jaw impression inserted into a bisected control ear cast from an open-jaw impression. The mold is at least 0.8 mm loose at the anterior ear wall. Since there are soft materials that can expand instantaneously, soft molds are unable to maintain an effective seal while mandibular movements stretch the ear canal wall. The likelihood of the acoustic seal being broken during jaw movements is similar for both soft and hard earmolds.8

Figure 3
Figure 3:
Earmolds made from closed-jaw impressions may be unable to maintain effective seal while mandibular movements widen the ear canal. The bisected control ear was cast from an open-jaw impression. A gap of 0.8 mm between the soft mold and the anterior ear wall is apparent.

A study conducted at a major earmold lab tried to determine whether hearing professionals fit soft or hard earmolds more successfully.9 A total of 2731 earmolds were investigated. Of these, 1318 were made from soft materials such as silicone, vinyl, and soft acrylic. The rest were hard acrylic molds.

The parameters of the molding process were monitored throughout the study to ensure consistent manufacturing. The study found that soft molds required 0.6% more remakes than hard molds. This clearly demonstrated that there is no reason to consider soft materials superior. This conclusion is consistent with my personal experience that if a hard-body earmold does not fit in the ear properly, the fit of a soft mold, in most cases, will not be any better.

In general, all these data indicate that earmold fit and acoustic seal are not enhanced by the use of soft materials. Rather, the accuracy of the earmold fitting is determined by the impression-taking technique, the viscosity of the impression material, and the parameters of the molding process.10,11

Comfort and safety

Theoretically, earmolds made from soft materials should be more comfortable than those made from hard materials. Practically, however, because most human ears appear to be softer than most earmolds, the softness of the mold is quite irrelevant. It is the ear tissue that has to conform to the earmold, not the earmold to the ear.

Results from custom hearing aid fittings support this opinion. The majority of shells for custom instruments are manufactured from either a hard acrylic or hard ultraviolet resin. A study found that with a proper impression-taking technique and competent manufacturing not more than 0.7% of custom in-the-ear hearing aids, including high-gain instruments, required a remake due to discomfort.12 These results would not be so good if soft materials were as critical for comfort as has been commonly claimed.

It may be worth mentioning that discomfort in earmold fittings, both hard and soft, often results from a loosely fitting mold. Even soft earmolds that require frequent pushing back into the ear may cause the ear tissue irritation. The resulting inflammation and discomfort are commonly, but incorrectly, interpreted by the dispensing professional as an indication of an excessively tight fit.

Soft earmold materials are recommended for children and for industrial noise-reduction earplugs, since the flexible material reduces the risk of injury from blows to the head.

Insertion

Difficulty in earmold insertion commonly results from one or more of the following: (1) the patient's limited manual dexterity, (2) inadequate in-lab impression trimming, (3) impression overwaxing, and/or (4) soft earmold material.

While inserting a soft earmold, the patient may have difficulty guiding the canal portion of the mold into the ear canal. As a result, the canal on the mold can bend backwards, interfering with proper insertion. Lubrication of either the ear aperture or the earmold may be helpful. However, lubrication may lead to partial or complete blockage of the earmold tube. In addition, some patients find earmold lubrication inconvenient.

Patient allergies

Hard and soft materials used to manufacture earmolds are biocompatible, so they generally do not cause irritation or sensitization to the skin in the ear. However, some patients exhibit high sensitivity to traces of certain chemicals, particularly those found in soft and hard acrylic resins, and as a result develop soreness of the ear tissue. For such patients the use of a hypoallergenic earmold material is recommended.

The challenge is that there is no single hypoallergenic material that is good for all patients with allergies. Commonly, clear ultra-violet resin (for hard molds) and clear silicone (for soft molds) are considered best. Unfortunately, some patients are also allergic to these plastics.

To determine which hypoallergenic earmold material is best for a given patient, the professional may administer a skin patch test in which small samples of earmold materials, provided by the earmold lab, are taped to the patient's arm or neck.13 The adverse reaction can range from dry itchy skin with a slight inflammation to a painful edema of the pinna, cheek, or neck area. The otoplastic material that does not result in an adverse reaction should be selected for the mold manufacturing.

Patients who are allergic to earmold materials usually have a history of allergies to other chemical agents. If the patient appears allergic to just his or her earmold, the irritation most likely has another cause. This situation is quite evident in binaural fittings where one earmold fits comfortably, but the other, according to the dispensing professional, requires a remake due to a severe “allergic” reaction. In such situations the ear inflammation is obviously caused by the poor fit of the earmold, which may include difficulty in the mold insertion, excessive tightness, or surface roughness. Ear inflammation can also result from constant tissue coverage, increased humidity in the ear canal, or lack of proper hygiene.

SUMMARY

All earmold materials, both hard and soft, have the same potential for sealing the ear adequately and preventing acoustic feedback, providing that an anatomically accurate ear impression is taken and the earmold is competently manufactured. Similarly, earmold comfort and ease of insertion depend more on the impression processing than on the actual softness of the earmold material.

REFERENCES

1. Pirzanski C: Unpublished data provided by earmold and chemical labs.
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6. Pirzanski C. An alternative impression-taking technique: The open-jaw impression. Hear J 1996;49(11):30–35.
7. Oliveira R: The dynamic ear canal. In Ballachanda BB, ed. The Human Ear Cana. San Diego: Singular Publishing Group, 1997: 83–111.
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10. Pirzanski C: Critical factors in taking an anatomically accurate impression. Hear J 1997;50(10):41–48.
11. Pirzanski C: Selecting material for impression taking: The case for standard viscosity silicones. Hear J 2000;53(10):43–50.
12. Maye V: Field return analysis by account. Starkey Labs Canada, internal study. 1997.
13. Valente M, Potts L, Lybarger E: Options: Earhooks, tubing, and earmolds. In Valente M, ed. Hearing Aids: Standards, Options, and Limitations. New York: Thieme Medical Publishers, 1996, 252–327.
Copyright © 2001 Wolters Kluwer Health, Inc. All rights reserved.