Clearly, we've come a long way since the old plaster ot Paris days, but for new hearing aid wearers, gelting an appropriate earmold is often the most challenging task they face.

For anybody wearing hearing aids, earmolds are an unfortunate necessity. Of course, there has to be some way to conduct sound into the ear from a behind-[he-ear (BTE) hearing aid, or as a case to house the electronic circuitry in all types of in-the-ear hearing aids.

But I say "unfortunate" because this necessary function is too often accompanied by discomfort, audible squeals (feedback) and unwanted electroacoustic changes. [Earmolds are also capable of producing acoustical changes, discussed in the July/August 1999 issue of Hearing Loss.]

For new hearing aid users, getting an appropriate earmold is often the most challenging task they face. During the initial hearing aid fitting procedures, new earmolds for BTE aids and re-casing of other types of aids are a frequent occurrence. Indeed, I've known a number of people over the years who, having perennial difficulty with earmolds, have decided to forgo hearing aids entirely. But this is even more unfortunate; the potential benefits of hearing aids are too important to let earmolds essentially determine success with amplification.

Before going into this topic in any more detail, it may be of interest to provide a little historical perspective. I well remember the first ear impressions taken of my ears overs 50 years ago. After inserting a protective tab in my ear canal (a small cotton ball tied with dental floss), the technician would coat the inside of the ear and his hands with mineral oil and proceed to hand insert a mixture of plaster of Paris into my ear. When the mixture got warm enough to be uncomfortable, that meant the impression had hardened and could be removed. Thanks goodness for the coating of mineral oil, otherwise he'd be taking some of my skin out with the impression! Later when I began working as an audiologist, this was how I made my first ear impressions also.

We've come a long way since then and this is clearly illustrated by a recent article in The Hearing Journal by Chester Pirzanski of Starkey Laboratories. The basic topic of the article was an examination of whether earmolds made of softer material are generally superior to those made of harder material. In the course of the article, he examines a number of other issues that relate to the performance of and satisfaction with earmolds. In addition to reviewing the points made in this article, I will also comment on another recent development regarding earmolds: the Laser Accurate Scan Replication method by Siemens.

Materials Used in Making Earmolds

Earmolds can be made with many different materials, ranging from hard acrylic (Lucite) to much softer materials, such as silicon or polyethylene. These materials vary considerably in the types of finishing techniques they undergo, as well as the extent of shrinkage during the "curing" process and then over a period of time.

All hard-body earmolds require sanding and polishing to facilitate insertion in the canal and for cosmetic purposes. The surface of soft earmolds is treated chemically for these same reasons. These finishing techniques may slightly alter the outer dimensions of the earmold. To compensate for this, earmold laboratories build-up ear impressions by coating them with wax before fabricating the earmolds. Both of these activities - the sanding/po]ishing of the earmold and the wax build-up of the ear impression - modify the physical dimensions of the final product to some extent. Even assuming an excellent impression, therefore, these post-impression activities can affect wearer comfort as well as susceptibility to acoustic feedback. Even a slight "pressure point" can produce a very uncomfortable sensation after a few hours of wear. In all the years that I've personally worn hearing aids, getting comfortable earmolds has always presented a greater problem for me than finding the appropriate hearing aids.

It has generally been believed that softer earmolds are more effective in ensuring an acoustic seal in the ear canal, thus minimizing feedback (I know this is the belief I've been operating on). However, as it turns out, this result has not been supported by the available research. In one study, the investigator (Macrae from the National Acoustic Laboratories in Australia) made four earmolds of varying softness from a single ear impression. He found no differences among the four different earmolds in ensuring acceptable acoustic seals. The extent of the acoustic seal was determined by injecting air through a tube in the earmold and determining the extent of air pressure in the ear canal before the air seal was broken.

Soft and hard earmolds appear to provide the same acoustic seal while being rated similarly in terms of comfort.

In a second study, Macrae investigated the impact of varying the thickness of the wax coating on the ear impressions First, he used a thin wax coating and checked the resulting acoustic seal in a silicon earmold made from this impression. Then he added another layer of wax coating, fabricated another silicon earmold, and again checked for the resulting acoustic seal. In all, he did this four times, with each earmold reflecting an additional layer of wax on the same ear impression. Generally, what he found was that the thicker the wax coating on the ear impression, the better the acoustic seal with consequently less acoustic feedback. Unreported, was the effect the thicker coating could have had upon wearer comfort.

However, the best results in the study were obtained with a multi-layered impression technique perfected by Douglas Fifield, also of the National Acoustic Laboratones. In this technique, after an initial impression is made and removed from the ear, additional layers of impression material are spread upon it and re-placed in the ear canal to harden. This has the effect of producing a fuller, more equal fit throughout the bowl of the ear and ear canal. Interested readers can find a full description of this technique in Volta Review, (1980), volume 82(1), pages 33-39. No wax buildup was used on this impression prior to it being used to make an earmold.

These different results were attributed to the fact that a heavy wax build-up on an ear impression simply follows the contours of the original impression. Any distortions in the original impression, therefore, simply become magnified with additional wax coatings. Thus, this action makes it more likely that an uneven stretching of the ear tissues would result when an earmold is inserted in the ear. This would produce both discomfort and make acoustic squeal more likely.

On the other hand, with the multi-impression technique, the build-up with additional impression material occurs right in the ear. Thus, this technique produces equal pressure and equal stretching throughout the bowl of the ear and the ear canal. And as it turned out, the resulting earmold with the multi-impression was actually less bulky than the earmold fabricated with a heavy wax build-up.

According to Pirzanski, the use of light impression materials is a common cause of loosely fitting earmolds, thus increasing the risk of acoustic feedback. And a loosely-fit earmold does not conform to the changing shape of the ear canal when someone talks or chews. This movement of the jaw will produce gaps between the wall of the ear canal and the earmold and potentially be a site for the sound leakage that produces acoustic feedback.

There have been efforts to fabricate earmolds with thermoplastic material. This type of material expands at body temperature within minutes after insertion into an ear While some people benefit from these self-Sealing earmolds, others do not. For most people, the assumption that the ear tissue stretches to conform to the expanding earmold proved to be unfounded. Many people reported that these thermoplastic earmolds were just too tight for comfort.

Allergic reactions may also dictate the nature of the material used in an earmold. Usually, both the hard and soft materials used to manufacture earmolds are biocompatible. Some people, however, exhibit a great sensitivity to some of the chemicals found in earmold material. These people may exhibit soreness or inflammation of the tissues within the ear and require earmolds made of hypoallergenic material.

The problem is that there is no single material that will be effective for all people. Much trial and error and very careful observation is required. For example, in a binaural fitting where only one ear is involved, it is unlikely that the cause of the problem is an allergic reaction. In this instance, the hearing aid dispenser must consider other possibilities, such as a poorly fitting earmold.

In the event of persistent allergic problems, I would recommend that an allergy specialist be consulted. (Incidentally, this is just the type of person that may be able to benefit from a middle ear implant described in the January/February 2001 issue of Hearing Loss).

In general, therefore, it does not appear that earmold fit, acoustic seal and comfort are enhanced by the use of soft material. Softer earmolds, however, are recommended for children (and athletes) because of the greater possibility of physical trauma to the head than the general population.

According to Pirzanski, ear impressions should be made while the person has his or her jaw slightly open using standard viscosity silicon material. In a personal communication, Brad Ingrao (a long-term SHHH supporter who has made a specialty of earmolds) tells me that he often starts with a closed-jaw impression, then after about 30 seconds, while the impression is still soft, he has the person slightly open his or her jaw. If, when the impression is removed from the ear, there is an indentation on it, he calls this anatomic feature to the attention of the earmold lab and asks them not to fill it in during the fabrication process. Summary What most determines the overall quality of the completed earmold is the initial accuracy of the ear impression, the viscosity of the impression material, and the specific care and techniques used to convert an impression into an earmold. This latter factor may be the most cogent, the technicians who perform this function need to be well-trained and carefully supervised. Even the best of impressions can be subtly altered during the fabrication process in ways that defeat the original excellence.

A new method, developed by the Siemens company essentially eliminates problems arising from poorly fabricated earmolds. In this method, which is termed LasR (for Laser Accurate Scan Replication), an ear impression is first made in the usual way. After this, howevei; there is a dramatic departure from conventional techniques. The ear impression is placed on support platform and then scanned with a laser beam. The scanning is programmed to "view" the impression from several positions to ensure a complete three-dimensional representation.

The results from the laser scan are transmitted to a computer program that controls an automated fabrication process. The computer program permits placement of the sound bore in any desired orientation and also creates the hollowed out bore into which the electronics are inserted. The completed earmold will, therefore, be a perfect replica of the original impression; provided that this is accurate, the earmold should fit comfortably.

According to company-sponsored research, consumers rated the LasR hearing instrument shells superior to conventional shells in respect to comfort, feedback reduction, and security in the ear.

The next step in the process is also under investigation. In this step, rather than scanning a completed impression, it is the three-dimensional cavity contours of the ear canal itself that are scanned with a hand-held laser probe. This information would then be transferred to the computer fabrication program for the next step in the process. In this way, any problems due to a poor earmold impression can also be eliminated. The dispenser need only wave a "magic wand" at the ear and, presto, a perfect earmold! Clearly, we've come a long way since the old "Plaster of Paris" days!

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From Hearing Loss, November/December, 2001. This column is supported, in part, by GRANT #133E980010 from the U.S. Department of Education, NIDRR, to the Lexington Center. Mention of products or companies by the author does not Indicate SHHH endorsement, nor should exclusion suggest disapproval. Since everyone's communication problems and needs vary, SHHH suggests consulting with your hearing health professional.