Tips for assessing the retention properties of impressions
|Chester Pirzanski, BSc, is senior supervisor with Oticon Canada, Kitchener, Ontario. He is also involved in earmold research with Oticon A/S in Denmark.|
Custom earmolds and hearing aid shells need to have the best possible fit in the patient’s ear. They are made from ear impressions individually taken from the patient, and ideally carry all the ear’s unique features.
In reality, some earmolds fall short of expectations and may readily work their way out from the ear. Research finds that this poor retention can be linked to an inappropriate style of earmold or hearing aid shell.1 Often, it is the spatial configuration of the ear canal and concha that does not support the custom product ordered.
The following article discusses objectives in custom product manufacturing, examines the common retention areas on ear impressions, and offers advice on the selection of earmold or hearing aid shell style for what is supported by the shape of the ear impression.
Objectives in Earmold Manufacturing
Currently, almost all hearing aid shells and a great number of hard earmolds are manufactured by major earmold labs with the computer aided design and manufacturing (CAD/CAM) method that involves digital impression scanning and virtual impression modeling.2,3 Modeling is the process where the technician (or modeler) modifies the virtual impression with 3D modeling software and creates a virtual earmold or hearing aid shell.
The main advantage of virtual modeling is that the modeler can try out different modifications or layouts of the earmold/shell before settling on the final plan. All changes are reversible until the modeler approves their design. The modeling files can be reviewed later to verify that all modelers adhere to the same modeling rules and the earpieces are the best possible for the customers. This review of the modeling files is also an excellent training tool, as impressions and earmolds of different shapes and sizes can be discussed and the best approach to modeling determined.
The virtual modeling has several objectives, which include:
1) Easy earmold insertion into the ear;
2) Secure fit (adequate retention);
4) Sufficient acoustic seal to prevent the occurrence of acoustic feedback;
5) Correct sound direction inside the ear canal; and
6) Discreet, cosmetically appealing fit in the ear.
Figure 1a shows an image of an ear impression with references to the anatomical features of the human ear, such as the concha, helix, and ear canal (we’ll get to Figure 1b in a minute). The ear canal has two bends that give the canal the distinctive pattern of parallel lines: the center line of the canal aperture appears parallel, or almost parallel, to the center line of the canal medial end.
The earline is an imaginary plane that runs through the deepest point of the tragus, antitragus, and helix. If the hearing aid shell or earmold fits within the earline limit (inside the concha), it will not protrude from the ear and will have an eye-pleasing discreet fit.
What Can Go Wrong in Earmold Ordering? Plenty!
Since human ears differ in size and shape, the above objectives have to be understood and applied to impression shaping. Requests to make the mold with a full helix or canal length may adversely affect the product integrity and fitting in the patient’s ear.
Figures 2a-f show examples of earmolds with a potential fitting problem. If the earmold (A) is modeled as a replica of the ear impression, it can be very difficult or even impossible to insert in the ear. Younger, more determined patients can manage insertion, but older patients—particularly those with dexterity problems—may fail. In contrast, if bullet shaping (B) is requested and all the details of the ear canal are removed, the mold may lack secure fit and slide out from the ear. A long canal earmold (C), preferred by some clinicians, may be uncomfortable if the canal on the mold touches the sensitive canal wall at the second bend. If a light impression material is used to take the ear impression, the earmold (D) can have a loose fit, allowing for uncontrolled sound leakage from the ear canal, and making the hearing aid susceptible to acoustic feedback. When an incomplete imprint of the canal is sent, such as with mold E, the sound bore may end up against the ear wall, thereby compromising the sound quality. Small ears may have the size of the concha inadequate to accommodate all the electronics within the earline limit, and the resulting hearing aid (F) may protrude.
A properly modeled earmold is shown in Figure 1b. The canal tip is trimmed for easy insertion and comfort; the canal aperture is adequately sealed; and the widening at the mold mid-section provides retention.
Retention Areas in the Ear Canal
Modeling an earmold that is easy to insert and has a secure fit may be tricky.5 Figure 3 shows an image of an ear impression and its cross section with the cut plane marked with a blue disc. The red line in Figure 3a outlines the gradual canal widening. This is the retention area. In general, retention for a canal-style earmold or hearing aid shell is created by a widening in the ear canal located deeper in the canal past the narrow canal aperture. Visually, if the canal wall offers support for the little man’s feet, the mold will fit well. However, if the canal area is modeled for maximum retention, as shown, the mold can be difficult to insert because the very wide canal portion of the mold will have to be forced through the narrow canal aperture.
Examples of alternative modelings with balanced insertion and retention areas are shown in Figure 3b-c. The modeling in Figure 3b is appropriate for a CIC or micromold; the modeling in Figure 3c is more appropriate for a canal-style or full-concha mold.
Figure 4 provides images of several ear impressions in both the anterior-posterior (front-back) and superior-inferior (top-down) views. Some offer very good in-canal retention, some not so good, and others none at all. Again, if the ear wall offers support for the little man’s feet, the resulting mold will have a secure fit.
The earmold’s ability to fit securely in the ear canal depends on the shape of the impression in both the anterior-posterior and superior-inferior views. Some impressions offer retention areas in both views, some in one view only, others in neither view. For example, in Figure 4, an imaginary impression that combines A and D has extensive retention areas; such an impression would require significant canal trimming to facilitate the mold insertion. An impression with shapes B and D, or B and F, or C and D will offer satisfactory retention and require less canal trimming as the canal end is not as bulky. An impression with C and F shapes will lack retention in the ear canal.
Retention Areas in the Concha
For larger style earmolds and hearing aid shells, additional retention areas at the ear concha and helix are utilized. Figure 5a shows an impression cross cut through the tragus and antitragus. The red lines outline the primary retention area for the canal lock, half shell, and full concha earmolds (and shells). If the earmold is secured between the tragus-antitragus constriction, it will fit well. Figure 5b shows a cross cut through the helix and antitragus. For the earmold to fit securely, it must rest against the retention areas. In this case, the earmold does not. This skinny earmold will look great in the ear but may not be appreciated by the patient due to poor retention.
Some clinicians request earmolds with a full helix for enhanced retention. Unfortunately, a full helix mold can be very difficult to insert and uncomfortable. Figure 6 illustrates that, when the patient attempts to insert the mold, the wide helix area on the mold will press and constrict the already narrow opening through which the mold is supposed to pass through. As a result, the earmold will not be fully inserted and the patient will wear it with the helix sticking out. The potential for discomfort and acoustic feedback becomes greatly increased.
Figure 7 shows how often a retention area at a given location in the ear canal and concha is present. As provided, more than half (64%) of ears offer retention in the anterior-posterior (A-P) view, and only 40% in the superior-inferior (S-I) view. Almost all ears have retention at helix, but few have it at the back of the concha. The antitragus area commonly offers good retention (80%) for the half shell and canal lock style earmolds.
As for the combination of both views of the ear canal, 70% of small earpieces—such as CIC, ITC, and micromold—will have adequate in-canal retention, 20% will fit better with a canal lock, and 10% will lack retention. This figure of 70% is less than the sum of figures 66% plus 40% because, as previously mentioned, some impressions have retention areas in both views, others in neither.
If the ear lacks in-canal retention, this does not mean that the mold will have a loose fit. Most earpieces will fit securely even in ears with little or no retention thanks to in-lab impression waxing (or surface offset in virtual modeling). Waxing or surface offset will give the mold a snug and secure fit.4
For earmolds that require a remake due to inadequate retention, a careful evaluation of the impression is the key to determine what remake action would be the most appropriate. The common request to lengthen the canal on the earmold or hearing aid shell may not be enough. A longer canal will not provide more retention for earmolds made from impressions illustrated in Figures 4a, 4c, 4e, and 4f. These impressions have retention areas close to the canal aperture or lack one completely. However, a longer canal would help earmolds made from impressions shown in Figures 4b and 4d, if the original earmolds had short canals.
Earmold and hearing aid fittings would be more successful if ear canals were not affected by mandibular movements. Unfortunately, 10% to 20% of patients fitted with custom earpieces have dynamic ear canals. When such patients open their mouths, the ear canal widens,6,7 which may render the earmold retention ineffective. Such a mold is illustrated in Figure 8a-b. While both impressions (A and B) have similar shapes, Impression A (the closed-mouth impression) has a narrower ear canal. Impression B—the open-mouth impression—is wider. An earmold made from Impression A will be loose and fall out from the ear when the canal widens with mouth opening. The solution for the earmold secure fit in dynamic ears is that the clinician takes an open-mouth impression with the use of a mouth prop detailed in a previous paper and available in the HR online archives.8
- Pirzanski, C. Factors in earmold style selection: Starting (and finishing) right. Hearing Review. 2001;8(4):20-24. Available at: www.hearingreview.com/issues/articles/2001-04_01.asp.
- Cortez R. Changing with the times: Applying digital technology to hearing aid shell manufacturing. Hearing Review. 2004;11(3):30-38. Available at: www.hearingreview.com/issues/articles/2004-03_03.asp.
- Sullivan R. Why morphology matters: Practical dispensing applications of 21st century shell design technologies. Hearing Review. 2008:15(3):38-89. Available at: www.hearingreview.com/issues/articles/2008-03_03.asp.
- Pirzanski C. Challenges of high-tech shell-making processes. Hear Jour. 2006;59(10):28-32.
- Pirzanski C. Earmolds and hearing aid shells: A tutorial, Part 4: BTE styles, materials, and acoustic modifications. Hearing Review. 2006;13(9):20-28. Available at: http://www.hearingreview.com/issues/articles/2006-08_04.asp.
- Oliveira R. The Dynamic Ear Canal and Its Implications. Hearing Review. 2005;12(2):18-19,82. Available at: .
- Pirzanski C, Berge B. Ear canal dynamics: Facts versus perception. Hear Jour. 2005;58(10):50-58.
- Pirzanski C. Earmolds and hearing aid shells: A tutorial, Part 2: Impression taking techniques that result in fewer remakes. Hearing Review. 2006;13(5):39-46. Available at: /issues/articles/2006-05_10.asp.
Correspondence can be addressed to HR or Chester Pirzanski at .