The hearing industry and the audiological fields have made steady progress in diagnostics and amplification over the past 20 years. Today, we have products that could only have been dreamed about in the 1980s: CICs, digital instruments, and remote control devices that are housed in watches.
Unfortunately, the process of reproducing the external ear via ear impressions to make the earmold or ITE housing (shell) has not changed substantially in the same 20 years. However, the process will change, and a custom ITE—without taking an ear impression—will someday become a reality.
The same digital technology that gave us digital hearing instruments is also providing the technical platform to eliminate the ear impression, replacing it with a digital scan of the external ear, which will capture the active area (ie, jaw open vs jaw closed) of the ear canal. Scanning the ear was always a dream in the hearing care profession, and we are now moving closer to the day when it will be a reality.
Although scanning the ear canal is still in the future, it is possible to start taking advantage of digital mechanics within the shell-making process today.
Digital Mechanical Processing and How It Works
The benefits of the new digital mechanical technology are only now beginning to be appreciated. This article, which reviews the Phonak NemoTech impression-making process, will introduce the reader to the concepts involved in using this technology and what can be expected from it in the future.
Step 1: The Impression. The process begins with an impression. Since the impression is the foundation of any custom-manufactured shell, it remains the key component for a proper fitting hearing instrument. Although this is an area that will see radical changes in the future, the impression-taking process remains the same at this time.
Figure 1. 3-D laser scanning captures the details of the impression down to the micron.
Step 2: 3-D Laser Scanning. Currently, the digital mechanical processing starts with the digitization of the shape of the ear impression (Figure 1). Using advanced laser-scanning methods, it is possible to produce an extremely accurate three-dimensional replica of the ear impression. The ear impression is scanned using lasers with specially created high-end optics that scan the impression, taking up to 100,000 data points, with precision on the level of microns. The shape of the impression is reconstructed in software, and these measurements are then transferred to a permanent database for storage.
Figure 2. The digitized impression is finished on-screen using a digital model of the shell.
Step 3: Computer Modeling. The digitized impression of the ear is finished on-screen by the shell technicians in order to optimize the fitting (Figure 2). The functional structures of each shell, such as wall thickness, venting paths, and mounting frames for electronic components, are designed by computer in virtual space. For quality purposes, the technician also has a model of the original ear impression for guidance in the detailing steps.
The shell can be viewed and modified in a three-dimensional space before manufacture. This detailed impression is stored in a database, and the original impression is kept for future comparison and retrieval.
Figure 3. The shell is produced by laser sintering. Thin layers of plastic are solidified with a laser.
Step 4: Shell Fabrication. The shell itself is produced by first heating a biocompatible nylon powder just below its melting temperature, and then a precise, computer-guided laser melts the contours of the shell, layer-by-layer (Figure 3). This technique is called laser sintering. This form of computer-aided manufacturing is designed to produce a shell of consistent thickness that is a perfect replica of the original impression. As with current shells, the shell material may be modified in the dispensing professional’s office.
Figure 4. The outer surface is completed with a textured finish.
Step 5: Surface Finish. The surface of the shell is matted to provide a skin-like texture (Figure 4). This textured finish provides a firm, comfortable fit with suitable retention in the ear. Yet, the material is not porous and it is designed for easy cleaning.
Present and Future Benefits
There are several advantages to this new shell-making procedure:
Figure 5. The new shell-making process allows recognition of factors, such as tightness of fit and points of irritation, before the shell is made.
Accuracy. Digitizing the image of the ear impression allows for a more accurate representation of the impression in the final shell. Assuming that the impression is accurate, the end result should be a more comfortable, better fitting instrument (Figure 5).
Durability. The plastic used in the sintered shell is designed to be much stronger than the material used in traditional shells. The medical-grade nylon material has been developed so that it will not wear thin or crack, even after several years of use.
Design Optimization. The computer-aided design process allows the technician to see how all the components will fit into the shell before it is made (Figure 2). Therefore, the shell can be designed to produce the smallest instrument possible that will still accommodate all of the necessary components.
Easy Reproduction, Comparison, and Replacement. Since the shell is made from a computerized three-dimensional representation, the data can be stored for later use. Remade shells can be compared to the original to identify where discrepancies in the fit may occur. This should remove some of the current “art” during the impression-making process and allow more decisions to be based on objective or scientific criteria. In addition, this information will allow knowledge-based and expert systems to be developed.
Lost instruments can be remade to original specifications from stored data. No new impressions are necessary.
Comfort and Fitting Issues. The shells are made from a hypoallergenic, medical-grade plastic. Its textured surface is designed to increase wearer comfort and to reduce slippage.
Clinical Trial Results
A clinical trial of the NemoTech shells is now under way. Pilot study data have been returned on 50 hearing instrument wearers and these data are summarized below.
The objective of the clinical trial is to compare existing UV shell technology with the new shell technology. Subjects were recruited based on their experience with hearing instruments. Since the goal is to evaluate the materials, the comparisons were made between shells that were identical in every way, except for the material and process used in manufacture.
The subjects for the clinical trial included 50 experienced hearing instrument wearers. All were wearers of custom hearing instruments, either full-shell or canal models. To be eligible for the study, they had to be wearing their hearing instruments successfully for at least 1 year. Although both monaural and binaural wearers were eligible, approximately 75% of the participants wore binaural amplification.
Each of the 50 subjects, via their hearing care professional, agreed to send their current hearing instruments to Phonak for duplication. The current hearing instruments were scanned, and exact replicas of the shells were manufactured using the digital shell-making process. Every attempt was made to accurately duplicate the electroacoustic characteristics of the original instruments. Both the old instruments and the new were returned to the wearer.
Wearers were instructed to complete a questionnaire regarding their current hearing instruments. They were asked to rate their satisfaction with their current hearing instrument shell on the following factors: ease of insertion, ease of removal, comfort at time of insertion, comfort after 1 to 2 hours, comfort after 4 to 8 hours, irritation, security, sound of their own voice, feedback, general appearance, and overall satisfaction. The rating used a 5-point Lickert scale: 1 was very dissatisfied, 5 was very satisfied.
Figure 6. Results from the pilot test of wearer acceptance of the shells. The new shell manufacturing technology produced results that were similar or superior to the UV manufacturing method: 84% of wearers chose to keep the new shells rather than the UV shell.
Upon being fit with the NemoTech shell, the subjects rated their satisfaction on these items. The survey was repeated after 6 and 12 weeks (Figure 6, page 49). Although the initial acceptance was good, the overall scores show little difference between the traditional UV shells and the new shell technology.
At the end of the trial period, the subjects were asked which shell they preferred. They had to keep one set, and return the other. Forty-two of the 50 subjects (84%) chose to keep the new shell.
In all cases, the wearer readily accepted the NemoTech shells. These shells were rated as good or better than the traditional shells on all categories tested.
It should be noted that the only thing tested in this pilot study was the acceptance of the shell material. Given the advantages to manufacturing and the ability to better match the requirements of the consumer with the new shell-making process, the acceptance of the new shell material is the key to allow this technology to move forward.
The Future of Digital Shell Making
The authors believe that the hearing industry is beginning to see the future of custom ITE hearing instrument manufacturing. Within the next few years, hearing care professionals will be able to use this innovation for three-dimensional visualizations of the custom instruments they order. They will be able to experiment with different shell styles, designs, and options via computers, and actually see the effects of those changes—before the hearing instrument physically exists.
Eventually, technology like this will enable the professional to scan the wearer’s ear directly and transmit these data electronically to the manufacturer. The ear impression will, at that point, become a thing of the past. All orders and files will be maintained electronically. In this way, digital mechanics represents a new frontier in the ordering, designing, and fitting of hearing instruments.