TY - JOUR
AU - MD, Nicolas Walsh,
AB - Abstract In this work an integrated system of computer-aided design and computer-aided manufacturing specifically designed to fabricate affordable transparent face masks is discussed. FaceScan©, a custom-designed software system, integrates shape capture, mask design, and pattern fabrication. The software controls a linear scan noncontact laser imager for facial topography acquisition and a milling machine for pattern fabrication. Compared with conventional methods of mask fabrication, this system is faster, more accurate, and less stressful for the patient and allows for greater control of the finished product. Masks for two subjects have been successfully fabricated using this system. The face is one of the most frequently burned areas of the body.1,2 The formation of hypertrophic scars and deforming contractures may lead to devastating facial disfigurement and functional problems. The patient may experience difficulty with vision, speech, and/or feeding along with a significant increase in the psychological stress associated with burn trauma. Nonsurgical and postsurgical management of facial scarring creates difficult clinical problems for occupational and physical therapists, who work to obtain the best possible functional and cosmetic outcomes for the patient.3,4 Techniques used to minimize hypertrophic scarring include elastic garments; elastic hoods; transparent face masks; and foam, silicone, or elastomeric inserts. Uniform compression provided by these devices help conform the scar and have minimal side effects.5 The use of pressure as a means to control hypertrophic scars has been reported as early as 1860, but it was not until the 1960s before it became a mainstream treatment modality.6 Several manufacturers of elastic garments send pressure garments for patients with burns to medical centers all over the world. Hypertrophic scars and contractures can be minimized by maintaining pressure until scar maturation, ideally 24 hours per day for up to 12 to 18 months.2,–4 Elastic garments seem to work well over tubular areas of the body. Garments do not provide uniform pressure to contoured areas of the body, resulting in the tendency of those areas to form hypertrophic scars.5 Foam or elastomeric inserts can lessen this problem. However, these appliances are usually opaque, which makes it acutely difficult to visually determine optimal fit.5 A transparent face mask (TFM) fabricated from an accurate pattern of the head eliminates many of the disadvantages of elastic hoods.7 The vascular blanching of scar beneath the TFM allows the therapist to ensure a proper fit acutely. Patients with facial scarring appear to accept the appearance of a TFM over the elastic hood because it is more socially acceptable as a result of the exposure of facial features in public.8 One disadvantage of the TFM is the lack of flexibility around the jaw. Fabricating a TFM by conventional means is labor intensive and requires a skilled technician. In a survey given to burn therapists at the 2001 American Burn Association Conference, 65% reported 6 to 10 hours of staff involvement in the fabrication of a TFM and 68% stated that the most difficult aspect of making a TFM was “casting” or “modifying the mold”.9 The survey also revealed that 86% of therapists needed to recast the patient as a result of improper fit. The survey concluded that it would be useful to develop more precise techniques to reduce the high incidence of recasting caused by an improper fit. The use of noncontact imaging for TFM fabrication was pioneered at Wright Patterson AFB in 1995.10 That study demonstrated that acquiring the shape of a patient's face is accurate, quick, and painless. This work has been continued at Total Contact, Inc., of Dayton, Ohio. Total Contact uses noncontact imaging to fabricate TFM, but the hardware involved is not easily transportable, expensive, and the software is not specific to the task. Patients need to be transported to their facility for a mask to be fabricated. Conventional manufacturing of face masks involves three general steps. First, an accurate cast is made of the patient's face. The second step is to make a plaster pattern from the cast for fabricating the mask. The third and final step is to mold the plastic over the plaster pattern. Dental alginate is typically used as a casting material. The alginate is poured over the patient's face and allowed to harden. Oftentimes straws are inserted in the nostrils, allowing the patient to breathe. Plaster strips are applied on top of the alginate to provide support. This casting procedure takes about 30 minutes. For the patient, creating a cast of the face is typically an uncomfortable, anxiety-provoking claustrophobic procedure. Children or anxious adults often require general anesthesia before undergoing the casting procedure.7,10 The finished cast is filled with plaster to make a positive pattern for molding the mask. The pattern is smoothed and plaster material is removed from the pattern to apply pressure to scarred areas. The area of the nose is typically built up to avoid excessive pressure on the bridge of the nose.11 There is very little subcutaneous tissue on the nose and therefore significant pressure can be applied to the nose before adequate pressure is achieved over the fleshy area of the cheeks. The plaster pattern is used to vacuum mold the mask. A variety of plastics have been used to make masks such as polycarbonate, copolyester, ethyl vinyl acetate, and cellulose acetate butyrate.5,10,11,12 The edges of the mask are trimmed and smoothed, and then the mouth, nostrils, and eyes are cut out of the mask and strapping is applied. METHODS At the University of Texas Health Science Center at San Antonio (UTHSCSA), the interest in face mask fabrication grew from the long-term program in prosthetic limb computer-aided design and computer-aided manufacturing (CAD/CAM). UTHSCSA has fabricated a wide range of laser-imaging devices as well as developed prosthetic socket CAD software.13,14 Currently, the first three steps of mask fabrication, measurement, design, and pattern fabrication are handled by CAD/CAM. An integrated software system, FaceScan©, was created specifically for mask design and fabrication. The final molding of the actual mask is completed by conventional vacuum forming. A noncontact laser-imaging device acquires the topography of the face. This desktop device is a modified prototype laser imager originally developed to capture the shape of residual limbs for prosthetics and orthotics (Figure 1). The imager uses a low-intensity laser to project a line on the face and uses two video cameras and a position encoder to determine the three-dimensional location of the projected line. Hardware incorporated on a custom computer plug-in card extracts the location of the projected line from the video signal and samples the position encoder.13 The camera geometry is optimized for scanning faces. Both cameras are directed upward at an angle of approximately 45 degrees to aid in capturing the contour beneath the chin and the eyebrow ridge. The scanning head glides on two parallel metal rods and is moved vertically by hand in a linear fashion from below the chin to above the hairline. For the patient, the process is quick and painless. The acquisition time is less than 5 seconds. Figure 1. View largeDownload slide UTHSCSA laser imager. Figure 1. View largeDownload slide UTHSCSA laser imager. The imager is operated by FaceScan© software, which was developed specifically for mask fabrication. FaceScan© is derived from Sockets© (Mignard Sarl Orthopedie-Prothese, Burck-sur-mer, France) software, the UTHSCSA prosthetic CAD package.13 FaceScan© integrates imaging, face mask design, and milling machine operation. The shape data acquisition takes place in real time. As soon as the scan is complete, the data are ready to begin computer-aided mask design. FaceScan© allows interactive local and global modification of the areas of the face. First, extraneous data are trimmed from the scan. Then, any voids in the data are filled by interpolation. Typically, a small amount of data smoothing is applied globally to the entire face whereas regions of hypertropic scaring are locally smoothed to a greater degree. Local regions are interactively defined by drawing an area of interest about the region (Figure 2). These regions can be reshaped as required. Regions can also be interactively defined for pressure relief. The region boundary is defined followed by definition of a control line within the bounded region. The control line is moved a selected distance from the face surface and the remainder of the bounded region is blended smoothly to the control line. Any smoothing or blending operation can be edited or deleted. Figure 2. View largeDownload slide FaceScan© design mode. Figure 2. View largeDownload slide FaceScan© design mode. The finished model is then mapped by FaceScan© to a volume of the foam blank used by the milling machine (Figure 3.). Positioning of the mask shape in the foam blank can be interactively modified. The software also performs interference checking to compensate for the size of the cutting tool, and the milling tool path can be previewed (Figure 4). The mapped data are sent directly via a serial connection to a three-axis milling machine. Figure 3. View largeDownload slide Facescan© milling preview. Figure 3. View largeDownload slide Facescan© milling preview. The three axis milling machine works in a cylindrical coordinate system (r, θ, z). Data are sent to the milling machine as a set of radial values at an angular resolution of 0.5 degrees and 1-mm z resolution. The milling machine interpolates four points between each z increment for greater smoothness. At this resolution, no additional smoothing of the pattern is required (Figure 4). The machine uses a one-quarter-inch ball endmill for cutting. The endmill has a 2-inch length of cut, which allows milling the pattern in a single pass. The milling process takes between 5 and 8 hours. Figure 4. View largeDownload slide Milled pattern. Figure 4. View largeDownload slide Milled pattern. The resulting face pattern is trimmed from the urethane foam blank. An initial sheet of 1/16-inch polypropylene is vacuum formed over the foam. This technique is used for two reasons. First, the foam has a tendency to stick to the plastic so the intermediate layer aids in mold release of the final mask. Second, the plastic in contact with the foam tends to pick up the texture of the foam. The outside of the intermediate layer provides a smooth surface to mold the final mask. The actual mask is then molded using copolyester over the plastic-coated pattern. A silicone mold release is used. The mask is trimmed and holes are cut for the eyes, nostrils and mouth. A six-point elastic harness is attached to the mask that makes for easy adjustment of mask pressure (Figure 5). Figure 5. View largeDownload slide Face mask with straps. Figure 5. View largeDownload slide Face mask with straps. RESULTS Two subjects with facial burns have been fitted to date (Figure 6). Each subject was scanned at the UTHSCSA Rehabilitation Computer Laboratory. They reported tolerating the procedure with ease and spent less than 10 minutes in the computer laboratory. Figure 6. View largeDownload slide Fitting patient with mask. Figure 6. View largeDownload slide Fitting patient with mask. There was a short learning curve for mask fabrication. Each of the two patients required a second mask. The first patient required a refitting because there was undue pressure on the bridge of the nose. Too much pressure was applied to the nose before sufficient pressure was applied to the fleshy areas of the cheeks. This problem was easily solved by building up the bridge of the nose a couple of millimeters using the FaceScan© software before the milling process. The second patient required a revised mask because the initial mask did not extend high enough on the forehead; his hair obscured the imager during the original scan. The patient was scanned a second time with a tight-fitting cap to hold back his hair. Both patients reported that the mask fit very well and that they tolerated wearing the mask. The first patient reported that he was encouraged to wear the mask when the raised scar on the right side of his face began to flatten within the first 2 weeks of wear. The scarring became flat and the patient reported no longer wearing the mask 9 months after his burn injury, although the burn team encourages patients to continue mask wear for at least 12 months. Accurate conformity led to a good fit and tolerable comfort. Both patients reported improved scar appearance and no contractures. DISCUSSION Within the past decade, the technique for applying pressure to the face has shifted from the elastic face mask to the plastic TFM. A mail survey conducted in 1990 revealed that 90% of therapists used elastic face masks over plastic face masks.4 In 2001, a random survey distributed to therapists at the American Burn Association conference reported that 81% used TFM to treat facial scars.9 The use of CAD/CAM for fabrication of TFM ensures optimal pressure and conformity. The noncontact nature of the laser imager combined with the speed of shape acquisition eliminates most of the drawbacks of convention fabrication. There are two factors keeping noncontact scanning technology from being widely adapted. First, the cost of the hardware prohibits burn facility procurement, so patients have to travel to a remote facility; second, traditional CAD/CAM software is not suited for the clinician, creating the need for additional technical support. The UTHSCSA FaceScan© CAD/CAM combines a low-cost imaging system with user-friendly software specific to face mask design. The laser-imaging system cost less than $10,000 to fabricate as a prototype. This is significantly less expensive than currently available laser imagers. Components were selected for expedience more than for price, so even lower costs are possible. An ordinary Microsoft© (Microsoft Corp., Redmond, WA) Windows-compatible computer is used to operate the system. The system is small enough to be easily transportable. The price is low because much of the work of the imager is integrated in the custom PC board. The imager is simple in that there are no motors; it is hand operated. Only the face topology is captured; no color information is captured. The software, FaceScan©, is user friendly and integrates the steps necessary for mask design and fabrication. It is possible to go from scanning a patient to sending data to the milling machine in less than 5 minutes. Currently, two prosthetic milling machines are supported by the software. The software can also export data in the IGES file format, which is supported by all major CAD/CAM vendors. It would be feasible for a burn care facility to have a low-cost imager along with task-specific software. The TFM would be designed at the facility by a therapist. Mask fabrication could be handled locally at the burn facility or by sending the data to a central fabrication facility. For in-house production, there are a variety of suitable milling machines available for less than $10,000. Central fabrication is common among prosthetics and orthotics providers, and many central fabrication facilities can accept data via e-mail. There are existing central fabrication facilities that could accept data from FaceScan©. The advantages of noncontact imaging for TFM fabrication include improved conformity and compression, decreased time involved by therapists, an easily transportable imager, and imaging software that is user friendly and requires minimal training. The chief virtue is that the topology of the face can be captured quickly and accurately without discomfort to the patient If a milling machine is available in house, then a mask can be available in just a few hours. A task that previously required a skilled technician for conventional fabrication or expensive hardware and a technician for CAD/CAM fabrication could be handled by the therapist. Should a mask require revision because of poor fit or excess pressure, the modification process and refabrication can be performed quickly so as not to delay appropriate therapy to a highly visible area of the body. REFERENCES 1. Richard R, Staley M, editors Burn care and rehabilitation: principles and practice.  Philadelphia: FA Davis; 1993. p. 409– 15. 2. Morgan RF, Nichter LS, Haines PC, Kenney JG, Friedman HI, Edlich RF Management of head and neck burns. J Burn Care Rehabil.  1985; 6: 20– 38. Google Scholar CrossRef Search ADS PubMed  3. Ward SR Pressure. 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Walsh NE, Lancaster JL, Faulkner VW, Rogers WE A computerized system to manufacture prostheses for amputees in developing countries. J Prosthetics Orthotics.  1989; 1: 165– 81. Google Scholar CrossRef Search ADS   14. Rogers B, Stephens S, Gitter A, Bosker G, Crawford R Double-wall, transtibial prosthetic socket fabricated using selective laser sintering: a case study. J Prosthetics Orthotics.  2000; 12: 97– 103. Google Scholar CrossRef Search ADS   Copyright © 2003 by the American Burn Association
TI - Computerized Manufacturing of Transparent Face Masks for the Treatment of Facial Scarring
JF - Journal of Burn Care & Research
DO - 10.1097/00004630-200303000-00006
DA - 2003-03-01
UR - https://www.deepdyve.com/lp/oxford-university-press/computerized-manufacturing-of-transparent-face-masks-for-the-treatment-vS0Kxfwpt0
SP - 91
EP - 96
VL - 24
IS - 2
DP - DeepDyve
ER -