Stereolithography and Prosthetic Test Socket Manufacture: A Cost/Benefit Analysis
Donald Freeman
Leslie Wontorcik, CP
ABSTRACT
Rapid prototyping technology has revolutionized the way parts are engineered and designed. Stereolithography (SL), a form of rapid prototyping, receives data from a CAD (computer-aided design) system and cures a liquid photopolymer resin with a laser to form a solid three-dimensional object.
The manufacture of prosthetic test sockets traditionally has been accomplished by vacuum forming a thermoplastic material over a casting. Stereolithography technology bypasses the need for casting and fabricates the socket directly from the mold. Two test sockets were fabricated using stereolithography and fit to a transtibial amputee. The manufacturing and capital costs were evaluated. Currently, the cost of using stereolithography is exceeding the cost of current methods of prosthetic test socket fabrication.
Stereolithography technology is in its infancy but changing rapidly. In the future, all prosthetic sockets and orthoses may be directly manufactured saving time and money while maintaining product quality.
Key Words: Prosthetic; Socket; CAD.
Introduction
Rapid prototyping produces a three-dimensional model or part by stacking many thin layers on top of one another (2). Stereolithography, the most widely used rapid prototyping and manufacturing technology, uses a CAD file to generate the solid three-dimensional object (3). Ultraviolet laser radiation is directed onto a pool of liquid photopolymer resin that hardens to a regulated depth, known as the layer thickness. The object is submerged into the liquid to add an additional layer of photopolymer resin, which will be cured by the laser once again. Layering the object is built through repeated passes of the laser until the part is completed (see Figure 1)
. The time it takes for the object to be cured by the laser is known as the build time. This is a function of the resin type, geometry and size, laser power and recoating speed.
The purpose of the study was to manufacture a prosthetic test socket using stereolithography and dynamically align it on a patient. The manufacturing time and cost/benefit was determined for a transtibial test socket eight inches (203 mm) in height.
Methods
Subject
We have been digitizing shapes for above-knee, proximal femoral focal deficiency, and below-knee sockets, and for various torso orthoses. Inspection of the frequency spectra of these shapes revealed the below-knee (BK) shapes consistently exhibited the most high-frequency content. Therefore, 55 pediatric and adult BK shapes were used as the data source. These shapes were digitized at Newington Orthotic and Prosthetic Systems, in Hartford, Conn., and at the corresponding institution.
Digitization
The mold was digitized on a Mind Seattle Provel® digitizer. The surface data points in the AOPA (American Orthotic and Prosthetic Association) CAD format were converted into a standard stereolithography ASCII file format recognized by rapid prototyping systems.
Manufacturing
The CAD image of the socket in the SL ASCII format was sent to a SLA-250/40 (Stereolithography Apparatus) for fabrication. Figure 2
shows a transverse view of the test socket submerged in the pool of liquid resin. Photopolymer epoxy resins were used to manufacture two test sockets. After the sockets were cured by the laser, a final curing process in an ultraviolet oven for two hours was performed. The resin is 80 percent cured by the laser in the SLA and the remaining 20 percent in the ultraviolet oven. Trim lines were added using a cast saw and standard finishing techniques.
The socket was bonded to a wood block using Siegelharz resin and preliminary alignment was done on endoskeletal components with a College Park TruStep Foot®. The completed test socket and components were then statically and dynamically aligned on the patient (see Figure 3)
.
Results and Discussion
An SLA-250/40 was used to manufacture two test sockets (see Table A)
. The manufacturing build times and total manufacturing costs are listed in Table B
.
The first test socket, as shown in Figure 4, with a build time of 58 hours, resulted in 0.0625 inch exterior grooves in the socket wall. The grooves were due to errors in the file conversion from the AOPA file format to the STL ASCII file format. These errors also caused the laser to cure the test socket wall in a random fashion increasing the build time. A one and one-half-inch check ring was built into the proximal edge of the first test socket to verify the accuracy of the digitized cast compared to the dimensions of the SL socket. Objects manufactured by SL have tolerances in the range of 60.0017 of an inch and were more than sufficient for a prosthetic test socket.
The total cost for the first test socket with a build time of 58 hours was $3,480 based on an average manufacturing cost of $60 per hour on a SLA-250/40. This includes machine preparation, resin and build time costs.
The second test socket, as shown in Figure 5, had a build time of 26 hours, which was approximately a 50 percent reduction in build time from the first test socket. This was a result of eliminating the exterior grooves in the socket wall by altering the STL ASCII file conversion software and removing the check ring. Also, a faster build time photopolymer resin also was used. The DuPont Somos 6110® resin, when cured by the laser, requires less energy to change the resin from a liquid to a solid state decreasing the build time. The socket wall thickness was reduced from 0.25 inches to 0.156 inches, slightly reducing the build time. The total cost of manufacture for the second test socket was $1,560.
3D Systems designs, develops, manufactures and markets SLA equipment (1). Three SLA machines available are the SLA-250/40, the SLA-350/10 and the SLA-500/40. Actual and projected build times for the two test sockets on these machines are listed in Table C
. The SLA-250/40 machine has the least expensive capital cost (Table D
) and had the slowest build time. The SLA-350/10 and the SLA-500/40 have a 45 percent and 65 percent faster build speed respectively than the SLA-250/40. This increase in build speed is due to greater laser power and the use of a Zepher® recoating arm that enables the photopolymer resin to be applied to the model at a faster rate. Table C
shows the projected build time of the two sockets manufactured on the faster SLA-350/10 and SLA-500/40. This decrease in build time on the faster SLA-350/10 and SLA-500/40 is significant and the lowest projected build time of nine hours is achieved on the SLA-500/40 machine with a total manufacturing cost of $900 based on an average $100/hour build time cost for the SLA-500/40.
The socket comfort and fit was similar to previous test sockets made of copolyester by traditional methods of vacuum forming. No material tests were conducted on the two test sockets. However, the photopolymer resins were strong enough to support the patient during ambulation for one hour. The strength of the two resins used appears to be adequate for a test socket fitting but is very brittle and would not be suitable for the high stess that occur in a definitive socket (4).
Conclusion
Prosthetic test sockets can be manufactured using stereolithography technology. The manufacturing and capital costs, however, exceed those of current methods of fabrication. The two photopolymer resins used in the study were semi-transparent and allowed for the viewing of the TEC® liner through the socket wall. The resins do not offer the necessary material properties for a definitive prosthetic socket but successfully were used in this study for a test socket fitting.
To make stereolithography a practical method of prosthetic test socket manufacture, the production time and cost must be reduced significantly. The manufacture of prosthetic test sockets by stereolithography has advantages such as eliminating the need for a positive mould, not restricting the shape complexity and allowing for a varied wall thickness.
Rapid prototyping technology is progressing at a rapid rate. Stereolithography, the most commonly used rapid prototyping process, will continue to be used by many industries worldwide. In the future, as resins emerge with new material properties and SLA machine speeds increase, the use of this growing technology may become a cost-effective method of manufacture for the orthotic and prosthetic industry.
References:
- 3D Systems. Solutions Network: 3D Systems. (www.3dsystems.com)
- Rovick JS. An additive fabricator for high speed production of artificial limbs. Proceedings of The Fifth International Conference on Rapid Prototyping, Dayton, Ohio, June 1994.
- Jacobs PF. Stereolithography: From art to part. Cutting Tool Engineering April 1993;45:3.
- Jacobs PF. Stereolithography and other RP&M technologies from rapid prototyping to rapid tooling. (passim)
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