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Thermoplastic Applications in Lower Extremity Prosthetics

C. Michael Schuch, C.P.O.

Introduction

The application of thermoplastic materials in lower extremity prosthetics dates back further than many current advocates of its use may remember.^1,2 Currently, for various reasons, its popularity and use in this area of prosthetics is on the rise. The purpose of this paper is to present advantages, disadvantages and technical considerations, all based on more than five years of clinical and/or research experience with various thermoplastic lower extremity prosthetic designs.

The author's initial experience with thermoplastics in custom fabricated prosthetic sockets was gained through research funded by the Veterans Administration at the University of Virginia Medical Center during 1984-1986, using flexible Surlyn below-knee (BK) sockets supported within semi-rigid polypropylene socket retainers.³ This experience made significant impressions on those prosthetists involved, primarily in two regards: first, the positive subjective patient feedback regarding socket flexibility and "forgiveness," coupled with the additional positive reaction to the significant weight reduction and, second, an interesting observation considering the rather limited availability of socket-pylon adapters at the time, was the significant fabrication advantages as compared to the traditional techniques of laminated sockets. Considerable experience has been gained with thermoplastics over the past five "ears, both individually and collectively.^3,4,5,6,7,8 Significant progress has been made as well. These advances include additional and better options for connecting the socket to the pylon, more component options that lend themselves to thermoplastic applications, and additional and better cosmetic finishing techniques. While it is recognized that this avenue of fabrication is not suitable for all prostheses, this author has become increasingly reliant upon thermoplastic variants in lower extremity prosthetics. The following subjective and anecdotal remarks regarding the advantages of and techniques in the use of thermoplastics includes experience in two different geographical and three different patient care settings over five years, involving more than 125 prosthetic client fittings as determined by careful record review.

Why Use Thermoplastics?

The answer to the above question, is, in short, that the advantages greatly outweigh the few disadvantages.

Patient acceptance of new techniques and materials should be at the top of any list of advantages for such techniques and/or materials. Patient reaction has been consistently positive regarding the flexibility and reduced weight of thermoplastic sockets. The author has found this to be the case, whether such patient response was obtained via subjective patient questionnaires used in research evaluation,³ or in normal clinical practice where such feedback is usually verbal and obtained with less formality.

The efficiency of thermoplastics in lower limb prosthetics is an additional advantage. Fabrication of thermoplastic prosthetic sockets is quick and relatively simple, as is the training of technical personnel. Both of these aspects of thermoplastic efficiency are noted as compared to the traditional lamination techniques which require precise mixing formulas and material layups, as well as additional fabrication time. When thermoplastic sockets are fabricated using any of the available modular component adaptors, efficiency is enhanced by the ease of assembly of socket and components as well as inherent dynamic alignment capabilities which eliminate the need for socket-component transfer procedures after dynamic alignment is established. A final measure of efficiency is durability. Excepting very early attempts with extremely thin Surlyn flexible sockets, this author has found thermoplastics to be significantly more durable than thermoset laminated sockets.

Frequently, disbelief has greeted such statements. It is suspected that the inherent flexibility and forgivingness of the polyethylene and polypropylene thermoplastics contribute to the durability experienced, whereas, the trend in laminated sockets has been to fabricate them increasingly thinner, with quite rigid, unforgiving reinforcement. Polyethylene for flexible sockets, and polypropylene for socket retainers or rigid sockets have been the author's choice of plastics since 1986. The only socket failures, or socket retainer failures recalled, are those that fail as the plaster model is being broken out and removed from the thermoplastic socket. Not a single instance of failure after delivery to a patient can be recalled. The advent of new and increasingly better thermoplastics should create even more confidence in their long-term durability.

The economy afforded by the use of thermoplastics in prosthetics is yet another advantage. Quicker fabrication and ease of training technical personnel, mentioned previously, offer obvious economic advantages. Lamination materials such as acrylic resins and carbon fiber are significantly more ex pensive then the earlier polyester resins used with dacron felt, nylon and fiberglass. Thermoplastic materials, whether precut for specific prosthetic socket size applications, or in large sheet size, offer significant material savings in addition to savings in fabrication time. Our profession is "being confronted with pressure for cost containment and it behooves us to respond responsibly. If the move to thermoplastics is one way to do so, then so be it."⁹

A final economic advantage to both prosthetist and patient alike makes use of the modular efficiency of thermoplastic sockets as described earlier. As opposed to the traditional practice of using several temporary or preparatory prostheses for mobilizing and training new amputees, in which simple pylon-foot systems are utilized and completely replaced with perhaps new and/or different components at time of delivery of the definitive prosthesis, the modular aspect of thermoplastic sockets with appropriate adaptors for use with modular-endoskeletal structural systems allows the patient to be provided with definitive components at the various levels indicated, including accessories such as rotators. The socket is the only component replaced as the patient's residual limb atrophies. Costs of such necessary and timely socket replacement are no greater than those associated with replacing the traditional preparatory limb and such replacements offer the advantage of training and early familiarization with the component functions that will remain definitively with the prosthesis. The modular concept of quick socket interchange and inherent alignment capability allows socket replacement and prosthesis realignment in a single patient visit (after molding and test socket visits) so that the patient does not have to be without the prosthesis.

A final and quite important advantage of thermoplastic applications in prosthetics is workplace safety. It is no secret that the materials and processes of laminations are hazardous. Additionally, the dust from grinding the various laminated resin sockets and/or polyurethane foam fillers is equally dangerous and hazardous. Vacuum forming of thermoplastic products is a much safer and cleaner technique. Grinding and sanding of these materials does not produce the dust particle problem associated with automated systems in prosthetics, the safety of the production environment can be further enhanced.

The Few Disadvantages

One cannot discuss advantages without including disadvantages. The disadvantages most commonly associated with thermoplastic sockets have been decreased durability because of material tears or splits, and imprecise fit because of shrinkage of thermoplastic materials.⁶ Experience with durability has already been addressed; an additional comment is relevant regarding both of these suggested disadvantages. Thermoplastic technology has advanced to the point that there is significant literature available from the various distributors of plastics with recommendations for proper application, specific to plastic type, thickness, oven temperature, heating time, cooling techniques, annealing techniques, etc., (Appendix A) ; if such recommendations are followed, especially in light of the continually improving quality of thermoplastic materials, some of which now claim zero percent shrinkage, then those problems previously associated with thermoplastic applications in prosthetics should become history.

Description of Techniques

Adaptor Plate: Our preferred technique for fabrication of thermoplastic sockets for use with modular-endoskeletal component systems is dependent upon a 2 x 2-inch aluminum plate, 1/4-inch thick, containing the standard modular four hole pattern, drilled and tapped to accept the Otto Bock 501S41 M6x2O counter sunk head Allen screw. (Appendix B, item 1 , Figure 1 , and Figure 2 ) This attachment plate is vacuum formed into the distal end of the socket, using one of several techniques described below. To prevent having to drill the hole pattern through the distal socket and risk damage to the threaded holes in the attachment plate, four dummy cap head screws, with the heads reduced to the diameter of the shaft of the screw, are placed in the threaded holes of the plate so that they project distally (Figure 3) . After the thermoplastic has cooled, the plastic over the four projecting dummy screws is easily cut open and the screws removed with the appropriate Allen wrench. Initially, we fabricated this attachment plate within our facilities, but due to increased volume in thermoplastic applications, we have found it less expensive and quicker to have the plates produced by a local machine shop. With almost five years' experience in several patient care settings using this type of adaptor plate, neither the author nor his associates have experienced any failures of the plate and related attachment technique.

In addition to the simple and inexpensive attachment plate described, there is a significant proliferation of attachment plate options and techniques, offering the prosthetist a wide range of choices for accomplishing the task of coupling modular prosthetic systems utilizing thermoplastic sockets.

Techniques For Incorporation of the Attachment Plate

The goal is to provide a transition from distal socket, which ideally provides a total contact interface with the residual limb, to the socket attachment plate. A smooth transition with no indentations or abrupt changes in the contour of the plastic is necessary for maximum strength and durability. The final goal is to have the attachment plate located in such a position so as to pre-align the socket angularly and linearly (Figure 4) . Described below are various alternatives for both below- (BK) and above-knee (AK) prostheses.

Below-Knee Alternatives

There are three alternatives commonly used for incorporating the attachment plate in BK thermoplastic sockets. If an insert liner is desired, such as Pe-Lite or silicone gel and leather, an exterior distal end buildup of 1/2-inch medium or firm density Pe-Lite is glued, heated and formed into placed using a latex rubber sleeve (Figure 5 and Figure 6 ). This Pe-Lite buildup is then flattened using a disc sander, paying attention to angular and linear alignment requirements and then blended into the shape of the attachment plate distally (Figure 7 and 8 ). Typically, 3/8-inch thick polypropylene is used in this, socket fabrication (Figure 9) .

In the case of a flexible socket, rigid socket-retainer system, the inner flexible socket is vacuum formed first. A distal buildup of the same type as described above is then added and blended into the attachment plate. Materials for the distal buildup have included 1/2-inch Pe-Lite or plaster of paris poured into a cup secured to the distal end of the socket. The plaster buildup is preferred because it is later removable for weight reduction. For such a socket design, we use modified or linear low density polyethylene of 1/4-inch thickness for the inner flexible socket and 3/8-inch thick polypropylene for the socket retainer. The polypropylene socket retainer may be fenestrated as desired.

An additional alternative is a hard socket technique in which no liner or flexible socket is used. The distal buildup of either Plastizote or Pe-Lite is applied directly to the distal end of the plaster model and blended into the model and the attachment plate. The result is a hard socket with an incorporated distal end pad. Polypropylene thickness of 3/8-inch is preferred.

Above-Knee Alternatives

The two AK alternatives commonly used are the flexible socket, rigid socket-retainer technique or the hard socket with Plastizote or Pe-Lite distal end technique, both fabricated in the same fashion as their BK counterparts. Plastic thicknesses for AK sockets are 1/4 to 5/16-inch for flexible sockets and 3/8 to 1/2-inch for hard sockets or socket retainers. (Suggested pre-vacuum thickness for both BK and A K sockets are for average size, average activity level adults. Thicknesses may vary depending on patient size, residual limb length, activity level, plastic vacuum-forming techniques, etc.)

Additional variables for the AK socket system depend on the socket length. In the case of shorter AK sockets, the distal thigh component, between the distal socket and the knee component, can be fabricated or provided in either of two ways. A modular component spacer can be incorporated, such as a pylon tube with adaptors, or, as we prefer, the Shamp Extend-A-Tube (Appendix B, item 2 , Figure 10 ). Alternatively, in the case of a flexible socket, rigid socket-retainer system, the socket retainer may extend the required length of the thigh and adapt directly to the knee attachment component, leaving a void between the flexible socket and the bottom of the retainer. The former alternative, use of a modular component, allows more adjustability and requires less precise measurements and fabrication. On the contrary, the latter alternative, extension of the socket retainer, requires precision and is limited in adjustability for height, but does allow for a decrease in total weight of the prosthesis. Frequently, we have used the former alternative to arrive at correct alignment and length, then transferred the modular spacer component, and refabricated the socket retainer to the correct length.

Conclusion and Summary

Discussion of experience with and advantages of thermoplastic applications in lower extremity prosthetics has been presented. Clearly, there is a case for their increased use in prosthetics. The variety of fabrication and socket design techniques possible further enhances their use. In the case of flexible socket, rigid retainer systems, the ease with which a flexible socket can be replaced has been suggested.⁸ Improvements in the thermoplastics designed specifically for prosthetic use have been noted. One should expect that as newer and better materials and techniques are identified, that this relatively new trend in prosthetic technology will continue to be of benefit to both prosthetist and patient alike.

C. Michael Schuch, C.P.O., is the manager of the J.E. Hanger Southeast Firms of Greenville Orthopedic Appliance Company in Greenville, S.C. and Friddle's Orthotic and Prosthetic Lab in Spartanburg, S.C.

References:

1. Wilson, A. Bennett, Jr. and M. Stills, C.O., "Ultralight Prostheses for Below-Knee Amputees," Prosthetics and Orthotics, 30:1, March 1976, pp.43-48.
2. Irons, et al., "A Lightweight Above-Knee Prosthesis with an Adjustable Socket, Orthotics and Prosthetics, 31:1, March 1977, pp.3-15.
3. Schuch, C. Michael, C.P.O. and A. Bennett Wilson, Jr., "The Use of Surlyn and Polypropylene in Flexible Brim Socket Designs for Below-Knee Prostheses," Clinical Prosthetics and Orthotics, 10:3, Summer 1986, pp.105-110.
4. Berry, Dale, C.P., "Flexible Above-Knee Socket Made From Low-Density Polyethylene Suspended By A Weight Transmitting Frame," JPOS-Composite Materials for Prosthetic Orthotic Application, April 1985.
5. Berry, Dale, C.P., IPOS-Flexible Socket, Case Study and Overview, April 10, 1985.
6. Kristinsson, Ossur, "Flexible Above-Knee Socket Made From Low Density Polyethylene, Supported by a Weight Transmitting Frame," Prosthetics and Orthotics, 37:2, June 1983, pp.2527.
7. Lehneis, HR., Ph.D., C.P.O., et al., `Flexible Prosthetic Socket Techniques," Clinical Prosthetics and Orthotics, 8:1, Winter 1984, pp.6-li.
8. Pritham, Charles H., C.P.O., et al., "Experience with the Scandinavian Flexible Socket," Orthotics and Prosthetics, 39:2, July 1985, pp. 17-32.
9. Pritham, Charles H., C.P.O., Technical Director, Durr-Fillauer Medical Inc., Editor, Journal of Prosthetics and Orthotics, personal communication, March 1990.