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Thermoplastics in Lower Extremity Prosthetics:Equipment, Components and Techniques

Charles H. Pritham, C.P.O.

Thermoplastics first found wide-spread application in lower extremity prosthetics some twenty years ago, initially for check sockets.¹ Today, the use of such sockets for static fitting and dynamic alignment, and with such auxiliary materials as alignate,^2,3 is an integral element in the armamentarium of the progressive prosthetist.

The next application of thermoplastic materials to materialize was for endoskeletal structural systems.^4,5 These offered the advantages of economy, light weight and ready alignment adjustability with heat. Unfortunately, they did not prove to be strong enough for routine use. However, Ohio Willow Wood Co. has revised the concept of a thermoplastic structural system in the Carbon Copy III.

The ultralight below-knee (BK) prosthesis, fabricated in exoskeletal form of polypropylene, gained considerable attention in the mid-70s.⁶ For a variety of reasons, it never achieved general acceptance.

One technique that has achieved acceptance is the concept of the adjustable socket. These were first demonstrated in an aboveknee (AK) version⁷ and subsequently, in a (BK) version.⁸

Another that has gained general interest is that of the flexible socket, SFS or ISNY, originally described by Ossur Kristinsson.^9,10,11

Quite aside from all these applications, thermoplastics have also found use in the fabrication of prostheses in general. This issue of the IPO chronicles the experiences of a number of practitioners with these developments. It is the specific goal of this article to summarize knowledge about the techniques, equipment and components used. The details involved in the fabrication of the specific devices enumerated above are not covered, for they are described adequately elsewhere.

Equipment

The specific vacuum forming technique (Table 1) used depends to a large extent on the equipment available and this seems a logical place to start. Vacuum forming machines for blister forming (Figure 1) are available from a variety of manufacturers and come with a variety of platens and frames (Figure 2) . They all utilize some method of regulating the flow of air so that air may be evacuated slowly at first. This gives the operator time to eliminate webs. Some have advocated use of circular frames, but it is the author's experience that square frames are perfectly satisfactory and cheaper to fabricate. It is advantageous to have frames in a wide range of sizes for use in instances when fine control over wall thickness is desired, i.e. flexible wall sockets. Equipment for drape forming (Figure 3) is similarly available.

Whichever technique is used, it is desirable to utilize a vacuum system that incorporates a surge tank. This acts in a manner analogous to a flywheel to smooth out the demand and response cycle. A vacuum system so equipped can evacuate the air trapped within the envelope of hot plastic faster that can a vacuum pump alone. This is essential when delays may be encountered in affecting a perfect seal, or if the seal is lost. This means that an otherwise lost job may be salvaged.

The other big variable is the oven. A wide variety of ovens may be used, but if the blister forming technique is to be employed, it is necessary to have an oven tall enough inside to allow for the sag of the plastic. This implies a convection oven. A baker's convection oven (such as are used in cookie booths at malls) that measures about 24 x 24 x 24 inches in its interior dimension, is suitable for most prosthetic applications. Optional features that are desirable are glass doors, interior light and an interrupt switch. This last item turns off the fan when the doors are open. This means that the oven stays up to working temperature and needs less time to recover once the doors are closed, and the room stays cooler.

Manuals describing vacuum forming and utilization of specific pieces of equipment are available from some of the equipment suppliers. ^12,13,14

Vacuum Forming Technique

When vacuum forming, it is worthwhile to keep a number of general points in mind. Thermoplastic materials are not particularly good transmitters of heat, and this means that the surface of the sheet of material being used will reach the proper working temperature before the core does. This can become particularly critical with thicker pieces. Either the core is not yet at its proper temperature when the piece is formed and undesirable stresses are created, or the surface can become too hot and begin to degrade leading to fumes, odors and loss of strength. The solution is to lower the temperature and to use a longer heat soak period to heat the sheet.

Models or metal components that are to be incorporated into the vacuum formed structure can rob the plastic sheet of the heat if they are too cold. The result is a loss of detail and definition and the creation of undesirable stresses in the plastic. At the very least, the model should be warm to the touch. Metal components should be warm, or if there is a good deal of intricate detail that the plastic needs to encapsulate, the component should be put in the oven at the same time that the plastic is. This also holds true if the metal piece is to be sandwiched between two layers of plastics.

The use of smaller pieces of plastic to reinforce critical areas of the structure being vacuum formed is worth bearing in mind. Such reinforcing pieces are heated with the main piece, removed from the oven first and positioned on the model and encapsulated in the main piece when it is positioned. The two molten pieces of plastic amalgamate into one under the pressure of the vacuum. Obviously, good technique and team work are necessary to ensure that this welding process occurs properly. It is used most frequently in this country in hip or knee disarticulation sockets to reinforce the area around the joint attachment components. The technique is also used to good effect in Japan in fabricating the thigh extension of the Tokyo Center (TC) AK prosthesis. is This thigh extension is drape formed with the seam running across the distal end and up the medial wall. It is reinforced distally with a piece of plastic, and in addition, about two-thirds the way up proximal from the distal end, the medial seam is reinforced with a strip about 1 to 1 1/2 x 2 1/2 to 3 inches in size. Presumably, this smaller piece of hot plastic is taken from the oven and secured to the model temporarily with adhesive of some sort while the main piece of plastic is draped overall. Resort to such an expedient would seem particularly necessary if the model to be vacuum formed were positioned with its longitudinal axis horizontal and if the plastic were formed with its mating seam down, on the under side of the model. It would seem necessary even if the model were positioned with its longitudinal axis vertical.

Once the molten plastic is positioned, it is critical to effect the vacuum seal and evacuate the trapped air as promptly as possible. Undesirable stresses can be induced in the part being formed if it is not drawn into the final shape before the temperature drops below the critical point. Similarly, it should be held under vacuum until it has thoroughly cooled.

Socket Configuration

The various configurations described in Table 2 can be used for either BK or AK prostheses. The options involving a laminated outer socket wall can be considered as a compromise or intermediate stage between a prosthesis that is fully laminated and one that incorporates no laminated elements. It is perhaps best indicated for use in instances when exoskeletal construction is most suitable for the needs of the patient.

A variety of options are available for use when fabricating an endoskeletal prosthesis with a vacuum formed socket. The particular option to be used perhaps depends upon the purpose to which the prosthesis is to be put. If it is to be used as a temporary prosthesis for a relatively short period, then one of the simpler configurations would seem most practical for reasons of speed and economy. If, however, the prosthesis is to be used as a definitive for a long time, then one of the more complex styles would seem suitable for aesthetic reasons if none other.

The central issue to be addressed when fabricating an endoskeletal prosthesis with vacuum formed socket is the matter of attaching the socket to the structural system. The technique that seems to have found general agreement involves the use of an attachment plate inside the socket (Figure 4) . The structural system is bolted to this through the bottom of the socket. A variety of such attachment plates (Figure 5) are available from a number of manufacturers.

While the specific details vary, in general, the plate is mounted on the distal end of the socket model with a buildup of some sort and the socket is then vacuum formed. As a convenience in locating the holes in plates that use one of the two common four-hole patterns (either the Otto Bock European pattern, or the USMC pattern) place four cap head screws in the holes before vacuum forming. The plastic will be higher over these screws than the surrounding area and may be readily ground away to expose the screw heads. The screws may then be removed with an Allen wrench. To avoid the inconvenience and aggravation of having to line the plate up with the holes and hold it in place while starting the screws during assembly of the prosthesis, it is worthwhile to cut a groove in the buildup just proximal to the proximal edge of the plate before vacuum forming. This will ensure that the plate will be trapped inside the socket and cannot move. The nylon hose can then be tied off in this groove.

The attachment plate must be positioned in proper orientation relative to the socket model. In some instances, this entails transferring the alignment from a prototype prosthesis (a prosthesis incorporating a transparent check socket and dynamic alignment unit and used for purposes of establishing proper fit and alignment of the definitive prosthesis before its fabrication), and in other instances, it involves establishing bench alignment of the socket model relative to the attachment plate and structural system to be bolted to it. In this latter instance, the bench alignment must be close enough to the eventual finished alignment of the prosthesis to take into account the range of adjustment in the alignment unit to be used. Static and dynamic alignment are then performed with the prosthesis before its being finished.

Conclusion

The basic factors involved in using thermoplastic techniques in lower extremity prosthetics have been reviewed. Prosthetists wishing to employ such techniques in their practice must consider a number of factors in deciding which of the various options to implement. The most fundamental of these is the availability of suitable equipment. They must also consider the purpose to which the prosthesis is to be put, and which of the available structural systems they wish to use. As is often the case in prosthetics, there is no one hard and fast solution to these questions and similar results may be obtained a number of different ways.

Charles H. Pritham, C.P.O., is Technical Coordinator for Durr-Fillauer Medical, Inc., located in Chattanooga, Tenn.

References:

1. Mooney, V. and R. Snelson, "Fabrication and Application of Transparent Polycarbonate Sockets," Orthotics and Prosthetics, 26:1, March 1972, pp.1-13.
2. Hayes, R., "A Below-Knee Weight-Bearing Pressure Formed Socket Technique," Orthotics and Prosthetics, 29:4, Dec. 1975, pp.37-40.
3. Hayes, R., "A Below-Knee Weight Bearing Pressure Formed Socket Technique," Clinical Prosthetics and Orthotics, 9:3, Summer 1985, pp.13-16.
4. Lehneis, H.R., "A Thermoplastic Structural and Alignment System for Below-Knee Prosthesis," Orthotics and Prosthetics, 28:4, Dec. 1974, pp.23-30.
5. Pritham, C.H., "Development of a Thermoplastic Below-Knee Prosthesis with Quick-Disconnect Feature," Orthotics and Prosthetics, 28:4, Dec. 1974, pp.31-36.
6. Wilson, A.B., C.H. Pritham, and M.L. Stills, Manual for an Ultra-light Below-Knee Prosthesis. REC, MRH, Temple University 1977.
7. "Irons, G., V. Mooney, S. Putnam, and M. Quigley, "A Lightweight Above-Knee Prosthesis with an Adjustable Socket," Orthotics and Prosthetics, 31:1, March 1977, pp.3-15.
8. Wilson, H.B., C.M. Schuch, and R.O. Mitschke, "A Variable Volume Socket for Below-Knee Prostheses," Clinical Prosthetics and Orthotics, 11:1, Winter 1987, pp.11-19.
9. Kirstinsson, 0., "Flexible Above-Knee Socket made from Low Density Polyethylene, Supported in a Weight Transmitting Frame," Orthotics and Prosthetics, 37:2, 1983, pp.22-27.
10. Pritham, C.H., C. Fillauer, and K. Fillauer, "Experience with the Scandinavian Flexible Socket," Orthotics and Prosthetics, 39:2, July, 1985, pp.17-32.
11. "Fabrication Procedures for the ISNY Above-Knee Flexible Socket," Jan., 1989.
12. "Mobile Vacuum Unit," Durr-Fillauer Medical, Inc., 1986.
13. "Technical Manual for Vacuum Forming of Plastics in Orthotics and Prosthetics" United States Manufacturing Co.
14. "Thermoforming Guide," Orthomedics, 1989.
15. Koike, K., Y. Ishikara, S. Kakuri, and T. Imamura, "The T.C. Double Socket Above-Knee Prosthesis," Prosthetics and Orthotics International, 5:3, Dec. 1981, pp.129-134.