An Adjustable Preparatory Prosthesis for Recent Below-Knee Amputations
Terry J. Supan, C.P.O. *
This work supported by V.A. Contract No. V101 (134)-P326 and presented at the 1978 AOPA National Assembly, San Diego, California, Nov. 3,1978.
Management of the below-knee (B/K) amputee is one of the largest areas of limb prosthetics. It is also one of the areas where prostheses are the most successful. It is our opinion that early management of B/K amputees can be improved and made less costly through the use of an appropriate preparatory prosthesis.
The goal was to develop a preparatory prosthesis for B/K amputees which would have a socket that may be changed in size as the limb shrinks. Presently, many new prostheses must be made over or lined extensively because of residual limb volume change. Much of this volume change occurs on the posterior and distal aspect of recently amputated limbs. The preparatory prosthesis designed to meet these needs consists of a low temperature thermoplastic interface, an outer polypropylene socket, a Polyvinylchloride (P.V.C.) pylon and a SACH foot (Fig. 1). The interface keys into the outer socket, and this fit remains relatively intimate because the volume changes are mostly restricted to one area of the socket.
Aquaplast* is used for the removable interface because it can be formed directly to the amputee's limb or to a modified cast (Fig. 2). The inherent memory of the material can be utilized to reduce the volume of the socket. During the forming procedure, the posteriodistal aspect of the interface is stretched to reduce its thickness. When this area is later reheated, it will tend to return to its original thickness, thus reducing the interface's volume.
The outer polypropylene socket is vacuum-formed, either directly over the Aquaplast interface or over a plaster model of the interface. Attachment of the P.V.C. pylon is made by means of a polypropylene extension  . A short length of P.V.C. pipe is temporarily attached to the interface or model in static alignment. Thus, when the socket is vacuum formed and the P.V.C. removed, the extension becomes an internal part of the socket. Ice-water quenching of the polypropylene for one minute is recommended because the material is flexed extensively during insertion and removal of the interface (Fig. 3).
Polyvinylchloride (P.V.C.) pipe-schedule 40, 1 1/4"—is used for the pylon, so alignment changes can be made as the amputee progresses from simple weight shifting to full ambulation (Fig. 4). Alignment changes are made by locally applying heat to the pylon . Initially, a hose clamp is used to maintain pylon attachment to the socket. Once basic alignment is achieved, this attachment is reinforced with either "pop rivets" or sheet metal screws. Attachment of the pylon and foot is made with a standard aluminum foot plug. The distal end of the pylon is chamfered internally, heated until softened, then forced over the foot plug. A hose clamp may be used to secure the pylon and plug, although this is not always necessary.
After the amputee has progressed to ambulation outside parallel bars, a rigid polyurethane foam is used to cover the pylon. All major alignment changes should have occurred by this time. Although the shaped foam improves the cosmetic appearance of the prosthesis, its main function is to reinforce the pylon and protect the amputee from a mechanical failure.
In the normal course of rehabilitation, an amputee's limb volume will decrease as ambulation increases. This decrease is usually accommodated by the addition of prosthetic socks and maintained at night by either elastic bandages or "shrinker" socks. Because the Aquaplast interface can be separated from the prosthesis, it may be used independently to accommodate the volume decrease and worn separately at night to maintain limb volume. When the amputee has progressed to a 15-ply sock, the posterior distal aspect of the interface is reheated (Fig. 5). The interface's volume will decrease because of the material's memory. This procedure is continued until the amputee's limb volume remains constant during one month of ambulation. At that time, the amputee is ready for the definitive prosthesis.
The fitting of nine individuals during its development has shown that the use of this type of preparatory prosthesis is effective in management of below-knee amputees with recent amputations. The cost in time and money for the preparatory prosthesis would be considerably less than the cost of a definitive prosthesis, extensive liners and adjustments, and possible new sockets within one year's time. Our experience indicates that upon completion of preparatory prosthesis use, the amputee's limb is "well seasoned" and ready for a definitive prosthesis which should last for several years.
References:
- Pritham, Co., Letner, I., and Knighton, D., "Use of Thermoplastic Components in Temporary Prostheses", Orthotics and Prosthetics, 30:4, 31-34, Dec. 1976.
- Pritham, Charles, Director, Prosthetics and Orthotics, Rehabilitation Engineering Center, Moss Rehabilitation Hospital-Temple University-Drexel University, Philadelphia, Pennsylvania 19141, Personnel Communications.
- Stupp, S.I., Supan, T.J., and Belton, D., "Ice-Water Quenching Technique of Polypropylene", Orthotics and Prosthetics, 33:1,16-21, March 1979.
- Lehneis, H.R., "A Thermoplastic Structural and Alignment System for B/K Prostheses", Orthotics and Prosthetics, 28:4, 23-29, Dec. 1974.
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