A Manual for Below-Knee Amputees

Fabrication of a Below-Knee Prosthesis

Whether the prosthesis is to be crustacean or endoskeletal (often called "modular") type, the prosthetist usually begins by wrapping the stump with plaster-of-Paris bandages to obtain a negative mold. A positive model is made by filling the negative mold with a mixture of plaster-of-Paris and water, and allowing it to harden.

After modification of the model to provide the proper characteristics to the finished socket, a plastic socket is formed over it. The first one is usually a test, or check, socket made of a transparent plastic to determine if further modifications are needed.

A new method being used by many prosthetists for obtaining a modified model of the stump involves use of a computer and automatic machinery. Known a CAD/CAM (Computer-Aided-Design/Computer-Aided-Manufacturing), this method permits prosthetists to modify the model more easily since it does not require making and carving an actual plaster model.

The socket is mounted on an adjustable leg for walking trials, and when both the prosthetist and the amputee are satisfied, the limb is ready for the finishing procedures. The exoskeletal shank may be of plastic-covered wood or all plastic. The endoskeletal type uses carved foam rubber over the supporting pylon and the entire prosthesis is encased in a either a latex or fabric stocking.

Steps in the fabrication of a plastic prosthesis for a below-knee (trans-tibial) amputee:

  1. A negative mold of the stump is made by wrapping it with a wet plaster-of-Paris bandage.
  2. A positive model of the stump is made by filling the cast with a mixture of plaster of Paris and water.
  3. After modifications have been made to the model by the prosthetist to make sure that the pressures m the socket will be correct, a test, or check socket, is made by forming a heated sheet of clear plastic over the model.
  4. The clear plastic socket is tried on to make sure that it fits properly.
  5. A new positive model is made by filling the clear socket with a mixture of plaster of Paris and water.
  6. The socket to be used on the definitive prosthesis is formed over the model by using either a mixture of plastic resin and cloth or by forming a heated sheet of plastic over the model.
  7. The definitive socket is attached to a pylon that can be adjusted for alignment and walking trials can be made.
  8. The finished prosthesis maybe either exoskeletal or endoskeletal.

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Copyright 1996 - Alvin L. Muilenburg and A. Bennett Wilson, jr.

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