Prosthetic sockets stabilized by alternating areas of tissue compression and release

Randall D. Alley, CP; T. Walley Williams III, MA; Matthew J. Albuquerque, CPO; David E. Altobelli, MD

biodesigns Inc, Westlake Village, CA; Liberating Technologies Inc, Holliston, MA; Next Step Orthotics & Prosthetics Inc, Manchester, NH; DEKA Research and Development Corp, Manchester, NH

INTRODUCTION

Traditional upper-limb prosthetic sockets share certain problems. Most sockets simply contain the tissue of the remaining limb. Since a prosthetist produces them by slightly modifying casts taken by wrapping plaster bandages around the limb, the sockets are usually circular in cross section and thus encapsulate the limb. The advent of myoelectric control led to new socket designs. Transradial (TR) sockets were needed that would stabilize the location of the electrodes, and the Muenster and Northwestern sockets were introduced. These sockets are self- suspending but nonetheless still display a number of problems. They do not permit the user to fully flex or extend the elbow, they do not prevent lost motion between the bones of the remaining limb and the distal prosthetic structure during active lifting, and they do not load the bone uniformly but rather concentrate the load near the ends.

Myoelectric control also changed transhumeral (TH) sockets with the introduction of the Dynamic Socket. It has a low lateral trim line to prevent the lifting of the electrodes during the extremes of flexion and abduction. It also has anterior and posterior wings that stabilize the prosthesis against rotation around the long axis. Similarly, the X-frame socket has replaced the full con- tact socket for amputations at the shoulder level, because it permits the user to bend forward and to move the shoulder while maintaining good contact with electrodes. It also stabilizes the prosthesis against rotation at its superior and inferior borders and covers far less surface area of the thorax for increased heat dissipation. In this article, we review the evolution of these designs with additional references by Lake.

LONGITUDINAL DEPRESSIONS AND RELEASE AREAS DEFINE COMPRESSION/RELEASE STABILIZED SOCKET

This article will introduce improved sockets for persons with TR, TH, and transfemoral (TF) amputations created with longitudinal depressions added in the socket walls with open release areas between the depressions that receive the displaced tissue. When the depressions and release areas are correctly located, they reduce motion of the underlying bony structures with respect to both the socket and the rest of the prosthesis. One can define the depressions and releases during cast-taking but only by radically changing the way casts are taken.

Traditionally, the prosthetist uses a plaster wrap to define the shape of the remaining limb. The typical plas- ter wrap results in a shell that is almost circular in cross section throughout most of its length. When the shell is filled with plaster, the prosthetist modifies the resulting positive model before creating a socket over it by laminating or by thermoforming plastic. The prosthetist then adds extra plaster to the model to create space in the socket to accommodate bony prominences and removes the plaster to tighten up the fit. The experienced prosthetist can speed up the rectification process by contouring the original cast while it is setting.

Creating a compression/release stabilized (CRS) socket requires one to apply selective pressure during cast-taking, but this pressure must be applied in a specific way. A definition of terms will help the reader to follow the discussion. We only briefly summarize the casting process here, because prosthetists must be fully trained and certified in the application of this design such that patients are not harmed because of an incomplete understanding of the process.

If during the cast-taking, the technician pushes inward toward the bone, he or she will create a depression in the resulting cast. When the depressed area is parallel to the length of the underlying bone, it will appear as a channel or longitudinal depression. Further use of the word depression in this article will describe any shape created by pushing inward and use of longitudinal depression will describe long depressions parallel to the bones underneath. If one pushes a substantial area inward while holding the limb of the amputee, this action will displace tissue in other areas outward to form bulges. When the cast is taken, the stretched plaster wrap over these bulges still applies some inward force. For a CRS socket to perform correctly, these areas should have little or no inward force where the tis- sues bulge. After all remaining force is removed between the longitudinal depressions, the areas between are called release areas. After we discuss the physics underlying the operation of a CRS socket in this article, we will briefly illustrate how each of the three socket designs (TH, TF, and TR) can be created using the plaster cast technique. The unique features of these sockets are the longitudinal depressions and the release areas. The release areas are critical to the functioning of this new socket design.