A new trim line concept for trans-tibial amputation prosthetic sockets.

Introduction Proximal trim lines for trans-tibial prosthetic sockets have traditionally been trimmed according to the mode of suspension to the limb. The supracondylar suspension, which retains the prosthetic socket to the limb by gripping above the femoral condyles, has a more proximal trim line compared to the more distal trim line used for a suspension with a patellar-tendon-bearing strap or a thigh corset. In more modern trans-tibial sockets other modes of suspension are used. A distal pin or coupling suspension used together with a roll-on silicone socket was introduced by Ossur Kristinsson in 1987. Carl Caspers introduced in 1991 a true total contact socket by using polyurethane with flow characteristics as an interface between the stump and the hard prosthetic socket. As a suspension mode for this socket an airtight sleeve was used with an expulsion valve fitted in the hard socket. This created a vacuum suspension (Board, Street and Caspers, 2001). This technique has partially been used earlier but not in connection with an expulsion valve. The technique, using an expulsion valve, should only be used together with a true total contact socket. The new improved suspensions open possibilities for the re-design of the proximal trim line. For 5 years the author has used a new trim line which provides better "freedom" for activities, maximum sitting comfort, and better cosmesis. It also improves the durability if a vacuum sleeve is used. This article describes the theory behind the