Amputations II – 4

AMPUTATIONS 4
BIOMECHANICAL CONSIDERATIONS

When amputated, the residual femur no longer maintains its normal anatomic (9o varus) or mechanical (3o varus) alignment. The shaft tends to abduct due to loss of adductor power and flex due to the relative strength of the flexors over the hip extensors. The flexor system is anatomically stronger than the extensor system no matter what the level of transection. Despite the tendency for flexion contracture, it is somewhat desirable to have the anterior subcutaneous protection of the quadriceps musculature anteriorly. The significant loss of adductor power is related to the transection of adductor magnus, which has the largest moment arm of the three adductor muscles, and the tendency to reattach the remaining adductor musculature back to the femur while it is in an abducted position. Preservation of adductor power is difficult to achieve technically, but the goal is to re-establish normal muscle tension when reattaching adductor magnus.

There is some philosophical disagreement amongst authors regarding the importance of limb alignment and prosthetic fit. Gottschalk has long been a proponent of adductor myodesis, and feels very strongly that the abducted position of the femur (particularly in midstance) contributes significantly to the gait disturbance (side lurching) and increased energy requirements. He furthermore demonstrates that the socket is unable to control the position of the femur within its soft tissue envelope, regardless of its shape. Schuch and Pritham, on the other hand, while not disagreeing with Gottschalk’s biomechanical argument, contend that proper configuration of the socket does in fact influence the position of the femur, and therefore is an important factor in the overall function of the limb. The truth is that optimal outcome probably depends on both.

TECHNICAL POINTS

The skin flaps are kept short – AP or sagittal flaps are acceptable. The adductor magnus is detached sharply off the adductor tubercle to preserve some tendon, then reflected medially. After osteotomizing the femur transversely, drill holes are placed in the lateral and AP directions for reattachment of adductor magnus and the quadriceps muscles. Adductor magnus is brought across the cut end of the femur while maintaining its tension; it is secured with suture while the femur is held in adduction. Quadriceps is then sutured to the posterior femur while the hip is in extension (to minimize flexion contracture) and the remaining hamstring muscles are anchored to the posterior area of adductor magnus. The technique of stabilizing opposing muscles by suturing them together over the end of the bone may lead to the muscles sliding back and forth over the tip, causing bursa formation, crepitus, and discomfort. Myodesis may not, however, be possible without sacrificing some length and devitalizing tissue – this should be considered in amputation in the context of ischemia.
POSTOPERATIVE STUMP MANAGEMENT

“Although rigid dressings control the edema and stump position better than soft dressings, they are cumbersome to apply and do not offer any great advantage in the long term in transfemoral amputations.” Gottschalk, F.A., 1996

Options include:
� elastic bandage applied as a hip spica with the hip extended
� rigid dressing (generally felt to be difficult to apply and maintain)
� soft dressings (problems with edema control and wound support)
� immediate postoperative prosthesis fitting
� Owen silk and sterile fluffed gauze over which an Orlon stump stocking is rolled and held in place on the remaining thigh and hip tissue by medical adhesive.

PROSTHETIC CONSIDERATIONS

Again, the longer the stump the greater the lever arm and the easier it is to propel the prosthesis.

� Socket: The socket must be properly contoured for functioning muscles. It should be in contact with skin in all areas to prevent swelling and increase proprioception and control. The socket is adducted (8-12o) and flexed slightly (5o) to put the abductors and extensors under stretch – this slight stretch allows them to contract at near maximum strength during stance phase. High pressure areas must be properly supported. The ischial tuberosity and gluteal muscles carry the majority of the weight and therefore must be well supported. The AP dimension is fashioned with anterior support to prevent the ischial tuberosity from falling into the socket during weightbearing. Also, support around the distal-lateral femur is important, as it is pulled against the lateral wall of the socket by the hip abductors during midstance. Suspension currently is done primarily by silicone sleeve, replacing the old belt suspension systems.