Suspension of the Below-Knee Prosthesis: Comparison of Supracondylar Cuff and Brim

DAVID LYTTLE, MB, FRCS (C)*Winnipeg, Manitoba

Young amputees make full use of their lower-limb prostheses, so much consideration should be given to the correct prescription of the socket and the foot and ankle components. Often the choice of suspension is not discussed at the interview with the prosthetic clinic team. Yet, suspension is very important; not only does it prevent the prosthesis from falling off but it must avoid pistoning, minimize shear forces on the skin of the amputation limb, and enhance axial and rotational stability.

The conventional below-knee prosthesis of twenty years ago was suspended by a leather thigh corset laced between metal side bars and attached to the socket brim by single-axis hinges. This provides superior security against losing one's prosthesis, but few activities demand such security.

Alternative methods of suspension have been developed. One very active and gymnastic amputee at our clinic prefers the silastic sleeve for superior stability even though frequent replacements are needed and we can demonstrate that it causes some venous congestion of the amputation limb. Most patients fitted at our associated adult clinic opt for the supracondylar cuff suspension which is our first preference for dysvascular amputees. A positive feature is that suspension is well maintained even when sitting, and the elderly spend most of their time sitting. The socket brim can be trimmed conveniently low, there is minimal skin friction at the femoral epicondyles, and a reasonably cosmetic appearance results. The circumferential cuff, if applied tightly, may impair circulation of lymphatic, venous and even arterial blood flow in those with poor peripheral circulation. In addition, a certain degree of strength and dexterity is required of the patient to apply and adjust the Velcro, sometimes difficult for elderly or arthritic individuals.

In our juvenile amputee clinic, the preference is for supracondylar/suprapatellar fitting (SCSP) with careful molding of the brim to the compressible soft tissues in the supracondylar region. A one-piece prosthesis is a great thing for youngsters who are apt to mislay bits and pieces at home or school. Variations in the SCSP design include the use of a microcellular foam liner with wedged brim or the use of a removal medial wedge. The latter is apt to be lost if the retaining strap breaks. Another type of wedge created by inflatable rubber pads has not found favor, although a few knee disarticulation patients use this method. Possible adverse effects to be considered in any suspension system include circulatory impairment; friction of the brim against the skin; restriction of knee motion in flexion, extension and rotation; fracture or ligamentous injury about the knee; and the inability of a particular patient to keep the suspension properly adjusted. A valgus stress applied to the knee causes the possibility of rupture of the medial collateral ligament, a danger greater in older adolescents than in young children. A prosthetic brim trimmed at knee joint level concentrates such forces on the collateral ligaments. The higher SCSP brim concentrates forces at the distal femoral growth plate; fractures there are much more common than ligamentous tears in young people. We have not encountered any such injuries in amputation limbs and very few have ever been reported. Probably a shock absorbing system in the prosthetic fitting minimizes this risk. Investigation of this concept was carried out in four patients in whom valgus and varus stresses were applied to each knee under radiographic control and with a strain gauge to standardize the stresses. Joint line separation on X-ray was slightly greater on the amputated than on the sound knee joint. A more dramatic finding was that the fibula scissored behind the tibia and allowed more apparent varus position of the prosthesis than actual knee joint strain. This fibular shock absorber may explain the low incidence of injuries to amputation limbs. It occurs whether the limb is supported in a prosthesis with a high brimmed socket or with a low brimmed socket.

Given their personal choice, patients will decide about suspension based on familiarity and loyalty to the first prosthesis fitted. Upon reaching the age of 18, patients display a definite tendency to opt for supracondylar cuff. The choice will also depend upon whether the lifestyle is more active than sedentary and upon hearsay and myths which circulate in sporting groups. For instance, many skiers are told by their fellow athletes to obtain thigh corset suspension to avoid losing suspension on the slopes. On the whole, most young people are best fitted with a prosthesis with suprapatellar/supracondylar brim. It is easy to don and doff, and provides good voluntary suspension. It is almost impossible for someone to pull the prosthesis off against the wishes of the wearer; suspension in mud and snow is excellent. Involuntary release when in flexion is probably a reasonable safety feature, as the prosthesis will come off if forces are applied during falls and tumbles. The SCSP provides excellent rotational stability for quick turns when running or ice skating, and for a first prosthetic fitting it enforces a good gait pattern by prohibiting knee extension beyond the desirable 5 degrees of flexion. The undesirable features of friction upon the skin at the brim and an obvious bump in one's trousers at the knee when sitting are the principal reasons why most young people decide to change when they reach adulthood.

*Medical Director, Rehabilitation Engineering Department and Director of Orthopaedics Rehabilitation Hospital, Health Sciences Center, 800 Sherbrook Street, Winnipeg, Man itoba R3A 1M4