Q. What do the attached images show and what is the mechanism of action of this prosthesis?

A.

The images show a hip disarticulation prosthesis. This consists of a moulded plastic waist band which gains purchase to (or suspension from) the trunk at the iliac crests and at the stump, as demonstrated in the diagram. This basic hip disarticulation prosthesis, the Canadian hip disarticulation prosthesis, is further described by Radcliffe.¹

The knee joint visible in this girl's prosthesis is of the four bar linkage type. Knee prosthetics have been previously described in this journal. ²

The loss of hip motors and pelvis stabilizers create a significant functional deficit. The tendency is for a patient to lift and swing the entire prosthesis forward during gait by lifting and internally rotating the pelvis. This intuitive action might also be thought to lock the knee in extension prior to heel contact and thus give a satisfactory gait pattern. In fact the correct gait pattern is the opposite and must be explained to the patient with reference to the mechanics of the prosthesis.

The prosthetic hip joint consists not of a ball and socket but rather a long transverse hinge joint, which offers more stablity. This joint is placed well anterior and distal to the position of the normal hip joint. This anterior placement of the prosthetic hip joint displaces the ground reaction line anterior to the knee joint, exerting on it an external extension moment and thus stabilizing the knee during the gait cycle. Without this the knee would tend to collapse into flexion during stance.

At the back of the hip joint and just distal to it there is a "bumper" which is attached to the pelvic element of the prosthesis. This rubber bumper is adjusted so as to contact the thigh and push the hip into flexion at the beginning of swing phase. The patient may, if she so wishes, sit down hard on the prosthesis and thus on this Bumper and (with proper adjustment) this will initiate hip joint flexion. The patient can learn to fine tune this event and by leaning back slightly may cause the pelvic element and bumper to engage the hip sooner. The flexion at the hip joint which occurs displaces the ground reaction line behind the knee joint which will then flex. Thus the patient can control knee flexion during gait not by lifting the pelvis and swinging the leg forward but by sitting down on the prosthesis and letting the bumper initiate hip and knee flexion.

Hip flexion is resisted to about 15 degrees in the basic prosthesis by the use of an elastic band. This band runs from just behind the bumper on the pelvic element to the front of the "tibial" element, both the hip and the knee joints and therefore resisting hip and knee flexion. However, once the hip flexes (via the bumper mechanism described), the elastic band will encourage knee flexion. (see diagram). 

The patient is encouraged to walk as follows. Because of the inherent stability of the prosthesis the patient is encouraged to put weight on the heel following heel contact. The knee will not flex. Forward bending by the patient will increase extension stability in the knee. She is encouraged to maintain full weight bearing throughout the stance phase as she rolls over the extended prosthesis.

At the end of stance phase the hip is in extension and the thigh will contact the bumper on the pelvic element of the prosthesis. Thus the hip and then knee flex, and ground clearance is achieved during forward swing of the leg without the need to lift the pelvis or internally rotate the pelvis. The elastic band limits hip flexion to 15 degrees and momentum restores knee extension at the end of swing, and the extended prosthesis is again stable in preparation for heel strike.

1. Radcliffe CW. The biomechanics of the Canadian-Type Hip disarticulation prosthesis. Artif Limbs 1957; 4:29.

2. Mc Cormack D. Prosthetic Knee Designs: Biomechanics and Functional classification. Irish Journal of Orthopaedic Surgery and Trauma. January 1997.