It is well accepted that any fluid-control mechanism (hydraulic or pneumatic) results in a smoother gait. Motion studies conducted at Northwestern University have confirmed that a more normal gait for the hip dis-articulation/transpelvic amputee is also produced. Gait analysis has demonstrated that utilization of a hydraulic knee in a hip disarticulation prosthesis results in a significantly more normal range of motion at the hip joint during the walking cycle than is possible with conventional knees. In addition, a more rapid cadence was also possible.
A third type that has proved advantageous for this level of amputation is the polycentric (four-bar) knee. Although slightly heavier than the previous two types, this component offers maximum stance-phase stability. Because the stability is inherent in the multilinkage design, it does not erode as the knee mechanism wears during use. In addition, all polycentric mechanisms tend to "shorten" during swing phase, thus adding slightly to the toe clearance at that time. Many of the endoskeletal designs feature a readily adjustable knee extension stop. This permits significant changes to the biomechanical stability of the prosthesis, even in the definitive limb. Because of the powerful stability, good durability, and realignment capabilities of the endoskeletal polycentric mechanisms, they are particularly well suited for the bilateral amputee. Patients with all levels of amputation, up to and including translumbar (hemicorporectomy), have successfully ambulated with these components.
In an effort to overcome this limitation, the hip flexion bias system was developed for the young, active amputee who wished to walk rapidly. At toe-off, kinetic energy from the coil spring is released, and the prosthetic thigh is thrust forward. Not only does this provide the amputee with a more normal-appearing gait, it also improves ground clearance. As a result, the prosthesis can be lengthened to a nearly level configuration in most cases (Fig 21B-5.). However, two potential problems have been noted with this approach. One is the development of annoying squeaks in the spring mechanisms after a few months of use, which sometimes tend to recur inexorably. A more significant concern is that as the spring compresses between heel strike and midstance, it creates a strong knee flexion moment. Unless this is resisted by a stance control knee with a friction brake or a polycentric knee with inherent stability, the patient may fall. Since the friction-brake mechanisms lose their effectiveness as the surface wears, the polycentric knee is the preferred component with this hip mechanism.