For many years, the use of fluid-controlled knee mechanisms for high-level amputees was considered unwarranted since these individuals obviously walked at only one (slow) cadence. The development of hip flexion bias mechanisms and more propulsive foot designs have challenged this assumption. Furthermore, a more sophisticated understanding of the details of prosthetic locomotion has revealed an additional advantage of fluid control for the hip-level amputee.
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.
In addition to foot mechanisms, several ankle components have recently reached the American market. These can be paired with most of the feet mentioned above to add additional degrees of motion as desired. Examples include the SwePro ankle from Sweden, The Blatchford (Endolite) Multi-Flex ankle from England, and the Seattle ankle.
If you were able to walk normally before your amputation and do not have other illnesses such as angina or breathing difficulties this will also make it more likely you will walk after your amputation.
Some studies have shown that in elderly patients undergoing major amputation (below or above knee) for hardening of the arteries, over half the patients fitted with an artificial leg never used it effectively, especially if rehabilitation was delayed for longer than two months after the amputation.
Torque-absorbing devices are often added to hip dis-articulation/transpelvic prostheses to reduce the shear forces transmitted to the patient and components. Ideally, they are located just beneath the knee mechanism (Fig 21B-9.). This increases durability by placing the torque unit away from the sagittal stresses of the ankle while avoiding the risk of introducing swing-phase whips (which can occur if it is placed proximal to the knee axis). The major justification for such a component is that the high-level amputee has lost all physiologic joints and, hence, has no way to compensate for the normal rotation of ambulation.
Finally, a number of new components have been developed recently that combine the characteristics of some of the above classes of knee mechanisms. For example, Teh Lin manufactures a "Graphlite" knee consisting of a polycentric unit with pneumatic swing-phase control in a carbon fiber receptacle. Such "hybrid" designs are expected to increase over the next few years.
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.
Other than the exception discussed above, knee mechanisms are selected by the same criteria as for transfemoral (above-knee) amputees. The single-axis (constant-friction) knee remains the most widely utilized due to its light weight, low cost, and excellent durability. Friction resistance is often eliminated to ensure that the knee reaches full extension as quickly as possible. A strong knee extension bias enhances this goal and offers the patient the most stable biomechanics possible with this mechanism. Although the single-axis type was proposed as the knee of choice for the Canadian hip disarticulation design, more sophisticated mechanisms have proved their value and are gradually becoming more common.
Finally, transverse-rotation units or positional rotators originally developed for the Oriental world have become available worldwide. Installed above the knee mechanism, these devices permit the amputee to press a button and passively rotate the shank 90 degrees or more for sitting comfort (Fig 21B-10.). They not only facilitate sitting cross-legged upon the floor but also permit much easier entry into restaurant booths and other confined areas. This component is particularly advantageous for entering and exiting automobiles.
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.
The most important part of any prosthesis is the socket, which provides the man-machine interface. During the initial assessment of the amputee, examination of postoperative radiographs and careful palpation of the pelvis are recommended. Some amputees present as "hip disarticulation" when they have a short femoral segment remaining or as "transpelvic" when part of the ilium, sacrum, or ischium remains. Unanticipated bony remnants can become a puzzling source of discomfort. On the other hand, they may sometimes be utilized to assist suspension or rotary control or to provide partial weight-bearing surfaces. Due to the success of ischial containment transfemoral sockets, the importance of precise contours at the ischium and ascending ramus is now more widely recognized. The same principles can readily be applied to hip disarticulation sockets to increase both comfort and control (Fig 21B-11.).