At first glance, a manual locking knee seems a logical choice. However, experience has shown that this is rarely required and should be reserved as a prescription of last resort. Only additional medical disabilities such as blindness will require this mechanism. Unlocking the knee joint in order to sit requires the use of one hand in the unilateral case; expecting a bilateral amputee to cope with dual locking knees and dual locking hips is unrealistic. Furthermore, in the event of a fall backwards, fully locked joints may prevent the amputee from bending his trunk to protect his head from impact.
Nearly 40 years ago, Hutter reported a single case of successful fitting of a transfemoral suction socket to a hip disarticulate with a mass of redundant tissue distal to the ischium. The senior author (T.v.d.W.) has personally fitted three such cases recently (Fig 21B-16.). Each was able to ambulate successfully and to retain full suction suspension with only the assistance of a thin, elastic Silesian belt despite the total absence of any femur. The major difference in socket configuration is the creation of a trough like channel to contain both the medial and lateral aspects of the ischiopubic ramus since no femur remains. Such fittings have been done on an experimental basis where there is sufficient residual muscle tissue to create both suction suspension and biomechanical "locking" to stabilize the socket on the patient. This approach warrants further follow-up and evaluation to determine its practicality.
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The recent advent of laminating silicone rubbers allows even more flexibility than do available thermoplastics. As was noted earlier, the resulting comfort and range of motion has been associated with significantly higher rates of prosthetic usage. Although the fabrication is complex and difficult and the finished result slightly heavier than thermoplastic designs, favorable patient response and good durability recommend further development and more widespread application of this technique. The senior author (T.v.d.W.) has fitted 35 silicone rubber sockets over the past 2 years. Amputees who have previously worn more rigid designs typically describe the rubberized sockets as feeling "more natural" or "more like a part of me" (Fig 21B-15.).
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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.).
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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.
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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.