TABLE OF CONTENTS We must now consider permanently implanted materials which are subject to considerable stresses, and which from time to time have to bear the full weight of the body, both at rest and in motion. These are replacement components for all or part of the different joints in the leg: the hip, knee, ankle and foot. Other such implants, which may have to suffer considerable stresses in use, but which normally do not have to bear the full body weight, are replacement joints in the arm:shoulder, elbow, wrist and hand
The major load-bearing joint replacements are generally used for older people, since those currently available tend to last for only about 15 to 20 years. Work is now under way to develop prostheses which can be implanted into younger, and consequently, more active people. These studies include determination of the most appropriate stress distribution in the femur and prosthesis, so as to give a close approximation to "normal" patterns.
One of the most interesting, and challenging, aspects of the design of artificial joints, is the behaviour of bone itself. Although, compared with other tissues in the human body, the bones seem to be quite hard and rigid, they are relatively easily deformed under pressure. There are numerous examples of enlarged or deformed organs, such as the liver, causing bone distortion, in which the bone say of the spine-is broken down and reformed into a new shape (resorbed). It is also, of course, like most other living tissue, constantly being destroyed and renewed.
