Others involve complex engineering. Arthritis not only limits joint movement but can be extremely painful. In the worst cases involving the hip, knee or elbow, it becomes necessary to replace the offending joint. Such replacements are reproductions, albeit ingenious ones, of the original. More sophisticated are the remarkable valves that organize the flow of blood between the various parts of the heart and control the rate at which it enters the large arteries leading away from the heart.
Modern valves are even tailor-made for the pressure and flow needs of a particular patient.
Not all implants are mechanical. For example, slow-release hormones for contraception and control of symptoms associated with the menopause can be embedded under the skin to obviate the need to take tablets daily. Still under research is an artificial pancreas for patients suffering from diabetes - a condition caused by a shortage of insulin, a hormone that regulates the level of sugar in the blood. The replacement pancreas would deliver insulin according to the sugar level. There are difficulties in developing this device, but there seems little doubt that one day the devilish problem of controlling blood sugar will be mastered - to the benefit of millions.