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  Prosthetics (Greek, ‘replacement’ or ‘spare parts’) is the artificial replacement of part of the body. This science probably goes back to the emergence of intelligent humans, whose survival depended upon being able to defend themselves and acquire food. With the accidental loss of a leg, he or she would have probably used a tree branch to act either as a crutch or a stump.

Wooden legs were first recorded in 500  BCE. At this time, the limbless were left to fashion crude devices for themselves, commonly known as peg-legs, and it was not until the 15th century that artificial limbs were made to order for knights. The design of limbs improved and some had joints that could move through the use of simple levers and springs.

The American Civil War and the Crimean War in the 19th century, and World War I and II in this century, gave tremendous impetus to the manufacture of limbs and their improvement. It is not only amputees who have benefited from new technology. New hip joints, pacemakers and plastic corneas, to name only a few, have transformed people\'s lives in a way that was not imagined, even at the start of this century.

A knowledge of the mechanics of walking has enabled the rational design of the socket of the leg prosthesis to be comfortable and transmit the locomotor forces to tissues not adapted to bear pressure. Knee mechanisms are fitted with self-acting brakes to allow the patient to bear load on a flexed knee as in descending stairs. Pneumatic or hydraulic dampers control the movements of the leg in the swing phase of walking. The constant complaint of the amputee is that his prosthesis is too heavy (although it may only be one third of the mass of the parts it replaces!). The use of aluminium and titanium alloys and carbon reinforced plastics have allowed leg prosthesis to be made much lighter. There are as yet no effective power sources capable of developing the loads necessary in leg prosthesis and having sufficient energy to last for an hour. Here is a field for further development.

Hand and arm prostheses pose different problems for the designer. Legs require strength but generally repeat the same movement patterns. Hands and arms tend to undertake a wide range of different function such as feeding, grooming, dressing and also complex functions such as writing or typing. Consequently a unilateral arm amputee will usually undertake most tasks with his or her ‘good’ arm relegating the prosthesis to simple tasks such as holding the plate while eating or the paper while writing. The major problem of the designer is that the position of the hand in three-dimensional space is defined by three linear dimensions, up, out and forward, and three corresponding angles. This basically requires six motors and twelve control points since each movement must be controlled for far forward and backward directions. Current generations of arm prostheses use either body power from movements of the shoulder or battery power driving one or two miniature electric motors. The control problem can only be dealt with by the amputee looking at his or her limb, since he or she cannot be aware of position, pressure or touch as in the normal way.

Internal prostheses are mostly in the orthopaedic field—exemplified by replacement joints for patients with arthritis. In Europe, in 1992, approximately 300,000 patients received replacement hip joints and about 150,000 replacement knees. The joints usually consist of a metal part moving against a polyethylene part with both being fixed by a kind of cement filler. There are records of patients still using hip joints 25 years after surgery, although at the present time problems seem to be arising due to wear particles in the period 10 to 15 years after the operation. This may require a second session of surgery to fit a new implant, which is usually more technically demanding.

Other prosthetic implants may be in such diverse areas as heart pacemakers, heart valves, cosmetic implants for ear, face or breast, stimulators for paralysed muscle or tubes for arterial replacement or treating hydrocephalus (water on the brain).

For all implants the responsiveness of the material to the body environment, its biocompatibility is of critical importance. Metals currently used involve specific types of stainless steel, titanium alloys and cobalt chromium alloys. Polyethylene and nylon types of plastics and aluminium or zirconium ceramics are also used. AA

Further reading A. Bennet Wilson, Limb Prosthetics.



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