An artificial replacement for a missing or diseased part of the body; for example, artificial limbs (see limb, artificial), heart valves (see heart-valve surgery), or glass eyes (see eye, artificial).
Development of such mechanical and biomechanical devices points the way to a much wider use of e?ective prostheses, enabling people who would previously have died or been severely handicapped to lead normal or near normal lives. The technical hazards that have already been overcome provide a sound foundation for future successes. Progress so far in producing prostheses should also ensure that organ replacement is free from the serious ethical problems that surround the use of genetic manipulation to cure or prevent serious diseases (see ETHICS).
Limbs These are best made to meet the individual’s requirements but can be obtained ‘o? the shelf’. Arti?cial joints normally comprise complex mechanisms to stimulate ?exion and rotation movements. Leg prostheses are generally more useful than those for arms, because leg movements are easier to duplicate than those of the arm. Modern electronic circuitry that enables nerve impulses to be picked up and converted into appropriate movements is greatly improving the e?ectiveness of limb prostheses.
Eyes Arti?cial eyes are worn both for appearance and for psychological reasons. They are made of glass or plastic, and are thin shells of a boat-shape, representing the front half of the eye which has been removed. The stump which is left has still the eye-muscles in it, and so the arti?cial eye still has the power of moving with the other. A glass eye has to be replaced by a new one every year. Plastic eyes have the advantage of being more comfortable to wear, being more durable, and being unbreakable. Research is taking place aimed at creating a silicon chip that stimulates the visual cortex and thus helps to restore sight to the blind.
Dental prostheses is any arti?cial replacement of a tooth. There are three main types: a crown, a bridge and a denture. A crown is the replacement of the part of a tooth which sticks through the gum. It is ?xed to the remaining part of the tooth and may be made of metal, porcelain, plastic or a combination of these. A bridge is the replacement of two or three missing teeth and is usually ?xed in place. The replacement teeth are held in position by being joined to one or more crowns on the adjacent teeth. A denture is a removable prosthesis used to replace some or all the teeth. The teeth are made of plastic or porcelain and the base may be of plastic or metal. Removable teeth may be held more ?rmly by means of implants.
Heart The surgical replacement of stenosed or malfunctioning heart-valves with metal or plastic, human or pig valves has been routinely carried out for many years. So too has been the insertion into patients with abnormal heart rhythms of battery-driven arti?cial pacemakers (see CARDIAC PACEMAKER) to restore normal function. The replacement of a faulty heart with an arti?cial one is altogether more challenging. The ?rst working attempt to create an arti?cial heart took place in the early 1980s. Called the Jarvik-7, it had serious drawbacks: patients had to be permanently connected to apparatus the size of an anaesthetic trolley; and it caused deaths from infection and clotting of the blood. As a result, arti?cial hearts have been used primarily as bridging devices to keep patients alive until a suitable donor heart for transplantation can be found. Recent work in North America, however, is developing arti?cial hearts made of titanium and dacron. One type is planted into the chest cavity next to the patient’s own heart to assist it in its vital function of pumping blood around the body. Another replaces the heart completely. Eventually, it is probable that arti?cial hearts will replace heart transplants as the treatment of choice in patients with serious heart disorders.
Liver Arti?cial livers work in a similar way to kidney dialysis machines (see DIALYSIS). Blood is removed from the body and passed through a machine where it is cleaned and treated and then returned to the patient. The core of the device comprises several thousand ?exible membrane tubules on which live liver cells (from pigs or people) have been cultured. There is an exchange of biological molecules and water with the ‘circulating’ blood, and the membrane also screens the ‘foreign’ cells from the patient’s immune system, thus preventing any antagonistic immune reaction in the recipient.
Nose The making of a new nose is the oldest known operation in plastic surgery, Hindu records of such operations dating back to 1,000 BC. Loss of a nose may be due to eroding disease, war wounds, gun-shot wounds or dog bites. In essence the operation is the same as that practised a thousand years before Christ: namely the use of a skin graft, brought down from the forehead. Alternative sources of the skin graft today are skin from the arm, chest or abdomen. As a means of support, the new nose is built round a graft of bone or of cartilage from the ear.
Health Encyclopedia