prostheses A prosthesis is an artificial replacement for a missing or non-functioning body part. It can be either functional or cosmetic, and can be attached to the body externally or implanted surgically. Probably the best known images of prostheses are fictional: from the fearsome hook and peg-leg of the seafarers of old (Captain Hook and Long John Silver) to the modern prosthetic supermen of television and film (the Bionic Man and Robocop). In the broadest sense, the area of prosthetics includes not only artificial limbs, joint replacements, heart valves, and dentures, but also hearing aids of various kinds, pacemakers, and breast and penile implants. The medical reasons for prosthesis use include congenital absence of body parts, other birth defects, trauma, amputation, radical surgery (often for cancer), disease, burns, and arthritis.

External prostheses have a long history. The Rig Veda, an ancient collection of hymns and one of the foundations of the Hindu religion, tells of Vispala, to whom the gods gave an iron prosthesis when she lost her leg in battle. According to Herodotus, the soldier Hegistratus of Elios used an artificial foot. Pliny, in his Natural History, writes of Marcus Sergius, who, during the Second Punic War, went into battle with an iron hand, fashioned to hold a shield, tied to his arm. Most prostheses of the early modern era were custom-built by armourers for their warrior masters. The German imperial knight Götz von Berlichingen, who in 1509 lost his hand in the Battle of Landshut and in 1773 was immortalized in a play by Goethe, commissioned an iron hand, which had four fingers and a thumb. This mechanical masterpiece was copied many times over the following two hundred years.

Advances in surgery, such as the use of anaesthesia and the tourniquet, eventually meant that a large number of patients could live through an amputation procedure, although surviving the almost inevitable infection afterwards was another matter. A significant market for limb prostheses was created by the first example of truly modern warfare, the American Civil War (1861–5), and by the US Government's commitment to supply prostheses to war veterans. From this time, companies began to supply limbs made from wood, metal, and leather. With simple construction and operation, these devices could be quite reliable. Feet were generally single units, usually with a simple hinge joint at the ankle. Knees locked when weight was placed upon them, and otherwise swung through when the wearer moved his hip forward. Many prosthetic hands were simple clamps or hooks driven by cables attached to harnesses slung across the shoulders and back of the wearer. Other hands had anatomic shapes but were of cosmetic value only.

The first externally powered prosthetic limbs were built in Germany around 1915, although it was not until the 1950s that electrically and pneumatically powered devices appeared for general use. The need to care for large numbers of amputee casualties from World Wars I and II led to significant advances in the field of prosthetics, especially in Europe, as later did the sudden temporary increase in babies born with limb abnormalities (resulting from maternal treatment with the drug thalidomide). Current motivations for further improvements in prosthetics include the need to help those suffering from traumatic loss of limbs caused by land mines and other explosive devices, and a desire to meet the ever-increasing demands of prosthesis users, many of whom wish to hold physically demanding jobs or to take part in sports.

The first use of a microprocessor in an external prosthesis occurred in the 1990s in a device that controlled the speed that an artificial knee could swing forward during walking, automatically matching the wearer's pace. More complex devices are now possible, due to a broad range of improvements in materials and electronics. Direct attachment of external prostheses to the skeleton has recently been achieved; this allows forces and vibrations to be transmitted directly to the bones, enhancing what the user can feel through the prosthesis, so-called osseo-perception.

Along with limb replacements, dentures, although kept within the mouth, are another form of external prosthesis with a long history. It is thought that the first set was made more than two thousand years ago by the Etruscans. In the modern era, a famous denture-wearer was the first President of the US, George Washington, whose dentures, although rumoured to be made of wood, were manufactured from hippopotamus ivory and cow's teeth, along with some metal and springs. By the late eighteenth century, porcelain had replaced ivory as the material of choice for teeth in dentures. It is now possible to be fitted with replacement teeth that sit on titanium posts implanted directly into the jaw bone. An important aspect of this and many other implant procedures is osseo-integration, or the achievement of firm fixation of the implant within bone.

The development of implantable prostheses is linked to advances in surgery. Until Lister introduced aseptic surgical techniques in the 1860s, infection after surgery was a great hazard, especially if any foreign material was inserted into the wound. By using carbolic acid to cleanse open wounds, Lister was able to achieve healing in cases where previously amputation was the only treatment. His techniques also made it possible to limit infection after an open operation. This paved the way for surgically implanted prostheses, although it was not until alloys, plastics, and ceramics with adequate inertness and mechanical properties were produced that real advances were made. This requirement of ‘biocompatibility’ of implant materials, in which the material exists in close harmony with the body without the bodily environment adversely affecting the material or the material adversely affecting the body, reduces the potentially usable implant materials to only a few dozen.

Even before Lister, orthopaedic surgeons were experimenting with metal wires and pins for repairing broken bones. In the first half of the nineteenth century, some of the first animal experiments to determine the suitability of materials for use in the body were carried out in dogs. After Lister's advances, widespread clinical investigations and further animal experiments were conducted to study the reactions of the soft tissues and bones to the presence of a foreign material in the body. It gradually became clear that steel possessed the best properties for highly stressed applications, although it was not until well into the twentieth century, with the introduction of stainless steel (containing molybdenum), Vitallium (an alloy of cobalt, chromium, and molybdenum), and tantalum, that implant surgery was truly able to advance.

Joint replacement by prosthetic means is one of the most successful operative procedures in modern medicine in terms of number of patients treated and quality-of-life improvement. The hip has historically received the greatest attention. In 1946, the Judet brothers introduced a hip prosthesis (replacing the upper part of the femur alone), which was the first femoral prosthesis designed on biomechanical principles and used on a widespread basis. It was also the first mass-produced surgical prosthesis in which a thermoplastic was used. Over time it suffered from serious problems of material degradation and inadequate fixation to bone, but it nevertheless stimulated considerable effort towards the improvement of joint replacement. In the 1960s, hip replacement was put on a firm basis by Charnley, who used polymethylmethacrylate cement to fix both the metal femoral component and the plastic pelvic component of the prosthesis into bone. It is now possible to replace almost all the joints of the body, including hips, knees, elbows, shoulders, ankles, and fingers.

Work on cardiovascular prostheses began after World War II, when techniques were developed to maintain circulation of oxygenated blood through the body during open-heart surgery. The main technical problems to be overcome are the tendency of blood to clot on a foreign surface and the need to maintain natural blood flow rates through the prosthetic devices. The first clinical use of a mechanical heart valve prosthesis was reported in 1954, but it was not until the 1960s that the caged-ball type valve (metal cage and rubber ball) met with major clinical success. Pivoting disc valves with pyrolitic carbon discs and bileaflet designs are now also on the market. The first report of a synthetic vascular graft appeared in 1952, when Voorhees described his initial work in dogs. Arterial prostheses made of Dacron became commercially available in 1957, while expanded polytetrafluoroethylene (ePTFE) grafts were introduced in the 1970s. In 1969, Cooley performed the first total artificial heart implantation in a human. However, despite early optimism, the rate of progress in this field has been disappointing. The primary use of the total artificial heart today is as a temporary support to allow some recovery of function or until a donor heart can be found.

Much of the progress in the field of prosthetics over the last fifty years has been due to cooperation between prosthetists, surgeons, engineers, and materials scientists. Replacement of almost every part of the human body has been attempted. The next generation of ‘spare parts’ will depend on the development of new materials which can adapt to the demands placed on them and on collaborations with cell biologists so that hybrid artificial organs made of both living elements and synthetic materials can be developed. The real challenges lie in replacing complex organs, such as the heart, lungs, kidneys, liver, and pancreas, and in being able to design prostheses that will last for decades.

Amy B. Zavatsky, and Peter J. Kyberd

Bibliography

Atkins, D. J. and Meier, R. H. (ed.) (1988). The comprehensive management of the upper-limb amputee. Springer-Verlag.
Black, J. (1999). Biological performance of materials: fundamentals of biocompatibility. Marcel Dekker, New York.
Childress, D. (1985). Historical aspects of powered limbs prostheses. Clinical prosthetics and orthotics, 9(1), 2–13.
Schurr, D. G. and and Cook, T. M. (1990). Prosthetics and orthotics. Appleton and Lange.
Silver, F. H. (1994). Biomedical materials, medical devices and tissue engineering: an integrated approach. Chapman and Hall.
Williams, D. F. and and Roaf, R. (1973). Implants in surgery. W. B. Saunders Company Ltd., London.

See also body decoration; cosmetic surgery; dentistry; hip replacement.