Artificial Limb

Background

Artificial arms and legs, or prostheses, are intended to restore a degree of normal function to amputees. Mechanical devices that allow amputees to walk again or continue to use two hands have probably been in use since ancient times, the most notable one being the simple peg leg. Surgical procedure for amputation, however, was not largely successful until around 600 b.c. Armorers of the Middle Ages created the first sophisticated prostheses, using strong, heavy, inflexible iron to make limbs that the amputee could scarcely control. Even with the articulated joints invented by Ambroise Paré in the 1500s, the amputee could not flex at will. Artificial hands of the time were quite beautiful and intricate imitations of real hands, but were not exceptionally functional. Upper limbs, developed by Peter Baliff of Berlin in 1812 for below-elbow amputees and Van Peetersen in 1844 for above-elbow amputees, were functional, but still far less than ideal.

The nineteenth century saw a lot of changes, most initiated by amputees themselves. J. E. Hanger, an engineering student, lost his leg in the Civil War. He subsequently designed an artificial leg for himself and in 1861 founded a company to manufacture prosthetic legs. The J. E. Hanger Company is still in existence today. Another amputee named A. A. Winkley developed a slip-socket below-knee device for himself, and with the help of Lowell Jepson, founded the Winkley Company in 1888. They marketed the legs during the National Civil War Veterans Reunion, thereby establishing their company.

Another amputee named D. W. Dorrance invented a terminal device to be used in the place of a hand in 1909. Dorrance, who had lost his right arm in an accident, was unhappy with the prosthetic arms then available. Until his invention, they had consisted of a leather socket and a heavy steel frame, and either had a heavy cosmetic hand in a glove, a rudimentary mechanical hand, or a passive hook incapable of prehension. Dorrance invented a split hook that was anchored to the opposite shoulder and could be opened with a strap across the back and closed by rubber bands. His terminal device (the hook) is still considered to be a major advancement for amputees because it restored their prehension abilities to some extent. Modified hooks are still used today, though they might be hidden by realistic-looking skin.

The twentieth century has seen the greatest advances in prosthetic limbs. Materials such as modern plastics have yielded prosthetic devices that are strong and more lightweight than earlier limbs made of iron and wood. New plastics, better pigments, and more sophisticated procedures are responsible for creating fairly realistic-looking skin.

The most exciting development of the twentieth century has been the development of myoelectric prosthetic limbs. Myoelectricity involves using electrical signals from the patient's arm muscles to move the limb. Research began in the late 1940s in West Germany, and by the late sixties myoelectric devices were available for adults. In the last decade children have also been fitted with myoelectric limbs.

In recent years computers have been used to help fit amputees with prosthetic limbs. Eighty-five percent of private prosthetic facilities use a CAD/CAM to design a model of the patient's arm or leg, which can be used to prepare a mold from which the new limb can be shaped. Laser-guided measuring and fitting is also available.

Raw Materials

The typical prosthetic device consists of a custom fitted socket, an internal structure (also called a pylon), knee cuffs and belts that attach it to the body, prosthetic socks that cushion the area of contact, and, in some cases, realistic-looking skin. Prosthetic limb manufacture is currently undergoing changes on many levels, some of which concern the choice of materials.

A prosthetic device should most of all be lightweight; hence, much of it is made from plastic. The socket is usually made from polypropylene. Lightweight metals such as titanium and aluminum have replaced much of the steel in the pylon. Alloys of these materials are most frequently used. The newest development in prosthesis manufacture has been the use of carbon fiber to form a lightweight pylon.

Certain parts of the limb (for example, the feet) have traditionally been made of wood (such as maple, hickory basswood, willow, poplar, and linden) and rubber. Even today the feet are made from urethane foam with a wooden inner keel construction. Other materials commonly used are plastics such as polyethylene, polypropylene, acrylics, and polyurethane. Prosthetic socks are made from a number of soft yet strong fabrics. Earlier socks were made of wool, as are some modern ones, which can also be made of cotton or various synthetic materials.

Physical appearance of the prosthetic limb is important to the amputee. The majority of endoskeletal prostheses (pylons) are covered with a soft polyurethane foam cover that has been designed to match the shape of the patient's sound limb. This foam cover is then covered with a sock or artificial skin that is painted to match the patient's skin color.

The Manufacturing
Process

Prosthetic limbs are not mass-produced to be sold in stores. Similar to the way dentures or eyeglasses are procured, prosthetic limbs are first prescribed by a medical doctor, usually after consultation with the amputee, a prosthetist, and a physical therapist. The patient then visits the prosthetist to be fitted with a limb. Although some parts—the socket, for instance—are custom-made, many parts (feet, pylons) are manufactured in a factory, sent to the prosthetist, and assembled at the prosthetist's facility in accordance with the patient's needs. At a few facilities, the limbs are custom made from start to finish.

Measuring and casting

• 1 Accuracy and attention to detail are important in the manufacture of prosthetic limbs, because the goal is to have a limb that comes as close as possible to being as comfortable and useful as a natural one. Before work on the fabrication of the limb is begun, the prosthetist evaluates the amputee and takes an impression or digital reading of the residual limb.
• 2 The prosthetist then measures the lengths of relevant body segments and determines the location of bones and tendons in the remaining part of the limb. Using the impression and the measurements, the prosthetist then makes a plaster cast of the stump. This is most commonly made of plaster of paris, because it dries fast and yields a detailed impression. From the plaster cast, a positive model—an exact duplicate—of the stump is created.

Making the socket

• 3 Next, a sheet of clear thermoplastic is heated in a large oven and then vacuum-formed around the positive mold. In this process, the heated sheet is simply laid over the top of the mold in a vacuum chamber. If necessary, the sheet is heated again. Then, the air between the sheet and the mold is sucked out of the chamber, collapsing the sheet around the mold and forcing it into the exact shape of the mold. This thermoplastic sheet is now the test socket; it is transparent so that the prosthetist can check the fit.
• 4 Before the permanent socket is made, the prosthetist works with the patient to ensure that the test socket fits properly. In the case of a missing leg, the patient walks while wearing the test socket, and the prosthetist studies the gait. The patient is also asked to explain how the fit feels; comfort comes first. The test socket is then adjusted according to patient input and retried. Because the material from which the test socket is made is thermoplastic, it can be reheated to make minor adjustments in shape. The patient can also be fitted with thicker socks for a more comfortable fit.
• 5 The permanent socket is then formed. Since it is usually made of polypropylene, it can be vacuum-formed over a mold in the same way as the test socket. It is common for the stump to shrink after surgery, stabilizing approximately a year later. Thus, the socket is usually replaced at that time, and thereafter when anatomical changes necessitate a change.

Fabrication of the prosthesis

• 6 There are many ways to manufacture the parts of a prosthetic limb. Plastic pieces—including soft-foam pieces used as liners or padding—are made in the usual plastic forming methods. These include vacuum-forming (see no. 3 above), injecting molding—forcing molten plastic into a mold and letting it cool—and extruding, in which the plastic is pulled through a shaped die. Pylons that are made of titanium or aluminum can be die-cast; in this process, liquid metal is forced into a steel die of the proper shape. The wooden pieces can be planed, sawed, and drilled. The various components are put together in a variety of ways, using bolts, adhesives, and laminating, to name a few.
• 7 The entire limb is assembled by the prosthetist's technician using such tools as a torque wrench and screwdriver to bolt the prosthetic device together. After this, the prosthetist again fits the permanent socket to the patient, this time with the completed custom-made limb attached. Final adjustments are then made.

Physical Therapy

Once the prosthetic limb has been fitted, it is necessary for the patient to become comfortable with the device and learn to use it in order to meet the challenges of everyday life. At the same time, they must learn special exercises that strengthen the muscles used to move the prosthetic device. When the patient has been fitted with a myoelectric device, it is sometimes true that the muscles are too weak to effectively signal the device, so again the muscles are exercised to strengthen them. Some new amputees are trained to wash the devices—including the socks—daily, and to practice getting them on and off.

A patient fitted with an artificial arm must learn to use the arm and its locking device as well as the hand. If the amputee lost an arm due to an accident and is subsequently fitted with a myoelectric device, this is relatively easy. If the loss of the limb is congenital, this is difficult. An instruction system has been developed to teach amputees how to accomplish many small tasks using only one hand.

Some patients fitted with an artificial leg also undergo physical therapy. It typically takes a new amputee 18-20 weeks to learn how to walk again. Patients also learn how to get in and out of bed and how to get in and out of a car. They learn how to walk up and down hill, and how to fall down and get up safely.

Quality Control

No standards exist for prosthetic limbs in the United States. Some manufacturers advocate instituting those of the International Standards Organization of Europe, particularly because U.S. exporters of prosthetic limbs to Europe must conform to them anyway. Others believe these regulations to be confusing and unrealistic; they would rather see the United States produce their own, more reasonable standards.

Lack of standards does not mean that prosthetic limb manufacturers have not come up with ways to test their products. Some tests evaluate the strength and lifetime of the device. For instance, static loads test strength. A load is applied over a period of 30 seconds, held for 20 seconds, then removed over a period of 30 seconds. The limb should suffer no deformation from the test. To test for failure, a load is applied to the limb until it breaks, thus determining strength limits. Cyclic loads determine the lifetime of the device. A load is applied two million times at one load per second, thus simulating five years of use. Experimental prosthetic limbs are usually considered feasible if they survive 250,000 cycles.

The Future

Many experts are optimistic about the future of prosthetic limbs; at least, most agree that there is vast room for improvement. A prosthetic limb is a sophisticated device, yet it is preferably simple in design. The ideal prosthetic device should be easy for the patient to learn how to use, require little repair or replacement, be comfortable and easy to put on and take off, be strong yet lightweight, be easily adjustable, look natural, and be easy to clean. Research aims for this admittedly utopian prosthetic device, and strides have been made in recent years.

Carbon fiber is a strong, lightweight material that is now being used as the basis of endoskeletal parts (the pylons). In the past it was used primarily for reinforcement of exoskeletal protheses, but some experts claim that carbon fiber is a superior material that will eventually replace metals in pylons.

One researcher has developed software that superimposes a grid on a CAT scan of the stump to indicate the amount of pressure the soft tissue can handle with a minimum amount of pain. By viewing the computer model, the prosthetist can design a socket that minimizes the amount of soft tissue that is displaced.

An experimental pressure-sensitive foot is also in the works. Pressure transducers located in the feet send signals to electrodes set in the stump. The nerves can then receive and interpret the signals accordingly. Amputees can walk more normally on the new device because they can feel the ground and adjust their gait appropriately.

Another revolutionary development in the area of prosthetic legs is the introduction of an above-knee prosthesis that has a built-in computer that can be programmed to match the patient's gait, thereby making walking more automatic and natural.

Where To Learn More

Books

Forester, C. S. Flying Colours. Little, Brown, 1938.

Sabolich, John. You 're Not Alone. Sabolich Prosthetic and Research Center, 1991.

Shurr, Donald G. and Thomas M. Cook. Pros the tics and Orthotics. Appleton and Lange, 1990.

Periodicals

Abrahams, Andrew. "An Amazing 'Foot' Puts Legless Vet Bill Demby Back in the Ballgame," People Weekly. April 4, 1988, p. 119.

Hart, Lianne. "Lives that Are Whole," Life. December, 1988, pp. 112-116.

Heilman, Joan Rattner. "Medical Miracles," Redbook. May, 1991, p. 124+.

"A Helping Hand for Christa," National Geographic World. November, 1986, p. 10.

"Off to a Running Start," National Geographic World. August, 1991, pp. 29-31.

—Rose Secrest

Artificial limb and joint

A limb or joint lost through accident, disease, or birth defect may be replaced with an artificial limb or joint. Such a replacement is called a "prosthesis," from the Latin word meaning "addition." Crude artificial limbs have been used since the earliest loss of an arm, leg, hand, or foot.

The Modern Era of Artificial Limbs

The modern era of artificial limbs began with the famous French surgeon Ambroise Paré (1517-1590; considered the "father of modern surgery"). Pare began his career as a barber-surgeon; in 1536 he became a battlefield surgeon. On the battleground his greatest challenge was developing ways to deal with gunshot wounds. The devastating nature of these wounds meant that soldiers' limbs often had to be amputated. After devising safer, more effective methods of amputation, Pare turned his attention to the design of artificial limbs to replace the ones he had surgically removed.

Paré exercised great ingenuity in his designs, always striving to simulate some degree of natural movement in his mechanical devices. An artificial leg pictured in Pare's Oeuvres ("Works") of 1575 featured a movable knee joint controlled by a string and a flexible foot operated with a strong spring. An artificial hand made by Pare had fingers that moved individually by means of tiny internal cogs and levers. When amputating a limb, Pare tried to leave enough stump so that it could be fitted with an artificial limb. Because of Pare's eminence, his ideas and designs for prostheses (plural of the word prosthesis), or artificial limbs, became well known.

Thalidomide Babies

Significant improvements were made in prosthetic design with the birth in the early 1960s of "thalidomide babies." These children were born with a variety of congenital (resulting from problems that occur while a baby grows in the womb) defects, including shortened or misformed limbs. The defects were caused when pregnant women took the drug thalidomide for relief of nausea and vomiting during the early months of a pregnancy. Artificial arms powered by carbon dioxide gas were eventually developed for these children. In the 1960s scientists in the former Soviet Union formed a prosthetic hand controlled by normal nerve impulses from the brain (the messages were picked up by electronic devices in the hand). More recently American scientists developed myoelectric ("myo" means muscle) prostheses. A myoelectric limb moves when it receives electrical impulses from nerves in the stump of the limb. Modern artificial limbs take advantage of plastics and fiberglass for enhanced strength and comfort.

Joint Replacement Surgery

Joints represent a special challenge for replacement. A joint is the place at which two bones come together, such as at the knee or shoulder. Replacement of joints began in the 1950s. Surgically installing artificial substitutes for joints that have become degenerated by disease, injury, or malformation is called total joint arthroplasty. Replacement of the hip and knee account for 80 to 90 percent of these operations. Other less frequently replaced joints are the shoulder, elbow, and small joints of the hands and fingers. The first total knee arthroplasty was performed in 1951; ten years later the first total hip replacement occurred.

Artificial joints are fastened to the bone either by cement or by a relatively new process called "bone ingrowth" in which the natural bone grows into the porous (full of small holes) surface of the prosthesis. Still, cementing is the favored technique for older patients. Some evidence claims that patients who get bone ingrowth replacement experience longer wear (more time before the artificial joint begins to wear out) than recipients of cemented joints. Artificial joint recipients must watch for signs of infection. Newer surgical techniques, including super-sterile operating rooms, are helping to minimize the risk of infection. Joint replacement does not usually restore normal function completely (for example, the replacement joint is not usually as flexible as the natural joint, and certain types of strenuous activities are limited). Nevertheless, joint replacement usually restores significant mobility and dramatically relieves the pain of problem joints.

artificial limb mechanical replacement for a missing limb. An artificial limb, called a prosthesis, must be light and flexible to permit easy movement, but must also be sufficiently sturdy to support the weight of the body or to manipulate objects. The materials used in artificial limbs include willow wood, laminated fibers and plastics, various metallic alloys, and carbon-fiber composites. One model of artificial leg is made of layers of stockinette cloth coated with plastic; it has duraluminum joints at the knee and ankle, rubber soles on the feet, and a leather cuff cushioning the stump. The cuff fits around the thigh like a corset, holding the artificial leg firmly in place, and connects to a leather belt around the waist. Often, spring joints are employed on foot pieces to give natural-looking movements. Microprocessors and an array of sensors are used to operate the mechanical and hydraulic system of some artifical legs, providing more natural locomotion. Other artificial legs sacrifice a natural appearance to achieve greater mobility, such as the C-shaped carbon-fiber Flex-Foot used by amputees to participate in track-and-field sports. Artificial legs may also be secured by suction between socket and stump.

Artificial arms, not having to support the weight of the body, may be made of lighter metals and plastics. They are usually strapped to the trunk and controlled by a shoulder harness. Bionic arms have been developed that permit a person to use thought to control the limited movements of the motorized prosthesis. The commands are transmitted through chest muscle that has been surgically connected to the remaining nerves associated with the lost limb; electrodes linked to the artificial arm convert the sensed electrical signals of the muscle into arm movement. Tests with monkeys have shown that robotic arms can also be controlled by the brain's electrical signals directly, using probes implanted in the brain and computer software to interpret the signals.

Artificial hands vary in structure and utility; research and development has resulted in devices that are both cosmetic and functional. For example, an artificial hand has been devised that utilizes a split hook resembling a lobster claw; this is enclosed within a flexible plastic glove that can be made remarkably lifelike, even having fingerprints. The biceps muscle can be attached to the prosthesis by a surgical procedure called cineplasty, which permits grasping in the terminal device while dispensing with shoulder harnesses. A more recent artificial hand has separate motors for each finger, allowing for a more natural and useful grip and movement; the prosthesis is controlled by electrical signals generated by the arm muscles that normally control the hand.

Artificial ligaments

Ligaments are bands of tough, elastic tissue that bind bones together at joints so that they can move. When a ligament is torn, it can either be repaired or replaced. Repair is the first choice, but often a torn ligament heals poorly and must be replaced. Most replacements come from connective tissues in the patient's own body (such as a knee tendon). Rehabilitation and return to full strength can take one to two years.

As anyone who participates in sports or other strenuous activities knows, the knee is very vulnerable to injury. When the knee is subjected to abrupt or progressive stress, one of its four ligaments is likely to tear. Repair or replacement of these ligaments is a major problem. To reduce rehabilitation time and provide greater strength, the W. L. Gore Company developed an artificial ligament made out of Gortex. Gortex is a porous (full of small holes) update of Teflon (a tough material invented in 1969 best known for its use in waterproof materials). The six-inch-long Gortex ligament consists of about 1,000 fibers braided together for strength. The ligament is attached to the bones above and below the knee with stainless steel screws and soon becomes naturally anchored as the bone grows into and through the Gortex.

Rehabilitation with the Gortex ligament can be as short as six weeks, and the procedure itself is usually done as outpatient arthroscopic surgery. The Food and Drug Administration (FDA) approved use of synthetic ligaments in humans in 1988. The approval, however, was only for patients who had tried and failed with a natural implant.

[See also Artificial hip ]