A limb prosthesis has three main parts:
The prosthesis attaches to the body at the interface. The interface consists of a socket and a rigid frame. The socket, which is made of plastic or laminated material, is where the working parts of the prosthesis (the components) attach to the person. The frame, which is made of graphite or similar materials, provides structural support for the socket and the components.
A liner is worn between the residual limb and the socket to provide cushioning and to make the fit tight. The liner is made of soft polyurethane or silicone, which clings to the skin without causing friction. Ideally, people should have two liners for each prosthesis. Alternating the liners from day to day can make them last longer. They maintain their elasticity and shape better when they are not worn on consecutive days.
A prosthetic sock may be worn instead of or with a liner. Socks are made of wool, nylon, or synthetic fabrics, sometimes with gel sandwiched between the layers of fabric. Socks are available in different thicknesses (plies). By putting on several socks or socks of different thicknesses or by taking socks off, a person can make the prosthesis fit better as the residual limb changes size, which it does normally throughout the day when activities, weather, and other factors change.
The interface may include devices that help hold the prosthesis on securely (called a suspension system). The following suspension systems are commonly used:
Suction valve: When the residual limb is put in the socket, air is forced out through an opening at the bottom of the socket. A one-way suction valve on the socket closes the opening and forms a seal that holds the prosthesis in place.
Liners with a locking pin: Most liners are locked into the bottom of the socket by a notched pin. Because the pin is pressed tightly against the residual limb, the parts of the residual limb near it can become irritated and inflamed, fluid may accumulate, and skin sores may develop.
Belts and harnesses: Sometimes the prosthesis is attached by a belt or harness. These devices may be used by people who have difficulty keeping the prosthesis on with a suction valve or locking pin or who cannot tolerate the pin. However, the harness can be relatively rigid and thus can be uncomfortable and cumbersome. It may also restrict movement.
Components are the working parts of the prosthesis. They include terminal devices (artificial fingers, hands, feet, and toes) and joints (wrists, elbows, hips, and knees), as well as metal or carbon fiber shafts, which function as bones.
Components that are controlled by microprocessors and powered myoelectrically are replacing the older hydraulic, body-powered models. Myoelectric prostheses create movement using the electrical charges naturally produced when a muscle contracts. The electrical charges are sent to an electric motor that moves the limb. These new components are more efficient and cause less stress to users.
Bionic components, which are just now becoming available, may enable people to function even better. Bionic components are moved as if by thinking, similarly to natural limbs. Such movement is made possible by rerouting the nerves that once went to the amputated limb and connecting them to healthy muscle in the body (for example, to chest muscle for an amputated arm). These nerves direct impulses once sent to the amputated limb through electrodes on the skin’s surface to microprocessors in the prosthetic limb and thus enable the user to move the limb.
Some people who wear a prosthesis choose to have the components enclosed by a cover. Prosthetic covers consist of flexible foam shaped by the prosthetist to look like the missing limb. The foam is often enclosed by a lifelike protective covering. How lifelike covers look depends on whether they are off-the-shelf or highly customized, designed by artisans to exactly match the user’s skin pattern. Some people—especially athletes during competition—prefer to omit the cover, leaving the components exposed.