The prosthesis attaches to the body at the interface, which consists of a socket and a rigid frame. At the socket (which is made of plastic or laminated material), the components attach to the user. The frame, which is made of graphite or similar materials, provides structural support for the socket.
A liner is worn between the residual limb (stump) 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, users should have 2 liners for each prosthesis. Alternating the liners from day to day helps maintain their elasticity and shape and makes them last longer.
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, users can make the prosthesis fit better as the residual limb varies in size, as it does normally throughout the day when activities, weather, and other factors change.
The interface may include a suspension system, which helps hold the prosthesis on securely. 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 limb near it can become irritated and inflamed, fluid may accumulate, and sores may develop.
Belts and harnesses: Sometimes the prosthesis is attached by a belt or harness. These devices may be used if keeping the prosthesis on with a suction valve or locking pin is difficult or the pin cannot be tolerated. However, the harness is relatively rigid and thus can be uncomfortable and cumbersome. It may also restrict movement.
Components include terminal devices (artificial fingers, hands, feet, and toes), and joints (wrists, elbows, hips, and knees). Metal shafts and customized carbon fiber structures, which function as bones, are used when extra strength, flexibility, and energy return are needed. For more advanced prostheses, there are control elements available that allow the user to move the prosthesis mechanically or electrically.
Components for upper extremity prostheses, which are controlled by microprocessors and powered myoelectrically, are replacing the older body-powered models. Myoelectric prostheses create movement using the electrical charges naturally produced when a muscle contracts. These electrical charges are picked up by surface electrodes and sent to an electric motor, which moves the limb. Components for microprocessor-controlled lower extremity prostheses utilize velocity, torque, and positioning sensors to help define function. These newer components are more efficient and require less effort to control the prosthesis.
Neural-integrated prosthetics, which are now in research and testing stages with upper-limb prosthetics, may enable people to function even better. The nerves that went to the amputated limb are rerouted to connect with healthy muscle (eg, 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; thus, the user can move the limb, as if by thinking, as with natural limbs.
Some users choose to have the components enclosed by a cover. Covers consist of 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 users—especially athletes during competition—omit the cover, leaving the components exposed.