A limb prosthesis has three main parts: the interface, the components, and the cover.

Interface: 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.

A liner is worn between the remaining part of the 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, 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 stump changes size, as 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:

Types of Prostheses

• Suction valve: When the stump 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 stump, the parts of the stump 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 is relatively rigid and thus can be uncomfortable and cumbersome. It may also restrict movement.

Components: 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 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 trauma 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 skin's surface to microprocessors in the prosthetic limb and thus enable the user to move the limb.

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Options for Prostheses Options Comments Foot Solid ankle, cushioned heel (SACH) A basic immovable foot made of rubber and wood Provides stability for the knee when the heel touches the ground because it has a soft heel that allows the whole foot to contact the ground Provides less stability when the user raises the heel and the opposite leg swings forward and thus results in uneven walking Requires more energy to use than other types of prosthetic feet Appropriate for people who are limited in their activities, not for active people Single-axis design Has an ankle joint that allows the foot to move up and down Allows the whole foot to quickly contact the ground after the heel touches the ground and the knee to straighten quickly Because of the above features, provides good stability for the knee, which is particularly important for people with an amputation above the knee Not appropriate for active people Multiple-axis (multiaxial) design Has an ankle joint that allows the foot to move up and down and the ankle to turn in and out and to rotate Enables users to walk on uneven terrain more easily With newer, lightweight models, minimal maintenance required Can be made to look lifelike Appropriate for active people Stored-energy (dynamic response) design Made of carbon graphite, which is lightweight and strong Enables users to walk smoothly and relatively naturally Requires less energy to use because the foot stores energy from when the heel touches the ground to when the toe pushes off, propelling the user forward May include a shock absorber to reduce the force of contact with the ground during walking Appropriate for active people Sport-specific A foot that can be customized for any specific sport For running (long-distance and sprinting), a foot designed with the foot bent downward toward the sole and with the capacity to store energy used to propel the user forward For swimming, an ankle designed to allow full range of motion in the water Knee Single-axis, constant friction design Simple in design, with only one pivot point (bends like a hinge) Durable, lightweight, and inexpensive Uses friction that does not vary to control the leg when it swings forward Allows users to walk normally at only one speed Relies on correct alignment by the prosthetist and muscle control by the user to provide stability Polycentric design Has several hinges with several pivot points that change as the knee moves, providing increased stability Shortens slightly when the knee is bent, so that the toe clears the ground more easily when the leg swings forward Provides stability for people with a short stump Appropriate for people whose leg was amputated at the knee joint, enabling users to sit more comfortably without the knee protruding Weight-activated stance control feature Locks the knee in a slightly bent position (to provide braking) when weight is put on foot Uses constant friction to control the leg when it swings forward but has a feature that helps swing the leg (called a knee extension aid) Requires users to walk at one speed Appropriate for people with weak muscles Manual lock feature Can be locked or unlocked by users as needed but requires a cable to do so Provides the most stability Requires more energy to use than other types of prosthetic knees Makes walking stiff and awkward Least desirable choice Fluid control system May use a compressed air (pneumatic system) or fluid (hydraulic system) to produce, store, and release energy as the knee bends and straightens Enable users to walk at different speeds May be equipped with a microprocessor Best choice for most people Microprocessor feature Has sensors that detect movement and can adjust the fluid control system accordingly Provides good control when the foot is on the ground and when the leg swings forward Can be programmed to compensate for stumbling and to enable users to descend stairs and ramps Reduces the energy need to use the prosthesis and enables users to walk more naturally Hand Precision (pincher) Enables users to pick up or pinch a small object Has a thumb that opposes the pad of the index finger Tripod Has a thumb that opposes the pads of the index and middle fingers Lateral Enables users to manipulate a small object (for example, turning a key in a lock) Has a thumb that opposes the side of the index finger Hook Enables users to carry objects with a handle Allows the thumb to be straightened (extended) and a finger to be curved (flexed) Spherical Enables users to grasp a round object (for example, a door knob or electric bulb) Allows the thumb and tips of fingers to be flexed Sport-specific Includes hands with a gripping device (for such sports as golf, archery, or weight-lifting) or a mesh pocket for catching a baseball Elbow Body-operated Lightweight Consists of a cable and harness, using movement of the shoulder and back to move the arm Less attractive than other options and sometimes bothersome Friction-operated Lightweight Raised or lowered by using the hand of the other arm Myoelectric Heavy but requires no cables Provides more function Options for Prostheses Options Comments Foot Solid ankle, cushioned heel (SACH) A basic immovable foot made of rubber and wood Provides stability for the knee when the heel touches the ground because it has a soft heel that allows the whole foot to contact the ground Provides less stability when the user raises the heel and the opposite leg swings forward and thus results in uneven walking Requires more energy to use than other types of prosthetic feet Appropriate for people who are limited in their activities, not for active people Single-axis design Has an ankle joint that allows the foot to move up and down Allows the whole foot to quickly contact the ground after the heel touches the ground and the knee to straighten quickly Because of the above features, provides good stability for the knee, which is particularly important for people with an amputation above the knee Not appropriate for active people Multiple-axis (multiaxial) design Has an ankle joint that allows the foot to move up and down and the ankle to turn in and out and to rotate Enables users to walk on uneven terrain more easily With newer, lightweight models, minimal maintenance required Can be made to look lifelike Appropriate for active people Stored-energy (dynamic response) design Made of carbon graphite, which is lightweight and strong Enables users to walk smoothly and relatively naturally Requires less energy to use because the foot stores energy from when the heel touches the ground to when the toe pushes off, propelling the user forward May include a shock absorber to reduce the force of contact with the ground during walking Appropriate for active people Sport-specific A foot that can be customized for any specific sport For running (long-distance and sprinting), a foot designed with the foot bent downward toward the sole and with the capacity to store energy used to propel the user forward For swimming, an ankle designed to allow full range of motion in the water Knee Single-axis, constant friction design Simple in design, with only one pivot point (bends like a hinge) Durable, lightweight, and inexpensive Uses friction that does not vary to control the leg when it swings forward Allows users to walk normally at only one speed Relies on correct alignment by the prosthetist and muscle control by the user to provide stability Polycentric design Has several hinges with several pivot points that change as the knee moves, providing increased stability Shortens slightly when the knee is bent, so that the toe clears the ground more easily when the leg swings forward Provides stability for people with a short stump Appropriate for people whose leg was amputated at the knee joint, enabling users to sit more comfortably without the knee protruding Weight-activated stance control feature Locks the knee in a slightly bent position (to provide braking) when weight is put on foot Uses constant friction to control the leg when it swings forward but has a feature that helps swing the leg (called a knee extension aid) Requires users to walk at one speed Appropriate for people with weak muscles Manual lock feature Can be locked or unlocked by users as needed but requires a cable to do so Provides the most stability Requires more energy to use than other types of prosthetic knees Makes walking stiff and awkward Least desirable choice Fluid control system May use a compressed air (pneumatic system) or fluid (hydraulic system) to produce, store, and release energy as the knee bends and straightens Enable users to walk at different speeds May be equipped with a microprocessor Best choice for most people Microprocessor feature Has sensors that detect movement and can adjust the fluid control system accordingly Provides good control when the foot is on the ground and when the leg swings forward Can be programmed to compensate for stumbling and to enable users to descend stairs and ramps Reduces the energy need to use the prosthesis and enables users to walk more naturally Hand Precision (pincher) Enables users to pick up or pinch a small object Has a thumb that opposes the pad of the index finger Tripod Has a thumb that opposes the pads of the index and middle fingers Lateral Enables users to manipulate a small object (for example, turning a key in a lock) Has a thumb that opposes the side of the index finger Hook Enables users to carry objects with a handle Allows the thumb to be straightened (extended) and a finger to be curved (flexed) Spherical Enables users to grasp a round object (for example, a door knob or electric bulb) Allows the thumb and tips of fingers to be flexed Sport-specific Includes hands with a gripping device (for such sports as golf, archery, or weight-lifting) or a mesh pocket for catching a baseball Elbow Body-operated Lightweight Consists of a cable and harness, using movement of the shoulder and back to move the arm Less attractive than other options and sometimes bothersome Friction-operated Lightweight Raised or lowered by using the hand of the other arm Myoelectric Heavy but requires no cables Provides more function

Cover: Some people who wear a prosthesis choose to have the components enclosed by a cover. Prosthetic 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 people prefer to omit the cover, leaving the components exposed.

Options: The prosthetist explains the available options and helps people choose the type of prosthesis and the options they need to accomplish their goals. For example, women who want to be able to wear shoes with different heel heights may prefer a prosthetic ankle that can adjust to different heights. People who swim can get a second prosthetic leg that is designed for swimming and can withstand water, salt, and sand. Runners can get prosthetic feet specifically designed for running.

Last full review/revision May 2007 by Erik Schaffer, CP