A limb prosthesis (see Fig. 1: Limb Prosthetics: Types of prostheses.) has 3 main parts: interface, components, and cover.

Fig. 1
Types of prostheses.

Interface

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 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 them 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 stump changes 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 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 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

Components 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 newer components are more efficient and cause less trauma to users.

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.

Cover

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 omit the cover, leaving the components exposed.

Options

The prosthetist explains the available options (see Table 1: Limb Prosthetics: Options for Prostheses) and helps users choose the type of prosthesis and options they need to accomplish their goals. For example, women who want to wear shoes with different heel heights may prefer a prosthetic ankle that can adjust to different heights. Swimmers can get a 2nd prosthetic leg that is designed for swimming and can withstand water, salt, and sand. Runners can get prosthetic feet specifically designed for running.

Table 1
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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 its soft heel allows the whole foot to contact the ground Provides less stability when the user raises the heel and the opposite leg swings forward, resulting in uneven walking Requires more energy to use than other types of prosthetic feet Appropriate for people who are limited in their activities, not appropriate for active people Single-axis design Has an ankle joint that allows dorsiflexion and plantarflexion of the foot 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 dorsiflexion and plantarflexion of the foot and inversion, eversion, and rotation of the ankle 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 has been 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 knee extension aid, which helps swing the leg 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 Does not provide swing-phase flexion, making walking stiff and awkward Least desirable choice Fluid control system May use compressed air (pneumatic system) or fluid (hydraulic system) to produce, store, and release energy as the knee bends and straightens Enables 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 hydraulic fluid or the magnetorheological 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 needed to use the prosthesis and enables users to walk more naturally Hand Precision (pincher) grip Enables users to pick up or pinch a small object Has a thumb that opposes the pad of the index finger Tripod (palmar) grip Has a thumb that opposes the pads of the index and middle fingers Lateral (key pinch) Enables users to manipulate a small object (eg, 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 thumb extension and finger flexion Myoelectric hook improves line of sight for functional grasp Spherical Enables users to grasp a round object (eg, door knob, electric bulb) Allows thumb and fingertip flexion Sport-specific Includes hands with a gripping device (eg, for golf, archery, or weight-lifting) or a mesh pocket for catching a baseball Elbow Body-operated Lightweight Consists of a cable and harness, using shoulder and back movement 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 its soft heel allows the whole foot to contact the ground Provides less stability when the user raises the heel and the opposite leg swings forward, resulting in uneven walking Requires more energy to use than other types of prosthetic feet Appropriate for people who are limited in their activities, not appropriate for active people Single-axis design Has an ankle joint that allows dorsiflexion and plantarflexion of the foot 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 dorsiflexion and plantarflexion of the foot and inversion, eversion, and rotation of the ankle 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 has been 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 knee extension aid, which helps swing the leg 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 Does not provide swing-phase flexion, making walking stiff and awkward Least desirable choice Fluid control system May use compressed air (pneumatic system) or fluid (hydraulic system) to produce, store, and release energy as the knee bends and straightens Enables 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 hydraulic fluid or the magnetorheological 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 needed to use the prosthesis and enables users to walk more naturally Hand Precision (pincher) grip Enables users to pick up or pinch a small object Has a thumb that opposes the pad of the index finger Tripod (palmar) grip Has a thumb that opposes the pads of the index and middle fingers Lateral (key pinch) Enables users to manipulate a small object (eg, 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 thumb extension and finger flexion Myoelectric hook improves line of sight for functional grasp Spherical Enables users to grasp a round object (eg, door knob, electric bulb) Allows thumb and fingertip flexion Sport-specific Includes hands with a gripping device (eg, for golf, archery, or weight-lifting) or a mesh pocket for catching a baseball Elbow Body-operated Lightweight Consists of a cable and harness, using shoulder and back movement 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

Last full review/revision February 2010 by Erik Schaffer, CP