artificial limb

The Columbia Encyclopedia, 6th ed.

artificial limb

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 be controlled by the brain's electrical signals directly, using probes implanted in the brain and computer software to interpret the signals, and in laboratory experiments a person has similarly controlled a robotic arm.

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. Software and electronics have improved sufficiently that some artificial hands can supply feedback to sensory nerves, enabling the user to feel the size, shape, and rigidity or flexibility of the object being handled.

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