Pam McNally, age 14, of Patterson, N.J., for her question:
HOW IS AN ARTIFICIAL LIMB FITTED?
Artificial limbs are used to replace arms or legs that have been lost as a result of injury, surgery or a birth defect. An artificial limb is called a prosthesis.
An individual prosthesis must be custom made for every patient. For several weeks after an amputation, the stump is wrapped tightly with elastic bandages to help it shrink to a firm, smooth surface. The tight wrapping ensures proper fitting of a prosthesis.
During the early stages, the patient exercises the muscles remaining in the limb to preserve their strength and movement. In some cases, the stump can be fitting with a temporary limb immediately after surgery.
In most cases, the stump is first wrapped in bandages soaked in plaster of Paris. After the plaster has hardened, the bandages are removed and the shape of the stump is preserved.
Liquid plaster poured into this mold hardens into a cast of the stump. The cast is used to produce a molded socket of plastic and fiberglass into which the stump will fit snugly and comfortably.
Light metal supports attached to the socket may contain an artificial joint to replace an elbow or a knee.
Some plastics used for leg prostheses soften at temperatures low enough to allow the socket to be molded into a stump covered only by thin layers of cloth and rubber. Such a socket is more flexible and more comfortable than one made from a plaster mold.
An artificial limb ends in a substitute hand or foot. A hand substitute may look like a real hand but most hand prostheses consist of a pair of metal hooks. The hooks act as tongs.
Metal hooks may be less attractive than a natural looking hand, but they have greater usability and strength.
Most artificial arms are controlled by an attached stainless steel cable that loops around the opposite shoulder. A movement of that shoulder produces movement at the wrist or elbow on the other side.
An artificial leg is controlled by body balancing movements that act through a shoulder harness or through a belt around the wrist.
No artificial limb can perform all the functions of a human limb. A prosthesis can carry out only a few movements that are both precise and powerful. It may be designed for either skillful or forceful movements.
A Boston arm, a prosthesis first used in 1968, has a small electric motor controlled by the muscles in the stump. Whenever any muscle contracts, it produces a thin electric current.
In the Boston arm, an amplifier increases the current which controls the motor in the prosthesis. A person wearing the Boston arm, which was developed by a team of Boston doctors and engineers, can "will" it to move.
His brain sends the same signals to the stump muscles that it would send if he wanted to move a real arm.