Betty McCarthy, age 15, of Willingsboro, N.J., for her question:
HOW IS AN ARTIFICIAL LIMB CONTROLLED?
Doctors and scientists have combined efforts
to produce artificial limbs that can replace arms or legs that have been lost as results of injuries, surgeries or birth defects. An artificial limb is called a prosthesis.
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 waist.
An individual prothesis must be custom made for each patient. For several weeks after an amputation, the stump is wrapped tightly with elastic bandages to help it shrink to a firm, smooth surface. Tight wrapping ensures proper fitting of a prothesis.
During this period, the patient exercises the muscles remaining in the limb to preserve their strength and movement. In some cases, the stump can be fitted 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 has been reproduced inside them. 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.
Some plastics used for leg protheses soften at temperatures low enough to allow the socket to be molded onto a stump covered only by thin layers of cloth and rubber. Such a socket is more flexible and comfortable than one made from a plaster mold.
In general, a leg doesn't have to perform so skillfully as an arm.
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. They are less attractive than a natural looking hand, but they have greater usability and strength.
A foot substitute is the same general shape as a normal foot.
We know, of course, that no artificial limb can perform all the functions of a human limb. But a prothesis can definitely carry out most movements that are both precise and powerful. It may be designed for either skillful or forceful movements, depending on the requirements of the individual patient.
The Boston arm, a prothesis first used in 1968, has a small electric motor controlled by the muscles in the stump. Whenever any muscle contracts, it produces a tiny electric current.
In the Boston arm, an amplifier increases this current, which controls the motor in the prothesis. A person wearing the Boston arm, which was developed by a team of Boston physicians 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. The power source for the motor is a small battery pack worn at the waist.