Dave checks out how nerves and muscles move prosthetic limbs.
Segment Length: 6:45

Prosthetic Limbs

How does a person control a prosthetic hand?

How does an artificial arm or leg work?
What kinds of artificial limbs are there?

Getting Started

Try to use a clothespin to substitute for fine motor hand functions like picking up coins or counting and moving sheets of paper. Could you unzip a zipper or tie a shoelace? Try using chopsticks. Both tools can be viewed as kinds of prostheses - not too different from the look of a prosthetic claw hand.

If you were born without your hands or lost them in an accident, how would you pick up things? If you lost a foot in an accident, how would you walk? What characteristics would the ideal artificial hand have - what would you need to be able to do with it to live normally? What about a foot?

What would your ideal prosthesis look like? How would it work? What kinds of prostheses do you think exist right now? Is it possible that you've seen someone with a prosthetic limb and haven't even noticed it? Why?

Overview

What would it be like to lose a hand, a foot, or even an entire arm or leg? Scary, that's for sure. How can amputees pick up things or walk or play soccer or write a letter? Although nothing is as good as the original flesh and bone, doctors can provide artificial replacements, called prostheses, for some damaged body parts. In addition to replacing lost functions, prostheses can result in cosmetic improvements for the patient and build self-confidence.

Simple prostheses like peg legs have been around for centuries. If they do not use sophisticated electronics, these artificial limbs are called static prostheses. One kind of artificial arm, for example, ends in a pair of hooks rather than a hand. The other end is attached to the remaining portion of the patient's arm, and then to a harness that straps over the shoulders. By moving the shoulder, the patient can pull on the harness, which in turn pulls on flexible cables to open and close the hooks, allowing the person to grasp objects. There is no sense of touch in this type of prosthesis, so the user has to watch closely what he or she is doing.

Dynamic prostheses, on the other hand, use sophisticated electronics. They can do this because the nerve and muscle systems in the human body are electrical. For example, an amputee with a myoelectric arm tenses his or her remaining arm muscle. Sensors detect this muscle electricity (myoelectricity) and transmit the signal to the artificial hand, powered by batteries, which then opens or closes.

Signals also can go from the environment to the patient, allowing an approximation of the sense of touch. For example, some prosthetic hands have sensors that can detect heat or cold and transmit that information to electrodes on the patient's skin.

Researchers are still improving prostheses. New materials allow artificial feet to press and spring on the ground very much like a real foot. One type of artificial foot transmits electronic information about pressure to amputees, allowing them to balance because they can tell whether their weight is on the toes, heels, or sides of the feet.

Connections

1. Artificial limbs are very expensive. What do you think would be the fairest way for everyone, rich or poor, to have access to the prostheses they need?
2. Make a list of some of the movies you have seen recently. Are there any amputees in them? How are they portrayed? Why do you think this is?

Resources

Artificial limbs: They look and work more like the real thing. (1995, Nov) Mayo Clinic Health Letter, p. 6.

Builder of hopes. (1995, Apr 17) Sports Illustrated, p. 5.

Field, R. (1993, Oct 15) Two big steps forward: With new prostheses, the paralyzed walk and amputees walk with feeling. Medical World News, p. 32.

Kelly, A. (1991, Feb-Mar) Advancement in prosthetics: New materials, high technology, and personal service benefit prosthetic patients. Independent Living, p. 42.

McClellan, D. (1993, July) Soft wear for artificial limbs. Technology Review, p. 22.

Ridley, K. (1994, Oct) Artificial sensations. Technology Review, p. 11.

Orthotics and Prosthetics online:
http://www.oandp.com/index.htm

Main Activity

Think of all the movements your hands make. These movements require sets of muscles that put opposing stress on the jointed bones in your fingers and palms. If one muscle in the set pulls harder, the hand opens. If another pulls harder, the hand closes. The muscles must work together to give you control over your movements. You can make a simple model of this process in your classroom.

Materials

• large, square block of wood with 12 hooks spaced equidistantly around the sides
• six 1.5-meter (5') lengths of light rope or clothesline
• masking tape and a marker
• large round table
• clock faces drawn on paper for recording data

Procedure

1. Mark off a large circle on a table with the masking tape (1.5 meters in diameter, maximum) and mark numbers on it from 1 to 12 to make a clock face.
   
   
2. Have six students sit around the table and place the block in the center of the circle.
   
   
3. Each time you want to add the ability to pull the block from a new direction, tie another length of rope onto a hook and hand the end to someone sitting around the circle. If you decide to use all six lengths of rope, you will end up with what looks like a wheel with spokes, with the block at the center.
   
   
4. Pass a clock face paper to each observer, who will write the data and results on the clock face with lines and arrows.
   
   
5. Figure out how much combined tension from different directions it takes to perform more and more complicated movements of the block. (The ropes are models of individual muscles and the block is a model of a finger.) Try moving steadily in straight lines at first; then starting and stopping; then starting, stopping, and reversing direction; then moving in a half circle, a full circle, and so on. Each participant may only pull the rope toward her or him, or allow it to be pulled away, in order to help make the block move in a desired way. No fair extending arms, grabbing anyone else's rope, starting a tug-of-war, or interfering with the block's movement. Work together to make the block move.

Questions

1. What is the fewest number of ropes required to make the block start and stop quickly and reliably?
2. Can you make the block travel in a circular arc with only two ropes attached to it?
3. What is the fewest number of ropes required to make a complete circle around the numbers on the clock?
4. When moving the block in a complete circle, who must pull the hardest on his or her rope and when?

Ask a prosthetist to come by and show several different types of prostheses-lower extremity, upper extremity, etc.

A thermometer is a sensor. Your eyes and skin are highly sophisticated sensors. How many other sensors can you list, both natural and invented?

Marionettes are related to prostheses, since you work their hands and feet from a distance. Construct a simple marionette out of paper, string, and paper fasteners. How easy is it to make working arms and legs?

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