Prosthetic limbs
Last updated: February 3, 2010.
Have you ever met anyone who has an artificial limb? Chances are
you have—without even knowing it. Modern prostheses (as replacement
limbs are properly known) work so effectively and look so convincing
that you wouldn't even know someone was wearing one. People with
prosthetic legs can often climb stairs, walk, swim, and run
as well as though they were using natural limbs, while
prosthetic arms and hands have advanced to the point where they give
individual control of all five fingers. You might think
they're boring or mundane, but prosthetic limbs are among the world's
truly great inventions: by boosting independence they boost optimism
and both these things dramatically improve people's lives. Let's take a closer look at how
these amazing devices work!
Photo: Left: Anything you can do... Prosthetic limbs help to give people back their independence and challenge stereotypes of "disability." Photo by Greg Mitchell courtesy of US Navy. Right: Anything you can do (part 2): Military amputees learn to surf at the Naval Medical Center San Diego. Photo by Jake Berenguer courtesy of US Navy.
What is a prosthesis?
A prosthesis (also called a prosthetic limb, artificial limb, or
limb replacement) is an artificially made substitute for a limb lost
through a congenital defect (present at birth), accident, illness, or wartime
injury. A type of prosthesis called a cosmesis is designed to
be purely cosmetic and has little or no function; artificial hands
often fall into this category. Other prostheses are highly functional
and have little or no cosmetic disguise; artificial legs, designed to
be covered by pants (trousers), are sometimes little more than
metal rods and wires.
Types of prosthesis
In theory, any part of the body—from your ear or nose to your finger
or toe—could be replaced by a prosthesis. In practice, there are four
common types of prosthetic limb, which replace either a partial or
complete loss of an arm or leg:
- Below the knee (transtibial): A replacement lower leg attached to an intact upper leg.
- Above the knee (transfemoral): A complete replacement leg.
- Below the elbow (transradial): A replacement forearm.
- Above the elbow (transhumeral): A complete replacement arm.
Photo: Right: A typical transfemoral prosthesis. The white part at the top is the socket: it fits snugly around the patient's residual limb. Photo by Greg Mitchell courtesy of US Navy.
How does a prosthesis work?
A prosthesis consists of a number of different parts. There's the
prosthetic limb itself, the socket (the joint between the prosthetic limb
and the patient's body), the attachment mechanism, and the control
system. Let's briefly look at these in turn.
The prosthetic limb
The construction of a prosthesis depends largely on the job it
will have to do. While cosmetic hands are concerned purely with
appearance, prosthetic legs are substitutes for a major structural
part of someone's body and have to bear their entire weight.
Typically, a replacement arm or leg is made from a strong and durable
but lightweight material such as carbon fiber, covered with foam
padding (for comfort) or flesh-colored plastic. The weight of
prosthetic limbs is a very important factor. We don't notice how much
our limbs weigh because they feel so natural and seem to move
themselves. But if you consider that your two legs amount to 30-40
percent of your total body weight, and your two arms about 10
percent, you can see why prosthetic limbs need to be much lighter:
you'd quickly get tired moving prosthetic limbs that weighed as much
as natural ones.
Photo: A pair of prosthetic racing "blades." Athletes who wear these use more conventional prostheses in everyday life!
Photo by Sara Wood courtesy of US Army.
The socket
The comfort and effectiveness of a prosthesis is largely governed
by how well it fits onto the remaining part of the patient's own
limb, which is called their residual limb
(or sometimes, informally, the "stump"). The connecting part of a prosthesis is called the
socket and it's carefully molded around a
plaster cast taken from the residual limb. The fit of a socket has to be precise or the new limb may
damage the residual one, causing discomfort or tissue damage and perhaps
even making it impossible to wear the prosthesis for a time. A
patient's residual limb is likely to change shape and size over months
and years so new sockets will be needed from time to time.
More precisely fitting sockets can now be made by scanning a patient's residual limb
with lasers.
The attachment mechanism
A well-fitting prosthesis is usually secured to a residual limb by
straps, harnesses, or even by suction. A snug, secure fitting is
vital for comfort—and ensures the limb can be properly controlled.
Often the socket is itself a part of the attachment: for example,
a prosthetic leg socket may consist of a large hollow plastic casing into
which the residual limb is inserted.
Photo: Right: This simple transradial (below-the-elbow) prosthetic arm attaches with a socket, straps, and a shoulder harness. Note the relatively crude "hand," which has been specifically designed to help the man do his job. Photo by courtesy of NASA Marshall Space Flight Center (NASA-MSFC).
The control system
Natural limbs are pulled back and forth by muscles stimulated by
our brains; in much the same way, the simplest functional prostheses
are operated by systems of cables that run through them, doing the
job of the muscles. One common type of prosthetic hand is a pincer,
sometimes worn inside a glove (for cosmetic reasons) that can be
opened or closed by pulling cables attached to the opposite shoulder.
Simple artificial legs work largely through gravity: the wearer
learns to walk in them, through practice, by alternately swinging
them into position and then balancing on them while they move their
unimpaired leg in its turn. More sophisticated prostheses are electrically
controlled and powered by rechargeable battery packs.
The most sophisticated (and
expensive) prostheses are myoelectric. They use electrodes to
sense muscular impulses in the residual limb (or elsewhere on the
patient's body); electronic control systems detect those signals
and amplify them to power electric motors that operate the prosthetic
limb as though it were real. In other words, the patient thinks "I want to move my
arm," their brain stimulates muscles in the residual arm, the electronic system commands the electric motors to move and the
prosthetic limb shifts accordingly.
Photo: Left: A prosthetist demonstrates a relatively simple prosthetic arm. Note the shoulder socket at the top (the light-colored plastic), the simple elbow joint, and the pincer-hand at the bottom controlled by cables running down from the shoulder. Photo by Greg Mitchell courtesy of US Navy.
Right: A much more sophisticated prosthetic arm and hand. This ultra-sophisticated i-LIMB™,
made by Touch Bionics, has five working fingers that give good enough grip to pick up marbles. Photo by Charmain Z. Brackett courtesy of US Army.
Learning to live with a new limb
Technology is only part of the story, of course. Getting used to a brand
new part of your body is a physical and psychological challenge involving a partnership
between the patient and their prosthetist. This hugely important
person works with you at every stage of the process, from choosing
the best prosthesis for your needs; through the process of measuring,
manufacture, and fitting; to helping you learn how to operate your
new limb; and tackle new challenges such as taking up a sport or activity.
Further reading
More information
- Blade Runner by Josh McHugh, Wired 15.03: An article about South-African runner Oscar Pistorius, who races on running-blade prostheses.
- Bionic man: The story of BBC producer Stuart Hughes and his artificial foot.
- Making artificial limbs for amputees: The fascinating and rewarding life of a prosthetist. By Jane Elliott, Health reporter, BBC News.
- Injured marine's guide to new limbs: A great video featuring British marine Mark Ormrod, who lost an arm and both legs in 2007.
- Step by Step: How amputees learn to walk. Another good story from BBC News.
- The Adventures of the i-LIMB: What's it like to share your life with an artificial hand? Inspiring blogger Darin Sargent tells us all about it.
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