Beaconsfield High School

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Prosthetic limbs

          

Prosthetic limbs go back a long way - there was a report published in 2000 that showed an Egyptian mummy with a wooden toe grafted to her foot. They are, quite honestly, remarkable. A replacement for a lost body part is an invaluable thing to have. You can get prosthetic parts for anything: eyes, noses, ears, arms, hands, legs, feet.

 This notwithstanding, there are only four options for artificial limbs: below the knee (transtibial), a prosthetic lower leg attached to an intact upper leg; above the knee (transfemoral), a prosthetic lower and upper leg, including a prosthetic knee; below the elbow (transradial), a prosthetic forearm; above the elbow (transhumeral), a prosthetic lower and upper arm, including a prosthetic elbow.

 There are four parts to a prosthetic limb: the socket, the limb, the attachment mechanism, and the control system.

The socket is the part of the limb that attaches to the remaining stump. A cast is taken of the stump to ensure no chafing, as this would be very painful. This is why it is so important for the cast and socket to be fitted by a prosthetist.

The limb is the actual piece of plastic and carbon fibre (this is what prosthetics are usually made of, as it is very light – your average pair of legs is about 40% of the overall bodyweight) that is a replacement for the actual limb. The aesthetics of this depends on where on the body it is going: a leg’s prosthetic is made purely for practicalities, but a hand is made to look as natural as possible.

The attachment mechanism is fitted to stump with a suspension system: this could be an elastic sleeve, suction socket (which is a kind of cup that fits around the stump), straps, or harnesses. Snug fitting is essential for comfort.

The control system varies a lot in complexity. The simplest prosthetics that are still functional are worked by cables that run through the middle of the limb, doing the job of the muscles. Simple artificial legs can operate mostly through gravity: the wearer learns to walk with the leg by swinging the prosthetic into position and then balancing on it while they move their undamaged leg in front of this one. Some of the more sophisticated prosthetics are controlled electronically.

Prosthetic limbs are hard to live with. There is no perception of pressure, so the wearer doesn’t know hard to hold something like a brick in relation to something like a glass decoration. This makes the experience even more unpleasant. It also makes driving very hard, for obvious reasons. There are hand controls for people with prosthetic legs, but not everyone is able to use them. However, there are solutions to these issues.

Some of the most sophisticated (and expensive) prosthetics are myoelectric. They function by using electrodes to the sense muscular impulses in the stump that are created by the brain trying to move the limb that is now replaced by the prosthetic. Electronic control systems perceive these impulses and magnify them to power electric motors that operate the prosthetic limb as though it were real. Basically, the wearer’s brain says, “I want to move my arm now”, and then when the muscular impulses come down the nerves, the control systems detect them, and move the prosthetic as though it were a real limb.

 Dr. Daniel Tan, of Cleveland Medical Centre, Ohio, has made a special type of prosthetic. It is similar to the myoelectric limb in some ways, such as the use of the brain’s signals to cause movement, but different in others. Dr. Tan ran a test, and the results were: by implanting electrodes around the stump of a wearer of a prosthetic limb and attaching these electrodes to a current generator, the wearer of the prosthetic had a sensation of feeling in the artificial limb. The nature of the feeling depended on the type of current that was applied. The simplest square wave current made a peculiar, electrical feeling. Using more intricate patterns, however, could recreate not only simple feelings like pressure and tapping on a table, but more subtle ones like hair brushing on an arm or finger. Delicate tasks are difficult for people with prosthetic limbs: when a group of subjects were asked to remove the stalks from cherries, only 42% were able to successfully do it without crushing the cherry. But when the electrodes attached to the people’s fingers were connected to the current generator, the success rate went up to 92%.

Another aspect that was good about this method was that the sensation of “phantom limbs” had vanished when the current was on. This feeling is where the person feels like their missing limb is still there. When the prosthetic was not attached to the generator, the artificial limb felt like an extra, unwanted part of the body; when it was, the limb was said to feel like an actual part of the body.

 

By Tabby

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