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Energy Requirements

Some prosthetics research is aimed at providing active devices, which do part of the work of the amputated limb, as opposed to passive devices that are controlled by the residual limb. An amputee with prostheses expends two to three times more energy than a nondisabled person to perform even the simplest activities, such as walking across a room or climbing stairs.

"A semi-active system, in which the limb itself performs part of the function, could reduce that energy requirement significantly," said Sabolich's Ortega. "And there would also be a psychological benefit, because the prosthesis would no longer be just a dead limb, but something that is helping."

Ortega said one area Sabolich is researching would provide sensory feedback from the prosthesis to the remaining limb. For instance, in an artificial leg, pressure sensors in the foot would send a mild electrical signal to the thigh muscles when there is pressure on the back, front or sides of the foot.

That kind of feedback would be similar to what they would get with the pressure of the ground against a natural foot, which would make their adjustment to the prostheses go more quickly, Ortega said.

Ortega said prosthetic designs are limited only by how large a power pack the amputee can carry.

"The crucial issue when it comes to trying to introduce any new prosthesis is the energy requirements," said Ortega. "Our muscles are so efficient, in terms of the power that they produce versus the fuel that they use, that we have a difficult time matching it."

Scientists are also working to build a better socket — the part of the prosthesis that attaches to the residual limb.

Dudley S. Childress, Ph.D., of Northwestern University's Rehabilitation Engineering Program, is working on applying the industrial practice known as rapid prototyping to socket production.

Sockets are now produced largely by hand. A cast is made of the residual limb, and plaster is poured into the cast to make a positive mold. The mold is then used to create a plastic or laminated polyester socket that fits over the residual limb.

Childress employs a computer-aided design program to measure the residual limb and design a socket. Then, using a modified "plastic deposition technology" called squirt shaping, a computer lays down small amounts of polypropylene to produce the desired shape, to very tight tolerances. In industry, the technology is used to quickly produce prototypes of everything from car parts to military weapons, to test them before starting mass production.

"Essentially, every socket is a prototype, and there are potentially some significant advantages to applying these techniques to prosthesis manufacture," Childress said. "We can make a socket in about 50 minutes, which isn't bad, but as people continue to work with the technology, it may be possible to get that down even faster."

The process would also allow manufacturers to make sockets out of different types of material than have been used in the past, or alter the thickness or characteristics of the material very quickly.

Another innovation being explored at Northwestern is powered prosthetic fingers. That might be difficult if you were going to match real fingers, he acknowledged, but most of the time that's unnecessary. Picking up a spoon and holding a book don't require much power, just control. Small motor technology and power storage capability have both improved vastly in recent years.

"If you want to do something like squeeze orange juice, you need force," Childress said. "But even for people without a prosthesis, that's tiresome, so we have all kinds of devices to do those jobs for us. So the intent of the powered fingers would be to provide prehensile [wrap-around] force."

Childress said his laboratory is also looking at devices that would improve the "feel" of prostheses over current devices. It would be comparable, he said, to the way power steering reduces the muscle power needed to steer a car, but you can still "feel" the road through the wheel.

Cables in artificial fingers and hands would connect to the muscles of the forearm, either through holes in the muscle that are surgically lined with skin, or tendons could be taken outside the residual limb and covered with skin. Either option would give the muscle the sense of how hard it is working and how fast it is moving.

Mundane, but Important, Needs

Joan E. Edelstein, Ph.D., director of Columbia University's physical therapy program, stresses the need for prosthetics research to focus not just on high-technology improvements, but to the more mundane but critical things such as fit, to make them as comfortable as possible, particularly among the elderly, whose needs may not be fully considered.

"Most patients are older people who have lost a limb because of diabetes, and the assumption is that they're going to be relatively undemanding of their prosthesis," Edelstein said.

Better prostheses for the elderly might prevent skin breakdown and infections, yet hardly any research dollars are being spent in that area, she said, explaining that, "It's not as glamorous as developing better prostheses for sport, or for children, and they are very difficult problems to overcome."

Research often proceeds along several courses at once, she noted, and you can never know which might yield the next major breakthrough.

Tags: Disabilities

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