09:30, 29 August, 2002
New Scientist Print
Two men paralysed on one side of their body can walk again, thanks to an ingenious implant that uses signals from a healthy leg to control a paralysed one.
Both men, aged 47 and 64, had been paralysed by strokes. Previously neither could walk unaided. But after sensors were placed over certain muscle groups on the healthy leg and stimulators implanted in the paralysed leg, they can now walk, stand and sit.
The unique therapy allows a patient to move their paralysed leg in a natural way without being aware that they are doing it, says Wenwei Yu, who developed the technique at Hokkaido University in Sapporo, Japan. But it could be another five years or more before the technology becomes available, he says.
In Yu's system, muscle sensors monitor signals from the patient's able leg. These are used to trigger pre-programmed electrical impulses in 11 electrodes implanted near nerves in the paralysed leg. This lets the paralysed leg do what the patient wants it to do - by taking its cue from the good leg.
Producing movement in limbs by electrically stimulating muscles or nerves is known as functional electrical stimulation. One of the difficulties of using conventional FES, says Paul Taylor, a clinical engineer at Salisbury District Hospital in Wiltshire, is overcoming "spasticity" - involuntary muscular spasms normally suppressed by the brain.
"So even if you had appropriate signals in the appropriate muscles it may not behave normally," he says. "There will also be stiffness, the muscles will be weak and activity from other muscles might be working against what you're trying to do," says Taylor.
Another problem with conventional FES, says Yu, is that patients have to activate the electrodes using their upper body, either through hand-held switches or sensors in their arms.
A certain wrist action, for example, could make a leg move. But this is far from practical, as they may want to make that same arm movement for other reasons. And while researchers have been trying out FES for many decades, much of the work on legs has focused on paraplegia, where both legs are paralysed. But hemiplegia, where only one leg is paralysed, is far more prevalent.
By taking advantage of the working leg to control the paralysed one, Yu avoids the problem of using the upper body to activate the electrodes. And he avoids any spasticity by tuning the electrical stimulations and their timing so that the muscles work in concert with each other to produce smooth coordinated movements.
Not only that, but the electrical stimulation itself has a therapeutic effect, preventing the leg muscles from getting stiff.
Gerald Loeb, an expert in FES at the University of Southern California in Los Angeles, says Yu and his team will have to ensure their technique is safe because many hemiplegic people are elderly. One fall and they could break a hip.
Yu acknowledges these dangers but says his system uses a learning program that tailors itself to the individual patient's muscle contractions. This means it can get almost perfect recognition of the patient's intentions, which should reduce the risk of falling.
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