Brain-machine interface set to improve life for amputees

Small study finds implanted electrodes lead to less stressful prosthetics. Barry Keily reports.

Existing prosthetics approximate “natural” leg movement but don’t convey complex information about pressure, angle and position.

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A newly designed brain-machine interface dramatically improves performance and reduces mental and physical stress for people with a leg amputated above the knee.

A proof-of-concept study involving two amputees, published in the journal Nature Medicine, found that a prosthetic that incorporates pressure and motion sensors in the foot and knee joint, connected to four electrodes implanted in the tibial nerve, significantly improved performance in both laboratory and real-world settings.

The addition of the sensors and their direct input into the nervous system overcomes the most significant problem involved in movement for above-the-knee amputees: lack of neurological feedback.

Existing prosthetics which incorporate knee and ankle joints satisfactorily approximate “natural” leg movement, but fail to convey complex information about pressure, angle and position. This means that users frequently have to check and consciously adjust how the artificial leg is performing, a process that generates considerable mental and muscular fatigue.

To alleviate this problem, researchers led by Francesco Maria Petrini from the Swiss Federal Institute of Technology recruited two volunteers. In each they implanted four multichannel electrodes into what was left of their tibial nerves.

These were then linked to the microprocessor-controlled sensors in the prosthetics, and the resulting “phantom” sensations calibrated through varying the strength of the power fed into the system.

“Short pulse trains of electrical current varying in intensity, duration and frequency were injected into each active site,” the researchers write.

“The volunteers described the sensation in terms of type, location, extent and intensity. Physiologically plausible sensations, that is, reported by the volunteers similarly to the ones perceived with the non-amputated leg, of touch, pressure, vibration and muscle activation were elicited over the phantom foot sole and lower leg.”

The volunteers were put through a series of tests with the feedback system turned off, and then repeated with it on. These were repeated over three months.

The researchers report that the sensor-electrode interface resulted in increased walking speeds, lower oxygen consumption, increased concentration, and an 80% decrease in reported pain.

Petrini and colleagues note that people with above-the-knee amputations carry a higher risk of cardiovascular trauma, associated with elevated levels of stress and oxygen usage. The new design, they suggest, could significantly improve these outcomes.

Before that can happen, however, longer trials involving a larger cohort will be necessary.

“Overall, this work paves the way for the development of a clinical tool that will significantly improve amputees’ health and quality of life,” they conclude.

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