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Natural nerves improve robotic leg performance

By Gene Emery

NEW YORK (Reuters Health) - Redirecting nerves from the thigh muscle to control an experimental robotic leg makes it easier to use and more like a natural leg, researchers report.

The leg's movements are guided by the patient's original nerves, which have been redirected to a small area of the thigh muscle. The robotic leg senses the unconscious muscle movements and translates them into movements for the knee and ankle of the 10-pound device.

"This technology offers a seamless transition to walking around, climbing stairs, and descending stairs and moving around on slopes and even repositioning the prosthesis without thinking about it. That's something no other device offers now," Levi Hargrove of the Rehabilitation Institute of Chicago told Reuters Health.

Zac Vawter, who lives in the Seattle area and lost his right knee following a motorcycle accident at age 31, has been testing the device and is the subject of a case report by Hargrove and colleagues in The New England Journal of Medicine.

In a news release, Vawter said the bionic leg "is a big improvement compared to my regular prosthetic leg" because it "responds quickly and more appropriately, allowing me to interact with my environment in a way that is similar to how I moved before my amputation."

The device is one of many ongoing attempts to develop powered artificial limbs that reproduce the complex process of walking - both to help restore motion to amputees or for people with other problems such as spinal cord injuries. But all have their limitations.

The robotic leg is being developed with an $8 million grant from the U.S. Army. There are more than one million lower leg amputees in the U.S.

Troy Turner, a member of the scientific advisory panel of The Amputee Coalition in Manassas, Virginia, said the project "represents the first true effort at letting someone control their prosthetic leg in a way that's very similar to biologic control."

Turner, who is a rehabilitation and human performance portfolio manager with the U.S. Army's Medical Research and Materiel Command in Fort Detrick, Maryland, estimated that only about four to six groups are researching the problem on this high level.

"I think this will be the first real successful effort in making it commercializable. It also lays the groundwork for a lot of continued efforts in taking this concept much further," he said.

A similar system has been used in robotic arms for years. But the challenge is greater in developing a robotic leg, according to Hargrove, because users of a robotic arm don't face a serious risk of falling if the signals are misread.

"If you're using a bionic arm and it misbehaves, the elbow may move slightly. If the prosthetic leg misbehaves … that could be quite a safety issue," Hargrove said.

In addition, "it has to be able to hold the person's weight, generate huge amounts of torque and power to help push a person along or lift them up the stairs. So strictly from a mechanical engineering standpoint, it's very very difficult to actually design a robotic leg," he said.

The leg does have limitations, the researcher said. "We need to make it lighter, quieter, have the battery life that lasts a bit longer" and reduce the movement error rate. The current model is good for about 5,000 steps. The Army's goal is 10,000 steps.

The work was done in conjunction with Vanderbilt University in Nashville and researchers from Northwestern University in Evanston, Illinois, and the University of Seattle.

SOURCE: http://bit.ly/15fq8aB New England Journal of Medicine, online September 25, 2013.

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