A better bionic leg, moved by mind control

Michelle Fay Cortez / Bloomberg News /

Published Sep 30, 2013 at 05:00AM

Zac Vawter, a software engineer who lives in the Seattle area, already knew about advances in bionic technology when a motorcycle wreck led to the amputation of his right leg just above the knee in 2009.

As doctors at Harbor View Medical Center in Seattle battled for three days to try to save his leg, Vawter asked about the method that uses the mind to move a prosthetic limb. The technology previously had been used only in arms.

Four years and an $8 million grant from the U.S. Army’s Telemedicine and Advanced Technology Research Center later, Vawter is considered the “test pilot” of the bionic leg that can tackle slopes, stairs and in-chair movement markedly better than existing devices. A team of researchers led by Levi Hargrove from the Rehabilitation Institute of Chicago’s Center for Bionic Medicine reported its results with the novel prosthetic in the New England Journal of Medicine.

“In my mind, it’s still the same thing in terms of moving my ankle down or up, or extending my leg forward or back,” Vawter said in a telephone interview. “It’s just walk like I would normally walk. It’s not special training or buttons or tricks. That’s a big piece of what I think is groundbreaking and phenomenal about this work.”

Additional refinements are needed to make the thought- controlled bionic leg commercially viable, Hargrove said in a telephone interview. Vawter is allowed to use the machine only a week at a time during visits every few months to the clinic in Chicago. Freedom Innovations LLC, a closely held company based in Irvine, Calif., is working on making the motorized machine smaller, quieter and more robust.

It’s been an evolutionary process. Most prosthetic legs work like a walking stick with springs, giving the patient something on which to balance. The next step up, robotic prosthetics, are further advanced with remote controls and embedded sensors that measure how much weight they must bear, the knee position and the way a person is turning, like mobile phones determine orientation. The thought-controlled device goes further, harnessing nerves that formerly regulated the leg’s movement to maneuver the prosthetic leg.

The new leg allows Vawter to seamlessly transition between walking and standing, with the biggest difference showing up when he is climbing stairs. With a standard prosthetic leg, Vawter always steps up first with his healthy left leg, then pulls the right leg along. With the thought-controlled leg, he is able to walk foot-over-foot, he said. Someone watching him climb wouldn’t know he had a prosthesis based on his gait, Hargrove said, though they may hear the motor whirring.

Much still to do

“It’s still a prosthetic, but it’s in between the leg I wear every day and prior to amputation,” said Vawter, who can’t yet jump to the rim of a basketball net with the robotic leg, as he could before the amputation. “It’s a dramatic improvement over my current prosthetic, but there is still a long way to go.”

The rate of errors, including the risk of falls, was shaved to just 1.8 percent with the new device, down from 12.9 percent with the standard robotic leg prosthesis.

The new device may be available within three to five years for the 1 million Americans with leg amputations, Hargrove said. The approach may benefit the 1,200 soldiers injured while serving in the U.S. military, many of whom are young and want to continue active lives. It may also help older people who want to remain at home, particularly those who have trouble standing and caring for themselves because of amputations, he said.

The researchers started with an advanced motorized knee and ankle prosthesis developed at Vanderbilt University. Their goal was to improve the “steering” of the device, using only the mind.

The first thing was to recreate the natural signaling process used to move, which was disconnected when the leg was severed. The signal in the brain that moves through the spinal cord, down the peripheral nerves and into the muscles remains intact until the spot of the amputation, Hargrove said.

The researchers “rewired” Vawter, redirecting two of the critical severed nerves into his hamstring, the muscle at the back of the leg. When he thinks about moving his knee or ankle, those nerves still fire, releasing a tiny burst of electricity.

Sensors taped on to the legs capture the signals. That data is added to a pattern-detection computer system that takes information from the robotic leg to predict the patient’s intended movement. While the researchers expected the additional information to make the leg operate more smoothly, the magnitude of the benefit was unexpected, they wrote in the New England Journal. Many errors weren’t even noticeable.

Computer prediction

The researchers measured nine muscles in the leg and analyzed the activities that were most important for regular function, Hargrove said. When Vawter performs any of those activities, the computer program predicts what he is doing.

Vawter became the test pilot of the device through his surgeon, Douglas Smith. A contributor on the paper, Smith is an expert on the use of targeted muscle reinnervation, when the nerves are repurposed to improve the control of a motorized arm prosthesis. He performed Vawter’s amputation, and identified him as a good candidate for helping develop the thought-controlled leg prosthesis.

The researchers adjusted the leg and the computer systems based on Vawter’s feedback. It has gone through major revisions of the hardware and many little changes to the software, said Vawter, who works at Engineered Software Inc. in Lacey, Wash.

Current prosthetic legs cost a few thousand dollars, with robotic devices as much as $100,000, Hargrove said. There is no price target yet for the thought-controlled bionic leg, he said.

“The value it will provide to the people who use it will be enormous,” Hargrove said. “We feel we’ve been able to eliminate the vast majority of safety critical errors. You can never predict how they will use these devices in their own lives, but we are making fantastic progress.”

A man who lost his leg in a motorcycle accident is considered the “test pilot” for a leg that can tackle slopes, stairs and in-chair movement markedly better than existing devices.