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 Harborview 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. "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. Vawter is allowed to use the machine only a week at a time during visits every few months to the clinic in Chicago.

Most prosthetic legs work like a walking stick with springs. The next step up, robotic prosthetics, has remote controls and embedded sensors that measure how much weight they must bear, the knee position and the way a person is turning.

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 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.

The rate of errors, including the risk of falls, was shaved to just 1.8 percent, 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. It may also help older people who want to remain at home.

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 critical severed nerves into his hamstring. When he thinks about moving his knee or ankle, those nerves still fire. Sensors taped on to the legs capture the signals. The data are 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.

Said Hargrove: "The value it will provide to the people who use it will be enormous."