New Bionic Leg and Surgical Procedure Allow People to Walk With More Control After Amputations

The experimental surgery connects two muscles in the legs of people with below-the-knee amputations, allowing them to control a prosthetic limb with their brain

A person with a prosthetic lower leg walks up a flight of stairs
The procedure allowed people to walk faster, climb stairs better and avoid obstacles more easily. Hugh Herr and Hyungeun Song

In a clinical trial, researchers enabled people with prosthetic legs to walk at the same speed as people without leg amputations. The experimental surgery involves reconnecting muscles in the person’s leg, which allows them to produce electrical signals that trigger the prosthetic limb’s motion.

The procedure may help people with leg amputations adapt to walking at varied speeds and moving on a range of terrains. The findings were published July 1 in the journal Nature Medicine.

“We were able to show the first full neural control of bionic walking,” Hyungeun Song, first author of the study and a researcher at MIT, tells the Washington Post’s Lizette Ortega.

“It gives the user such a high flexibility that is much closer to how the biological leg works,” Tommaso Lenzi, a biomedical engineer at the University of Utah who did not contribute to the findings, says to Nature News’ Miryam Naddaf.

Amy Pietrafitta, a para-athlete, got the surgery as a study participant in 2018. “The surgery stands on its own,” she tells MIT Technology Review’s Sarah Ward. “I feel like I have my leg back.”

Currently, prosthetic legs rely on robotic controls for walking. The prostheses respond to the environment and the person’s gait with predefined commands that dictate movement. This means people don’t have full control over the prosthetic limb, according to a statement from MIT.

“When I walk, it feels like I’m being walked, because an algorithm is sending commands to a motor, and I’m not,” Hugh Herr, a co-author of the study who creates bionic limbs at MIT, says to the Washington Post. Herr uses two robotic prostheses, since his legs were both amputated below the knee after a climbing accident.

Herr and the other researchers have been working to develop a surgical procedure and bionic limb that together provide a functionality similar to what people without amputations experience. In the new study, they gave seven people with below-the-knee amputations the experimental surgery, in which they connected a pair of muscles that work together to control the leg.

The procedure allows the muscles to communicate with each other. In general, muscles work in pairs: one contracts and the other lengthens to create movement. But traditional below-the-knee amputations disrupt these interactions, interfering with a person’s ability to sense exactly where their prosthetic limb is located.

In addition, connecting these muscles allows them to produce electrical signals linked to leg movement that are very similar to the signals in an intact leg. The participants used a bionic leg with a powered ankle and electrodes that can detect these signals, allowing the technology to respond to commands from the brain.

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To test the effectiveness of their surgery, the researchers then compared these seven participants to seven other people who received traditional below-the-knee amputations. All participants used the same bionic limb with electrode sensors.

The comparison showed that people who received the new surgical procedure increased their walking speeds by 41 percent, compared to people with the traditional procedure, allowing them to move at peak speeds similar to people without amputations when walking on flat ground. They could also better avoid obstacles and climb stairs and slopes more naturally. And they experienced less pain and muscle atrophy following the surgery.

“I actually found it remarkable that with so little learning, they were able to achieve such good results,” Levi Hargrove, a neural engineer at Northwestern University who was not involved in the research, tells Nature News. “They would see even more benefit with a longer accommodation period, wearing the device.”

“What’s super cool about this is how it’s leveraging surgical innovation along with technological innovation,” Conor Walsh, who develops wearable assistive robots at Harvard University and did not contribute to the findings, tells the Washington Post.

Researchers still need to see how the limbs work outside of a laboratory setting. But so far, about 60 people have received this type of surgery.

The procedure works best when it is performed at the same time as the amputation, though it still worked well in people who had received amputations a long time ago, according to the Financial Times’ Clive Cookson. Herr hopes the product will be commercially available in about five years, per the publication.

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