Mind-controlled prosthetic advances ability to walk more naturally

Prosthetic limbs have been around since at least 300 B.C., when a Roman nobleman used a leg made of bronze, iron and wood. And they’ve improved in ways that would have seemed nothing short of miraculous to him, becoming more flexible, versatile and close to a natural limb.

Many prosthetic limbs are not tied directly into neural pathways. Research and design improvements by various groups have helped those with cutting-edge prosthetic limb legs walk with a close-to-nature step, often using robotic sensors and controllers that are designed to achieve specific gaits.

This week, researchers at MIT and Brigham and Women’s Hospital announced the latest innovation: an approach that weds a specific surgical technique with great new technology, allowing someone who lost a lower leg what Hoodline.com describes as “more natural and obstacle-free movement.”

And it’s controlled by their thoughts through neural feedback.

Per the article, “The typical robotic prosthetics on the market, while advanced, have not been able to offer the full neural control that the body’s own nervous system can. These devices usually employ a combination of sensors and predefined algorithms to mimic natural gait patterns. What the MIT team has managed to do is, effectively reconnect muscles within the residual limb, which in turn make it possible to quite directly control a prosthetic leg, delivering a significant upgrade in proprioceptive feedback, which is the awareness of where one’s limb is in space.”

The surgical amputation technique is based on how muscle pairs work — in this case, part of the shin and part of the calf. By combining a neuroprosthetic interface with reconnected muscles in the residual limb, the patients get what an MIT news release calls “proprioceptive feedback about where the prosthetic limb is in space.”

When seven patients at Brigham and Women’s Hospital who had the surgery and interface were compared to seven patients using the identical prosthesis but without the interface, they found those who received it could walk faster, avoid obstacles and climb stairs quite naturally, the release said.

Both groups in the study used the same bionic limb: “A prosthesis with a powered ankle as well as electrodes that can sense electromyography signals” from the calf and shin muscles, according to MIT News . The signals go into a robotic controller that helps determine the bend of the ankle, as well as the torque and the power needed to complete the physical action.

“This is the first prosthetic study in history that shows a leg prosthesis under full neural modulation, where a biomimetic gait emerges. No one has been able to show this level of brain control that produces a natural gait, where the human’s nervous system is controlling the movement, not a robotic control algorithm,” Dr. Hugh Herr, co-director of the K. Lisa Yang Center for Bionics at MIT, an associate member of MIT’s McGovern Institute for Brain Research and the senior author of the new study, said in a written statement.

Stretching and contracting

Limb movement typically occurs as paired muscles stretch and contract, but most below-knee or below-elbow amputations sever the connections, which the researchers said makes it hard for the nervous system to sense muscle position and movement. The brain has to know that to move the limb well, so most prosthetics use robotic controllers in the artificial limbs.

The surgery developed at MIT, called agonist-antagonist myoneural interface, or AMI, connects muscle remnants from the shin, which contracts to move the ankle upward, and the calf, which counteracts the motion as one walks or climbs. The two muscles continue to talk to each other inside what’s left of the limb. “The surgery can be done during a primary amputation or the muscles can be reconnected after the initial amputation as part of a revision procedure,” the release said.

Those with the AMI surgery could walk about as fast as people who had not had an amputation, and get around obstacles more easily. Their movements were more natural, too, and easier to coordinate with their intact limb. The release noted that about 60 people total worldwide have had that type of surgery, which also works with lower arm amputations.

Research has shown that the preserved and reconnected muscles produce electrical signals similar to the signals made by a limb that is intact. “These natural behaviors emerged even though the amount of sensory feedback provided by the AMI was less than 20% of what would normally be received in people without an amputation,” the researchers noted.

The study, published in Nature Medicine , found the interface between the limb, the muscles and the brain produces “near natural” ability to walk and avoid obstacles. Those with the interface could walk faster in different situations, including across level ground, down a ramp, up a slope, on stairs and on level surfaces.

For many patients, that could be a very new world. While artificial limbs help mobility and other tasks, they often don’t feel natural, Herr said. “When you ask a patient ‘What is your body?’ They don’t include the prosthesis,” Herr told MIT Technology Review .

The article said, “The work is personal for him: He lost both his lower legs in a climbing accident when he was 17. He says linking the brain to the prosthesis can make it feel more like part of someone’s anatomy, which can have a positive emotional impact.”

Artificial brain connection

Researchers have found many ways to connect the brain to parts of the body that aren’t working well or that are missing. A sampling of the many innovations:

• Duke University reported in November on technology under development to help those who cannot talk regain the ability through a brain-computer interface.
• University of Michigan researchers have developed the Regenerative Peripheral Nerve Interface to help patients who lost a hand use mind control to move their prosthetic. It uses a small graft of muscle tissue that’s attached to the end of a severed nerve in the arm that has been amputated. “While other neural interfaces are harmful to nerves, the RPNI promotes healthy nerve growth and acts as a bioamplifier, converting faint neural signals sent from the brain into large, recordable, muscle signals that remain stable for years. Combined with machine learning algorithms, these signals enable intuitive, real-time mind control of advanced robotic prosthetic hands,” a news release on that technology reported.
• The University of Utah has long worked on improving prosthetics and on neural connections, as Deseret News has reported . For example, a biological/digital interface developed there “allows a high-tech prosthetic to not only be controlled by the wearer’s thoughts, but also feeds back a previously unattainable ‘touch sense.’”