Researchers develop brain-driven prosthetic limb for people with leg amputations

“We were able to demonstrate the first complete neural control of robotic walking,” said Hyungyeon Song, first author of the study and a postdoctoral researcher at MIT.

Most modern bionic prosthetics rely on pre-programmed robotic commands rather than the user's brain signals. Advanced robotic technologies can sense the environment and repeatedly activate a pre-set leg movement to help a person navigate this type of terrain.

But many of these robots work best on flat terrain and have difficulty navigating common obstacles like bumps or puddles. And the person wearing the prosthetic often has no say in adjusting the prosthetic once it’s moved, especially in response to sudden changes in terrain.

“When I walk, I feel like I’m walking because the algorithm is sending commands to the motor, and I’m not,” said Hugh Herr, the study’s lead author and a professor of media arts and sciences at MIT. A pioneer in biomechatronics, a field that blends biology with electronics and mechanics, Herr had both of his legs amputated below the knee several years ago due to frostbite, and he uses advanced robotic prosthetics.

“There is a growing body of evidence [showing] “When you connect the brain to a mechatronic prosthesis, embodiment occurs where the individual views the prosthesis as a natural extension of their body,” Hare added.

The researchers worked with 14 study participants, half of whom underwent below-knee amputations through an approach known as agonist neuromuscular interface (AMI) while the other half underwent traditional amputations.

“What’s so cool about this is how you can leverage surgical innovation alongside technological innovation,” said Conor Walsh, a professor at Harvard’s School of Engineering and Applied Sciences who specializes in the development of wearable assistive robots and was not involved in the study.

The AMI amputation procedure was developed to address the limitations of traditional leg amputation surgery, which severs important muscle connections at the amputation site.

Movements are made possible by the way muscles move in pairs. One muscle – known as an agonist – contracts to move a limb, while another muscle – known as an antagonist – lengthens in response. For example, during a biceps curl, the biceps is the antagonist because it contracts to lift the forearm upward, while the triceps is the antagonist because it lengthens to enable the movement.

When surgical amputation results in muscle pairs being severed, the patient's ability to feel muscle contractions after surgery is affected and, as a result, his ability to accurately and well sense where the prosthesis is in space is affected.

In contrast, AMI reconnects muscles in the residual limb to replicate the valuable muscle feedback a person gets from the intact limb.

The study is “part of a movement for next-generation prosthetic technologies that address sensation, not just movement,” said Eric Rombukas, an assistant professor of mechanical engineering at the University of Washington who was not involved in the study.

The AMI procedure for below-knee amputation is named after peter ewing After Jim Ewing, the first person to undergo the procedure in 2016.

Patients who underwent an Ewing amputation experienced less muscle atrophy in their remaining limb and less phantom pain, the sensation of discomfort in a limb that no longer exists.

The researchers fitted all participants with new prosthetics consisting of an artificial ankle, a device that measures electrical activity from muscle movement and electrodes placed on the surface of the skin.

The brain sends electrical impulses to the muscles, causing them to contract. Contractions produce their own electrical signals, which are detected by electrodes and sent to small computers attached to the prosthesis. Computers then convert these electrical signals into force and movement for the prosthesis.

Amy Pietravetta, a study participant who underwent an Ewing amputation after suffering severe burns, said the prosthetic gave her the ability to steer her feet and perform dance moves again.

“Being able to have that kind of curvature made it more realistic, and it felt like everything was there,” Petravetta said.

Thanks to enhanced muscle sensations, participants who underwent the Ewing amputation were able to use their bionic limbs to walk faster and with a more natural gait than those who underwent traditional amputations.

When a person has to deviate from normal walking patterns, they usually have to work harder to get around.

“This expenditure of energy … causes our heart to work harder and our lungs to work harder … and can lead to gradual destruction of the hip joints or lower spine,” said Matthew J. Carty, a plastic and reconstructive surgeon at Brigham and Women’s Hospital and the first doctor to perform the myocardial infarction procedure.

Patients who underwent Ewing's amputation and the new prosthesis were also able to move easily on slopes and stairs. They were able to smoothly adjust their feet to propel themselves up stairs and absorb shocks while descending.

The researchers hope that the new prosthetic will be commercially available in the next five years.

“We are beginning to glimpse this glorious future where someone can lose a significant portion of their body, and there is technology available to rebuild that aspect of their body back to full function,” Herr said.