“We were able to show the first full neural control of bionic walking,” said Hyungeun Song, first author of the study and a postdoctoral researcher at MIT.
Most state-of-the-art bionic prostheses rely on pre-programmed robotic commands instead of the user’s brain signals. Advanced robotic technologies can sense the environment and repeatedly activate predefined leg movements to help a person navigate such terrain.
But many of these robots work best on flat terrain and try to overcome common obstacles like bumps or puddles. The person wearing the prosthesis often has little say in adjusting the prosthetic limb once it is in motion, particularly in response to sudden changes in terrain.
“When I walk, it feels like someone is walking me because the algorithm is sending motor commands and I’m not,” said Hugh Herr, the study’s principal investigator and a professor of media arts and sciences at MIT. a pioneer in the field of biomechatronics, a field that combines biology with electronics and mechanics. Herr had his legs amputated below the knee due to frostbite several years ago and uses advanced robotic prostheses.
“There is a growing body of evidence. [showing] that when you connect the brain to a mechatronic prosthesis, there is an embodiment that occurs where the individual views the synthetic limb as a natural extension of their body,” Herr said.
The authors worked with 14 study participants, half of whom underwent below-the-knee amputations through an approach known as agonist-antagonist myoneural interface—AMI—while the other half underwent traditional amputations.
“What’s great about it is how it uses surgical innovation along with technological innovation,” said Conor Walsh, a professor at the Harvard School of Engineering and Applied Sciences who specializes in the development of wearable assistive robots and was not involved in the study.
AMI amputation was developed to address the limitations of traditional leg amputation surgery, which sever important muscle connections at the amputation site.
Movements are made possible by how muscles move in pairs. One muscle – known as the agonist – contracts to move the limb, and another – known as the antagonist – lengthens in response. For example, during a biceps curl, the biceps muscle is the agonist because it contracts to lift the forearm up, while the triceps muscle is the antagonist because it lengthens to allow movement.
When a surgical amputation severs muscle pairs, the patient’s ability to feel muscle contractions after surgery is impaired, and as a result, their ability to accurately and finely sense where their prosthetic limb is in space is compromised.
In contrast, the AMI procedure reconnects the muscles in the remaining limb to replicate the valuable muscle feedback a person receives from the intact limb.
The study “is part of a movement for a new generation of prosthetic technologies that deal with sensation and not just movement,” said Eric Rombokas, an assistant professor of mechanical engineering at the University of Washington, who was not involved in the study.
The AMI below-the-knee amputation procedure was named the Ewing amputation after Jim Ewing, the first person to undergo the procedure in 2016.
Patients who underwent an Ewing amputation experienced less muscle atrophy in the residual limb and less phantom pain, a feeling of discomfort in a limb that no longer exists.
The researchers fitted all participants with a new bionic limb that consisted of a prosthetic 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. The contractions produce their own electrical signals, which are detected by the electrodes and sent to small computers on the prosthesis. Computers then convert these electrical signals into force and movement of the prosthesis.
Amy Pietrafitta, a study participant who received Ewing’s amputation after severe burns, said the bionic limb gave her the ability to aim both legs and perform dance moves again.
“Being able to have that type of flex made it a lot more real,” Pietrafitta said. “It looked like everything was there.
Due to their heightened muscle sensations, participants who underwent Ewing amputation were able to use their bionic limb to walk faster and more naturally than those who underwent traditional amputation.
When a person has to deviate from their normal walking patterns, they usually have to work harder to get around.
“That energy expenditure … makes our heart work harder and our lungs work harder … and it can lead to gradual destruction of our hip joints or our lower spine,” said Matthew J. Carty, a reconstructive plastic surgeon at Brigham and Women’s Hospital. and the first physician to perform an AMI procedure.
Patients who received an Ewing amputation and a new prosthetic limb were also able to navigate ramps and stairs with ease. They smoothly adjusted their stance to push themselves up the stairs and absorb the shock as they went down.
The researchers hope that the new prosthesis will be commercially available within the next five years.
“We’re starting to look at this glorious future where a person can lose a large part of their body, and the technology is available to reconstruct that aspect of their body to full functionality,” Herr said.