“We were able to show the first complete neural control of bionic walking,” said Hyungeun Song, the study’s first author and a postdoctoral researcher at MIT.
Most state-of-the-art bionic prostheses rely on preprogrammed robotic commands instead of the user’s brain signals. Advanced robotic technologies can sense the environment and repeatedly activate a predetermined leg movement to help a person navigate that type of terrain.
But many of these robots work best on flat ground and struggle to navigate common obstacles like bumps or puddles. The person wearing the prosthesis often has little say in adjusting the prosthetic limb as it is in motion, particularly in response to sudden changes in terrain.
“When I walk, it feels like I’m walking because an algorithm is sending commands to a motor, and I’m not,” said Hugh Herr, the study’s principal investigator and professor of media arts and sciences at MIT and a. pioneer in the field of biomechatronics, a field that unites biology with electronics and mechanics. Herr had his legs amputated below the knee several years ago due to frostbite 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 when the individual sees the synthetic limb as a natural extension of their body,” Herr said.
The authors worked with 14 study participants, half of whom received below-the-knee amputations through an approach known as Agonist-antagonist Myoneural Interface – AMI – while the other half underwent traditional amputations.
“What’s really interesting about this is how it’s leveraging 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.
AMI amputation was developed to address the limitations of traditional leg amputation surgery, which severs important muscle attachments 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, and another—known as an antagonist—will lengthen 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 enable the movement.
When surgical amputation severs muscle pairs, the patient’s ability to feel muscle contractions after surgery is impaired and as a result, compromises their ability to accurately and finely sense where their prosthetic limb is located in space.
In contrast, the AMI procedure rewires the muscles in the remaining limb to replicate the valid muscle response that a person receives from an intact limb.
The study “is part of a next-generation movement of prosthetic technologies that address sensation and not just movement,” said Eric Rombokas, 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 receive the procedure in 2016.
Patients who underwent Ewing Amputation experienced less muscle atrophy in their remaining limb and less phantom pain, the sensation of experiencing discomfort in a limb that no longer exists.
The researchers fitted all participants with a new bionic limb, which 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 in the prosthesis. Computers then convert those electrical signals into force and movement for the prosthesis.
Amy Pietrafitta, a study participant who received an Ewing Amputation after severe burn injuries, said the bionic limb gave her the ability to straighten both of her legs and perform jumping movements again.
“Being able to have that kind of flex made it that much more real,” Pietrafitta said. “It felt like everything was there.”
With their enhanced muscle sensations, participants who underwent the Ewing Amputation were able to use their bionic limb 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 away.
“This energy expenditure … makes our heart work harder and our lungs work harder … and can lead to gradual destruction of our hip joints or our lower back,” said Matthew J. Carty, a reconstructive plastic surgeon. at Brigham and Women’s Hospital. and the first physician to perform the AMI procedure.
Patients who received the Ewing Amputation and the new prosthetic limb were also able to easily navigate ramps and stairs. They seamlessly adjusted their base to climb up the stairs and absorb the shock as they descended.
The researchers hope that the new prosthesis will be available on the market in the next five years.
“We’re starting to see a glimpse of this glorious future where a person can lose a large part of their body and have technology available to rebuild that aspect of their body back to full functionality,” Herr said.