“We were able to demonstrate the first complete neural control of bionic walking,” said Hyungeun Song, first author of the study and postdoctoral researcher at MIT.
Most advanced bionic prosthetics rely on pre-programmed robotic commands instead of the user’s brain signals. Advanced robotic technologies can sense the environment and repeatedly activate a predefined leg movement to help a person navigate that type of terrain.
However, many of these robotics work best on flat surfaces and have difficulty navigating common obstacles such as bumps or puddles. The person wearing the prosthesis often has little say in adjusting the prosthetic limb once it is in motion, especially in response to sudden changes in terrain.
“When I walk, it feels like I’m being walked because an algorithm is sending commands to a motor, and that’s not the case,” said Hugh Herr, the study’s lead researcher and a professor of media arts and sciences at MIT and a pioneer in the field of biomechatronics, a field that combines biology with electronics and mechanics. Herr’s legs were amputated below the knee several years ago due to frostbite, and he uses advanced robotic prosthetics.
“There is growing evidence [showing] that when you connect the brain to a mechatronic prosthesis, an embodiment occurs where 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 underwent a below-the-knee amputation via an approach known as the Agonist-Antagonist Myoneural Interface (AMI), while the other half underwent a traditional amputation.
“What’s so super cool about this is how surgical innovation is combined 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 research.
The AMI amputation was developed to address the limitations of traditional leg amputation surgery, which involves severing key muscle connections at the amputation site.
Movements are made possible by the way muscles move in pairs. One muscle – known as the agonist – contracts to move a 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 upward, while the triceps muscle is the antagonist because it lengthens to make the movement possible.
When pairs of muscles are severed during a surgical amputation, the patient’s ability to feel muscle contractions after surgery is reduced. This means he or she is less able to accurately and accurately sense where the prosthesis is in space.
In contrast, the AMI procedure reconnects the muscles in the remaining limb to reproduce the valuable muscle feedback a person gets from an intact limb.
The study “is part of a movement of next-generation prosthetic technologies that focus on 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 research.
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 Ewing amputation experienced less muscle atrophy in their remaining limb and less phantom limb pain, the feeling of 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 movements, 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. The computers then convert those electrical signals into force and movement for the prosthesis.
Amy Pietrafitta, a study participant who underwent the Ewing amputation after severe burns, said the bionic limb gave her the ability to point with both feet and perform dance moves again.
“Being able to have that kind of flexion made it so much more realistic,” Pietrafitta said. “It felt like everything was there.”
Thanks to 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 pattern than those who underwent traditional amputations.
When a person has to deviate from their normal walking pattern, they usually have to work harder to get around.
“That energy expenditure… makes our heart have to work harder and our lungs have to work harder… and it 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 prosthesis were also able to navigate ramps and stairs with ease. They smoothly adjusted their feet to push themselves up the stairs and absorb shock as they went down.
The researchers hope that the new prosthesis will be commercially available within five years.
“We’re starting to glimpse this glorious future where a person can lose a large part of their body and there’s technology available to make that aspect of their body fully functional again,” Herr said.