“We were able to demonstrate the first complete neural control of bionic walking,” said Hyungeun Song, first author of the study and a postdoctoral researcher at MIT.
Most advanced 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 predefined leg movement to help a person navigate this type of terrain.
But most of these robots work best on flat terrain and struggle to navigate common obstacles like bumps or puddles. The person wearing the prosthesis often has little say in how the prosthetic limb adjusts once it’s in motion, especially in response to sudden changes in terrain.
“When I walk, it feels like someone is walking because an algorithm is sending commands to a motor, which is not the case,” says Hugh Herr, the study’s lead researcher and a professor of media arts and sciences at MIT who is a pioneer in the field of biomechatronics, a field that combines biology, 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 a growing body of evidence showing that when you connect the brain to a mechatronic prosthesis, embodiment 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 received below-knee amputations using an approach known as agonist-antagonist myoneural interface (AMI), while the other half underwent traditional amputations.
“What’s super cool is how this leverages surgical innovation as well as technological innovation,” said Conor Walsh, a professor at the Harvard School of Engineering and Applied Sciences who specializes in developing wearable assistive robots and was not involved in the study.
The AMI amputation 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 the agonist – contracts to move a limb and another – known as the antagonist – lengthens in response. For example, in a biceps curl, the biceps muscle is the agonist because it contracts to lift the forearm, while the triceps muscle is the antagonist because it lengthens to enable the movement.
When surgical amputation severs pairs of muscles, the patient’s ability to feel muscle contractions after surgery is impaired and, therefore, compromises their ability to accurately and precisely sense where their prosthetic limb is located in space.
In contrast, the AMI procedure reconnects the muscles of the remaining limb to replicate the valuable muscular feedback a person receives from an intact limb.
The study “is part of a movement of the next generation of prosthetic technologies that address sensation and not just movement,” said Eric Rombokas, an assistant professor of mechanical engineering at the University of Washington, who did not participate in the study.
The AMI procedure for below-the-knee amputation was named Ewing amputation in honor of Jim Ewing, the first person to benefit from the procedure in 2016.
Patients who underwent an Ewing amputation experienced less muscle atrophy in their remaining limb and less phantom pain, the feeling of experiencing discomfort in a limb that no longer exists.
The researchers equipped all participants with a new bionic limb, consisting of a prosthetic ankle, a device measuring the electrical activity of 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 installed on the prosthesis. Computers then convert these electrical signals into force and movement for the prosthesis.
Amy Pietrafitta, a study participant who underwent an Ewing amputation after severe burns, said the bionic limb gave her the ability to point both feet and perform dance moves again.
“Being able to have that type of bending made it a lot more real,” Pietrafitta said. “It felt like everything was there.”
Thanks to their enhanced muscle sensation, 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 their normal walking habits, they usually have to exert more effort to get around.
“This energy expenditure … forces our heart and lungs to work harder … and it can lead to the progressive 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 doctor to perform the AMI procedure.
Patients with Ewing amputations who received a new prosthesis were also able to move easily on ramps and stairs. They smoothly adjusted their support to climb the stairs and absorbed shock during the descent.
Researchers hope the new prosthesis will be commercially available within the next five years.
“We’re starting to get a glimpse of this glorious future in which a person can lose a significant part of their body, and there is technology available to rebuild that aspect of their body until it’s fully functional,” he said. Herr said.