“For the first time, we’ve demonstrated complete neural control of bionic walking,” said Hyungwoong Song, lead author of the study and a postdoctoral researcher at MIT.
Most cutting-edge bionic prosthetics rely on pre-programmed robot commands rather than the user’s brain signals. Advanced robotics technology can sense the environment and execute pre-defined leg movements repeatedly to help a person navigate any type of terrain.
But many of these robots work best on flat ground and struggle to overcome common obstacles like bumps and puddles, and once the prosthetic is in motion, the wearer often has little control over adjusting the prosthesis, especially in response to sudden changes in the terrain.
“When you walk, the algorithm is sending instructions to the motors, so it feels like you’re walking, but you’re not,” said Hugh Herr, the study’s lead researcher, a professor of Media Arts and Sciences at MIT and a pioneer in the field of biomechatronics, which combines biology with electrical and mechanical engineering. Herr had a below-knee amputation due to frostbite several years ago and uses an advanced robotic prosthetic leg.
“The evidence is growing [showing] “When you connect your brain to a mechatronic prosthetic limb, it learns to perceive it as a natural extension of your body,” Herr says.
The authors surveyed 14 study participants, half of whom had undergone below-knee amputation using a technique called AMI (agonist-antagonist myoneural interface), and the other half had undergone traditional amputation.
“What’s exciting about this work is that it combines surgical innovation with technological innovation,” said Connor Walsh, a professor at Harvard University’s School of Applied Sciences who specializes in developing wearable assistive robots, who was not involved in the research.
AMI amputation was developed to address the limitations of traditional leg amputation surgery, which severs vital muscle connections at the amputation site.
Movement is made possible by muscles working in pairs: one muscle (the agonist) contracts to move a limb, while the other (the antagonist) stretches in response. For example, in a biceps curl, the biceps is the agonist because it contracts to lift the forearm, and the triceps is the antagonist because it stretches to make the movement possible.
The severed muscles caused by surgical amputation impair the patient’s ability to feel muscle contractions after surgery, and therefore their ability to accurately and finely sense where the prosthesis is in space.
In contrast, AMI surgery involves reconnecting the muscles of the remaining limb to recreate the valuable muscular feedback available from the healthy limb.
The research is “part of a movement toward the next generation of prosthetic technology that takes into account not just movement but also sensation,” said Eric Lombokas, an assistant professor of mechanical engineering at the University of Washington who was not involved in the study.
AMI surgery for below-knee amputation is Ewing amputation It is named after Jim Ewing, the first person to undergo the procedure in 2016.
Patients who had a Ewing amputation experienced less muscle atrophy in their remaining limbs and less phantom limb pain (the uncomfortable sensation of a limb that is no longer there).
The researchers fitted all participants with the new bionic prosthesis, which consists of an artificial ankle, a device that measures electrical activity from muscle movements, and electrodes attached to the surface of the skin.
The brain sends electrical pulses to the muscles, causing them to contract. The contractions generate unique electrical signals that are picked up by electrodes and sent to the prosthetic’s tiny computer, which converts the signals into force and movement for the prosthetic.
Study participant Amy Pietrafitta, who underwent a Ewing amputation after suffering severe burns, said the prosthetic limbs have allowed her to straighten her legs and dance again.
“Having those bends made it more realistic,” Pietrafitta says. “It felt like it was all there.”
The enhanced muscle sensation allowed the Ewing amputee participants to walk faster and with a more natural gait using their bionic limbs than participants with traditional amputees.
When a person has to deviate from their normal walking pattern, they usually have to exert more effort to move.
“That energy expenditure puts increased strain on the heart and lungs and can lead to the gradual destruction of the hip joint and lower spine,” said Matthew J. Carty, a reconstructive surgeon at Brigham and Women’s Hospital and the first doctor to perform the AMI procedure.
With a Ewing amputation and a new prosthesis, the patient was able to navigate ramps and stairs with ease, smoothly adjusting his footing to ascend stairs and absorbing shocks when descending.
The researchers hope that the new prosthesis will be available commercially within the next five years.
“We are beginning to get a glimpse of a bright future where people can lose large parts of their bodies and the technology exists to reconstruct them and make them fully functional,” Herr said.