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Amy Pietrafitta learned to walk seven times.
First as a child, then after an industrial burn that led to the amputation of her left leg in 2018. Since then, she has taken her “first steps” with prosthetic legs for running, water, high heels, rowing and daily walking.
Her latest “first steps” were different. This time, she was fitted with a bionic leg that was fully connected to her brain, allowing her to walk and move around like everyone else. With this new leg, she says, she couldn’t even tell the difference.
“I didn’t feel like my leg had been amputated,” said the 47-year-old from Massachusetts. “It was the happiest moment of my life.”
Pietrafitta participated in a study at the Massachusetts Institute of Technology on a new neuroprosthesis that is entirely controlled by the nervous system. His leg was amputated using a new muscle-pairing technique called agonist-antagonist myoneural interface, or AMI, which preserves signaling between muscles and the brain.
The neuroprosthesis uses sensors placed between the reconstructed amputation site and the bionic leg to transmit electrical signals from the brain. This allows the prosthesis to detect its position and movement and send this information back to the patient, allowing a sense of proprioception: the brain’s ability to sense its own movements and location in space.
The study, published Monday in the journal Nature Medicine, found that participants who underwent a specialized amputation and neuroprosthesis increased their walking speed by 41 percent, which is consistent with the abilities and ranges of motion of people without a leg amputation. The findings also suggest that the bionic limb could be controlled using just 18 percent of natural proprioception.
This is the first bionic leg fully controlled by the human nervous system to demonstrate natural walking speeds and gait patterns, according to Dr. Hugh Herr, principal investigator and senior author of the study and co-director of the Yang Center for Bionics at MIT, where the research took place.
“It’s natural, as if the limb were made of flesh and bone. It’s as if the central brain is not aware that the limb is amputated because it feels normal sensations,” said Herr, who had to have both legs amputated below the knee after a climbing accident in 1982.
Courtesy of Dr. Hugh Herr
Dr. Hugh Herr in his lab. He had both legs amputated below the knee after a climbing accident in 1982.
Herr embarked on the project after observing a trend in which prosthetics are increasingly controlled by robotic algorithms rather than the human nervous system.
“We want to rebuild the human body,” he said. “We want to rebuild and give people back what they’re looking for. We don’t want to just build more and more sophisticated tools or robotic devices.”
To test the interface, 14 participants were divided into two groups and fitted with bionic leg prostheses. Seven of them had undergone surgery for acute myocardial infarction, and seven had not.
The results suggest that people who had AMIs moved better in real-world environments, including slopes, stairs, obstructed paths and even an “American Ninja Warrior”-style course, according to Pietrafitta and Dr. Matthew J. Carty, lead surgeon on the first AMI procedure and co-author of the new study.
“This is a fundamental paradigm shift in how we think about amputation in conjunction with technology: treating it as a restorative procedure rather than a failure,” Carty said.
The technology also allows users to change gears without changing prosthetics. This reduces the hassle of switching legs, avoids unwanted attention and restores a sense of normalcy, Pietrafitta said.
“It was normal… I was ready to run. I kept wanting to let go of my hands (of the supports), get out and start moving,” she said.
Getting back to normal isn’t just about getting from point A to point B. According to Pietrafitta, it’s about reintegrating into society.
“It meant my life could start over. (…) I could go out and live the way I wanted to,” she said. “When you’re at home and you’re bedridden, your family and friends start to drift away. It’s too hard for people to come and see you suffer.”
For John Rheinstein, a specialist in upper and lower limb prosthetics and director of the Hanger Clinic in New York, the physical prosthesis is only part of the success of an amputation.
“Care has improved over the years (in part) because of advances in understanding and treating the emotional impacts of limb loss,” he wrote in an email.
According to a 2018 report from the federal Agency for Healthcare Research and Quality, about 1.9 million Americans are living with limb loss, a number that is expected to double by 2050, largely due to rising rates of diabetes, a known risk factor.
Herr says his team aims to commercialize fully neuro-controlled prosthetics within five years. However, the 2018 report estimates that less than half of amputees receive a prescription for prosthetics.
Amputees can struggle to get prescriptions and insurance coverage for prosthetics, especially for options with more advanced technology. Rheinstein cites insurance coverage for prosthetic limbs—particularly the lack of coverage for activity-specific prosthetics—as one of the most challenging aspects of her patients’ rehabilitation journey.
Courtesy of Dr. Hugh Herr
A study participant walks up stairs using a new prosthetic leg.
“Demonstrating to payers the value of the outcomes we deliver and how outcomes-based prosthetic care reduces overall healthcare costs” is one of the biggest challenges facing the industry today, he said.
For example, the BionX emPOWER prosthesis is the only powered prosthesis on the market, according to Herr, and many people who have lost a limb cannot access it.
Although AMI neuroprostheses are not available to the public, AMI surgery has been performed on about 60 people worldwide, according to Herr.
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At Brigham and Women’s Hospital in Boston, where Carty is a surgeon in the division of plastic surgery, AMI is now the standard of care for amputations, Carty said.
“The hospital’s ethics committee determined that it was unethical to offer the standard amputation because we now know that the AMI amputation is very beneficial,” Herr said. “We hope it becomes the standard not only in Boston, but at clinical sites around the world.”
Rheinstein says the more surgeons, prosthetists, researchers and patients work together on new developments, the better. He hopes the AMI technique will “focus attention on the importance of carefully planned amputation surgery for a patient’s successful rehabilitation.”
“Hollywood has given us visions of these remarkable bionic humans for decades…but the scientific reality has been slower than the science fiction,” Herr added.
“We are finally producing systems that we have seen in the movies.”