How a Breakthrough Surgery Enables the Brain to Control a Bionic Leg

One problem with traditional amputations is that they leave agonists and antagonists without that bone connection. What was in essence the muscles’ means of communication or coördination is gone. But Carty and Herr, in close collaboration with Shriya Srinivasan and Tyler Clites, who were then graduate students in Herr’s lab, started to envision ways of functionally reconnecting those agonist and antagonist muscles.

After a traditional amputation, the neural signals from severed muscles are only about three per cent of what they once were—insufficient for effective communication with a neurally controllable prosthesis. If the connections between agonist and antagonist muscles could be restored, however, the neural signals might be strengthened and clarified. The limb could then keep the brain informed about where it is and what it’s doing; the brain in turn might become better at controlling the muscles in a natural way. In other words, a person’s prosthetic limb could potentially be brought into close alignment with their phantom limb.

Many years of research went into this new approach to amputation. At one point, Clites spent about a year developing a way of stitching together agonist and antagonist muscles with tendons, which could slide back and forth along a bone-mounted titanium pulley. Clites told me that he had “all the ‘i’s dotted and all the ‘t’s crossed, and then that didn’t work at all.” In an experimental animal surgery, the muscles scarred down and became immobilized. “We had to ask ourselves, first, is the concept even good?” Clites told me. The titanium, which was not native to the body, seemed a likely culprit for the failure.

After many discussions, Herr, Carty, Srinivasan, and Clites went with a design that fashioned a pulley from a part of the ankle joint which in traditional amputations is basically tossed out. The idea looked good in theory, and the team presented it at a plastic-surgery conference. “The predominant feedback from surgeons was ‘That’s a really cool idea, but it will scar down and it will not move,’ ” Clites recalled.

The team tried out the surgery on human cadavers and animal models and thought that it might be working. “But you can’t get a rat to tell you what they are feeling,” Carty said. Did movement feel natural? How much could the animal sense its phantom limb or prosthesis? Did the prosthesis move in accordance with its thoughts? To answer these questions, the researchers needed a human—someone who was healthy but needed an amputation, and who was willing to receive a novel procedure. As Carty put it, they were looking for a “first astronaut.”

In the years after Herr’s accident, he had done numerous climbs with Ewing, his roommate. When they were about twenty, Ewing had “Life sucks” written on his left shoe; on the right, he had “And then you die.” On one climb, part of the way up the rock face, Herr asked him, “Does life really suck, Jim?” Ewing eventually married, had a child, and became a mechanical engineer, but he kept mountaineering. One day, in 2src14, he was scaling a limestone cliff in the Cayman Islands with his daughter. He slipped, fell, stopped a couple of times, and then fell again—this time all the way down, about fifty feet. Somehow, he survived.

Ewing’s left foot was so badly injured that putting any pressure on it caused excruciating pain, even a year later. “As an engineer, I was researching all kinds of different things to rebuild my ankle,” he said. But he couldn’t find anything that would let him climb again. Even walking was very difficult. “I was super depressed,” he said. He knew that Herr was leading a biomechatronics lab, so he got back in touch to inquire about anything new and experimental—and, alternatively, to hear what life with an amputation might be like. He remembers Herr saying, “Well, funny that you ask—we’ve just developed this new amputation protocol.” Herr directed him to Carty, and a couple of months later Ewing decided on amputation. He would be the first person with an “agonist-antagonist myoneural interface”—AMI for short.

Precisely when people began to make and use prostheses is unknown. A prosthetic leg, fashioned from poplar and tipped with a horse’s hoof, was found in a two-millennia-old grave in present-day China, along the Silk Road. A Roman general is said to have received an iron replacement for his right hand, to allow him to hold a shield. Ambroise Paré, a sixteenth-century French military barber-surgeon, devised a mostly metal leg with a knee joint, which could bend when a person was walking and lock when he was standing. Paré also worked on innovative surgical approaches to amputation, such as saving skin and muscle.

Throughout the years, prostheses have been reimagined in creative ways. Still, for a long time, the main difficulty for soldiers who needed amputations was surviving long after the operation. During the American Civil War, when infections killed more soldiers than artillery did, it was said that a soldier was lucky to have a limb shot off rather than cut off by a battlefield surgeon; field surgeons were unlikely to be working with a clean blade. (Advertisements from the time offered a type of ankle prosthesis that contained no metal and had a socket made from polished ivory and vulcanized rubber. It was touted as “EXTREMELY LIGHT; MUCH LIGHTER THAN ANY OTHER.”) For people who needed amputations, the greatest advance in care arguably was not superior prostheses but more modern surgical practices.

In the century that followed, amputation remained a neglected area of medicine. “When I was a medical student, amputations were sometimes given to the most junior member of a surgical team, and it was a contest to see how fast you could get the limb off,” David Crandell, a physiatrist in the Department of Physical Medicine and Rehabilitation at Harvard Medical School, told me. To this day, surgeons performing amputations too often have little sense of what happens to a patient in the years after recovery.

“Part of the problem was that amputation was thought of as a failure,” Carty told me. “The thinking was, Either you salvage the limb or you fail to salvage the limb.” He brought up Ewing’s case to demonstrate how that approach can be misguided. “He had this preserved foot, but it’s painful all the time,” Carty said. “He stops climbing—he stops doing all these things that matter most to him.” For Ewing, an amputation and a fitting with a prosthesis could be more restorative than keeping the foot.

Carty and his colleagues were confident that the AMI amputation would be safe, and that Ewing would be able to use a conventional prosthesis without trouble. “Still, when you’re doing something for the first time, you’re freaking out the whole time, because you’re wondering what you’ll find,” Carty said. They were not sure that the surgery would allow the muscles to move more freely, which was essential for a strong neural connection to a prosthesis.

According to a description of what would become known as the Ewing amputation, the surgeon makes a “stairstep incision” over the shin using a scalpel. The relevant part of the limb is “exsanguinated.” A flap of skin is peeled back to expose the leg muscles. Care is to be taken, the account notes, to isolate the saphenous vein and a nearby nerve. This is only the beginning of what is simultaneously a delicate, gruesome, and revolutionary surgical procedure; one of the required tools is a bone saw.

On July 19, 2src16, Ewing spent more than five hours in the operating room. “Things went pretty well for me,” he recalled. Two weeks after the surgery, even before he had healed enough to have a prosthesis fitted, he went to a local climbing gym. “I remember feeling very liberated,” he said. “I was using just one leg, but I felt free from pain. I could propel myself up that wall dynamically.”

A few weeks later, Ewing went to the lab at M.I.T. The first thing the team wanted to know was whether the connected agonist and antagonist muscles in the amputated limb could move. An ultrasound probe showed that they could. “For a scientist, that’s Christmas morning,” Clites, who is now an assistant professor at the U.C.L.A. School of Engineering, said. “That was the big wow.” The research team then worked on picking up electrical signals from the muscle, measuring the strength of those signals, and using them to guide the movement of a prosthetic leg.

Ewing amputations are now the standard of care at Brigham and Women’s, and are performed at many hospitals. Carty frequently teaches the method to other surgeons, sometimes even by Zoom. Footage from one of Ewing’s later visits to the lab shows the first time that the research team connected the prosthesis directly to his leg. “It’s really cool to feel it through my knee,” he says in the video. “Feels like there’s a foot there.” At first, he moves the prosthesis slowly. Later, he observes, “Literally within minutes of having it all connected, it starts becoming part of me.” We see him sitting cross-legged, with the prosthesis on top, fidgeting the foot by flexing and pointing it repeatedly—a moment Carty remembers as astonishing. “I said, ‘Jim, do you know you’re doing that?’ ” Carty recalled. Ewing replied, “No, I was just hanging out.”

One of the many eerie elegances of our bodies is that we manage to walk without thinking much about it. We never have to study a user’s guide to our legs in order to coördinate the contraction of one muscle and the relaxation of another. Almost all of that labor is done unconsciously. I sometimes think of the conscious mind as a clueless factory boss who spends her time daydreaming while the workers on the floor operate all the necessary machinery. Every so often, the self-important boss is startled into action and sends down a message like “Step around that puddle!” or “Run faster!” But only the workers know all the detailed adjustments required to carry out the order. “Even now, we don’t fully understand walking—which surprises people,” Herr told me. His lab has spent thousands of hours filming, assessing, scanning, and mathematically modelling people as they walk.

Even the most sophisticated robotic leg prostheses are engaged in merely a rough approximation of human locomotion; they “know” only what the current science knows about how we walk or run or jump, which leaves out a considerable amount. They have microprocessors that make thousands of calculations a second, and they can convey a burst of energy that, even in the absence of a calf muscle, enables a person to lift their prosthetic heel with the appropriate amount of energy. But on uneven ground, for example, they don’t allow a person to move in a truly biomimetic way. This means that an almost incomprehensibly complex technology effectively knows less than a child.

When Clites was a Ph.D. student in Herr’s lab, he worked closely with Ewing to “tune” the prosthesis to Ewing’s perception of movement. The sensors for electromyography (EMG), which is like an EKG for muscles outside the heart, were taped to his residual limb and detected the electrical activity in his leg muscles. (The team is also researching an approach to detecting muscle movements that involves small implanted metal spheres.) If Ewing was asked to lightly flex his foot but the prosthetic foot flexed intensely, the system could be adjusted. “Maybe one philosophical concept here is that, if the amputation is done well and the interface is done well, then the best possible prosthetic device is a really stupid one,” Clites told me. “It is one that doesn’t have to think very much at all . . . because the person’s brain and spinal cord are doing all the thinking.” Herr described this in a clarifying way: “There’s no real algorithm on the robot. It’s all from biological computation. That’s cool, because the person is in control.”