People whose limbs have been amputated are often left with phantom sensations or pain in the missing appendage. Prosthetics don't feel anything like the real thing. And people with artificial limbs have to keep looking down, because they can’t feel where their artificial arm or leg is in space.
MIT Media Lab professor Hugh Herr knows these problems all too well. A double-leg amputee from a climbing accident in high school, Herr has struggled with prosthetics his whole adult life. “I’m wearing two bionic legs,” says Herr, a biophysicist who co-directs the MIT Center for Extreme Bionics. “When my bionic ankles move, I have no feeling of that movement.”
Now Herr and his colleagues have made an advance they hope will address these problems. In a paper published this week in Science Robotics the team attached two bits of a rat’s muscle, taken from elsewhere in its body, to the end of severed nerves. This agonist-antagonist muscle pair allowed the body to sense the stretch, torque and speed of a muscle in a realistic way, electrical measurements showed, and in theory, would allow more control over a prosthetic limb. Although the study is only in rodents, Herr’s surgical colleagues have begun to show similar results in humans.
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If the work can be fully substantiated in people, “it would be groundbreaking,’ says Rickard Brånemark, an associate professor of orthopedics at the University of Gothenburg in Sweden, who was not involved in the research. “If this [method] is really working, it has a great new potential to create control,” he says. “Today’s prosthetic control is very poor.”
Not having a sense of where an artificial limb is in space—known as proprioception—is a huge disadvantage of artificial limbs. Someone with a prosthetic leg, for instance, can’t close their eyes and balance on the artificial limb. Those with an artificial arm can’t know whether their elbow is bent or where their hand is without looking at it. They also don’t feel like their artificial limb belongs to them, which can lead to so-called “phantom” sensations or pain.
Providing paired muscle tissue at the end of patients’ severed nerves will allow them to sense stretch and force with each movement. That should restore their proprioception, as well as a sense of connectedness to the artificial limb, says Herr.
The findings may also help transform amputation surgery, which has remained essentially unchanged for 2,000 years—with the exception of anesthesia, which not only made the surgery more bearable for patients but also allowed surgeons to take more time at their unpleasant task.
Instead of sawing off a limb in a straight line, the new research and related work suggests surgeons should carefully dissect it to leave bits of muscle capping off the nerves. Nerves that still have muscle tissue attached are less likely to trigger pain, and their signals are easier for prosthetics to detect, according to the research.
“I think we are at a very exciting time in which we are fundamentally reassessing the way in which we link the human body to technology,” says study co-author Matthew Carty, a staff surgeon at Brigham and Women’s Hospital in Boston. “I think our work represents a contribution to this and has the potential, with others, to reinvent the way we think about a very, very old operation.”
Carty, who is also an associate professor at Harvard Medical School, says he has performed the more complicated amputation method on three patients so far. “It’s been exceedingly successful,” he says, adding that he’s currently writing up his results for publication.
All three patients have been experimenting in Herr’s lab with new prosthetics designed to read electrical signals from their preserved muscles. Carty is also screening potential candidates with older amputations for surgery to restore muscle pairs at the ends of their severed nerves. The paired muscles should give them more control of their prosthetics, as well as allow the devices to wirelessly “read” nerve signals. This should permit someone with a prosthetic foot to step on a gas pedal, for instance, without watching it, says Carty.
Dr. Paul Cederna, chief of plastic surgery at the University of Michigan, pioneered the idea of leaving muscle tissue at the ends of nerves during amputations, and has performed 130 such experimental amputations in people over the last three years. His patients have had very good results, he says, with less pain both wearing prosthetics and from the “phantom” sensations that the body can feel when a limb is cut off. He has also had success, he says, in attaching muscle to the end of nerves severed in long-ago amputations.
Cederna’s surgeries have involved leaving one muscle attached. The insight from Herr’s team’s to leave two complementary muscles should make a huge difference, Cederna says, both in giving the amputee a sense of their artificial limb in space, and in helping next-generation prosthetics work better. “It’s an absolutely great idea,” he says. He and Herr plan to collaborate on future research.
Researchers have been trying for years to transmit nerve signals directly to prosthetics. But these signals are on the order of microvolts, and most prosthetics have trouble distinguishing something so faint. Muscles amplify nerve signals by orders of magnitude. Wirelessly reading these muscle signals should allow more effective signaling, says Herr, who has undergone some preliminary testing himself to see if he’s a good candidate for the procedure.
“With this approach, we’re very confident that the human will actually feel position, will actually feel speed, will actually feel force,” he says. “It’ll completely feel like their own limb.”
Photo credit US Navy