Medicine has progressed a lot since the Civil War, but amputations haven’t. Once a limb is sliced off, surgeons wrap muscle around the raw end, bury nerve endings, and often attach a fixed prosthesis that is nowhere near as agile as the flesh-and-blood original. Better robotic limbs are available, but engineers are still figuring out how to attach them to people and give users fine motor control. Now, a team of researchers and clinicians has developed a simple surgical technique that could lead to prosthetics that are almost as responsive as real limbs.
“It’s a very clever model,” says Melanie Urbanchek, a muscle physiologist at the University of Michigan in Ann Arbor. “[It makes] use of what the body naturally has to offer.”
The biggest barrier to lifelike limbs is that signals can no longer travel in an unbroken path from the brain to the limb and back. Scientists have developed several ways to bridge the gap. The simplest is to place electrodes on remaining muscle near the amputation site. For finer control, doctors can use severed nerves themselves to relay the signals, through electronic attachments. But when they aren’t rejected by nerve tissue, such attachments tend to receive weak signals. A stronger signal comes from attaching nerve endings to small muscle grafts that amplify the signal and relay it using electrodes. But even this method fails to take advantage of a simple biological solution for joint control: the pairing of agonistic and antagonistic muscles. When you contract your biceps to bend your elbow, for example, your triceps on the other side of the joint stretches, providing resistance and feedback. Together, such opposing muscle pairs let you fluidly adjust a limb’s force, position, and speed.
The new technique, developed at the Massachusetts Institute of Technology (MIT) in Cambridge, creates such a pairing for prosthetic joint control. It respects “the fundamental motor unit in biology, two muscles acting in opposition,” says Hugh Herr, a biophysicist at MIT and co-developer of the method.
Let’s say you lost your leg above the knee. Surgeons would take two small muscle grafts from somewhere in your body, each a few centimeters long, and suture them together end-to-end to form a linear pair. They would place the pair under the skin near the amputation site. Then they’d suture the two ends to the tissue under the skin, so that when one half of the muscle graft contracts, the other stretches. Finally, they’d connect severed nerve endings to the graft and allow the nerves to grow into it.
Once the graft is healthy and connected, the researchers would use electrodes to connect each muscle to a smart prosthetic leg. The severed nerves that would normally tell the ankle to extend, for example, would instead go to one of the grafted muscles, which would contract, sending a signal to the robotic ankle to extend. As the grafted muscle contracts, its mirror opposite would stretch, sending a signal back to the brain. The grafts would receive additional electrical feedback from the smart prosthesis, indicating the ankle joint’s position and force, allowing for finer adjustments. Additional grafts could be added to control other joints in the prosthesis.
The new technique, called an agonist-antagonist myoneural interface, was tested in rodents. The MIT team operated on seven rats, severing muscles and nerves in the back right leg of each. Researchers then grafted on a pair of muscles about 3 centimeters long, connected severed nerves, and let the rats heal for 4 months. When electrodes were attached, the grafted muscles worked in tandem, one contracting and the other stretching. They also emitted electrical signals in proportion to the stimulation. That response suggests that the technique could allow for fine-grained control of a human prosthetic, the researchers report today in Science Robotics. What’s more, inspection under a microscope showed that the grafts healed well and were populated with new nerves and blood vessels and healthy neuromuscular junctions.
“This is fairly low-risk. It’s minor surgery,” says Rickard Branemark, an orthopedic surgeon and prosthetics researcher at the University of California, San Francisco. Even without adding a prosthesis, growing severed nerves into muscle grafts could prevent painful neuromas, or abnormal nerve growth. With the new method and a smart prosthesis, “there’s every expectation that the human will feel position, will feel speed, will feel force in the same way that they once felt when they had a limb,” says Herr, who lost his own legs below the knees to frostbite while ice climbing, and is in line to get the procedure. He says they’ll have results from human trials within the next 2 years.