New Surgery May Enable Better Control of Prosthetic Limbs

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Researchers at Massachusetts Institute of Technology (MIT) have invented a new type of amputation surgery that can give users of prosthetic devices better control their residual muscles and a restored sense of proprioception. The team found that reconnecting severed muscle pairs allows them to retain their normal push-pull relationship, offering much better sensory feedback.

"Both our study and previous studies show that the better patients can dynamically move their muscles, the more control they're going to have. The better a person can actuate muscles that move their phantom ankle, for example, the better they're actually able to use their prostheses," said Shriya Srinivasan, PhD, lead author of the study.

In the study, 15 patients who received the new surgery, known as agonist-antagonist myoneural interface (AMI), could control their muscles more precisely than patients with traditional amputations. The AMI patients also reported feeling more freedom of movement and less pain in their affected limbs.

"Through surgical and regenerative techniques that restore natural agonist-antagonist muscle movements, our study shows that persons with an AMI amputation experience a greater phantom joint range of motion, a reduced level of pain, and an increased fidelity of prosthetic limb controllability," said Hugh Herr, PhD, the senior author of the paper.

Most muscles that control limb movement occur in pairs that alternately stretch and contract. One example of these agonist-antagonist pairs is the biceps and triceps. When you bend your elbow, the biceps muscle contracts, causing the triceps to stretch, and that stretch sends sensory information back to the brain.

"When one muscle contracts, the other one doesn't have its antagonist activity, so the brain gets confusing signals," said Srinivasan. "Even with state-of-the-art prostheses, people are constantly visually following the prosthesis to try to calibrate their brains to where the device is moving."

Instead of severing each muscle, the new amputation technique connects the two ends of the muscles so that they still dynamically communicate with each other within the residual limb. Since the pre-clinical studies, about 25 people have undergone the AMI.

For the study, the researchers measured the precision of muscle movements in the ankle and subtalar joints of 15 patients who had AMI transtibial amputations performed. These patients had two sets of muscles reconnected during their amputations: the muscles that control the ankle, and those that control the subtalar joint, which allowed the sole of the foot to tilt inward or outward. The study compared these patients to seven people who had traditional transtibial amputations.

After measuring the activity of specific muscles in both legs of the participants, the researchers compared the electrical signals coming from the muscles in the amputated limb with those from the intact limb and found that for AMI patients, they were very similar. They also found that patients with the AMI amputations were able to control the muscles of their amputated limbs much more precisely than the patients with traditional amputations. Patients with traditional amputations were more likely to perform the same movement over and over in their amputated limbs, regardless of how far they were asked to flex their ankle.

The researchers also discovered an effect they did not anticipate: AMI patients reported much less pain and a greater sensation of freedom of movement in their amputated limbs.

"Our study wasn't specifically designed to achieve this, but it was a sentiment our subjects expressed over and over again. They had a much greater sensation of what their foot actually felt like and how it was moving in space," Srinivasan says. "It became increasingly apparent that restoring the muscles to their normal physiology had benefits not only for prosthetic control, but also for their day-to-day mental well-being."

The research team has also developed a modified version of the surgery that can be performed on people who have already had a traditional amputation. This process, which they call "regenerative AMI," involves grafting small muscle segments to serve as the agonist and antagonist muscles for an amputated joint.

"We're learning that this technique of rewiring the limb, and using spare parts to reconstruct that limb, is working, and it's applicable to various parts of the body," Herr said.

The study, "Neural interfacing architecture enables enhanced motor control and residual limb functionality postamputation," was published in the Proceedings of the National Academy of Sciences.

Editor's note: This story was adapted from materials provided by MIT.

Photograph courtesy of MIT.