MIT researchers have invented a new type of amputation surgery that can help amputees better control residual muscles and detect where their “ghost limb” is in space. The researchers claim that this restored sense of proprioception should translate into better control of the prosthetic limbs, as well as a reduction in limb pain.
In most amputations, the muscle pairs that control the affected joints, such as the elbows or ankles, are cut. However, the MIT team has found that reconnecting these muscle pairs, which allows them to maintain their normal push-pull relationship, offers people much better sensory feedback.
“Both our study and previous studies show that the more patients can move their muscles dynamically, the more control they will have. The better a person acts on the muscles that move the phantom ankle, for example, the better they will be able to use the prostheses, ”says Shriya Srinivasan, an MIT postdoctoral fellow and lead author of the study.
In a study that will appear this week in the Proceedings of the National Academy of Sciences, 15 patients who received this new type of surgery, known as the agonist-antagonist mioneural interface (AMI), could control the muscles more accurately than patients with traditional amputations. Patients with AMI also reported feeling more freedom of movement and less pain in the affected limbs.
“Through surgical and regenerative techniques that restore natural agonist-antagonist muscle movements, our study demonstrates that people with AMI amputation experience a greater range of motion of the phantom joint, a reduced level of pain, and greater fidelity. of controllability of prosthetic limbs, ”says Hugh Herr, professor of media arts and sciences, head of the Media Lab’s biomechatronics group and lead author of the work.
Other authors of the article are Samantha Gutierrez-Arango and Erica Israel, senior research support collaborators at the Media Lab; Ashley Chia-En Teng, an MIT college student; Hyungeun Song, a graduate student in the Harvard-MIT program in health science and technology; Zachary Bailey, former visiting researcher at the Media Lab; Matthew Carty, visiting scientist at the Media Lab; and Lisa Freed, a research scientist at the Media Lab.
Restoring the feeling
Most muscles that control limb movement occur in pairs that stretch and contract alternately. An example of these agonist-antagonist pairs are the biceps and triceps. When you bend your elbow, the biceps muscle contracts and causes the triceps to stretch, and this stretch returns the sensory information to the brain.
During a conventional limb amputation, these muscle movements are restricted, cutting off this sensory feedback and making it much harder for amputees to feel where their prosthetic limbs are in space or to perceive forces applied to those limbs.
“When one muscle contracts, the other doesn’t have its antagonistic activity, so the brain receives confusing signals,” says Srinivasan, a former member of the Biomechatronics group who now works at the Koch Institute for Integrative Cancer Research. from MIT. “Even with state-of-the-art prostheses, people are constantly visually following the prosthesis to try to calibrate their brain to where the device is moving.”
A few years ago, the MIT Biomechatronics group invented and scientifically developed in preclinical studies a new amputation technique that maintains the relationships between these muscle pairs. Instead of cutting each muscle, they connect the two ends of the muscles so that they still communicate dynamically with each other within the residual limb. In a 2017 rat study, they showed that when animals contracted one muscle in the pair, the other muscle stretched and sent sensory information to the brain.
Since these preclinical studies, about 25 people have undergone AMI surgery at Brigham and Women Hospital, performed by Carty, who is also a plastic surgeon at Brigham and Women Hospital. In the new PNAS In this study, the researchers measured the accuracy of muscle movements in the ankle and subtalar joints of 15 patients who underwent AMI amputations below the knee. These patients had two groups of muscles reconnected during their amputation: the muscles that control the ankle and those that control the subtalar joint, which allows the sole of the foot to tilt inward or outward. The study compared these patients with seven people with traditional amputations below the knee.
Each patient lying down with their legs resting on a foam cushion was assessed, allowing the feet to extend into the air. Patients did not wear prosthetic limbs during the study. The researchers asked them to flex their ankle joints (both intact and “ghost”) 25, 50, 75, or 100 percent of their full range of motion. The electrodes attached to each leg allowed the researchers to measure the activity of specific muscles as each movement was performed repeatedly.
The researchers compared the electrical signals from the muscles of the amputated limb to those of the intact limb and found that for patients with AMI they were very similar. They also found that patients with AMI amputation were able to control the muscles of their amputated limb much more accurately than patients with traditional amputations. Patients with traditional amputations were more likely to perform the same movement over and over on their amputated limb, regardless of the extent to which they were asked to flex their ankle.
“The ability of patients with AMI to control these muscles was much more intuitive than those with typical amputations, which largely had to do with how the brain processed how the phantom limb moved,” says Srinivasan.
In a document that appeared recently in Scientific translational medicine, the researchers reported that brain scans of AMI amputees showed that they obtained more sensory feedback from residual muscles than patients with traditional amputations. In the work now underway, researchers are measuring whether this ability translates into better control of the prosthetic leg while walking.
Freedom of movement
The researchers also discovered an effect they did not anticipate: patients with AMI reported much less pain and a greater sense of freedom of movement in the amputated limbs.
“Our study was not specifically designed to achieve this, but it was a feeling that our subjects expressed over and over again. They had a much greater sense of how the foot felt and how it moved in space, ”says Srinivasan. “It became increasingly clear that restoring the muscles to their normal physiology had benefits not only for prosthesis 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, called “regenerative AMI,” involves grafting small muscle segments to serve as the agonist and antagonist muscle of an amputated joint. They are also working on developing the AMI procedure for other types of amputations, including above the knee and above and below the elbow.
“We’re learning that this limb reassembly technique and the use of spare parts to rebuild that limb works, and it’s applicable to various parts of the body,” Herr says.
The research was funded by the MIT Media Lab Consortium; the National Institute of Child Health and Human Development of the National Institute of Health and the National Center for Medical Rehabilitation Research; and congressional-led medical research programs by the U.S. Department of Defense.