MIT researchers have invented a new type of amputation surgery that can help amputees better control their remaining muscles and indicate where their “phantom limb” is in space. This restored sense of proprioception should be translated into better control of prosthetic limbs, as well as a reduction of limbs, the researchers say.
In most amputations, muscle pairs that control the affected joints, such as elbows or ankles, are cut off. However, the MIT team found that reconnecting these muscle pairs, enabling them to maintain their normal push-pull ratio, provides people with much better sensory feedback.
“Both our study and previous studies show that the better patients can move their muscles dynamically, the more control they will have. The better a person can move muscles that move their phantom ankle, for example, the better they are actually able to use their prostheses, ‘says Shriya Srinivasan, an MIT postdoctoral fellow and lead author of the study.
In a study published this week in the Proceedings of the National Academy of Sciences, 15 patients who underwent this new type of surgery, known as agonist-antagonist myoneural interface (AMI), were able to control their muscles more precisely than patients with traditional amputations. The AMI patients also reported that they had more freedom of movement and less pain in their affected limb.
“Through surgical and regenerative techniques that restore natural muscle movements of agonist-antagonists, our study shows that individuals with an AMI amputation experience a greater range of motion of the ghost joint, a reduced level of pain and an increased fidelity of the controllability of the prosthetic limb, “says Hugh Herr, a professor of media arts and sciences, head of the Biomechatronics group in the Media Lab, and senior author of the article.
Other authors of the paper include Samantha Gutierrez-Arango and Erica Israel, senior research supporters at the Media Lab; Ashley Chia-En Teng, an MIT undergraduate student; Hyungeun Song, a graduate student in the Harvard-MIT program in health sciences and technology; Zachary Bailey, a former visiting researcher at the Media Lab; Matthew Carty, a visiting scientist at the Media Lab; and Lisa Freed, a research scientist from the Media Lab.
Restore sensation
Most muscles that control limbs 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.
During a conventional amputation of the limbs, these muscle movements are restricted, which disables the sensory feedback and makes it much more difficult for amputators to feel where their prosthetic limbs are in space, or to feel the forces applied to the limbs. .
“When one muscle contracts, the other one does not have its antagonistic activity, so the brain receives confusing signals,” says Srinivasan, a former member of the Biomechatronics group now working at MIT’s Koch Institute for Integrative Cancer Research. “Even with the latest prosthesis, people are constantly following the prosthesis visually to calibrate their brains to where the device is moving.”
A few years ago, the MIT Biomechatronics group invented a new amputation technique and developed it scientifically in the preclinical studies that maintain the relationship between the muscle pairs. Instead of separating each muscle, they connect the two ends of the muscles so that they still communicate dynamically with each other within the remaining limb. In a 2017 study of rats, they showed that when the animals get one muscle from the pair, the other muscle will stretch and send sensory information back to the brain.
Since these preclinical studies, approximately 25 people have undergone AMI surgery at Brigham and Women’s Hospital, performed by Carty, who is also a plastic surgeon at Brigham and Women’s Hospital. In the new PNAS study, the researchers measured the accuracy of muscle movements in the ankle and subtalar joints of 15 patients who underwent knee AMI amputations. These patients have two sets of muscles that are reconnected during their amputation: the muscles that control the ankle, and those that control the subtalar joint, which causes the sole of the foot to tilt inward or outward. The study compared these patients with seven people who had mastered traditional amputations.
Each patient was evaluated while propping his legs on a foam pillow so that their feet could stretch into the air. Patients did not wear prosthetic limbs during the study. The researchers asked them to bend their ankle joints – both the intact and the “phantom” – with 25, 50, 75 or 100 percent of their full range of motion. Electrodes attached to each bone enabled the researchers to measure the activity of specific muscles as each movement was performed repeatedly.
The researchers compared the electrical signals of the muscles in the amputated limb with those of the intact limb and found that it was very similar for AMI patients. They also found that patients with the AMI amputation could control the muscles of their amputated limb 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 limb, no matter how far they were asked to bend their ankle.
“The ability of the AMI patients to control these muscles was much more intuitive than those with typical amputations, which largely had to do with the way their brains processed the ghost bone processing,” says Srinivasan.
In a paper recently published in Science Translational Medicine, the researchers reported that brain scans of the AMI amputees showed that they received more sensory feedback from their remaining muscles than patients with traditional amputations. In work currently underway, the researchers measure whether this ability translates into better control of a prosthetic leg while walking.
Freedom of movement
The researchers also discovered an effect they did not expect: AMI patients reported much less pain and a greater sense of freedom of movement in their amputated limbs.
‘Our study was not specifically designed to achieve this, but it was a sentiment that our subjects repeatedly expressed. “They had a much bigger feeling how their foot actually feels and how it moves in space,” says Srinivasan. “It has become increasingly clear that the recovery of the muscles to their normal physiology not only has benefits for prosthetic control, but also for their daily mental well-being.”
The research team also developed a customized 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 the grafting of small muscle segments to serve as the agonist and antagonistic muscles for an amputated joint. They are also working on developing the AMI procedure for other types of amputations, including above the knee, above and below the elbow.
“We learn that this technique of rewiring the limb and using parts to reconstruct the limb works and that it applies to different parts of the body,” says Herr.
The research was funded by the MIT Media Lab Consortia, the National Institute of Child Health and Human Development, the National Center for Medical Rehabilitation Research, and the U.S. Department of Defense’s Congressional Medical Research Programs.