MIT researchers, working with surgeons at Harvard Medical School, have developed a new type of amputation surgery that allows amputees better control of their remaining muscles and receive sensory feedback. Image credit: MIT

MIT researchers have invented a new type of amputation surgery that allows amputees to better control their remaining muscles and see where their “phantom limb” is in space. This restored sense of proprioception should result in better control of the prosthetic limbs as well as a reduction in limb pain, the researchers say.

Most amputations involve severing pairs of muscles that control the affected joints, such as the elbows or ankles. However, the MIT team found that reconnecting these pairs of muscles, which allows them to maintain their normal push-pull relationship, gives people 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. For example, the better a person can manipulate muscles that move their phantom ankles, the better they are actually able to using their prostheses, “says Shriya Srinivasan, MIT postdoc and lead author of the study.

In a study that will appear in the Proceedings of the National Academy of Sciences this week, 15 patients who received this new type of surgery known as the myonural agonist-antagonist interface (AMI) could have more precise muscle control than Patients with conventional amputations. The AMI patients also reported more freedom of movement and less pain in their affected limbs.

“Using surgical and regenerative techniques that restore natural muscle movement between agonists and antagonists, our study shows that people with an AMI amputation have more phantom joint range of motion, less pain, and greater accuracy in the controllability of the prosthesis,” he says Lord, professor of media arts and sciences, head of the biomechatronics group in the media lab, and lead author of the paper.

Other authors on the paper include Samantha Gutierrez-Arango and Erica Israel, Senior Research Support Associates at the Media Lab; Ashley Chia-En Teng, MIT student; Hyungeun Song, a PhD student in Harvard-MIT’s health sciences and technology program; Zachary Bailey, former visiting researcher at the Media Lab; Matthew Carty, visiting scholar at the Media Lab; and Lisa Freed, a scientist in the Media Lab.

Restore sensation

Most of the muscles that control the movement of the limbs come in pairs that alternately expand and contract. An 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 traditional limb amputation, these muscle movements are restricted, cutting off this sensory feedback and making it much more difficult for amputees to feel where their prostheses are in space or to feel forces being exerted on those limbs.

“When one muscle contracts, the other has no antagonist activity, so the brain receives confusing signals,” says Srinivasan, a former member of the biomechatronics group now at MIT’s Koch Institute for Integrative Cancer Research. “Even with state-of-the-art prostheses, people are constantly following the prosthesis visually to try and calibrate their brains to the position of the device.”

A new operation can allow better control of the prosthesis

MIT researchers, working with surgeons at Harvard Medical School, have developed a new type of amputation surgery that allows amputees better control of their remaining muscles and receive sensory feedback. Image credit: MIT

Several years ago, in preclinical studies, the MIT Biomechatronics group invented and scientifically developed a new amputation technique that maintains the relationships between these muscle pairs. Instead of severing each muscle, they connect the two ends of the muscles so that they still communicate dynamically within the remaining limb. In a 2017 study on rats, they showed that the other muscle would stretch and send sensory information back to the brain when the animals contracted with one muscle in the couple.

Since those preclinical studies, about 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, researchers measured the precision of muscle movements in the ankle and subtalar joints in 15 patients who had AMI amputations below the knee. In these patients, two sets of muscles were reconnected during amputation: the muscles that control the ankle and the muscles that control the subtalar joint, which allows the sole of the foot to tilt in or out. The study compared these patients to seven people who had traditional amputations below the knee.

Each patient was examined while laying their legs on a foam cushion and holding their feet in the air. The patients did not wear prostheses during the study. The researchers asked them to flex their ankle joints – both intact and “phantom” – for 25, 50, 75, or 100 percent of their total range of motion. Electrodes attached to each leg allowed researchers to measure the activity of specific muscles as each movement was performed repeatedly.

The researchers compared the electrical signals from the muscles in the amputated limb with those of the intact limb and found that they were very similar in AMI patients. They also found that patients with AMI amputation were able to control the muscles of their amputated limb much more precisely than patients with conventional amputations. Patients with traditional amputations were more likely to repeat the same movement in their amputated limb over and over, regardless of how far they were asked to bend their ankle.

“AMI patients’ ability to control these muscles was much more intuitive than that of typical amputations, which was largely to do with how their brain processed the movement of the phantom limb,” says Srinivasan.

In a recent article 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 conventional amputations. In the ongoing work, the researchers are measuring whether this ability leads to 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 wasn’t specifically designed to achieve this, but it was a feeling our subjects kept expressing. They had a much better sense of how their foot actually felt and how it moved around the room.” says Srinivasan. “It has become increasingly clear that restoring the normal physiology of the muscles has benefits not only for prosthetic control but also for daily 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 the transplantation of small segments of muscle to act as agonist and antagonist muscles for 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 learn that this technique of rewiring the limb and using spare parts to reconstruct that limb works and is applicable to different parts of the body,” says Mr.

Prostheses feel more natural

More information:
Shriya S. Srinivasan el al., “The neural interface architecture enables improved motor control and post-amputation of residual limb functionality”, PNAS (2021). Provided by the Massachusetts Institute of Technology

Quote: A new operation may allow better control of the prostheses (2021, February 15), accessed on February 15, 2021 from has been

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