MIT researchers have invented a new type of amputation operation that can help amputees better control their residual muscles and feel where the “ghost limb” is in space. This restored feeling of proprioception should translate into better control of the prosthetic limbs, as well as a reduction in limb pain, the researchers say.
In most amputations, the pairs of muscles that control the affected joints, such as the elbows or ankles, are cut. However, the MIT team found that reconnecting these muscle pairs, allowing 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. The better a person can act on the muscles that move their phantom ankle, for example, the more they are able to use their prostheses, ”says Shriya Srinivasan, an MIT postdoctoral fellow and lead author of the study.
In a study that will appear this week in The works of the National Academy of Sciences, 15 patients who received this new type of surgery, known as the mioneural agonist-antagonist interface (IMA), could control their muscles more precisely than patients with traditional amputations. Patients with AMI also reported that they felt 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 shows that people with AMI amputation experience a wider range of motion of joint phantoms, a lower level of pain and increased fidelity of prosthetic limb controllability,” he says. Hugh Herr, professor of media arts and sciences, head of the Biomechatronics group at Media Lab and lead author of the paper.
Other authors of the paper include Samantha Gutierrez-Arango and Erica Israel, research associates at Media Lab; Ashley Chia-En Teng, an MIT student; Hyungeun Song, a Harvard-MIT graduate student in health science and technology; Zachary Bailey, former researcher visited at the Media Lab; Matthew Carty, scientist visited at Media Lab; and Lisa Freed, a researcher in the Media Lab.
Restoring the sensation
Most of the muscles that control the movement of the limbs appear in pairs that stretch and contract alternately. 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 conventional limb amputation, these muscle movements are restricted, interrupting this sensory feedback and making it much more difficult for the amputees to feel where their prosthetic limbs are in space or to feel the forces applied to these limbs.
“When one muscle contracts, the other has no antagonistic activity, so the brain receives confused signals,” says Srinivasan, a former member of the Biomechatronics group now working at MIT’s Koch Institute for Integrative Cancer Research. “Even with state-of-the-art prostheses, people are constantly visually tracking the prosthesis to try to calibrate their brains 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 those muscle pairs. Instead of cutting each muscle, they connect the two ends of the muscles so that they communicate dynamically with each other inside the residual limb. In a 2017 study of rats, they showed that when the animals contracted one muscle of the pair, the other muscle would 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. Again PNAS The researchers measured the accuracy of muscle movements in the ankles and subtalar joints of 15 patients who had AMI amputations below the knee. These patients had two sets 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 who had traditional amputations below the knee.
Each patient was evaluated while lying with their feet resting on a foam pillow, allowing their 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 “phantom” – with 25, 50, 75 or 100% of the entire range of motion. The electrodes attached to each leg allowed the researchers to measure the activity of certain muscles, as each movement was performed repeatedly.
The researchers compared the electrical signals from the muscles of the amputated limb with those from 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 precisely than patients with traditional amputations. Patients with traditional amputations were more likely to perform the same movement several times in their amputated limb, no matter how much 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 were largely related to how their brain processed the way the phantom limb moved,” says Srinivasan.
In a paper recently published in Science of Translational MedicineThe researchers reported that brain scans of AMI amputees showed that they received more sensory feedback from their residual muscles than patients with traditional amputations. In ongoing work, researchers are measuring whether this ability translates into better control of a prosthetic foot while walking.
Freedom of movement
The researchers also found 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 again and again. They had a much bigger feeling than what their foot actually felt and how it moved in space “, says Srinivasan. “It has become increasingly clear that restoring muscles to their normal physiology has benefits not only for prosthetic control, but also for their daily mental well-being.”
The research team also developed a modified version of the surgery that can be performed on people who have already had a traditional amputation. This process, which I call “regenerative AMI,” involves grafting small muscle segments to serve 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 limb rewinding technique and the use of spare parts to rebuild that limb works and is applicable to different parts of the body,” says Herr.
The research was funded by the MIT Media Lab Consortium; The National Institute of Health of the National Institute of Child Health and Human Development and the National Center for Research in Medical Rehabilitation; and congressional-led medical research programs within the U.S. Department of Defense.