Nervous-system-driven prostheses give amputees greater control

Marvel’s “Bucky” Barnes, Mad Max: Fury Road’s Furiosa, and Gazelle from the Kingsman franchise. What do these fictional movie characters have in common? Bionic limbs they’re able to wield with great precision. Unfortunately, real life hasn’t caught up with Hollywood’s portrayal of ultra-responsive prostheses.

Current prosthetic legs don’t provide feedback to the nervous system like an intact limb would, instead relying on robotic sensors and controllers that move according to a predefined gait algorithm. But new research by MIT in collaboration with Brigham and Women’s Hospital has brought us a step closer to the kind of prosthetic control we’ve so far only seen in movies.

“This is the first prosthetic study in history that shows a leg prosthesis under full neural modulation, where a biomimetic gait emerges,” said Hugh Herr, professor of Media Arts and Sciences at the MIT Media Lab, co-director of the K. Lisa Yang Center for Bionics at MIT, an associate member of MIT’s McGovern Institute for Brain Research and the study’s corresponding author.

Walking is a complex process of neuromechanics, an interaction between nerves and muscles. On the nerve side of things, afferent neurons carry sensory information from the skin (or other organs) up the spinal cord to the brain, where association neurons decide how to respond. Then efferent neurons send information from the brain down the spinal cord and out towards the muscles, telling them what motion to perform.

Then, there’s proprioception, which is the body’s ability to sense movement, action, and location, an awareness of the body in space. In intact limbs, proprioception is facilitated by biological sensors in muscle pairs acting in opposition to each other, taking turns contracting (called the agonist) and stretching (antagonist). In conventional below-the-knee amputations, these muscles are severed, disrupting proprioception and making it difficult for people to control their prosthetic limb because they can’t accurately sense where the limb is in space.

Several years ago, Herr and his colleagues began developing a new surgical procedure called an agonist-antagonist myoneural interface (AMI) to replicate proprioception in an amputated limb and restore a normal gait to amputees. Rather than severing the natural pairing of agonist and antagonist muscles, surgeons connect their ends so they can communicate with one another.

How the bionic prosthesis interacts with the surgically reconstructed lower leg muscles
How the bionic prosthesis interacts with the surgically reconstructed lower leg muscles

Song et al.

“With the AMI amputation procedure, to the greatest extent possible, we attempt to connect native agonists to native antagonists in a physiological way so that after amputation, a person can move their full phantom limb with physiologic levels of proprioception and range of movement,” said Herr.

A 2021 study demonstrated that in people with below-the-knee amputations, the new surgical technique provided more precise control of the amputated limb, and the remaining muscles produced electrical signals very similar to those produced in the intact limb. Unexpectedly, the AMI patients also reported far less pain and a greater sensation of freedom of movement in their amputated limbs. The present study explored whether those electrical signals could generate commands for a prosthetic limb and generate proprioceptive feedback that would allow the user to choose to adjust their gait as needed.

Seven people who’d had the AMI surgery were compared with seven who’d had traditional below-the-knee amputations. All were given the same bionic limb, a prosthesis with a powered ankle and skin-mounted electrodes that sense electromyography (EMG) signals from the tibialis anterior muscle, which runs along the outer aspect of the lower leg, and the gastrocnemius or calf muscle (in AMI patients, these two muscles had been surgically joined). The EMG signals were fed into a robotic controller that helped the prosthesis calculate how much to bend the ankle, how much torque to apply, or how much power to deliver.

Helping people with amputation walk naturally

Study participants were tested in different walking scenarios: across a 10-meter (33-ft) level pathway, up a slope, down a ramp, up and down stairs, and on a level surface while avoiding obstacles. Those with the AMI neuroprosthetic interface fared better, walking at about the same rate as someone without an amputation and navigating obstacles more easily. Their movements were more natural, and they could better coordinate the movements of their prosthetic and intact limbs. They could also push off the ground with the same force as someone without an amputation.

“Because of the AMI neuroprosthetic interface, we were able to boost that neural signaling, preserving as much as we could,” said Hyungeun Song, a postdoctoral researcher in MIT’s Media Lab and the study’s lead author. “This was able to restore a person’s neural capability to continuously and directly control the full gait, across different walking speeds, stairs, slopes, even going over obstacles.”

These results were observed even though the sensory feedback provided by the AMI was less than 20% of what people without amputations would receive.

“One of the main findings here is that a small increase in neural feedback from your amputated limb can restore significant bionic neural controllability, to a point where you allow people to directly neurally control the speed of walking, adapt to different terrain, and avoid obstacles,” Song said.

It’s a step towards Herr’s lab’s goal of better integrating a person and their prosthesis rather than just focusing on creating increasingly more technologically advanced bionic limbs.

“The problem with that long-term approach is that the user would never feel embodied with their prosthesis,” said Herr. “They would never view the prosthesis as part of their body, part of self. The approach we’re taking is trying to comprehensively connect the brain of the human to the electromechanics.”

Worldwide, about 60 patients have had the AMI surgery so far, which can also be used in people with arm amputations, according to the researchers.

The study was published in the journal Nature Medicine.

Source: MIT

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