State-of-the-art prosthetic limbs, while helpful, don’t offer users complete neural control. They depend on robotic sensors and controllers that utilize pre-programmed algorithms to facilitate movement. However, a team at MIT, working with Brigham and Women’s Hospital, has achieved a breakthrough. Through a novel surgical procedure and neuroprosthetic interface, they’ve demonstrated that a prosthetic leg can be entirely controlled by the body’s nervous system, resulting in a natural walking motion.
The surgical amputation, known as the agonist-antagonist myoneural interface (AMI), involves reconnecting muscles in the remaining limb. This allows patients to experience “proprioceptive” feedback, meaning they can sense the location of their prosthetic limb in space.
In a study involving seven patients who underwent this surgery, the MIT team observed significant improvements in their ability to walk. Compared to individuals with traditional amputations, these patients could walk faster, navigate obstacles, and climb stairs more naturally.
Hugh Herr, a professor of media arts and sciences, 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 senior author of the study, stated, “This is the first prosthetic study in history that shows a leg prosthesis under full neural modulation, where a biomimetic gait emerges. No one has been able to show this level of brain control that produces a natural gait, where the human’s nervous system is controlling the movement, not a robotic control algorithm.”
Beyond improved mobility, patients who underwent the AMI surgery experienced less pain and muscle atrophy. This procedure, applicable to both arm and leg amputations, has been performed on approximately 60 patients globally.
Hyungeun Song, lead author of the paper published in NatureMedicine and a postdoc in MIT’s Media Lab, explained, “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.”
Traditional below-the-knee amputations disrupt the natural interaction of muscle pairs responsible for stretching and contracting, making it difficult for the nervous system to perceive muscle position and contraction speed. This sensory information is crucial for the brain to control limb movement. Consequently, individuals with such amputations often struggle to control their prosthetic limbs, relying on robotic controllers and sensors for adjustments.
The AMI surgery, developed by Herr and his colleagues, addresses this issue by preserving the connection between muscle pairs. This allows for continued dynamic communication within the residual limb, enabling a more natural interaction with the prosthetic. A 2021 study by Herr’s lab demonstrated that patients who had undergone the AMI surgery exhibited greater control over their amputated limb muscles. The electrical signals produced by these muscles were remarkably similar to those observed in their intact limb.
Building on these findings, the researchers investigated whether these electrical signals could be harnessed to control a prosthetic limb while providing the user with proprioceptive feedback. This feedback loop would allow users to consciously adjust their gait based on their perception of the limb’s position.
The results, published in Nature Medicine, confirmed their hypothesis. The sensory feedback provided by the AMI interface translated into a smoother, more natural walking ability and improved obstacle avoidance.
Song elaborated, “Because of the AMI neuroprosthetic interface, we were able to boost that neural signaling, preserving as much as we could. 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.”
The study compared seven individuals who had undergone the AMI surgery with seven who had traditional below-the-knee amputations. All participants used the same bionic limb, equipped with a powered ankle and electrodes to detect electromyography (EMG) signals from specific muscles. These signals were then processed by a robotic controller to adjust ankle bending, torque, and power delivery.
Across various tasks, including level-ground walking, slope and ramp navigation, stair climbing, and obstacle avoidance, individuals with the AMI neuroprosthetic interface consistently outperformed those with traditional amputations. They walked faster, exhibited more natural movements, and demonstrated better coordination between their prosthetic and intact limbs.
“With the AMI cohort, we saw natural biomimetic behaviors emerge,” Herr noted. “The cohort that didn’t have the AMI, they were able to walk, but the prosthetic movements weren’t natural, and their movements were generally slower.”
Remarkably, these natural behaviors emerged despite the AMI providing less than 20 percent of the sensory feedback typically experienced by individuals without amputations. Song highlighted the significance of this finding, stating, “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.”
Matthew Carty, a surgeon at Brigham and Women’s Hospital, associate professor at Harvard Medical School, and a co-author of the paper, emphasized the importance of such collaborations, saying, “This work represents yet another step in us demonstrating what is possible in terms of restoring function in patients who suffer from severe limb injury. It is through collaborative efforts such as this that we are able to make transformational progress in patient care.”
This shift towards user-driven neural control represents a significant step towards Herr’s lab’s vision of “rebuilding human bodies.” Instead of relying solely on sophisticated robotic controllers and sensors, this approach aims to create a more seamless integration between the user and the prosthetic.
Herr articulated this vision, stating, “The problem with that long-term approach is that the user would never feel embodied with their prosthesis. 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.”
This groundbreaking research was supported by the MIT K. Lisa Yang Center for Bionics and the Eunice Kennedy Shriver National Institute of Child Health and Human Development.
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