Nervous system-driven prosthesis returns mobility to amputees

4th July 2024
Harry Fowle

A newly developed surgical procedure from MIT has provided individuals with enhanced neural feedback from their residual limbs. As a result of this nervous system-driven prosthesis, seven patients have been able to walk more naturally and navigate obstacles more effectively.

Advanced prosthetic limbs can help individuals with amputations achieve a natural walking gait, but they do not offer full neural control over the limb. These prosthetics rely on robotic sensors and controllers to move the limb using predefined gait algorithms.

MIT researchers, in collaboration with colleagues from Brigham and Women’s Hospital, developed a new surgical intervention and neuroprosthetic interface that enabled a natural walking gait with a prosthetic leg controlled entirely by the body’s nervous system. This surgical procedure reconnected muscles in the residual limb, allowing patients to receive proprioceptive feedback about their prosthetic limb's position.

In a study involving seven patients who underwent this surgery, the MIT team discovered that these individuals could walk faster, avoid obstacles, and climb stairs more naturally compared to those with a traditional amputation.

Hugh Herr, Professor of Media Arts and Sciences, Co-Director of the K. Lisa Yang Centre for Bionics at MIT, an Associate Member of MIT’s McGovern Institute for Brain Research, and the Senior Author of the new study commented: “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.”

Patients also experienced reduced pain and less muscle atrophy following this surgery, known as the agonist-antagonist myoneural interface (AMI). To date, approximately 60 patients worldwide have undergone this procedure, which can also be performed on individuals with arm amputations.

Achieving sensory feedback

Limb movement is typically managed by pairs of muscles that alternate between stretching and contracting. A traditional below-the-knee amputation disrupts these interactions, complicating the nervous system's ability to sense muscle position and contraction speed — essential sensory information for the brain to control limb movement.

People with such amputations often struggle to control their prosthetic limbs due to the inability to accurately perceive the limb's spatial position. Instead, they rely on the robotic controllers and sensors within the prosthetic limbs, which can detect and adjust to slopes and obstacles.

To help individuals achieve a natural gait under full nervous system control, Herr and his team began developing the AMI surgery several years ago. Rather than severing the interactions between agonist-antagonist muscle pairs, this surgery connects the muscle ends, allowing dynamic communication within the residual limb. This procedure can be performed during the initial amputation or as a revision after the initial surgery.

Herr 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.”

In a 2021 study, Herr’s lab discovered that patients who underwent this surgery could more precisely control the muscles of their amputated limb. The muscles produced electrical signals similar to those from their intact limb.

Building on these promising results, the researchers explored whether these electrical signals could generate commands for a prosthetic limb while providing users with feedback about the limb’s spatial position. The wearer could then use this proprioceptive feedback to adjust their gait as needed.

In the new Nature Medicine study, the MIT team confirmed that this sensory feedback translated into a smooth, nearly natural ability to walk and navigate obstacles.

Hyungeun Song, a PostDoc in MIT’s Media Lab and Lead Author of the paper, stated: “Because of the AMI neuroprosthetic interface, we were able to boost that neural signalling, 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.”

A natural walking gait

In this study, researchers compared seven individuals who had undergone AMI surgery with seven who had traditional below-the-knee amputations. All participants used the same type of bionic limb: a prosthesis equipped with a powered ankle and electrodes capable of sensing electromyography (EMG) signals from the tibialis anterior and gastrocnemius muscles. These signals were fed into a robotic controller, which helped the prosthesis determine the appropriate ankle bend, torque, and power delivery.

The subjects were tested in various scenarios: walking on level ground across a 10-meter pathway, walking up a slope, descending a ramp, navigating stairs, and walking on a level surface while avoiding obstacles.

In all these tasks, those with the AMI neuroprosthetic interface walked faster—at about the same speed as individuals without amputations—and navigated obstacles more easily. They exhibited more natural movements, such as pointing the prosthetic toes upward when climbing stairs or stepping over obstacles. They also better coordinated their prosthetic and intact limbs and could push off the ground with the same force as non-amputees.

Herr commented: “With the AMI cohort, we saw natural biomimetic behaviours emerge. The cohort that didn’t have the AMI could walk, but the prosthetic movements weren’t natural, and their movements were generally slower.”

These natural behaviours emerged even though the sensory feedback provided by the AMI was less than 20% of what non-amputees would normally receive.

Song stated: “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 and associate professor at Harvard Medical School, who is also an author of the paper, noted: “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.”

Enabling neural control by the limb user aligns with Herr’s lab’s goal of “rebuilding human bodies,” moving away from reliance on increasingly sophisticated robotic controllers and sensors—tools that, while powerful, do not feel like part of the user’s body.

Herr explained: “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.”

The research received funding from the MIT K. Lisa Yang Centre for Bionics and the Eunice Kennedy Shriver National Institute of Child Health and Human Development.

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