Cutting-edge prosthetic limbs have the power to empower amputees with a natural walking gait. However, they fall short of providing full neural control over the limb, relying on pre-defined algorithms.
In partnership with Brigham and Women’s Hospital, MIT researchers have pioneered a groundbreaking surgical intervention and neuroprosthetic interface that enables a prosthetic leg to be fully driven by the body’s own nervous system.
A group of seven patients underwent a surgical procedure, and a team from MIT found that they were able to walk faster, avoid obstacles, and climb stairs much more naturally than people with a traditional amputation.
“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,” says Hugh Herr, a professor of media arts and sciences, co-director of the K. Lisa Yang Center for Bionics at MIT, the senior author of the new study.
Additionally, patients reported experiencing reduced pain and muscle atrophy following the surgery, known as the agonist-antagonist myoneural interface (AMI). Approximately 60 individuals worldwide have undergone this type of surgery, which is also applicable to those with arm amputations.
The traditional below-the-knee amputation disrupts the interactions of paired muscles, making it difficult for the nervous system to sense muscle position and contraction speed, which is crucial for limb movement. As a result, controlling a prosthetic limb becomes challenging, and reliance on robotic controllers built into the prosthetic limb increases.
The innovative AMI surgery aims to address these issues by maintaining dynamic communication between muscles within the residual limb. By opting for this surgical approach, individuals may have the opportunity to achieve a more natural gait under full nervous system control.
The MIT team found this sensory feedback did indeed translate into a smooth, near-natural ability to walk and navigate obstacles.
For this study, researchers conducted a comparison between seven individuals who underwent AMI surgery and seven who had traditional below-the-knee amputations. Both groups used a bionic limb equipped with a powered ankle and EMG-sensing electrodes, which fed signals into a robotic controller to assist in ankle bending, torque application, and power delivery.
The subjects were tested in various scenarios, including level-ground walking, walking up and down slopes and stairs, and navigating obstacles. The individuals with the AMI neuroprosthetic interface demonstrated faster walking speeds akin to those without amputations and exhibited enhanced obstacle navigation.
Their movements were more natural and showed improved coordination between their prosthetic and intact limbs. Additionally, they were able to apply the same amount of force while pushing off the ground as individuals without amputations.
“With the AMI cohort, we saw natural biomimetic behaviors emerge,” Herr says. “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.”
Even though the AMI provided less than 20% of the usual sensory feedback, these natural behaviors still emerged in individuals with amputations.
“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 says.
Journal reference:
- Hyungeun Song, Tsung-Han Hsieh, Seong Ho Yeon, Tony Shu, Michael Nawrot, Christian F. Landis, Gabriel N. Friedman, Erica A. Israel, Samantha Gutierrez-Arango, Matthew J. Carty, Lisa E. Freed & Hugh M. Herr. Continuous neural control of a bionic limb restores biomimetic gait after amputation. Nature Medicine, 2024; DOI: 10.1038/s41591-024-02994-9