Modern Prosthesis Allows Natural Walking

Modern prosthetic limbs help people with amputations walk naturally but don’t give full neural control. They use robotic sensors and pre-set algorithms to move.

MIT researchers, with Brigham and Women’s Hospital, developed a new surgical method and neuroprosthetic interface. This allows a prosthetic leg to be controlled entirely by the nervous system. By reconnecting muscles in the remaining limb, patients get feedback on the position of their prosthetic limb.

In a study with seven patients who had this surgery, the MIT team found they could walk faster, avoid obstacles, and climb stairs more naturally than those with traditional prosthetics.

“This is the first study to show a leg prosthesis fully controlled by the nervous system, resulting in a natural walking pattern. No one has demonstrated this level of brain control before, where the movement is directed by the nervous system, not robotic algorithms,” says Hugh Herr, a professor and senior author of the study from MIT.

Patients also had less pain and muscle loss after the surgery, called the agonist-antagonist myoneural interface (AMI). So far, about 60 patients worldwide have had this surgery, which can also be done for arm amputations.

Input from the Prosthesis Sensor Feedback

Most limb movements rely on pairs of muscles working together, with one stretching while the other contracts. In a standard below-the-knee amputation, this muscle pairing is disrupted, making it hard for the nervous system to detect the muscle’s position and contraction speed. This sensory information is vital for the brain to decide how to move the limb.

People with this type of amputation often struggle to control their prosthetic limb because they can’t accurately sense its position. Instead, they depend on robotic controllers and sensors built into the prosthetic to detect and adjust to slopes and obstacles.

To help achieve a more natural gait controlled by the nervous system, Herr and his team developed the AMI surgery. Instead of cutting the natural muscle interactions, they connect the ends of the muscles so they can still work together within the remaining limb. This surgery can be done during the initial amputation or later as a revision procedure. This approach is somewhat like finding a person’s purpose in life—just as reconnecting muscles helps achieve a natural gait, finding one’s purpose can lead to a more fulfilling and harmonious life.

Restoring Natural Movement with AMI Surgery

“With the AMI amputation procedure, we connect the original muscle pairs in a natural way so that after the amputation, a person can move their phantom limb with normal sensation and range of motion,” says Herr.

In a 2021 study, Herr’s lab found that patients who had this surgery could control the muscles of their amputated limb more precisely, and the electrical signals from these muscles were similar to those from an intact limb.

Encouraged by these results, the researchers investigated whether these electrical signals could control a prosthetic limb and provide feedback about the limb’s position. This feedback allows the person to adjust their movement as needed.

In the new Nature Medicine study, the MIT team found that this sensory feedback resulted in a smooth, nearly natural walking ability and obstacle navigation.

“Thanks to the AMI neuroprosthetic interface, we were able to enhance neural signaling, preserving as much as possible. This restored a person’s ability to continuously and directly control their walking, even over different speeds, stairs, slopes, and obstacles,” says Song.

Achieving a Natural Walk with the Modern prosthesis

In most cases, movements of our limbs controlled by pairs of muscles that work together by stretching and contracting alternately. When someone has a below-the-knee amputation, these muscle pairs can’t work together as they should. This disrupts the nervous system’s ability to sense how a muscle positioned and how fast it’s moving — crucial information for the brain to coordinate limb movements.

People with this type of amputation often struggle to control their artificial limbs because they can’t feel exactly where the limb is in space. Instead, they rely on robotic controllers embedded in the prosthetic limb. These limbs also have sensors that detect and adjust to different terrains like slopes and obstacles.

To help people achieve a more natural walking pattern controlled by their nervous system, Herr and his team started developing the AMI surgery several years ago. Instead of cutting off natural muscle interactions, this surgery reconnects the ends of muscles so they can still communicate effectively within the remaining limb. This procedure may take place during the initial amputation or as a follow-up surgery later on.

AMI Operation

“With the AMI surgery, we observed natural, lifelike movements,” Herr explains. “In contrast, those without AMI could walk, but their prosthetic movements were less natural and generally slower.”

Even though the sensory feedback from AMI was less than 20% of what’s normal for people without amputations, it significantly improved the ability to control the prosthetic limb neurally. This allowed users to control walking speed directly, adapt to different surfaces, and navigate around obstacles.

Matthew Carty, a surgeon at Brigham and Women’s Hospital and associate professor at Harvard Medical School, who co-authored the study, emphasized that this research marks significant progress in restoring function for patients with severe limb injuries.

Herr’s lab aims to integrate neural control directly into prosthetic limbs, moving away from reliance solely on sophisticated robotic controllers and sensors. This approach aims to make prosthetics feel more like a natural part of the user’s body, enhancing embodiment and self-connection.

Read the Original Article MIT News

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