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Researchers at MIT have created an innovative bionic knee that assists individuals with above-the-knee amputations in walking more swiftly, ascending stairs, and maneuvering around obstacles more efficiently compared to conventional prosthetic devices.

In contrast to prostheses where the residual limb fits into a socket, this new mechanism is seamlessly linked with the individual’s muscle and bone structure. This design affords enhanced stability and provides the user with significantly greater control over the prosthesis’s movements.

Participants in a limited clinical trial indicated that the limb felt much more integrated with their own body, particularly in comparison to individuals utilizing traditional above-the-knee amputations.

“A tissue-integrated prosthesis — fixed to the bone and directly regulated by the nervous system — is not just a lifeless, detached apparatus, but a system that is intricately connected to human physiology, delivering a higher level of prosthetic integration. It’s more than just a tool that an individual uses; it’s a fundamental aspect of self,” states 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 newly published study.

Tony Shu PhD ’24 serves as the primary author of the article, which is released today in Science.

Enhanced control

In recent years, Herr’s lab has been developing advanced prosthetic solutions that can retrieve neural signals from muscles remaining after an amputation, utilizing this information to guide a prosthetic limb.

During a standard amputation, pairs of muscles that alternate between stretching and contracting are typically severed, disrupting the usual agonist-antagonist dynamic of the muscles. This disturbance complicates the nervous system’s ability to perceive the muscles’ position and contraction speed.

With the innovative surgical method formulated by Herr and his team, called the agonist-antagonist myoneuronal interface (AMI), muscle pairs are reconnected during surgery to maintain their dynamic communication within the residual limb. This sensory feedback assists the prosthesis wearer in determining how to move the limb and generates electrical impulses that can be harnessed to control the prosthetic device.

2024 study, researchers demonstrated that individuals with below-the-knee amputations who underwent AMI surgery could walk faster and navigate around obstacles in a much more natural manner than those with standard below-the-knee prosthetics.

In the latest research, they adapted this method to better assist individuals with above-the-knee amputations. Their objective was to create a system capable of not only interpreting signals from muscles via AMI but also integrating into the bone for enhanced stability and improved sensory feedback.

To accomplish this, the researchers formulated a technique to insert a titanium rod into the residual femur at the site of amputation. This implant provides superior mechanical control and load-bearing capabilities compared to traditional prosthetics. Additionally, the implant features 16 wires that gather data from electrodes positioned on the AMI muscles within the body, enabling more precise conversion of the signals derived from the muscles.

This bone-integrated system, referred to as e-OPRA, relays AMI signals to a newly designed robotic controller tailored specifically for this study. The controller utilizes this data to compute the torque required to maneuver the prosthesis as desired by the user.

“All components work cohesively to enhance the transfer of information in and out of the body and to better mechanically interact with the device,” Shu explains. “We’re directly applying load to the skeleton, which is meant to bear weight, instead of utilizing sockets that can be uncomfortable and lead to frequent skin infections.”

In this study, two subjects received the combined AMI and e-OPRA system, known as an osseointegrated mechanoneural prosthesis (OMP). These individuals were compared to eight participants who had undergone AMI surgery without the e-OPRA implant, and seven individuals who had neither AMI nor e-OPRA. All subjects had a turn using an experimental powered knee prosthesis developed by the lab.

The researchers evaluated the participants’ capabilities in performing various tasks, including bending the knee to a specific angle, ascending stairs, and stepping over obstacles. For the majority of these tasks, individuals with the OMP system outperformed those with AMI surgery alone and showed significantly better results than users of traditional prosthetic devices.

“This article signifies the realization of a vision the scientific community has held for an extended period — the implementation and demonstration of a fully physiologically integrated, volitionally controlled robotic leg,” remarks Michael Goldfarb, a professor of mechanical engineering and director of the Center for Intelligent Mechatronics at Vanderbilt University, who was not involved in the study. “This is exceptionally challenging work, and the authors deserve immense recognition for their success in achieving such a challenging objective.”

A sense of integration

In addition to analyzing gait and other movements, the researchers posed questions aimed at assessing participants’ sense of embodiment — specifically, the degree to which their prosthetic limb felt like an inherent part of their body.

Questions incorporated whether the patients felt as if they possessed two legs, sensed the prosthesis as an extension of themselves, and felt in command of the prosthetic limb. Each inquiry aimed to evaluate the participants’ feelings of agency, ownership of the device, and body representation.

The research team found that over time, the two individuals with the OMP experienced significantly greater increases in feelings of agency and ownership compared to the other subjects.

The AMI procedure is now routinely performed on patients with below-the-knee amputations at Brigham and Women’s Hospital, and Herr anticipates it will soon be adopted for above-the-knee amputations as well. The combined OMP system will require larger clinical trials to secure FDA approval for commercial deployment, which Herr estimates could take around five years.

This research received support from the Yang Tan Collective and DARPA.

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