For decades, above-the-knee amputees have had to live with stiff, uncomfortable prosthetics that felt more like rigid tools than natural limbs. As a result, climbing stairs, stepping over objects, or even just walking at a decent pace becomes a constant struggle.
However, MIT researchers have developed a bionic knee so well-integrated with the body that users begin to feel it as part of themselves, not just mechanically, but neurologically too. In early clinical trials, users with this new prosthesis walked faster, navigated obstacles more naturally, and even reported a sensation of owning the limb, as if it were truly their own.
“A prosthesis that’s tissue-integrated, anchored to the bone, and directly controlled by the nervous system is not merely a lifeless, separate device. It’s not simply a tool that the human employs, but rather an integral part of self,” Hugh Herr, one of the researchers and a professor at MIT, said.
This innovation could mark a turning point in bionics, a field that has long promised lifelike prosthetics but often failed to match the human body’s complexity.
Integrating robotic parts with the human body
The MIT team developed a system called the osseointegrated mechanoneural prosthesis (OMP), which is the result of years of effort to restore not just movement, but the feeling of having a real leg again.
Traditional prosthetic limbs rely on sockets that wrap around the residual limb. These sockets are often uncomfortable, can cause skin infections, and offer limited stability or control. More importantly, they do not tap into the nervous system’s natural signals, so the prosthesis remains just a passive, lifeless tool.
OMP, on the other hand, makes use of a surgical technique known as the agonist-antagonist myoneuronal interface (AMI), which helps reconnect muscle pairs in the residual limb to improve communication with a bionic knee. During a typical amputation, pairs of muscles that once worked together, stretching and contracting in coordination, are severed. This destroys the muscle-to-muscle communication that the brain depends on to sense movement.
In the AMI technique, these muscle pairs are reconnected during surgery, allowing them to work dynamically even after amputation. This not only restores internal feedback to the brain, helping users feel their muscles again, but also creates a reliable stream of electrical signals that can be used to control a robotic limb.
They also added another key feature, a titanium rod implanted directly into the femur bone, replacing the need for a socket. This method, known as osseointegration, provides a stable, load-bearing base for the prosthetic limb, closer to how a real bone supports the body.
Moreover, this titanium implant houses 16 wires connected to electrodes embedded in the AMI muscles. These electrodes collect neural signals and send them to a custom-built robotic controller, which calculates how much torque is needed to move the knee the way the user intends. This entire setup—muscles, nerves, bone, implant, and robotic limb—creates a seamless flow of information between the body and the bionic knee.
“All parts work together to better get information into and out of the body and better interface mechanically with the device. We’re directly loading the skeleton, which is the part of the body that’s supposed to be loaded, as opposed to using sockets, which is uncomfortable and can lead to frequent skin infections,” Tony Shu, lead researcher and a graduate student at MIT, said.
Testing the tissue-integrated bionic knee
To test their bionic implant, the researchers worked with 17 participants. Two had both the AMI surgery and the e-OPRA bone implant (the full OMP system), eight had AMI alone, and seven had neither.
All participants tried out the same experimental powered knee developed by MIT. In movement tests, such as climbing stairs, stepping over barriers, or bending the knee to precise angles, the OMP users consistently outperformed the others. What’s even more exciting is that those with the OMP system reported a much higher sense of agency and ownership, saying the prosthesis felt like a natural part of their body.
“When you ask the human user what their body is, the more it’s integrated, the more they’re going to say the prosthesis is actually part of the self,” Herr notes.
These results could reshape how prosthetics are designed, moving away from the idea of external tools toward fully integrated body systems. For amputees, it offers more than just faster walking or better balance; it brings back something far more personal, which is the feeling of having a leg.
The next step is to test the full OMP system on a larger group of participants and receive FDA approval. This process could take a few years, but given the OMP’s potential, the wait may well be worth it.
The study is published in the journal Science.
