MIT develops bionic knee to help amputees restore natural movement

By Stephen Beech

The Six Million Dollar Man is a step nearer reality after scientists successfully integrated a bionic knee into tissue to restore natural movement.

American researchers developed a new bionic knee that can help people with above-the-knee amputations walk faster, climb stairs, and avoid obstacles more easily than they could with a traditional prosthesis.

Unlike prostheses in which the residual limb sits within a socket, the new system is directly integrated with the user’s muscle and bone tissue.

That enables greater stability and gives the user much more control over the movement of the artificial limb, according to a study published in the journal Science.

Participants in the clinical study also reported that the limb felt more like a part of their own body, compared to people who had more traditional above-the-knee amputations.

Senior author Dr. Hugh Herr, of Massachusetts Institute of Technology (MIT), said: “A prosthesis that's tissue-integrated - anchored to the bone and directly controlled by the nervous system - is not merely a lifeless, separate device, but rather a system that is carefully integrated into human physiology, offering a greater level of prosthetic embodiment.

"It’s not simply a tool that the human employs, but rather an integral part of self.”

In the cult 1970s TV show pilot Steve Austin is fatally injured in an airplane crash.

But the government spends six million dollars on bionic implants in order to rebuild Austin, giving him superhuman abilities.

Dr. Herr’s lab has been working on new technology that can extract neural information from muscles left behind after an amputation - and use that information to help guide a prosthetic limb.

He explained that, during a traditional amputation, pairs of muscles that take turns stretching and contracting are usually severed, disrupting the normal "agonist-antagonist" relationship of the muscles.

Dr. Herr says the disruption makes it "very difficult" for the nervous system to sense the position of a muscle and how fast it’s contracting.

Using the new surgical approach developed by Dr. Herr and his team - known as agonist-antagonist myoneuronal interface (AMI) - muscle pairs are reconnected during surgery so that they still dynamically communicate with each other within the residual limb.

He explained that the sensory "feedback" helps the wearer of the prosthesis to decide how to move the limb, as well as generating electrical signals that can be used to control the prosthetic limb.

In a 2024 study, the researchers showed that people with amputations below the knee who received the AMI surgery were able to walk faster and navigate their way around obstacles much more naturally than people with traditional below-the-knee amputations.

In the new study, the team extended the approach to better serve people with amputations above the knee.

They wanted to create a system that could not only read out signals from the muscles using AMI but also be integrated into the bone, offering more stability and better sensory feedback.

The researchers developed a procedure to insert a titanium rod into the residual femur bone at the amputation site.

They say the implant allows for better mechanical control and load bearing than a traditional prosthesis.

The implant also contains 16 wires that collect information from electrodes located on the AMI muscles inside the body, which enables more accurate transduction of the signals coming from the muscles.

This bone-integrated system - known as e-OPRA - transmits AMI signals to a new robotic controller developed specifically for the study.

The controller uses the information to calculate the torque necessary to move the prosthesis the way that the user wants it to move.

Study lead author Dr. Tony Shu said: “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.”

Two people received the combined AMI and e-OPRA system, known as an osseointegrated mechanoneural prosthesis (OMP), as part of the new study.

They were compared with eight who had the AMI surgery but not the e-OPRA implant, and seven others who had neither AMI nor e-OPRA.

All the participants took a turn at using an experimental powered knee prosthesis.

The researchers measured the participants’ ability to perform several tasks, including bending the knee to a specified angle, climbing stairs, and stepping over obstacles.

In most of the tasks, participants with the OMP system performed better than the people who had the AMI surgery but not the e-OPRA implant, and much better than users of traditional prostheses.

The researchers also asked questions designed to evaluate the participants’ sense of "embodiment" - the extent to which their prosthetic limb felt like a part of their own body.

The research team found that as the study went on, the two participants with the OMP showed "much greater" increases in their feelings of agency and ownership than the others.

Dr. Herr added: “Another reason this paper is significant is that it looks into these embodiment questions and it shows large improvements in that sensation of embodiment.

“No matter how sophisticated you make the AI systems of a robotic prosthesis, it’s still going to feel like a tool to the user, like an external device.

"But with this tissue-integrated approach, when you ask the human user what is their body, the more it’s integrated, the more they’re going to say the prosthesis is actually part of self.”

The AMI procedure is now done routinely on patients with below-the-knee amputations at Brigham and Women’s Hospital, and Dr. Herr expects it will soon become the standard for above-the-knee amputations as well.

He believes the combined OMP system could be in full commercial use within five years.