Ever since Hugh Herr lost both his legs to a rock-climbing accident, he’s been on a quest to design replacement limbs that feel like the real thing.
It’s now possible to engineer light-weight custom lower legs with flexible ankles that fit onto a residual limb, commonly known as the stump. These prostheses allow people with lower-limb amputations to walk again. But they tend to fall more often and struggle to climb stairs, walk on uneven surfaces, or simply get out of a chair. The limbs also feel like attachments, rather than natural extensions of the body.
These problems are worse for people with above-knee amputations. Our knees support our body weight but can also bend and twist when jumping, twirling, hopping, or running.
This month, Herr and his team at MIT introduced a “bionic” leg capable of complex movements. The system is attached to the user’s leg bone for stability and records electrical activity from nearby muscles to translate intent into smooth movement.
Compared to people with traditional above-the-knee prostheses, those fitted with the bionic knee were far more agile. The two volunteers could easily stand up from a sitting position, step over stacked blocks, and kick a ball with minimal upper body support.
More importantly, both said the bionic leg felt like part of their body.
“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,” said Herr in a press release. “It’s not simply a tool that the human employs, but rather an integral part of self.”
Multiple Upgrades
Your knee is like the Grand Central Station of nerves and muscles. Signals from the brain to walk, jump, or climb activate opposing groups of muscles involved in movement while keeping the joint stable. These connections form what’s called proprioceptive sensation. They tell us where our lower limbs are in space, even when our eyes are closed, and are vital for balance when standing or walking. But amputation often severs the link, making it hard for the brain to sense how much each muscle is contracting, its exact location, and if it’s in balance with others.
“This perception is lost after amputation,” wrote Lee Fisher at the University of Pittsburgh, who was not involved in the study. “A lower-limb amputation removes the coupling between muscle and bone that is necessary for driving the neural activity associated with proprioception.”
To get around this, Herr’s group updated their approach to surgical amputation, which for the most part, hasn’t changed since the turn of the 19th century. The new technique—called AMI, for agonist-antagonist myoneural interface—reconnects severed muscle pairs in the residual limb. A small piece of muscle is taken from another part of the body and attached to nerves in the stump. It’s a bit like adding a new cable to connect two broken ones. When the muscle pairs twitch, the new circuit sends out electrical signals. These signals are captured by electrodes and used to control the movements of the prosthetic limb.
In a 2014 study, seven people with below-the-knee amputations underwent AMI surgery. Their proprioception improved, and they were able to increase their walking speed to the point that they roughly matched people without amputation.
Hardware was the next part of the solution. Most above-knee prostheses have a custom socket fitting the mechanical leg to the stump. The socket must limit pressure, which could rub and break down the skin, while maintaining a solid mechanical connection.
“Amputees often recognize improved socket fit as a major priority for enhancing their quality of life,” wrote Fisher.
A popular alternative attaches the prosthesis to a metallic rod embedded in the stump’s residual bone. This more rigid connection makes the prosthesis more stable.
Although it’s more invasive, “this approach has been increasingly adopted by patients because it requires fewer follow-up visits and achieves better walking ability,” wrote Fisher.
The new prosthetic leg combines both of these advances into a bionic leg with more stability and better sensory feedback. The team inserted a titanium rod into the upper leg bone of two people who had already undergone AMI surgery. The leg’s sixteen electrodes capture muscle activity and send it to a computer, which calculates how to move the leg as intended.
The two volunteers tested out their new limb by climbing stairs, kicking balls, sitting and standing, and stepping over obstacles. They handled these tasks far better than those with older hardware or only AIM surgery.
Less Pain, More Gain
A common problem after amputation is phantom pain, where people feel their missing limb is still there, causing pain. It’s difficult to treat with painkillers. Surgically reconnecting amputated nerves to healthy muscles has had some early success, but the surgery is difficult. Surprisingly, the bionic leg eased pain in both participants, likely due to its connection with their muscles.
The team also heavily focused on how natural the leg felt. They asked all participants, including those who only underwent AMI surgery and those with traditional prosthetics, if the artificial leg felt like their own. Did it feel like they had two functional legs, for example, and did they feel in control of their prosthetics? Both people outfitted with the new bionic leg had a greater sense of embodiment—the prosthesis felt like part of their body—than the other two groups.
“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,” said Herr. “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.”
AMI surgery is already approved for below-knee amputation. The team is testing their new design in more patients and aiming for approval of their above-knee bionic leg within five years.
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