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ATKnee: low cost prosthetic knee

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The Global Engineering and Research – GEAR Lab – focuses on the marriage of mechanical design theory and user-centered product design to create simple, elegant technological solutions for use in highly constrained environments. Our technologies are aimed at making a positive impact on the world and elucidating novel scientific/engineering knowledge. Professor Amos Winter, director of this giant and made history as the main inventor of the LFC, the Leveraged Freedom Chair – an off-road wheelchair designed with the noble aim of generating awareness and continue the war for the elimination of architectural barriers to the benefit of people with disabilities so that they can move freely outside the home – he has always been interested in promoting research areas that focus on emerging markets in developing countries.

web.mit.edu

Last work of GEAR Lab, at a very high social impact lies in the realization of ATKnee – All Terrain Knee – a prosthetic knee high performance and low cost totally. The focus of the ATKnee project is to create a low-cost, high-performance prosthetic knee that uses only passive mechanical elements to generate a normal walking gait. The ATKnee is being designed to meet the mobility and stability needs of above-knee amputees in developing countries and offer improved performance over locked and free-swinging joints. The project includes investigating the fundamental biomechanics of transtibial amputees and codifying how changes in lower leg and foot mass affect desired knee torque and hip energy output throughout the gait cycle. With this insight, we are optimizing simple, passive mechanical elements, such as springs and dampers, to create the correct knee torque to induce desired gait kinematics. Our aim is to provide similar levels of performance as high-end, active-controlled knees at a fraction of the cost, and make a prosthetic technology that will be adopted in developing and developed markets.

web.mit.edu

In the last two decades, prosthetic limb technology has grown by leaps and bounds. Today, the most advanced prostheses incorporate microprocessors that work with onboard gyroscopes, accelerometers, and hydraulics to enable a person to walk with a normal gait. Such top-of-the-line prosthetics can cost more than $50,000. Amos Winter is aiming to develop a passive, low-tech prosthetic knee that performs nearly as well as high-end prosthetics, at a fraction of the cost.
“We’re going after this disruptive opportunity,” says Winter, an assistant professor of mechanical engineering at MIT. “If we can make a knee that delivers similar performance to a $50,000 knee for a few hundred dollars, that’s a game-changer.”

web.mit.edu

Now Winter and his colleagues have taken a significant step toward that goal. In a paper published in IEEE’s Transactions on Neural Systems and Rehabilitation Engineering, the team reports that it has calculated the ideal torque that a prosthetic knee should produce, given the mass of the leg segments, in order to induce able-bodied kinematics, or normal walking.
Using the paper’s results, the group has built a prototype of a prosthetic knee that generates a torque profile similar to that of able-bodied knees, using only simple mechanical elements like springs and dampers. The team is testing the prototype in India, where about 230,000 above-knee amputees currently live.
“In places like India, there’s still stigma associated with this disability,” Winter says. “They may be less likely to get a job or get married. People want to be incognito if they can.”
The paper’s co-authors include graduate student Murthy Arlekatti and Yashraj Narang, a PhD student at Harvard University.

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Most amputees in developing countries wear passive prostheses — simple, cheap designs with no moving parts. “When you see people walk in them, they have a pretty distinctive limp,” Winter says. In part, that’s because passive prostheses do not adjust the amount of torque exerted as a person walks. For instance, in normal walking, the knee flexes slightly, just before the foot pushes off the ground — a shift in torque that keeps a person’s center of mass steady. In contrast, a stiff, unbending prosthetic knee would cause a person to bob up and down with each step.
Winter reasoned that in order to produce a passive prosthetic knee that mimics normal walking, he would have to also mimic the changing forces, or torque profile, during normal walking. He and his team looked through the scientific literature for data on normal walking, and found a complete dataset that represented one person’s gait, including the angle of their joints, the weight of each leg segment, and the ground reaction force — the force between the ground and the foot — during a single step, or gait cycle. The researchers used the measurements to calculate a torque profile — the amount of torque generated by the knee during normal walking. As prostheses are generally one-third to one-half as heavy as human legs and feet, the researchers adjusted the torque profile to apply to lighter leg segments.
“If you applied healthy levels of torque to a much lighter limb, your kinematics would get all screwed up,” Winter says. “Robotic limbs are designed to dial that torque back. Our challenge was, how do you tune the torque profile to get able-bodied motion, with a passive prosthetic knee?”

web.mit.edu

The researchers then looked at whether they could build a prosthetic knee to replicate the adjusted torque profile, using simple mechanical elements. Currently, the group has engineered a simple prototype that includes a spring and two dampers that act as brake pads. The spring allows the knee to bend just before the foot pushes off the ground. At the same time, the first damper engages to prevent the leg from swinging back. The second damper engages as the leg swings forward, in order to slow it down just before the heel strikes the ground.
Matthew Major, an assistant professor of physical medicine and rehabilitation at Northwestern University, says the challenge for prescribing clinicians and prosthetic designers is to customize prosthetics for individual users’ kinematics. Winter’s work, he says, “enhances our ability to provide more personalized health care through robust and cost-effective technology that is available to a vast percentage of the world’s population living with lower-limb loss.”
Winter’s team is now testing the design with volunteers in India.
“This was a quick and dirty prototype, but so far, we’re seeing good indicators of natural gait,” Winter says. “I’m not ready to claim victory yet, but [this paper] lays out a roadmap that is very different than what’s been done before, which will enable us to achieve very high performance at low cost. And that’s what we’re going after.”

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