In the last two decades, prosthetic limb technology has grown, with the most advanced prostheses incorporating microprocessors that work with onboard gyroscopes, accelerometers, and hydraulics to enable a person to walk with a normal gait. Amos Winter, PhD, an assistant professor of mechanical engineering at Massachusetts Institute of Technology (MIT) 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.
“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,” said Winter.
Winter and his colleagues said they have taken a significant step toward that goal. In a paper published in IEEE 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. Using the paper’s results, the group built a prototype of a prosthetic knee that generates a torque profile similar to that of able-bodied knees using only simple mechanical elements. The prototype 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.
Most individuals with amputations in developing countries wear passive prostheses. “When you see people walk in them, they have a pretty distinctive limp,” Winter said. 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 causes a person to bob up and down with each step.
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 said. “I’m not ready to claim victory yet, but [this paper] lays out a road map that is very different than what’s been done before, which will enable us to achieve very high performance at low cost….”
This research was funded, in part, by the National Science Foundation. The paper’s co-authors include graduate student Murthy Arlekatti and Yashraj Narang, a doctoral student at Harvard University.
Editor’s note: This story was adapted from materials provided by the Massachusetts Institute of Technology.