Cheap prosthetic knee designed by Indian-origin scientist

Scientists, including those of Indian-origin, have designed a cheap prosthetic knee that mimics normal walking motion and are testing the prototype in India, where about 230,000 above-knee amputees currently live.

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 USD 50,000.

Amos Winter, an assistant professor of mechanical engineering at Massachusetts Institute of Technology, is developing a low-tech prosthetic knee that performs nearly as well as high-end prosthetics, at a fraction of the cost.

Winter and his colleagues have 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.

In a paper published in IEEE Transactions on Neural Systems and Rehabilitation Engineering, the team reported that it 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 prototype is being tested in India, where about 230,000 above-knee amputees currently live, researchers said.

“In places like India, there’s still stigma associated with this disability. They may be less likely to get a job or get married,” Winter said.

The paper’s co-authors include graduate student Murthy Arlekatti and Yashraj Narang, a PhD student at Harvard University.

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 said.

In part, that is 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 centre 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.