"We're going after this disruptive opportunity," says Winter. "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."
The team has built a prototype of a prosthetic knee that it claims 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."
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 during a single step.
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?"
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.
"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 roadmap that is very different than what's been done before, which will enable us to achieve very high performance at low cost."
