Massachusetts Institute of Technology (MIT) engineers have developed a simple, low-cost, passive prosthetic foot that can be tailored to an individual. Given a user’s body weight and size, the researchers can tune the shape and stiffness of the prosthesis, such that the user’s walk is similar to an able-bodied gait.
In 2012, soon after Amos Winter, PhD, an associate professor of mechanical engineering, joined the MIT faculty, he was approached by Jaipur Foot, a manufacturer of passive prosthetic feet, based in Jaipur, India. The Jaipur Foot is geared toward amputees in developing countries. It is rugged, suitable for outdoor use, and relatively life-like, but it is heavy and the handmade internal structure results in a big variation in product quality, said Winter. The organization asked Winter whether he could design a better, lighter foot that could be mass-produced at low cost.
Instead of designing a prosthetic foot to replicate the motions of an able-bodied foot, he and former graduate student Kathryn Olesnavage sought to design a prosthetic foot that would produce lower-leg motions similar to those of an able-bodied person’s lower leg as they walk. The team looked for ways to relate how foot mechanics relate to how the lower leg moves while the foot is in contact with the ground. To do this, the researchers consulted an existing dataset comprising measurements of steps taken by an able-bodied walker with a given body size and weight. With each step, previous researchers had recorded the ground reaction forces and the changing center of pressure experienced by a walker’s foot as it rocked from heel to toe, along with the position and trajectory of the lower leg.
Winter and his colleagues developed a mathematical model of a simple, passive prosthetic foot that describes the stiffness, possible motion, and shape of the foot. They plugged into the model the ground reaction forces from the dataset, which they could sum up to predict how a user’s lower leg would translate through a single step. With their model, they then tuned the stiffness and geometry of the simulated prosthetic foot to produce a lower-leg trajectory that was close to the able-bodied swing.
The team then sought to identify an ideal shape for a single-part prosthetic foot that would be simple and affordable to manufacture, while still producing a leg trajectory very similar to that of able-bodied walkers. The resulting foot shape looked similar to the side-view of a toboggan. Prototypes were made from machined nylon, a material chosen for its energy-storage capability.
“What’s cool is, this behaves nothing like an able-bodied foot — there’s no ankle or metatarsal joint — it’s just one big structure, and all we care about is how the lower leg is moving through space,” Winter says. “Most of the testing was done indoors, but one guy ran outside, he liked it so much. It puts a spring in your step.”
Going forward, the team has partnered with Vibram, an Italian company that manufactures rubber outsoles. Vibram is designing a lifelike covering for the prosthesis that will also give the foot some traction on muddy or slippery surfaces.