Two generations of the single-part, low-cost prosthetic foot made from nylon.
Photograph courtesy of the study’s authors.
A research team at the Massachusetts Institute of Technology (MIT) has produced a customized prosthetic foot that can be inexpensively mass-produced and mimics an able-bodied gait. The shape and stiffness of the prosthetic foot is based on the user’s height and weight, and additional fine tuning by a prosthetist produces a gait that is similar to someone without an amputation.
Instead of designing a prosthetic foot to replicate the motions of an able-bodied foot, the researchers looked to design a prosthetic foot that would produce lower-leg motions similar to an able-bodied person’s lower leg as they walk. The team calculated how varying the mechanics of the foot affected movement of the lower leg. The mathematical model describes the stiffness, possible motion, and shape of the foot by calculating the lower leg trajectory error (LLTE). (The LLTE is the root-mean-square error between the lower-leg trajectory calculated for a given prosthetic foot’s deformed shape under typical ground reaction forces, and a target physiological lower-leg trajectory that they used from published gait data that monitored steps of people without amputations.) The authors’ prior work has shown that feet with optimized LLTE values can accurately replicate physiological kinematics and kinetics.
To determine the ideal fit, the prosthetist would use height and weight data from a table to choose the initial model and then adjust it to the user’s preferred level of stiffness or compliance.
“Particularly in limb fitment camps, where a patient may interact with a technician for only a matter of minutes to have a foot fitted, there is a lot of value in a lookup table where the technician can choose a foot based on the patient size and bodyweight,” Amos Winter, PhD, an assistant professor in the Department of Mechanical Engineering at MIT, told the Alliance of Advanced BioMedical Engineering. “But we also have to account for user-centered preferences. Some people may prefer a stiffer or more compliant foot, so there might be some iteration in the process. The lookup table could provide an accurate first guess, and then the prosthetist could use their experience and patient feedback to dial in the correct stiffness.”
The team conducted load testing of the foot and tested prototypes on six people with transtibial amputations in India—the target users for the technology; the feedback confirmed for the researchers that the foot is ready for extended field trials, which are expected to begin in 2019.
“The size of the foot is dictated by the user; we want the foot length to match their physiological foot length,” Winter said. “The stiffness (and thickness) of the foot is dictated by our design framework, where we can tune the compliance of the foot, so it bends enough during walking to facilitate near able-bodied motion and loads.”
The single-part prosthesis is made of nylon 6/6, which the researchers say can be cost effectively manufactured with injection molding, extrusion, or 3D printing.
“In wealthier markets, we are confident we can still make pretty low-cost, specifically customized feet if they are made of plastic…,” Winter said. “For higher segments of the market we could possibly also offer custom carbon fiber feet, which would be lighter but obviously more expensive.”
The project began in 2012 when the makers of the Jaipur Foot asked Winter to design a better, lighter foot that could be mass-produced at low cost. In 2017, he received a $500,000 five-year grant from the National Science Faculty Early Career Development (CAREER) Program for the project. A coauthor of the study, PhD candidate Kathryn Olesnavage, received a $15,000 2017 Lemelson-MIT Student Prize for her work on the design.
The results are published in ASME’s Journal of Mechanical Design.
Editor’s note: This story was adapted from materials provided by MIT.
