Design of a Novel Mechatronic System to Test Prosthetic Feet Under Specific Walking Activity Loads and Evaluate their Lower Leg Trajectory Error

Abstract

Lower limb amputees, numbered at more than 40 million globally, are challenged with limited mobility due to prosthetic devices that do not fully restore the functionalities of their biological limbs. While commercially available energy storage and return feet do restore some of the functionalities of a missing limb, the development and use of these prosthetic devices are limited by the current design, evaluation, and prescription processes. This is because the connection between the combined mechanical characteristics of a foot and user outcomes, such as mobility, comfort, and walking effort, is not fully understood.

The lower leg trajectory error (LLTE) is a novel prosthetic foot performance metric that provides a quantitative connection between the mechanical characteristics of a foot and the expected gait of an amputee. Thus far, the LLTE value of a foot has only been calculated via simulation, which limits the practical use of the metric in prosthetic foot design, evaluation, and prescription. One way to systematically measure the LLTE value of a physical prosthetic foot would be through a mechanical bench test, but the capabilities of existing bench testing devices are insufficient due to limited degrees of actuation and reported accuracy.

The purpose of this work was to design the Prosthetic Foot Testing Device (PFTD), a mechatronic testing device that could apply specific and uncoupled GRFs to any CoP on a foot and measure its deflection, through which it could measure the LLTE value and thus predict walking performance of any passive prosthetic foot. First, we determined high-level functional requirements of the PFTD, including the ranges of reference loads and prosthetic foot deflections as well as the LLTE measurement accuracy, such that the PFTD could meaningfully measure the full range of commercially available prosthetic feet. Second, we derived the relationships between the variables used to calculate the LLTE metric and those controlled or measured by the PFTD. Third, we used these relationships to design the PFTD and perform sensitivity analysis to ensure it could meaningfully and accurately measure the LLTE value of any passive prosthetic foot. In future work, the PFTD will be built, validated, and used to measure and compare the LLTE values of various prosthetic feet. The PFTD and theory presented herein may become a new tool in the prosthetics industry to systematically and amputee-independently measure and compare the performance of prosthetic devices using the LLTE value as a universal metric, which could ultimately improve the development and prescription processes of prostheses.