Development and Validation of a Passive Prosthetic Foot Design Framework based on Lower Leg Dynamics

Abstract

People with lower limb amputations face considerable challenges to everyday mobility that affect their quality of life. This is especially the case in low and middle income countries (LMIC) where the lack of affordable high-performance prosthetic devices forces people to use inadequate limbs that require more effort and exhibit unnatural walking motions. This thesis develops methods for designing customized, high-performance, low-cost, and durable passive prosthetic feet that enable users to replicate able-bodied walking patterns.

The current development process of prosthetic feet relies on extensive user testing and iterative design rather than a predictive and quantitative design methodology that would facilitate the development of improved prosthetic devices. Here, we further developed the lower leg trajectory error (LLTE) framework, a novel design methodology that connects the mechanical characteristics of a prosthetic foot to the user's walking pattern. We extended the methodology to describe the entire prosthetic step for multiple walking activities and foot architectures, including durability requirements, and efficient constitutive modelling of prosthetic foot designs. These developments resulted in more than a two-fold improvement in the walking performance of LLTE-designed prosthetic feet that fulfilled the international standards durability requirements, and a ten-times reduction in computational time compared to the original LLTE methodology. The LLTE design framework and foot architectures described in this work should provide designers, engineers, and clinicians with a practical, predictive, and quantitative tool for designing and evaluating prosthetic feet.

Using the LLTE framework, low-cost, customized passive prosthetic feet prototypes were designed and clinically evaluated for level ground walking against conventional carbon fiber prostheses. The LLTE feet performed as predicted with no iteration for a wide variety of patients. In addition, these prosthetic feet demonstrated 14% closer replication of able-bodied walking motion, 46% higher propulsion, 13% lower peak leg loading, and higher user preference compared to a standard commercial carbon fiber foot for less than a tenth of its cost. These results suggest that the LLTE framework can be used to design customized, low-cost prostheses that enable able-bodied walking pattern, with reduced effort and risk of long-term injuries.

A systematic sensitivity investigation of five foot prototypes designed using the LLTE framework showed that users' most closely replicated the target able-bodied walking pattern with the predicted LLTE-optimal foot, experimentally demonstrating that the predicted optimum was a true optimum. In addition, the predicted LLTE performance of the prototype feet was correlated to the user’s ability to replicate the target walking pattern, user's preference, and conventional clinical outcomes. This sensitivity study illustrated the utility of the LLTE framework as an systematic and robust evaluation methodology for prosthetic feet, potentially improving the development and prescription of prosthetic devices.

A rugged prosthetic foot with a cosmetic overmold was also designed using the LLTE framework to accommodate the economic, environmental, and cultural requirements for users in India. The foot was distributed to 16 prosthetic users in India to be used for several months. Users walked 16% faster with the foot compared to their daily-use prosthesis, the Jaipur foot, and commented on the reduced effort of walking. The rugged foot endured one million cycles of fatigue testing, and the wear and tear of daily living without alterations in its mechanical performance. This mass-manufacturable, high-performance rugged foot could replace conventional feet used in low-resource settings and significantly improve the mobility and quality of life of LMIC prosthesis users.