Development and validation of a novel framework for designing and optimizing passive prosthetic feet using lower leg trajectory

Abstract

This thesis presents a novel framework to optimize the design of passive prosthetic feet to best replicate physiological lower leg trajectory under typical ground reaction forces. The goal of developing this framework is ultimately to design a low cost, mass manufacturable prosthetic foot for persons with amputations living in the developing world. Despite a vast body of literature on prosthetic foot design, there is a dearth of knowledge regarding how the mechanical characteristics of passive prosthetic feet affect their biomechanical performance. Without understanding this relationship, the design of a prosthetic foot cannot be optimized for peak performance as measured by gait symmetry, metabolic cost of walking, or subjective feedback. The approach to designing prosthetic feet introduced here involves predicting the lower leg trajectory for a given prosthetic foot under typical loading and comparing this modeled trajectory to target physiological gait kinematics with a novel metric called the Lower Leg Trajectory Error (LLTE). The usefulness of this design approach was demonstrated by optimizing three simple conceptual models of prosthetic feet, each with two degrees of freedom. An experimental prosthetic foot with variable ankle stiffness was built based on one of these analytical models and tested by a subject with unilateral transtibial amputation in a gait lab under five different ankle stiffness conditions. Across five prosthetic-side steps with each of the five ankle stiffness conditions, the constitutive model used in the optimization process accurately predicted the horizontal and vertical position of the knee throughout stance phase to within an average of 1.0 cm and 0.3 cm, respectively, and the orientation of the lower leg segment to within 1.5°. After validating the theory behind this approach with the simple conceptual foot models, a method was developed to implement the same approach in optimizing the shape and size of a single-part compliant foot, resulting in a lightweight, easy to manufacture, low cost prosthetic foot. The optimal prosthetic foot design was built and tested qualitatively on six subjects in India with unilateral transtibial amputations with promising preliminary results..