Passive Prosthetic Foot Shape and Size Optimization Using Lower Leg Trajectory Error

Abstract

A method is presented to optimize the shape and size of a passive, energy-storing prosthetic foot using the lower leg trajectory error (LLTE) as the design objective. The LLTE is defined as the root-mean-square error between the lower leg trajectory calculated for a given prosthetic foot's deformed shape under typical ground reaction forces (GRFs), and a target physiological lower leg trajectory obtained from published gait data for able-bodied walking. Using the LLTE as a design objective creates a quantitative connection between the mechanical design of a prosthetic foot (stiffness and geometry) and its anticipated biomechanical performance. The authors' prior work has shown that feet with optimized, low LLTE values can accurately replicate physiological kinematics and kinetics. The size and shape of a single-part compliant prosthetic foot made out of nylon 6/6 were optimized for minimum LLTE using a wide Bezier curve to describe its geometry, with constraints to produce only shapes that could fit within a physiological foot's geometric envelope. Given its single part architecture, the foot could be cost effectively manufactured with injection molding, extrusion, or three-dimensional printing. Load testing of the foot showed that its maximum deflection was within 0.3 cm (9%) of finite element analysis (FEA) predictions, ensuring the constitutive behavior was accurately characterized. Prototypes were tested on six below-knee amputees in India—the target users for this technology—to obtain qualitative feedback, which was overall positive and confirmed the foot is ready for extended field trials.

Biomechanical Evaluation of Prosthetic Feet Designed Using the Lower Leg Trajectory Error Framework

“…The LLTE framework presented here enabled the analysis of prosthetic feet throughout the entire stance phase, streamlining the design of customized prostheses for specific body size and desired walking activity compared to traditional design processes 12,47 . Integrating the structural analysis with the optimization process within MATLAB and extending the LLTE framework to the entire stance enabled the design of ESR prosthetic feet with both a flexible heel and keel, and improved LLTE values, compared to previous LLTE-optimized prostheses 19,20 . LLTE values of prosthetic feet optimized with the upgraded framework over the entire stance phase were 56% lower than when optimized over only mid-stance, resulting in improved anticipated walking performance over previously designed prostheses 19,20 .…”

“…Integrating the structural analysis with the optimization process within MATLAB and extending the LLTE framework to the entire stance enabled the design of ESR prosthetic feet with both a flexible heel and keel, and improved LLTE values, compared to previous LLTE-optimized prostheses 19,20 . LLTE values of prosthetic feet optimized with the upgraded framework over the entire stance phase were 56% lower than when optimized over only mid-stance, resulting in improved anticipated walking performance over previously designed prostheses 19,20 . The LLTE framework is not restricted to the parametric foot architecture used in this study and could be applied to any prosthetic foot architecture for which a structural model can be built, such as jointed passive prosthetic feet or powered prostheses.…”

“…1) 18 . Here we extended the calculation of the LLTE metric from the portion of step where the foot is flat on the ground 19 , mid-stance, to the entire step by including the heel-strike and late-stance portions of a step (Fig. 1d).…”

“…The lower leg trajectory error (LLTE) framework 18 is a novel design methodology to deterministically design user-specific prostheses by quantitatively connecting the mechanical characteristics of a prosthetic foot to the gait of an amputee. This methodology enables the systematic tuning of the mechanical properties of passive prosthetic feet (geometry and stiffness) to yield a desired biomechanical response, meet a target cost, and satisfy specific manufacturing requirements 19 . For a given user, a reference kinetic and kinematic walking dataset is scaled to the person's body characteristics (mass, height and foot length).…”

Design and Prototyping of a Shape-Changing Rigid-Body Human Foot in Gait

“…As previously stated, the primary purpose of this work was the embodiment of a data already exists. While methods of characterizing the stiffness of a prosthetic foot at off-joint locations utilizing mathematical models or previously generated data exist [52,53], these methods are generally better suited for design of single-body compliant feet.…”