A Novel Framework for Quantitatively Connecting the Mechanical Design of Passive Prosthetic Feet to Lower Leg Trajectory

Abstract

This paper presents a novel framework that quantitatively connects the mechanical design of a prosthetic foot to its anticipated biomechanical performance. The framework uses kinetic inputs (ground reaction forces and center of pressure) to predict kinematic outputs of the lower leg segment by knowing the geometry and stiffness of the foot. The error between the predicted and target kinematics is evaluated using a root-mean-square error function called the Lower Leg Trajectory Error (LLTE). Using physiological kinetic inputs and kinematic targets, three model foot architectures were optimized to minimize the LLTE. The resulting predicted lower leg kinematics were compared to those of the same foot architectures optimized for physiological roll-over geometry. The feet with minimized LLTE had lower leg kinematics closer to physiological than those optimized for roll-over geometry. A prosthetic foot that exactly mimics physiological roll-over geometry may result in gait kinematics that differ greatly from physiological, as roll-over geometry omits information about the foot-ground contact constraint, lower leg orientation, and temporal progression of a step. The LLTE-based framework is agnostic to specific foot designs provided their constitutive behavior can be characterized, and it can accept alternate inputs and targets depending on what performance and clinical objectives are desired.

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

“…The LLTE value is then calculated as the error between the simulated and reference lower leg trajectory (Eq. 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.…”

“…During mid-stance, the position and orientation of the lower leg are fully defined by the position of the CoP along the ground, the applied GRFs, and the prosthetic foot mechanical characteristics, assuming a no-slip condition and the foot to be tangent to the ground at the CoP location 18 (Fig. 1c).…”

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

“…Thus, roll-over geometry does not fully describe the functionality of a given prosthetic foot. Our previous work has demonstrated that it is possible for two different prosthetic feet to have identical roll-over geometries but yield very different lower leg kinematics under the same ground reaction forces (GRFs) [18][19][20]. Therefore, roll-over geometry is insufficient to be used in optimizing the design of prosthetic feet, as ideal kinematics cannot be ensured.…”

“…We have created a novel design objective, called the lower leg trajectory error (LLTE), that quantifies how closely the position of the lower leg segment for a given prosthetic foot is able to replicate target physiological lower leg position, in both space and time, throughout the course of a step [18][19][20]. LLTE is defined as…”

“…(1), produces a foot that best replicates physiological gait kinematic and kinetic data can be found in Refs. [18] and [20]. However, the framework could similarly be employed with a different cost function to optimize the foot for other goals.…”