Design and Testing of a Prosthetic Foot Prototype With Interchangeable Custom Rotational Springs to Adjust Ankle Stiffness for Evaluating Lower Leg Trajectory Error, an Optimization Metric for Prosthetic Feet

Abstract

A prosthetic foot prototype intended for evaluating a novel design objective for passive prosthetic feet, the Lower Leg Trajectory Error (LLTE), is presented. This metric enables the optimization of prosthetic feet by modeling the trajectory of the lower leg segment throughout a step for a given prosthetic foot and selecting design variables to minimize the error between this trajectory and target physiological lower leg kinematics. Thus far, previous work on the LLTE has mainly focused on optimizing conceptual foot architectures. To further study this metric, extensive clinical testing on prototypes optimized using this method has to be performed. Initial prototypes replicating the LLTE-optimal designs in previous work were optimized and built, but at 1.3 to 2.1 kg they proved too heavy and bulky to be considered for testing. A new, fully-characterized foot design reducing the weight of the final prototype while enabling ankle stiffness to be varied is presented and optimized for LLTE. The novel merits of this foot are that it can replicate a similar quasi-stiffness and range of motion of a physiological ankle, and be tested with variable ankle stiffnesses to test their effect on LLTE. The foot consists of a rotational ankle joint with interchangeable U-shaped constant stiffness springs ranging from 1.5 Nm/deg to 16 Nm/deg, a rigid structure extending 0.093 m from the ankle-knee axis, and a cantilever beam forefoot with a bending stiffness of 16 Nm2. The prototype was built using machined acetal resin for the rigid structure, custom nylon springs for the ankle, and a nylon beam forefoot. In preliminary testing, this design performed as predicted and its modularity allowed us to rapidly change the springs to vary the ankle stiffness of the foot. Qualitative feedback from preliminary testing showed that this design is ready to be used in larger-scale studies. In future work, extensive clinical studies with testing different ankle stiffnesses will be conducted to validate the optimization method using the LLTE as a design objective.