Biomechanical design framework for prosthetic feet: Experimentally validated non-linear finite element procedure

Performance evaluation of prosthetic feet during their design is typically performed experimentally, which may be time and cost intensive. This work presents a first-of-its-kind application of a numerical procedure for the a priori determination of various stance phase biomechanical parameters of a prosthetic foot, such as its roll-over characteristics, centre of pressure trajectory, ankle flexion moment arm and ankle range of motion, to aid in its design. The numerical procedure is based on finite element analysis, which includes geometric, material and contact non-linearity. Boundary conditions emulating the rocker-based inverted pendulum model were employed to evaluate the biomechanical parameters. The finite element model was validated by employing an inverted pendulum-based apparatus using the structurally complex Ottobock Solid Ankle Cushioned Heel (SACH) prosthetic foot as the test device. A comparison of the numerical and experimental results showed low magnitude of errors. For example, the percentage error of the radius of curvature of the roll-over shape was ~0.1%. The differences found appear to be clinically insignificant, which substantiates the reliability of the model. The proposed numerical model can be employed to obtain detailed a priori insights into the biomechanical parameters influencing a prosthetic foot's characteristics during gait, which can better inform the design, analysis and prescription of prosthetic feet.