Modulation of internal tissue stresses of the knee via control of variable-stiffness properties in a 3D-printed footwear: A combined experimental and finite element analysis.

AIMS: Therapeutic footwear with variable-stiffness properties has been shown to be effective in reducing the external knee adduction moment. However, the exact changes of internal tissue stresses following intervention with variable-stiffness footwear remains unknown. The objective of this study is to construct a finite element model of the foot-ankle-knee complex to investigate the therapeutic efficacy of the variable-stiffness footwear in modulating internal knee joint stress distributions.

METHODS

Footwear with lateral-to-medial stiffness ratio of 1.00, 1.33, 1.49 and 1.77 were designed and 3D-printed as lattice structures at different porosity levels. A three-dimensional finite element model of the foot-ankle-knee complex was constructed with cartilage-meniscus structures characterized by magnetic resonance imaging. Ground reaction forces and muscular loads estimated from gait experiments with 3D-printed variable-stiffness footwear were used to drive the model, and effects of footwear intervention with four stiffness ratios on tibiofemoral joint loads were evaluated during the simulated instant in gait.

RESULTS

Compared with the constant-stiffness design, the variable-stiffness footwear reached a maximum reduction of the first peak knee adduction moment up to 17.1%. As the lateral-to-medial stiffness ratio increases, the peak stresses at the meniscus in the medial compartment of the knee were reduced by 5.0%, 7.9%, and 16.4%, respectively. The stress-modifying effects were less pronounced for the lateral compartment of the knee.

CONCLUSION

The variable-stiffness footwear design produced stress relieving to the cartilage and meniscus structures, and the fact that lower stiffness ratio had reached significantly less effects, suggesting a favourable stress-relief effect should consider footwear with a higher lateral-to-medial stiffness ratio.