Computational models of flatfoot with three-dimensional fascia and bulk soft tissue interaction for orthosis design

The finite element (FE) method has been widely used to investigate the internal force of plantar fascia, which could reveal the relationship between plantar fascia dysfunction and flatfoot deformity during weight-bearing conditions. However, for most foot FE models, plantar fascia utilized truss elements or three-dimensional geometry that did not consider the interaction between plantar fascia and bulk soft tissue. These configurations could ignore the impact of superoinferior loading induced by arch support and underestimate the plantar fascia loading. This study aims to investigate how the fascia-bulk soft tissue interaction affects the internal foot biomechanics in the flatfoot FE analysis with a three-dimensional plantar fascia model, which included both fascia-bone and fascia-bulk soft tissue interactions (3DBPT). To evaluate the effect of fascia-bulk soft tissue interaction on internal foot mechanics, this study compared the 3DBPT model with the other two plantar fascia models, including linear fascia (BPL) and three-dimensional plantar fascia without fascia-bulk soft tissue interaction (3DBP). The predicted foot contact pressure in the 3DBPT model was compared with the measured value obtained by the F-Scan pressure measurement system in balanced standing. Peak von Mises stresses in the plantar fascia and foot ligaments were reported. The stress of the plantar fascia in the 3DBPT model was higher than that of 3DBP. In the 3DBPT model, the superoinferior loading exerted on the bulk soft tissue could be directly transferred to the plantar fascia. The proposed model, including the plantar fascia and bulk soft tissue interaction, could reveal relatively reliable plantar fascia loading in flatfoot deformity, thereby contributing to the development of orthotic designs for the flatfoot deformity.