Histological analysis, viscoelastic characterization, and modeling of human plantar fascia

The human plantar fascia is a soft connective tissue that extends from the calcaneal tuberosity to the metatarsal region, providing static support to the foot and functioning as a shock absorber. Its roles and pathology are closely linked to its collagen fiber composition. Therefore, in-depth investigation of the mechanics of this tissue, an area with limited prior research, is crucial, especially for developing accurate in silico foot models for clinical and surgical purposes. This study aims to address this gap by combining experimental procedures, microscopic analysis of collagen fibers, and constitutive modeling to accurately describe the behavior of the plantar fascia. Uniaxial extension tests, conducted with a multi-step stress relaxation protocol until failure, show behavioral differences between samples cut along the proximal-distal and lateral-medial directions. This is supported by histological analysis, which shows that collagen fibers predominantly align along the proximal-distal axis. The experimental data, together with histological findings, are used to calibrate the parameters of a nonlinear viscoelastic, fiber-reinforced model through an inverse analysis process. This model captures the mechanical response of the plantar fascia, with the fiber component described using a generalized structural tensor formulation. The new insights gained provide a valuable understanding of the mechanical behavior of the human plantar fascia and pave the way for improvements in existing computational models. Statement of Significance: The human plantar fascia has received limited attention, with few studies partially addressing its mechanical behavior. This work provides a comprehensive characterization of the tissue by integrating mechanical testing, histology, and constitutive modeling. The histological assessment of samples collected along different tissue directions enables the quantification of collagen fiber orientation and dispersion, offering new insights into the anisotropy of the tissue. Mechanical experiments are employed to develop and calibrate an anisotropic visco-hyperelastic model, capable of capturing the full complexity of the tissue's mechanics. Completed by a validation of the proposed model, this study delivers the essential components needed for the development of accurate in silico foot models, with potential applications in clinical and surgical evaluation of the plantar fascia.