Biomechanical behaviour of heel pad tissues: constitutive formulation and analysis

The peculiar structure of heel pad is considered with regard to its biomechanical function as a dumping and pressure resistant cushion. According to a honeycomb configuration, heel pad tissue is organized as dense strands of fibrous tissue, characterized by circular or cone-shaped septa. The septa bound chambers filled with fat cells. The chambers are reinforced by further transverse and diagonal fibers. The fibrous tissue strands are firmly attached to the underface of the calcaneum and extending to the subcutaneous tissues. The structural configuration of heel pad, as a network of fibrous components binding fat cells, entails the capability to withstand impacts and prolonged pressure loads. A specific visco-hyperelastic constitutive model is developed taking into account the typical features of heel pad tissue mechanical response, as large displacements and strains, almost incompressible behavior, non linear stress-strain relationship and time-dependent effects. Preliminarily, constitutive parameters are evaluated using a comparative analysis of model results and data from in vitro mechanical tests, according to stochastic-deterministic optimisation procedures. In order to interpret the mechanical response of heel pad living tissues, which is deeply influenced by interaction and connection with surrounding tissues and structures, constitutive parameters must be updated by considering data from specific additional experimental tests. Analytical and numerical models are provided to interpret the different experimental situations. Good agreement is achieved between experimental data and model results, confirming the mechanical coherence between the proposed constitutive formulation and heel pad tissues mechanics.