In silico tools of human joints in healthy and pathological conditions: a combined experimental and computational approach

Every day, the human foot and knee are subjected to several stresses resulting from daily repetitive actions, such as work, sports, static standing, or simply locomotion. Unfortunately, musculoskeletal pathologies—mainly caused by diabetes and osteoarthritis—or injuries can impair the physiology of both soft and hard tissues, leading thousands of people each year to undergo foot and knee surgery or, in the worst cases, as complications of diabetes, to be subjected to partial or complete foot amputation. This scenario is responsible for the patient’s worsening quality of life and for an economic and social impact that is not negligible. Thus, due to the complexity of these joints, analysing the structure, function, and mechanical properties of their tissues is essential to determine the possible involvement of the tissue in the development and progression of the disease and to study the differences in tissue behaviour between healthy and pathological conditions. Furthermore, these outcomes are crucial for the development of in silico joint models, which are valuable tools (emerged in recent decades) that apply the principles of computational mechanics within the medical field. In light of this, the present work aims to investigate and characterise the mechanical behaviour of the foot and knee tissue, compare their behaviour in healthy and pathological conditions, with the final goal of paving the way for the development of an in silico model of the human joints. The initial step involves examining the tissue's anatomical and histological micro- and macrostructures, which is essential for choosing the most suitable experimental tests and establishing the appropriate constitutive model to describe the mechanical behaviour of foot and knee tissues. In particular, attention is focused on the soft tissues of these two joints. Then, after performing multiple experimental tests on both healthy and pathological tissue samples (revealing non-linearity, time dependence, and isotropic/anisotropic behaviour), and defining the proper constitutive model for each (considering hyperelasticity, viscosity, and fibre arrangement), parameter identification and validation are carried out. Comparing healthy and diseased tissues, specifically those affected by diabetes in the foot and osteoarthritis in the knee joints, enabled us to highlight the disease's influence on foot biomechanics and its potential role in contributing to the development of the pathology. After this, the results are used to develop in silico joint models by integrating anatomical geometry with tissue mechanics. While the 3D model of the tissues is defined using CT/MRI image segmentation and the anthropometric information, their mechanical behaviour is defined through the mechanical parameters previously determined. The final model achieved can be a valuable tool in support of non-invasive and cost-effective clinical applications, aiding, for example, in the simulation of orthopaedic surgery procedures and the design of specific orthotic devices to prevent, e.g., the onset of diabetic foot ulcers.