ABSTRACT
Cartilage has the task of creating a joint connection that is as frictionless and damping as possible. It is a complex multi-phase material consisting of a liquid saturated collagen fiber-reinforced porous solid. The mechanical properties of cartilage are highly dependent on the interaction of the isotropically distributed, electrically negative charged proteoglycans and the highly anisotropic and inhomogeneously distributed collagen fiber network. The negatively charged ions of the proteoglycans lead to an electrical gradient which in turn causes an osmotic negative pressure. This causes a swelling inside the cartilage whereby the collagen fibers are mechanically prestressed and thus reducing the total pressure acting on the cartilage matrix. A major cartilage disease that currently affects more and more people especially in developed countries is osteoarthritis. It changes the composition of the proteoglycans which results in a decrease in osmotic pressure, leading to increased abrasion. In addition, a defibration of the collagen fibers occurs which causes a decrease of tensile strength and viscoelasticity in fiber direction. In order to account for these age-related degeneration processes a model using the Theory of Porous Media as a homogenization approach is developed. The model describing the mechanical behavior of articular cartilage is based on the biphasic model presented in Pierce et al. 2013. It includes the incompressible poroelastic solid matrix reinforced with collagen fibers and the incompressible fluid in the pores. We aim to incorporate the influences of osmotic pressure (Wang et al. 2018) in order to determine the correct initial stress state under imaged configuration.
