Articular cartilage with intra- and extrafibrillar waters: a chemo-mechanical model

A three-phase multi-species electro-chemo-mechanical model of articular cartilage is developed that accounts for the effect of two water compartments, namely intrafibrillar water stored in between collagen fibrils and extrafibrillar water covering proteoglycans. The collagen fibers constitute the solid phase while intrafibrillar water and dissolved NaCl on one hand and extrafibrillar water, ions Na+ and Cl- and proteoglycans on the other hand form the two fluid phases. Chemical equilibrium between the fluid phases is assumed and only the mechanical aspects of the behaviour are considered. Although mass transfer occurs here, the theory is time-independent. The complete picture that includes time-dependent mass transfer between the two fluid phases, diffusion of water and ions and electrical field is postponed for a future study. Constitutive functions are motivated and material parameters calibrated. Simulations of confined compression and oedometric free swelling are compared with available data. The evolution of internal entities, e.g. mass and molar fractions of intra- and extrafibrillar waters and ions, is also documented during chemical and mechanical loading processes. (C) 2003 Elsevier Ltd. All rights reserved.