The role of osmotic pressure and tension-compression nonlinearity in the frictional response of articular cartilage

Articular cartilage is the bearing material of diarthrodial joints such as the knee, hip, or shoulder. Some studies of cartilage lubrication have hypothesized that pressurization of its interstitial fluid may contribute predominantly to reducing the friction coefficient at the contact interface of articular layers. This study introduces a formulation for the dependence of the frictional response of articular cartilage on interstitial fluid pressurization, which accounts for the osmotic pressure in cartilage as well as the tissue's tension-compression nonlinearity, and is based on the theory of mixtures for soft hydrated charged tissues. Theoretical predictions of this model are obtained for the configuration of unconfined compression creep. It is observed from theory that increasing the salt concentration of the tissue's bathing solution reduces the minimum friction coefficient that can be achieved, relative to its equilibrium value; the model also predicts that increasing the applied load can similarly reduce the minimum friction coefficient. Physical interpretations of these phenomena are provided by the model. Experimental results are presented which support these theoretical findings and produce time-dependent responses in good agreement with model predictions. Furthermore, it is observed that the equilibrium friction coefficient does not remain constant under various loads or salt concentrations, and correlation analyses suggest that the equilibrium value depends in part on the compressive strain in the tissue.