Boundary lubrication of articular cartilage: The role of interstitial fluid pressure, surface porosities and equilibrium friction coefficient

This thesis work presents the development and verification of a boundary lubrication model for articular cartilage in diarthrodial joints. This model indicates that even in the absence of fluid film lubrication, the friction coefficient between articular cartilage surfaces may remain very low because cartilage interstitial fluid pressurization can contribute more than 90% of the fluid load support, thus minimizing the interfacial load supported by the collagen-proteoglycan solid matrix and consequently reducing the friction coefficient.; Using the biphasic theory for cartilage, a theoretical solution was found for the frictionless rolling contact of cylindrical biphasic articular cartilage layers. It is concluded from this study that, the interstitial fluid pressurization may support more than 90% of the total applied load. Based on this study and the numerous theories and experiments in the literature, a boundary lubrication model was formulated, which accounts for the interstitial fluid pressurization. as well as the equilibrium friction coefficient, {dollar}musb{lcub}eq{rcub},{dollar} the friction coefficient when the interstitial fluid pressure is zero. Apparently, {dollar}musb{lcub}eq{rcub}{dollar} is directly related to the presence of a boundary lubricant at the cartilage surface. In this work, {dollar}musb{lcub}eq{rcub}{dollar} was first investigated under steady shear. It was found that {dollar}musb{lcub}eq{rcub}{dollar} is a function of sliding speed and infinitesimal compressive strain. {dollar}musb{lcub}eq{rcub}{dollar} was also found to increase when more than 85% of the chondroitin sulfate in proteoglycan was digested with chondroitinase ABC.; This friction model was shown to be qualitatively consistent with a number of observed experimental friction measurements. To validate this model experimentally, a new biaxial friction testing apparatus was built. The transient and equilibrium frictional response of articular cartilage were measured under time-varying displacement and loading conditions. The predicted time-dependent response or the effective friction coefficient (i.e., the measured friction coefficient, {dollar}musb{lcub}eff{rcub}){dollar} was found to correlate very well with the corresponding experimental measurements.; Finally, the effect of cartilage osteoarthritis on the interstitial fluid pressurization was investigated, which sheds lights on the degenerative process which might influence the frictional response of diarthrodial joints.