Effect of load variation on the friction properties of articular cartilage

Synovial joints are known for their remarkable load bearing capacity while maintaining very low friction and wear over their lifetime. Although other joint components play a role in the lubrication mechanisms, articular cartilage with its unique biphasic characteristics is believed to be largely responsible for the observed low friction properties in these joints. In the current study, a pin-on-plate machine was used in a cartilage loaded against cartilage configuration to study the effect of load variation on cartilage friction properties. Both static and dynamic loading conditions were tested under three different contact stress levels - 0.2, 0.3, and 0.4 MPa with phosphate buffered saline as the lubricant. Indentation experiments were performed and modelled through finite-element (FE) method to determine the biphasic material properties of cartilage. A second FE model adopting the derived biphasic properties of cartilage was then used to simulate the static friction tests and determine the interstitial fluid load support in the cartilage tissue under different loads. An increase in contact stress was found to reduce the friction levels significantly (p < 0.05) between the articulating cartilage surfaces in both dynamic and static models, while the percentage of fluid load support within the cartilage was almost identical. It was shown that while interstitial fluid pressurization of cartilage plays the major role, another extraneous factor or mechanism that influences the friction between cartilage surfaces is needed to completely explain the results observed.