The natural synovial joint: A finite element investigation of biphasic surface amorphous layer lubrication under dynamic loading conditions

Despite much research, the lubrication mechanisms operating in healthy synovial joints, particularly in the boundary or mixed regimes, are still not clear. This work continued the previous research by the authors in using a finite element model to investigate the effect of a soft, biphasic, amorphous surface layer on the lubrication characteristics of articular cartilage. This model was extended and the 'biphasic surface amorphous layer (SAL) effect' was investigated under repetitive, dynamic loading conditions. The results of this model showed that the lubrication benefits and stress shielding conferred by the soft, biphasic SAL remained almost entirely effective, despite incomplete cartilage swelling during unloading. The reduction in friction and improved lubrication were shown to be more beneficial during rapid dynamic loading and adversely effected by slow, static loading. The very rapid recovery of the SAL was shown to play an essential role in the mechanism and was important in helping the tissue to sustain low friction under conditions of repeated dynamic loading.