A functional effect of the superficial mechanical properties of articular cartilage as a load bearing system in a sliding condition

The structure and composition of articular cartilage show depth-wise inhomogeneity and anisotropy. In particular, the dense collagen network covers and reinforces the superficial tangential zone of the tissue. It is thought that this peculiar structure offers the excellent tribological property of articular cartilage. The purpose of this study was to investigate the functionality of the superficial tangential zone (STZ) of articular cartilage as a load bearing system. The 2-dimensional finite element (FE) model was accepted for sliding configuration with sufficient extent of sliding distance. The standard model as a control was carried from our previous study, which included depth-dependent Young׳s modulus of the solid phase, fiber reinforcement with strain-dependency and permeability with compaction effect. The mechanical property of the superficial layer was modified for a parametric study of its functionality. According to research results in the past, the tangential stiffness of the fiber reinforcement of the STZ model was enhanced, and the following anisotropic permeability was also modified. The stationary contact condition and the migrating contact condition were examined to compare the effect of the superficial tangential layer. The result showed that the significant reduction of friction coefficient was found in migrating contact condition of the STZ model. In the observation of field output of FE analysis, the contacting surface formed a thin low permeability layer, which would enable the high fluid pressure and the low fluid flow at the same time. It seemed that the stiffening of the fiber reinforcement of the superficial layer promoted the formation of the low permeability layer. Beyond the effectivity of the fiber reinforcement of biphasic matrix on the interstitial fluid pressurization, the findings of this study indicated that the compaction effect on the permeability would involve a quite complex phenomenon in long term migrating contact sliding.