Influence of proteoglycan on time-dependent mechanical behaviors of articular cartilage under constant total compressive deformation

Articular cartilage has biphasic property based on high water content. It is generally believed that the proteoglycan supports the compressive load, but the detailed loading mechanism has not yet been clarified. In this study, first we observed the changes in compressive stress and strain of articular cartilage under constant total compressive deflection. We evaluated the changes in modulus of elasticity, which was estimated from the stress-strain relation in equilibrium state. To examine the role of proteoglycan in compressed articular cartilage, we compared the time-dependent viscoelastic behaviors in both the intact cartilage and the cartilage treated with chondoroitinase ABC under constant total compressive deformation. We could confirm that the peak stress after compression and the modulus of elasticity at equilibrium were reduced after the digestion of proteoglycan. Next, we observed the changes in local strain in both articular cartilage specimens with and without chondroitinase treatment by monitoring the position of stained chondrocyte in the confocal laser scanning microscope. These visualized images indicated that the local strain changed time-dependently and depth-dependently. The digested cartilage showed the quicker change in movement and larger thinning in surface layer than the intact cartilage. These results indicate that the proteoglycan contributes to the compressive load-carrying capacity and controls the permeability.