Biphasic creep and stress relaxation of articular cartilage in compression: Theory and experiments

Articular cartilage is a biphasic material composed of a solid matric phase ( approximately equals 20 percent of the total tissue mass by weight) and an interstitial fluid phase ( approximately equals 80 percent). The solid matrix was assumed to be intrinsically incompressible, linearly elastic and nondissipative while the interstitial fluid was assumed to be intrinsically imcompressible and nondissipative. Further, it was assumed that the only dissipation comes from the frictional drag of relative motion between the phases. However, more general constitutive equations, including a viscoelastic dissipation of the solid matrix as well as a viscous dissipation of interstitial fluid were also developed. It was concluded that 1) a nonlinearly permeable biphasic model, where the permeability function is given by an experimentally determined empirical law: k equals A(p) exp left bracket alpha (p)e right bracket , can be used to describe more accurately the rheological properties of articular cartilage, and 2) the frictional drag of relative motion is the most important factor governing the fluid/solid viscoelastic properties of the tissue in compression.