LAUSR

Biphasic poro-viscoelastic constitutive material model (BPVE) captures both the fluid flow dependent and independent behavior of cartilage under stress relaxation type indentation. A finite element model based on BPVE formulation was developed to explore the sensitivity of the model to Young's modulus, Poisson's ratio, permeability and viscoelastic constitutive parameters expressed in terms of Prony series coefficients. Then we fit the numerical model to experimental force versus time curves from stress relaxation indents on bovine tibial plateaus to extract the material properties. Measurements were made over the period of two days to capture the material property changes that resulted from trypsin-induced degradation. We measured spatial and temporal changes in mechanical properties in the cartilage. The areas of degradation were characterized by an increase in both permeability and summation of Prony series shear relaxation amplitude constants. These findings suggest that cartilage degradation reduces the intrinsic viscoelastic properties of the tissue's solid phase in addition to impairing its capacity to offer frictional drag to the interstitial fluid flow (permeability). The changes in material properties are measurable well before structural degradation occurs.