Nonlinear viscoelastic properties of articular cartilage in shear.

The strain dependence of the intrinsic viscoelastic properties of the cartilage matrix in shear was investigated. Stress relaxation experiments were performed on bovine articular cartilage at shear strains ranging from approximately 3% to 16%. The tissue was found to exhibit nonlinear strain-dependent viscoelastic behavior, with the nonlinearity occurring primarily in the short-time transient during stress relaxation. In addition, the equilibrium stress was found to fit a quadratic relation with strain. This relationship was noted to be nearly linear with strain from 3% to 16%. The instantaneous stress was seen to be highly nonlinear, and followed a cubic relationship with applied shear strain. Fung's quasilinear theory can be used to describe the stress relaxation response over the range of strains examined when a nonlinear regression is performed to determine an "average" normalized relaxation function. Alternately, strain dependence can be incorporated into the model to describe and predict more accurately the strain-dependent stress relaxation response.