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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.

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The objective of this study was to investigate the effects of mechanical fatigue in the form of cyclic shear strain on articular cartilage. Three millimeter diameter full-thickness plugs were cored from the lateral aspect of bovine tibial plateaus. Sinusoidal shear strains of +/- 5, +/- 10, and +/- 15% were applied to the specimens at 100 Hz for 3 h (a total of 108 x 10(4) cycles). The mechanical shear properties of the tissue (loss and storage moduli) were determined as a function of the number of applied strain cycles. A rapid, irreversible decrease of approximately 35% of initial modulus was found to occur in both loss and storage modulus during application of the first 90,000 cycles. Further decay in the moduli was found to occur from 90 x 10(3) to 108 x 10(4) cycles, but was of considerably smaller magnitude than the initial decrease. The moduli remained relatively constant beyond application of 108 x 10(4) cycles. No consistent change in proteoglycan content was found to be associated with the fatigue process when comparing tested specimens with fresh, untested tissue, and with experimental controls. In addition, no structural defects in the mechanically altered tissue were revealed by scanning electron microscopy.