Effect of Collagen Viscoelasticity on the Mechanical Response of Articular Cartilage

INTRODUCTION Articular cartilage is known to exhibit strong transient mechanical behavior. When subjected to high strainrate loading, its apparent stiffness can be 10 times greater than its stiffness at equilibrium. However, the mechanism of this strain-rate dependent response is not completely understood. While some studies have shown evidence of a fluid-flow dominant mechanism of the strain-rate dependence [8,11,15], others indicate a significant influence from intrinsic viscoelasticity (referred to as viscoelasticity hereafter) on the response [1,3,4,6,9]. Theories have experienced difficulties in determining the roles of fluid-flow and viscoelastic properties in the mechanical behavior of articular cartilage. Collagen fibers in rat tail tendon have been found to be viscoelastic [2,13]. The intrinsic viscoelasticity of proteoglycans has not been well documented. In fact, the viscoelastic shear modulus of cartilage has been reported to increase with collagen cross-links [3], indicating that the collagen fibers account for the viscoelasticity of cartilage. Therefore, it was assumed in the current work that, collagen fibers in articular cartilage are viscoelastic, but the proteoglycan matrix is elastic. The possible role of fiber viscoelasticity in the transient response of articular cartilage was then explored.