Viscoelastic behavior of osteoarthritic cartilage.

We have studied the incremental stress-strain behavior of human articular cartilage in tension in an attempt to understand the molecular basis for fibrillation and fissure formation in osteoarthritis. Our results indicate that the elastic spring constant for collagen in the direction per pendicular to the cleavage line pattern is about 1.6 GPa (2.3 GPa after correction for the collagen content) and the collagen fibril length is between 0.558 pm at low strains and 1.24 pm at high strains for normal cartilage. Values for the elastic spring constant and collagen fibril length were both found to decrease in OA. The value of the elastic spring constant for collagen perpendicular to the cleavage line pattern is similar to that calculated based on stress-strain curves reported by Kempson. Our results indicate that the elastic spring constant for collagen and the collagen fibril length decrease as the extent of fibrillation and fissure formation increase. Decreases in the elastic spring constant of collagen are consistent with loss of the superficial layer, degradation of proteoglycans and collagen, and subsequent mechanical fatigue. However, changes in the polymer volume fraction are consistent with enzymatic degradation preceding mechanical disruption. It is concluded that osteoarthritic changes to cartilage involve enzymatic degradation of matrix components and fibril fragmentation that is promoted by subsequent mechanical loading.