Viscoelastic behavior of osteoarthritic cartilage

We have studied the incremental stress-strain behavior of human articular c artilage in tension in an attempt to understand the molecular basis for fib rillation and fissure formation in osteoarthritis. Our results indicate tha t the elastic spring constant for collagen in the direction perpendicular t o 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 mum a t low strains and 1.24 mum 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 perpe ndicular to the cleavages line pattern is similar to that calculated based on stress-strain curves reported by Kempson([1]). Our results indicate that the elastic spring constant for collagen and the collagen fibril length decrease as the extent of fibrillation and fissure f ormation increase. Decreases in the elastic spring constant of collagen are consistent with loss of the superficial layer, degradation of proteoglycan s 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 cart ilage involve enzymatic degradation of matrix components and fibril fragmen tation that is promoted by subsequent mechanical loading.