Compositional and metabolic changes in damaged cartilage are peak-stress, stress-rate, and loading-duration dependent.

The first objective of this study was to determine if the cumulative effects of impact or smoothly arising compression would damage the matrix of articular cartilage. Canine cartilage explants were subjected to repeated impacts or to smoothly arising compressions of as much as 20 MPa at 0.3 Hz for as long as 120 minutes. An increase in the water content of the loaded core compared with the surrounding ring was considered indicative of matrix damage. The results showed that damage to cartilage required repeated impacts with a peak stress of at least 2.5 MPa and a stress rate of at least 30 MPa/sec for 2 minutes or longer. This suggested that impact damage is cumulative and stress-rate dependent. The second objective was to identify biosynthetic and compositional changes in impact-damaged cartilage over a period of time after loading. Accordingly, canine cartilage explants were subjected to repetitive impacts of 5 MPa at 0.3 Hz for 2, 20, and 120 minutes. The loaded explants were then cultured for as long as 10 days. The increase in water content (1.9-3.8%) in the core region relative to the surrounding ring persisted during the 10-day culture. A significant increase in fibronectin synthesis (22-47%) was found in the core region of impact-damaged cartilage. Proteoglycan synthesis was increased by 41-104%. An increase in denatured collagens (11-70%) in the loaded cores substantiated damage to the collagen network. Denatured collagens stained with COL2-3/4m monoclonal antibody were consistent with the compositional findings and were mainly located near the articular surface and in the deep zone. These changes were consistent with early osteoarthritis and suggested the induction of the initial stages of osteoarthritis in the impact-damaged cartilage.