Composition and transport properties of human ankle and knee cartilage

The incidence of osteoarthritis is significantly higher in the knee as compared to the ankle, suggesting that differences in the properties of cartilage from these joints may contribute to the development of osteoarthritis. As an avascular tissue, articular cartilage depends primarily upon diffusion for molecular transport. The goal of this study was to determine if differences in the structure and composition between ankle and knee cartilage were also reflected as differences in solute transport properties. The diffusion coefficient and partition coefficient of a 70-kDa dextran molecule were measured in human ankle and knee articular cartilage using fluorescence recovery after photobleaching (FRAP) and were compared to the proteoglycan, collagen, water, and DNA contents within each zone. The mean partition coefficient was significantly lower in the ankle compared to the knee (0.010 ± 0.002 vs. 0.022 ± 0.003, p < 0.01), but no differences in the diffusion coefficients were observed (34.6 ± 0.9 μm²s^-1 vs. 35.4 ± 2.4 μm²s^-1, p=0.70). Ankle cartilage exhibited higher proteoglycan content as well as a trend toward lower water content, suggesting that ankle cartilage has a smaller effective pore size than knee cartilage. These findings suggest that differences in the composition of ankle and knee cartilage contribute to a difference in the partition coefficient. The results of this study provide further support for the hypothesis that the transport properties of cartilage may play a role in the differences in the incidence of osteoarthritis in these joints by altering the effective concentration of growth factors and cytokines to which chondrocytes are exposed.