The increased swelling and instantaneous deformation of osteoarthritic cartilage is highly correlated with collagen degradation

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Abstract
Objective To provide evidence for the hypothesis that the loss of tensile strength of osteoarthritic (OA) cartilage (resulting in swelling—the hallmark of OA) is due to an impaired collagen network and not to loss or degradation of proteoglycans. Methods The amount of degraded collagen molecules, the fixed charge density (FCD) on a dry‐weight basis, the degree of swelling in saline, and the instantaneous deformation (ID; a test reflecting the tensile stiffness of the collagen network) were measured in full‐depth OA femoral condyle samples. In addition, levels of the crosslink hydroxylysylpyridinoline (HP), the amount of degraded collagen molecules, and the degree of swelling were determined in the 3 zones (surface, middle, and deep) of OA cartilage. We also compared the ID of normal and OA cartilage. Results In full‐depth OA cartilage, a close relationship was found between swelling and ID. Swelling and ID correlated strongly with the amount of degraded collagen molecules, and were not related to FCD. OA cartilage showed the same zonal pattern in HP levels as normal cartilage (i.e., an increase with depth). No relationship was found between collagen crosslinking and swelling of the surface, middle, and deep zones. In all 3 zones, swelling was proportional to the amount of degraded collagen molecules. Compared with that of normal cartilage, the change in ID of OA cartilage was most pronounced at the surface in a direction parallel to the direction of the collagen fibrils. Conclusion The decreased stiffness of the OA collagen network (as measured by swelling and ID) is strongly related to the amount of degraded collagen molecules. The anisotropy in ID parallel and perpendicular to the direction of the fibrils revealed that the impairment of strength resides mainly in, and not between, the fibrils. Proteoglycans play only a minor role in the degeneration of the tensile stiffness of OA cartilage.