Collagen fibril stiffening in osteoarthritic cartilage of human beings revealed by atomic force microscopy

Objective: This study aimed to characterize the in-situ mechanical property and morphology of individual collagen fibril in osteoarthritic cartilage using indentation-type atomic force microscopy (IT-AFM). Methods: The specimens with intact articular cartilage (AC), mild to severe degenerated cartilage from osteoarthritis (OA) were collected with informed consent from the postmenopausal women who underwent hip or knee arthroplasty. The fresh specimens were cryo-sectioned by layers with 50 μm thick for each from the articular surface to calcified cartilage, and then processed for AFM imaging and nanoindentation test. For each layer, a total of 20 collagen fibrils were randomly selected for testing. AFM tips with the nominal radius less than 10. nm were employed for probing the individual collagen fibril, and the obtained cantilever deflection signal and displacement were recorded for calculating its elastic modulus. Results: An intact AC exhibited a gradation in elastic modulus of collagen fibrils from articular surface (2.65 ± 0.31. GPa) to the cartilage-bone interface (3.70 ± 0.44. GPa). It was noted in mildly degenerated OA cartilage that the coefficient of variation for mechanical properties of collagen fibers, ranging from 25% to 48%, significantly increased as compared with intact one (12%). The stiffened collagen fibrils occurred at either articular surface (3.11 ± 0.91. GPa) or the cartilage-bone interface (5.64 ± 1.10. GPa), accompanied by loosely organized meshwork with advancement of OA cartilage degeneration. It was echoed by histological findings of OA cartilage, including fibrotic changes of surface region and tidemark irregularities. Conclusion: The stiffened collagen fibrils in AC occurred with OA onset and progression, not only at articular surface but also the cartilage-bone interface.