ABSTRACT
OBJECTIVE: The calcified cartilage layer is thought to be integral to force transmission between the compliant articular cartilage (AC) above and underlying stiff bone. This study aims to determine how such a stiffness gradient across the calcified cartilage and bone changes with joint degeneration and how different scalar levels of testing are correlated. METHOD: Using a bovine model of early osteoarthritis (OA), multiple samples of calcified cartilage on subchondral bone (SB) from sixteen bovine patellae, displaying a range of cartilage states from intact (healthy) to moderately degenerate, were tested using macroscopic three-point bending, microhardness indentation, and nanoindentation. Mechanical properties were correlated to cartilage health and microstructural morphometric measurements obtained from high resolution imaging using Differential Interference Contrast (DIC) Microscopy. RESULTS: There was a significant decrease in the moduli obtained from tests done at increasing scalar levels. The macroscale average modulus obtained from three-point bending showed that the SB was 10 times stiffer than the calcified cartilage in healthy tissue, 5 times in tissue displaying mildly degenerate AC and 8 times with moderate degeneration. Microhardness testing of multiple points from the calcified cartilage to the SB revealed that there was a monotonic gradual increase in the mean modulus. The moduli obtained from nanoindentation testing indicated that the SB was about twice the stiffness of the calcified cartilage. CONCLUSION: The mechanical transition from calcified cartilage to SB involves a graded continuum of increasing material stiffness. This stiffness gradient is altered in association with early degenerative change in the overlying AC, detectable only at the macro level.
