Wide bandwidth nanomechanical assessment of murine cartilage reveals protection of aggrecan knock-in mice from joint-overuse

Aggrecan loss in human and animal cartilage precedes clinical symptoms of osteoarthritis, suggesting that aggrecan loss is an initiating step in cartilage pathology. Characterizing early stages of cartilage degeneration caused by aging and overuse is important in the search for therapeutics. In this study, atomic force microscopy (AFM)-based force-displacement micromechanics, AFM-based wide bandwidth nanomechanics (nanodynamic), and histologic assessments were used to study changes in distal femur cartilage of wildtype mice and mice in which the aggrecan interglobular domain was mutated to make the cartilage aggrecanase-resistant. Half the animals were subjected to voluntary running-wheel exercise of varying durations. Wildtype mice at three selected age groups were compared. While histological assessment was not sensitive enough to capture any statistically significant changes in these relatively young populations of mice, micromechanical assessment captured changes in the quasi-equilibrium structural-elastic behavior of the cartilage matrix. Additionally, nanodynamic assessment captured changes in the fluid-solid poroelastic behavior and the high frequency stiffness of the tissue, which proved to be the most sensitive assessment of changes in cartilage associated with aging and joint-overuse. In wildtype mice, aging caused softening of the cartilage tissue at the microscale and at the nanoscale. Softening with increased animal age was found at high loading rates (frequencies), suggesting an increase in hydraulic permeability, with implications for loss of function pertinent to running and impact-injury. Running caused substantial changes in fluid-solid interactions in aggrecanase-resistant mice, suggestive of tissue degradation. However, higher nanodynamic stiffness magnitude and lower hydraulic permeability was observed in running aggrecanase-resistant mice compared to running wildtype controls at the same age, thereby suggesting protection from joint-overuse.