Nanoindentation differentiates tissue-scale functional properties of native articular cartilage

Cartilage mechanical properties are typically tested at the macroscale. To demonstrate the ability of nanoindentation to characterize in situ articular cartilage properties at the tissue scale, we investigated the local structure-property relationships of intact articular cartilage of a normal rabbit metacarpophalangeal joint. We calculated the mechanical parameters of stiffness, S, resistance to penetration, R, and volumetric creep strain, dV/V, from nanoindentation of the articular surface at specific regions of interest. We measured morphological parameters of superficial zone thickness, middle zone thickness, total uncalcified thickness, and cell density from corresponding regions with light and polarized light microscopy. Mechanical parameters were compared to morphological parameters. There were significant positive correlations (r = 0.98, p < 0.05) between superficial zone thickness and both S and R. However, we found no significant correlation between dV/V and the zone sizes. There were moderate, negative correlations between cell density and both S and R, suggesting an effect of cell volume on cartilage behavior at the tissue scale. We opine that the superficial zone plays important role in load support, as evidenced by correlations between zone size and intact cartilage mechanical properties.