NANOINDENTATION OF SOFT TISSUES AND OTHER BIOLOGICAL MATERIALS - Nanoindentation with Biological Applications - page 284

cartilage tissue. 19 Interestingly, the intact cartilage exhibited an increase
in contact stiffness as a function of depth, while the repair cartilage,
which was shown through histological analysis to be fibrocartilage, did
not show a dependence on penetration depth. This difference in behavior
was attributed to differences in structure in the two tissues, with the
intact cartilage having the graded structure typical of articular cartilage
with parallel-aligned collagen in the superficial layer, while the
fibrocartilage was more uniform in structure and composition. 19 The
results of this study reinforced the conclusions of the previous cartilage
repair study, again demonstrating the importance of mechanical
assessment when interpreting the functionality of repair tissue, and
showing that nanoindentation can be used to perform that assessment. In
addition, the comparison of PFA-fixed and unfixed healthy cartilage
clearly demonstrated the importance of sample hydration and choice of
hydrating fluid on mechanical properties.
In a study by Li et al. , 20 mechanical parameters measured through
nanoindentation in specific regions of rabbit metacarpophalangeal
(finger) joint cartilage were correlated with morphological parameters
such as cell density and thickness of the superficial zone. This study
followed the methodology of the cartilage repair study by Ebenstein
et al. , 18 performing indents using a 100 micrometer radius of curvature
conopherical diamond probe tip and reporting the functional parameters
of contact stiffness, resistance to penetration, and volumetric creep strain
to avoid assuming a specific contact model. 20 Indents were performed at
predetermined regions of interest on the joint surfaces expected to have
different properties due to different loading conditions in vivo . The
functional mechanical properties were compared to morphological
parameters obtained with light and polarized light microscopy to
correlate local cartilage structure with mechanical properties. Both
stiffness and resistance to penetration were found to correlate strongly
with superficial zone thickness, suggesting that the superficial zone,
where collagen is aligned parallel to the surface, plays a significant role
in supporting load. 20
All three of the above studies could not have been performed using
bulk mechanical testing techniques due to the small size of the regions of
interest in the tissue samples. In all three cases, the cartilage was tested