NANOINDENTATION OF SOFT TISSUES AND OTHER BIOLOGICAL MATERIALS - Nanoindentation with Biological Applications - page 282

articular cartilage. 21 Another study has also recently been performed to
assess the effect of indenter size on the equilibrium modulus of bovine
patellar cartilage measured by nanoindention, for future application to
measuring mechanical properties in cartilage in murine models. 22 These
studies are summarized in Table 9-1 , and each will be discussed in more
detail below. A schematic illustration of in situ cartilage indentation with
a spherical tip is shown in Fig. 9-1 .
In one of the early cartilage indentation studies performed using a
commercial nanoindenter, Ebenstein et al. 18 used nanoindentation to
compare the mechanical properties of regions of repair cartilage and
control cartilage in the rabbit knee, and to correlate those properties with
histological composition. Quasi-static indentation was performed with a
100 μm radius of curvature conospherical tip to generate load-
displacement curves in each tissue region. As illustrated in Fig. 9-2 ,
indents in tissue regions with different histological features (healthy
cartilage vs. hyaline-like repair cartilage vs. fibrocartilage) showed
significant variation in load-displacement behavior. Because of concerns
over the validity of the traditional indentation contact models for this
application, the authors did not use the compliance method 6 to calculate
reduced modulus values. Instead, the authors reported functional
parameters of contact stiffness, resistance to penetration, and volumetric
creep rate to quantify the indentation load-displacement behavior in
each tissue region. 18 Using these functional parameters, a correlation
was found between the mechanical behavior and the histological
classification of each tissue region tested. The results showed that even
when repair cartilage had developed some histological features of
healthy cartilage (hyaline-like repair), the mechanical behavior was not
fully restored. These results demonstrated the importance of mechanical
assessment of repaired cartilage in addition to histological analysis for
determining the functional behavior of repair tissue.
In a more recent cartilage repair study in miniature pig knees by
Franke et al. , 19 a correlation was again found between histological
markers of tissue composition and mechanical properties measured
by nanoindentation. In this study, indentation was performed using the
traditional Berkovich (3-sided pyramidal) tip, and the penetration depths