NANOINDENTATION OF BONE - Nanoindentation with Biological Applications - page 211

anisotropy but will be weighted toward the properties in the direction of
the applied load. Swadener et al. (2001) demonstrated that, through
integration of measured elastic constants over the indented plane, an
anisotropy ratio (based on elastic modulus E ) can be determined
that accounts for the indentation modulus ( M ) ratio in anisotropic
materials. 56,120 For bone, the E ratio of 1.75 (ratio of longitudinal to
transverse moduli) corresponds to a M ratio of 1.4. 120 This approach, in
combination with testing bone of experimentally varied compositions,
was used to demonstrate a potential mechanism for anisotropic material
behavior. 106 Bone mineral therefore was implied to have greater
connectivity in the longitudinal versus the transverse directions.
3.2. Heterogeneity
The three-dimensional structure and composition of bone is impressively
complex. Nanoindentation is an ideal tool for studying the mechanical
behavior within individual millimeter-scale structures, such as in
trabecular bone, or micrometer-sized tissue-level features, such as
lamellae. Further, probing at the nanometer scale permits the study of
bone's compositional heterogeneity at the tissue-level and the
relationships between the three phases of bone material.
Nanoindentation overcomes many of the difficulties that exist in
accurately mechanically testing small volumes of bony structures.
Attempts have been made to isolate individual osteons, lamellae, and
single trabeculae for micromechanical testing. 121-124 Such examinations
are complicated by our inability to accurately dissect tissues at very
small scales without causing damage to the test specimen. Testing is also
complicated by the geometry of the structural features of interest, such as
in the case of a single trabeculae, where the cross-section varies in
diameter and geometry along its length. 125 In addition to sample
preservation and preparation, micromechanical testing is complicated by
adequately gripping the sample, slippage, edge effects, and high
measurement uncertainty—especially for custom-built mechanical
testing systems. Such factors may profoundly influence testing results.
Nanoindentation avoids such complications by providing a means of
testing the bone material in situ .