NANOINDENTATION OF BONE - Nanoindentation with Biological Applications - page 220

indentation modulus of interstitial bone is the highest followed by
osteonal and then trabecular bone. The nanoindentation modulus of dry
trabecular bone samples measured by nanoindentation ranged from 8.2 to
19.4 GPa, whereas cortical (both interstitial and osteonal) bone was
significantly higher (18 - 26 GPa). 45,57,59,119 Osteonal bone is generally
reported to have a lower modulus value than the higher mineral content,
older interstitial bone. 60 The measured moduli for human bone depend
greatly on a vast number of experimental, compositional, structural, and
anatomical parameters. Additionally, the values reported above are for
dry bone and thus do not represent true in vivo values. Despite the
variability, general property relationships between types of bone are
maintained and thus may be investigated using nanoindentation.
Nanoindentation tests to shallow depths can be used to test the
mechanical properties of smaller structural features such as individual
lamellae. For example, using a ramp-and-hold test to 500 N (100 nm),
Gupta et al . found alternating regions of high and low stiffness due to the
lamellar structure. 60 Less mineral dense regions (in thin lamellae)
showed decreased stiffness. 60 Rho et al. found a decrease in mean
modulus of the thick lamellae with radial distance outward from the
center of the osteons. 44 Combined nanoindentation mapping with 2D
Raman mapping, of similar spatial resolution, showed a homogenous
distribution of the phosphate band in osteons suggesting a uniform
mineral distribution. 168,169,170 Collagen fibers, not the degree of
mineralization, may not be the dominating faction in elastic modulus
values altering with lamellae. 170 X-ray microprobe analysis demonstrated
that the thick lamellae had 10-15% higher calcium and phosphorus
content than thin lamellae, 171 thus indicating that the mineral content may
have a profound effect on the mechanical properties. Most likely, both
collagen orientation and the degree of mineralization contribute to the
mechanical variations seen between thick and thin lamella. While further
studies are needed to fully understand the relationships between
structure, function, and composition; nanoindentation, in combination
with adjunctive techniques, is an ideal tool to elucidate the role of
microstructural variations in bone.