Page 336 - Vibrational Spectroscopic Imaging for Biomedical Applications

310 Cha pte r T e n

framework for fracture risk assessment, for determining how the
increasing propensity for fracture with age can be prevented, and for
developing new synthetic materials whose toughening behavior
mimics that of natural cortical bone. From a more general perspec-
tive, mechanical factors are known to affect bone remodeling as well
as an increased mechanical demand results in bone formation (i.e.,
while decreased demand results in net bone resorption). Current the-
ories clearly suggest that mechanical stress plays an important role in
31
bone modeling and remodeling. Therefore, microscopic informa-
tion of the stress patterns developed in bone may lead to a better
understanding of how bone functionality and crack healing can be
affected by mechanical loading.
The objective of our Raman studies has been that of investigating
how macroscopically applied external loading similar in magnitude
to that occurring in vivo during bone fracture manifest at the micro-
scopic level in the bone matrix. 2,15 Using the PS approach applied to
−1
the 980 cm Raman band of hydroxyapatite, a direct evaluation of
the stress tensor trace < σ ∗ > has been obtained and spatially resolved
ij
maps of microscopic stress experimentally determined around the tip
of a propagating crack. It was found that stress patterns were highly
heterogeneous and strongly related to the locations of the observed
microdamages (cf. scanning electron micrograph and stress map in
Fig. 10.5a and b, respectively), indicating that the resulting stress field
is significantly altered by the microdamages. In a recent study,
32
Thompson et al. reported that a recoverable bond in the collagen
molecules might contribute to conspicuous energy dissipation in the
postyield deformation of bone. To elucidate the role of collagen in the
postyield deformation of bone, microdamage accumulation has been
proposed to lead to surface energy dissipation during bone deforma-
tion, whereas the degradation and plastic deformation in the collagen
network are the major mechanisms in the inelastic and viscoelastic
energy consumption. 33–35 We have also confirmed the occurrence of
2
collagen stretching mechanism in a previous study. In Fig. 10.5a, it
can be seen that a cloud of microcracks is formed along a constant
direction. In addition, in correspondence of locally microcracked
areas, the crack-tip stress was released (cf. Fig. 10.5b). As a conse-
quence, the stress field around the crack tip assumed a peculiar stripe-
like morphology. In other words, bone is capable to partly release the
stress intensification at the tip of a propagating crack through the occur-
rence of a self-damaging mechanism. It should be noted that the use of
a confocal probe enabled us to minimize in depth averaging of the
Raman signal, so that a relatively sharp map could be obtained. In the
crack-tip experiments shown in this study, the confocal probe was
shifted below the sample-free surface by about 10 μm in order to
minimize surface effects. It is interesting to compare the crack-tip
stress field developed in cortical bone with that recorded in synthetic
polycrystalline alumina (scanning electron micrograph and stress