Bone Microdamage and Its Contributions to Fracture - Skeletal Aging and Osteoporosis: Biomechanics and Mechanobiology - page 93

4 The Contribution of Microdamage to the Biomechanical
Properties of Aging Bone
Bone accumulates fatigue microdamage in vivo due to cyclic loading from everyday
activities. However, as mentioned previously, microdamage repair may be reduced
with the deterioration of the bone remodeling process, leading to an excessive
accumulation of microdamage. This in vivo microdamage accumulation contributes
to deterioration of bone's mechanical integrity [ 2 , 44 ] where accumulated micro-
damage as well as its morphology affect both the elastic and post-yield properties of
bone [ 10 , 45 , 46 ].
In particular, microdamage accumulation in the form of linear microcracks is
correlated to loss of material stiffness, or modulus reduction [ 46 - 50 ]. It has been
shown that bone's elastic modulus decreases after cyclic loading due to accu-
mulation of microdamage, where the modulus loss has a linear relationship with
the amount of diffuse damage and a quadratic relationship with extent of linear
microcracks [ 46 ]. It has also been demonstrated that microdamage accumulates
only above a certain threshold of modulus degradation after approximately 15%
stiffness loss [ 46 ]. Bone may undergo significant modulus degradation even before
microcracks are evident, and the mechanical properties of bone can be compro-
mised even before substantial microcrack accumulation can be observed on the
microscopic level [ 46 , 51 , 52 ]. These data suggest that the presence of micro-
damage at the sublamellar level may contribute to the deterioration of bone's
mechanical properties. Consistent with this notion a recent study shows that,
similar to the loss of whole bone material properties, both nano- and micro-
mechanical properties are significantly lower in damaged bone compared to
controls [ 50 ].
Bone's post-yield and fracture properties are more directly assessed through
variables that describe bone's fracture resistance including strength, toughness,
and crack propagation parameters. Particularly, literature shows that cortical bone
toughness is negatively associated with microdamage accumulation [ 44 , 53 ].
There is a two- to three-fold increase in microdamage accumulation under sup-
pressed remodeling by bisphosphonates. This accumulation is associated with
approximately 20% decrease in bone toughness without any changes in bone
strength [ 54 ]. In contrast to strength, toughness provides a measure of the amount
of energy bone can absorb per volume before failure, independent of the shape or
size of the bone (see Appendix). Currey et al. showed that microdamage accu-
mulation affects bone toughness more significantly than strength [ 55 ].
Crack density, size, and propagation parameters measured during or after
fatigue loading also provide useful information about bone's fracture resistance
[ 14 , 46 , 56 ]. Literature shows a negative correlation between microdamage density
and fracture toughness [ 53 , 57 - 59 ]. It has been proposed that bone regions where
cracks easily initiate but do not propagate are more fracture resistant than bone
regions where cracks cannot easily initiate but propagate quickly once formed
[ 23 , 60 , 61 ]. Consequently, fatigue-resistant materials derive their properties