Biomedical Engineering Reference
In-Depth Information
In addition to these features of the mechanical stimulus, the mode of loading
influences damage formation. In particular, whether a moiety of bone is
exposed to compressive or tensile strains influences the type of damage
that forms.
180, 181
compressive loading produces few but long microcracks,
suggesting that cracks in compression are difficult to initiate but propagate
easily.
181
They are thought to result from the shearing of longitudinally oriented
osteons.
182
In contrast, tensile loading primarily produces diffuse damage,
which suggests that cracks in tension have a lower initiation threshold but
do not readily propagate.
181
Questions have been raised regarding whether microdamage in bone
is good or bad. The simple answer is that it is both. Microdamage can be
good, as bone is unique from non-biological structural materials in that it is
capable of self-repair through targeted remodeling (Fig. 2.12B). This was
demonstrated by Burr and colleagues
183, 184
who found new bone BMus
to be four- to six-times more likely to be associated with fatigue-induced
microcracks than by chance alone. Furthermore, when bone remodeling is
suppressed by treatment with bisphosphonates, the amount of microdamage
significantly increases.
185-187
Thus, damage serves as a stimulus that activates
bone remodeling, with the initiating stimulus possibly being osteocyte
apoptosis.
188, 189
The remodeling response ensures relative homeostasis
between damage formation and its repair and maintains skeletal mechanical
competence. It also enables a bone to adapt over time to its mechanical
environment such that it can meet increasing levels of loading.
On the other hand, microdamage in bone may be bad as it has been
associated with a pathology continuum that includes stress fractures and
complete bone fractures.
190-192
damage serves as a stimulus that activates
bone remodeling which normally removes damage approximately as fast as it
occurs. however, remodeling is time dependent with the time required to reach
a new equilibrium following the development of damage being in the order
of one remodeling period (approximately 3-4 months).
125, 126
If insufficient
time is given to adapt to a mechanical stimulus, further damage may occur
with additional loading. although a remodeling reserve exists that allows
increased activation of remodeling units in response to increases in damage
formation, an increase in the number of active remodeling units removes bone
temporarily, reducing bone mass and potentially increasing the chance for
damage initiation when loading is continued. This feed forward results from
the fact that resorption precedes formation in the remodeling process so that
an increase in the number of currently active remodeling units is associated
with an increase in bone porosity. This reduces the elastic modulus of the
bone, which in turn increases strain and, subsequently, the rate of damage
formation. Thus, microdamage can be viewed as bad as it can be associated
with a progressive loss of stiffness and strength as microcracks coalesce
forming larger macrocracks until eventual fatigue failure is reached.
