Biomedical Engineering Reference
In-Depth Information
focussing on the solely mineralization process [
112
,
163
], the mechanisms by
which charges and piezo-electric properties affect osteoblasts and osteoclasts
responses are more difficult to be identified [
11
,
107
]. Moreover, this piezo-electric
craze faded away in the 1980s when more compelling mechanisms related to the
interstitial fluid movement began being studied [
55
]. Only recent studies proposed
that piezo-electricity could induce an increase in the electric charge of the bone
tissue and thus engenders an opposite electro-osmotic flow limiting the total
interstitial flow, and thus increasing the apparent stiffness and the mass transport
properties of bone tissue [
3
,
78
]. Moreover, it has been recently proposed that the
mineral crystals of the bone matrix could act as an electric storage system [
106
].
Notwithstanding this renewed interest in bone piezo-electricity, important com-
ponents of this hypothesis have to be demonstrated and several questions remain.
In particular, due to the poro-mechanical coupling, the stress (or strain) field of the
solid phase of bone cannot be separated of concomitant interstitial fluid movement.
Spurred on by the pioneering paper of [
121
] in the late 1970s, the mainstream idea
in the bone community became that the mechano-transduction of bone remodelling
was a flow-induced phenomenon [
35
].
2.5.2 Bone Fluid Flow Signals
It had been for a long time believed that the sole function of bone interstitial fluid
movement in the lacuno-canalicular pores was to provide nutrients and remove
wastes. The strain induced micro-flows were first proposed by Piekarski and
Munro [
121
]. However, these lacuno-canalicular micro-flows have only been
experimentally observed 20 years later by tracer studies [
67
-
69
,
155
]. This diffi-
culty to carry out convenient in vivo experiments to measure hydraulic fluid
velocities and interstitial fluid pressure within bone tissue motivated the model-
driven investigations of the bone behaviour.
• Evidence of the fluid flow and stretch in bone cell activity Since it forms the
immediate environment of bone cells and the pathway for nutrient supply and
waste removal, the role of bone interstitial fluid in bone activity is evident [
66
].
In vivo, when comparing the effect of static [
72
] and cyclic mechanical loading
[
135
], only the later mechanical condition positively influences bone formation
for peak strains of 0
:
1 %
:
The difference between these two loading types is that
the dynamical loading induces interstitial fluid movement. In parallel, several
in vitro studies proved that bone cells were sensitive to neighboring fluid flow
[
54
,
96
,
161
]. Thus, several studies demonstrated that bone cells are more
responsive to fluid flow than to mechanical strain. For instance, the strain-
induced osteoblastic response measured by Binderman et al. [
14
] is 6 times
lower than the flow-induced response observed by Reich and Frangos [
127
,
128
]. When focussing on the osteocytes, if their in vivo sensitivity to the
mechanical loading was known in the late 1980s [
143
], the strong influence of
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