Multiscale Approach to Understand the Multiphysics Phenomena in Bone Adaptation - Multiscale Computer Modeling in Biomechanics and Biomedical Engineering - page 40

level would cause bone fracture. This paradox reinforces the idea that the direct
mechano-sensation of the mechanical loading by the bone surface cells (OBL,
BL and OCL) is certainly not the main sensing pathway.
• Micro-cracks Notwithstanding the reversible micro-strains, physiological
observations of bone tissue exhibit that normal bone presents micro-cracks
[ 2 , 114 ]. These cracks originate within the bone cortex and tend to merge and
propagate along the cement lines that form the outer layer of the osteons [ 113 ,
140 ]. This indicates that one role of the cement line could be to deflect the bone
micro-cracks propagation and thus limit the fracture risk. In the wake of these
imaging results, new scenarios of the bone remodelling initiation have been
proposed. For instance, the stress concentration phenomenon that is inherent to
these micro-cracks has been proposed to be the key textural phenomenon
inducing the remodelling process [ 62 ]. Sites of remodelling in cortical bone
have been indeed shown to occur in conjunction with micro-cracks [ 20 ]. In
particular, it has been observed experimentally that a strong association between
microdamage, osteocyte viability and modulation of remodelling activity does
exist [ 151 ]. This supports the idea that osteocyte apoptosis may play a role in the
signalling mechanisms by which bone is remodelled after microcrack formation
[ 111 ]. It has also been suggested that a micro-damage occurring inside the
osteonal volume may generate a cell transducing mechanism based on ruptured
osteocyte processes [ 48 ]. Concomitantly, micro-cracks are likely to alter the
fluid flow and convective transport through the bone tissue and thus modify the
hydraulic behaviour of the fluid in the vicinity of the sensitive cells [ 40 , 73 , 103 ,
108 , 110 ]. As shown hereafter, the fluid environment of osteocytes plays also a
crucial role of bone mechano-sensation.
• Bone piezo-electricity: a 60 years old idea From the birth of electrophysiology
in the wake of Galvani's work in the late eighteenth century [ 120 ] to the con-
temporary electromagnetic medicine, the action of electricity on living tissue
fascinates. Focussing on bone electricity, the year 2012 is the sixtieth anni-
versary of the discovery of the piezo-electricity of bone which was reported by
Dr. Yasuda from Kyoto, Japan [ 70 ].
The piezo-electric properties of dry bone are not due to the apatite crystals,
which are centrosymmetric and thus non-piezo-electric, but to collagen molecules
[ 37 ]. Collagen exhibits the polar uniaxial orientation of molecular dipoles in its
structure and can so be considered as a sort of dielectric material. In the 1960s,
electric measurements in bone tissue [ 164 ] motivated the hypothesis that bone
adaptation could be explained thanks to collagen piezo-electricity. Historically, it
was argued that a mechanically loaded bone induces compression on its concave
side and tension on its convex side [ 10 ]. Due to the piezo-electricity of bone
collagen, negative charges are visible on its compressive side and positive charges
on its tensile parts [ 95 ]. Thus, it is stipulated that, in the electro-negative zone,
osteoblasts would be stimulated, increasing bone formation, whereas osteoclasts
activity, and therefore bone resorption, would be improved in the electro-positive
zone. If the polarization implications seem to be quite easy to understand when