Multiscale Approach to Understand the Multiphysics Phenomena in Bone Adaptation - Multiscale Computer Modeling in Biomechanics and Biomedical Engineering - page 39

2.4.2 The Osteocyte: A Good Candidate for Mechano-Sensation
On the other hand, many authors stipulate that osteocytes are the bone mechano-
sensing and mechano-transducing cells. Indeed, osteocytes are the most abundant
cells in bone tissue, since there are 10 times more osteocytes than osteoblasts,
which are themselves more numerous than osteoclasts. Furthermore, the special
spatial organization of the osteocytes within the bone cortex forms a tridimen-
sional network connected to the remodelling actors located at the bone surface.
Cell-to-cell communication of electrical signals and small molecules through
gap junctions has been demonstrated in osteoblasts. There is evidence for similar
gap junctions in osteocytes, and it seems likely that they participate in such
communication with osteoblasts, and bone-lining cells as well.
2.5 Bone Remodelling Signals
There has been considerable speculation that osteocytes produce a signal pro-
portional to mechanical loading by sensing different remodelling signals within
bone tissue through stretch-activated ion channels, interstitial fluid flow, electrical
potentials, or some other phenomenon. In this subsection, the main stimuli that can
induce bone remodelling are presented.
2.5.1 Stimuli Originating in the Solid Matrix of Bone
According to the Wolff's law [ 162 ] that roughly states that bone is preferentially
deposited in the area characterized by a high mechanical solicitation and removed
where it is not mechanically needed, the first research of the bone remodelling
were linked to the skeleton deformations.
• Bone matrix micro-strains Considering different physical activities, micro-
strains of the human skeleton have been measured thanks to micro-gauges
exhibiting values ranging between 0 : 04 and 0 : 3 % [ 21 , 71 ]. The key parameters
that directly influence the biological response of bone tissue have been shown to
be the strain amplitude [ 136 ] and the strain rate [ 13 , 49 , 72 , 134 ]. In particular, it
is known that bone adaptation is proportionally governed by the strain rates
[ 116 , 149 ].
• Hiatus between in vivo and in vitro micro-strains In physiological conditions
typically corresponding to normal locomotion activities, the bone tissue strains
that can be measured in vivo remain rather small since quantitative data
obtained for running horses and men and fast flying birds present maximal
values around 0 : 2 0 : 3% [ 21 , 133 ]. These measurements are paradoxical when
compared to the in vitro necessary strains that induce a cellular response which
are one or two order higher, from 1 to 10 % [ 34 , 135 ]. In vivo, such a huge strain