Multiscale Approach to Understand the Multiphysics Phenomena in Bone Adaptation - Multiscale Computer Modeling in Biomechanics and Biomedical Engineering - page 63

Even if huge steps have been made since the early works of Julius Wolff, the
current bone remodelling paradigm still draws from the vision of the German
surgeon: the general point of view consists in analyzing the different biophysical
phenomena in the light of the mechanical state of the skeleton. Although in vivo
chemical transport seem to be crucial in the behaviour of bone tissue and despite
the efforts done to properly describe the biochemical autocrine and paracrine
cascade leading to the removal and formation of tissue [ 28 ], mass transport at the
organ scale remains the poor relation of the bone remodelling paradigm. The
peculiar role of electrically charged species in this biochemical remodelling
pathway is obvious, with the calcium ions at the top of the list.
If we follow up the possibilities emerging from this work, a step toward a new
paradigm of bone adaptation can be made. Indeed, adopting a multiscale vision,
we are able to include significant innovative elements in the representation of the
remodelling process linked to transport phenomena within bone volume. As
illustrated by Fig. 11 , we could propose that the formation and resorption of bone
are mainly controlled by the ability to provide in situ calcium ions from the blood
supply to the osteocytes inside the bone system. This mechanism could be regu-
lated by ionic permselectivity induced by the electric surface charge of the lacuno-
canalicular pores. The cationic/anionic permselectivity is the ratio of cations/
anions based on the total number of ions that pass through the selective nano-
porous lacuno-canalicular material. In response to a mechanical solicitation of
bone and the concomitant piezo-electric effects of the collagen-apatite matrix, the
tensile part of the tissue, respectively the compressive one, would generate a
positively charged environment that decreases the cationic flux, respectively a
negatively charged one increasing this flux [ 95 ]. As a result, the calcium transport
from the blood supply toward the osteocytes environment would depend on the
local loading conditions. This asymmetry would engender different extracellular
calcium concentrations, modifying the osteocytic signaling pathway and so the
bone adaptation [ 83 ]. Thus, the in vivo tracer experiments of Tami et al. [ 147 ]
could be a clue of this permselectivity effect in the lacuno-canalicular transport. In
this study, it was shown that the load-induced transport mechanisms of negatively
charged and neutral dextran particles are different. If the neutral species remained
confined in the vasculature, the anions did penetrate inside the extravascular pores.
If these authors also indicate that this trend is more pronounced in the tension area
of bone, no information is given concerning the compressive zone of bone. This
has to be done now to reinforce our proposal.
By giving some answer to recurrent questions in bone biology, a reexamination of
the common point of view of bone remodelling models can be proposed, raising up
new questions. In particular, the focus should be made on the phenomena occurring at
the cellular scale. We hope that the concomitant advances in bone modelling at the
micro- and nanoscale (homogenization, molecular dynamics, etc.) and in micro-
scopic experiments (imaging process, micro-sensors, etc.) will result in an inflexion
of the current paradigm of bone adaptation replacing stress-controlled transport
phenomena in the heart of the problem. Thus, through such an interscale approach,