Multiscale Approach to Understand the Multiphysics Phenomena in Bone Adaptation - Multiscale Computer Modeling in Biomechanics and Biomedical Engineering - page 31

shear sensitivity of the osteocytes?; v. What are the consequences of the micro-
scopic physico-chemical properties of the bone microstructure on the mass
transport within the lacuno-canalicular system? Finally, from these simple
model-driven observations, we propose a new perspective to alter the current bone
adaptation paradigm.
1 Introduction
The mechano-transduction process consists of a chain of three main functions:
mechano-reception, mechano-transmission and mechano-activation. Whether they
are biological or more generally industrial, these functions make that some sys-
tems act as mechano-sensors and/or senso-actors. Thus, these systems are able to
gather and transmit information on their neighborhood and on themselves, mod-
ifying so their properties in response to various physico-chemical solicitations.
The living materials, which are intrinsically active systems, are typical adaptive
systems that may change their response in function of the context. Their activity
generates changes in mass, composition and shape. This phenomenon is called
remodelling.
As such bone tissue can sense, react and adapt itself to its environmental
vicinity. For instance, when submitted to a cycling loading generated by walking
activity, bone is able to change its morphology and its mechanical properties by
forming, respectively resorbing, bone matter in the high, respectively low, stress
zones. For instance, it is well known that the bone quality decreases during space
flight [ 22 , 101 ].
Thus bone formation and resorption is the result of a series of events trans-
forming a physical information into a biological response. This process including
all the phenomena characterizing the bone cell ability to sense mechanical stimuli
and possibly to reply is called the mechano-transduction of bone remodelling. As
sketched in Fig. 1 , the cycle of the mechano-transduction of bone remodelling can
be coarsely summarized as: a macroscopic external physical stimulus (i) is prop-
agated within the bone tissue (ii), and then sensed at the microscale by sensitive
cells (typically the osteocytes, OCY) (iii), that induce signals emission (iv) to
activate effector cells that will resorb (osteoclasts, OCL) old tissue and create
(osteoblasts, OBL) new one (v), thus modifying the macroscopic properties of the
organ (vi).
This adaptation results in the optimization of bone morphology to obtain the
best mechanical resistance using the minimum mineral quantity. This was already
postulated in the nineteenth century by the surgeon Julius Wolff [ 162 ]. The Wolff's
law is part of the general trend in the biological community of the late nineteenth
century. This trend can be crystallized by the statement of Spencer: ''Life is
definable as the continuous adjustment of internal relations to external relations''
[ 145 ]. If this theory has been roughly checked thanks to experiments and clinical
observations, many avenues of research are still open [ 36 , 137 ].