Multiscale Approach to Understand the Multiphysics Phenomena in Bone Adaptation - Multiscale Computer Modeling in Biomechanics and Biomedical Engineering - page 37

tissue. By observing bone tissues, resorption and formation stages of a basic
multicellular unit (BMU) have been described [ 9 ]. Note that some authors prefer
calling it bone modelling unit. The work of this BMU consists in resorbing and
synthesizing bone material to generate a secondary osteon. Bone is so constituted
by billions of modelling units likely to create new osteons. Since each BMU is
chronologically and geographically separated from the others, it is possible to
adapt the quantity and the architecture of bone. This tacitly suggests that the
adaptation process depends on systemic or local factors. Indeed, the cellular events
responsible for remodelling are locally controlled, certainly by autocrine or par-
acrine factors generated in the bone micro-environment [ 64 , 87 , 159 ]. Classically,
the bone remodelling process is decomposed into 6 phases. Basically, this phe-
nomenon sequentially occurs as summed up hereafter [ 51 , 92 ].
2.3.1 Activation
In response to biochemical messages due to external physical stimuli, a BMU is
activated. The stimuli that initiates bone remodelling have not been yet identified
and a possible scenario is explored in the following of this article. The current
concept of bone remodelling is based on the hypothesis that osteoclastic precursors
become activated and differentiate into osteoclast. Thus, the lining cells are joined
by the mono-nucleate precursors of osteoclasts or pre-osteoclasts.
2.3.2 Resorption
The bone resorption cascade involves a series of steps to obtain the removal of
both the mineral and organic constituents of bone material. Roughly, the osteo-
clastic differentiated cells resorb old bone material. These cells ultimately undergo
apostosis.
2.3.3 Reversal Phase
After the maximum eroded depth has been achieved by the osteoclasts, there is a
reversal stage that lasts around 9 days. During this stage, osteoblasts counteract the
osteoclats activity; resorption is stopped at a surface corresponding to the cement
line that surrounds the future osteon.
2.3.4 Formation
This stage consists in the bone matrix formation in response to a complex cascade
of events. The osteoblasts synthesize the organic extracellular matrix called
osteoid and composed by collagen fibers arranged in concentric lamellae.This