Biomedical Engineering Reference
In-Depth Information
7
Cortical Bone Remodeling under
Mechanical
Stimulus
7.1 Introduction
As an extension of the formulation presented in Chapter 6, this chapter
describes a model of bone cell population dynamics for cortical bone remod-
eling under mechanical stimulus. In the mathematical model presented here,
the interstitial fluid shear stress retaining the lacuno-canalicular porosity
structure caused by mechanical loading is considered [1,2] as the physical
mediator of mechanotransduction by osteocytes, along with the bone mech-
anosensitivity defined as the function of loading frequency, the number of
loading cycles during a loading day, the rest time between loading bouts,
and the length of loading period. The extended Hill equation is described to
study the case where two ligands bind to their respective receptors on the
same cell.
Three rate equations describing changes of osteocytes, nitric oxide (NO),
and prostaglandin E2 (PGE
2
) are included in this model. A standard bone
fracture energy (BFE) is introduced for comprehensively assessing the bio-
mechanical significance of mechanical bone remodeling and is shown to be
more appropriate than traditional methods such as bone mineral content
(BMC) in terms of experimental results. The results from this model are in
good agreement with experimental observations extracted from the litera-
ture, including the rapid release of NO after loading, the percentage of BMC
increase for different loading regimes, and increased percentage of BFE. The
chapter presents a brief description of the dynamics model presented in ref-
erences 1-4.
At the cellular level, bone remodeling is an organized process where
osteoclasts remove old bone and osteoblasts replace old bone with newly
formed bone. The osteoclasts and osteoblasts work in a coupled manner
within a BMU, which is a mediator mechanism bridging individual cel-
lular activity to whole bone morphology [1,5] that follows an activation-
resorption-formation sequence [6].
Bone is a metabolically active tissue capable of adapting its structure and
mass to the biological and mechanical environment and repairing damaged
171
