Biomedical Engineering Reference
In-Depth Information
Compact bone structures are susceptible to failure when subjected to
cyclic loadings, which often generate microfractures. So the remodeling pro-
cess should have a function of repairing the damage in osteonal bone. It is
known that the bone resorption function is mainly attributed to osteoclasts
and bone formation to osteoblasts. When bone tissue is damaged, osteo-
clasts remove necrotic osteocytes. Growth factors such as BMP or TGF-β
exist in latent forms in osteocytes. They are activated at the site of bone
resorption by osteoclasts and released into the bone fluid. Osteoblasts are
then stimulated by these growth factors to form bone and fill up resorption
cavities.
It has been proposed that under normal circumstances the generation of
damage by loading and its repair by remodeling can reach an equilibrium
state in which the damage burden waiting to be repaired is tolerable [37].
If the loading increases, more microcracks are generated and more osteo-
cytes are removed. This results in more growth factors in the bone fluid to
accelerate bone formulation and maintain the equilibrium state. It has also
been observed that when loadings are excessive, accelerated remodeling not
only removes damage at a higher rate, but also increases the rate of damage
production [47]. In contrast, a decrease in loading can also result in fewer
microcracks and consequently lower presence of growth factors. It can thus
be seen that bone tissue can remodel itself well to protect itself from damage
and keep its mass unchanged.
But where do the two strain thresholds come from and why can the
bone tissue change its mass and structure? Qu et al. [2] hypothesized that
when the electrical signals change within a certain range, the quantities of
growth factors hidden in osteocytes remain unchanged. When the electri-
cal signals exceed this range, the quantities of growth factors in osteocytes
will increase or decrease. Then the bone tissue begins to model itself. If the
growth factors increase, more new bone tissue can be deposited and the
bone mass will also increase. This can be considered as
MESm.
Similarly,
MESr
comes from the decrease of growth factors. The theory developed
based on this hypothesis could be used to analyze the magnetoelectro-
mechanical behavior of bone tissues in the modeling and remodeling
processes.
5.3 A Mechanistic Model for Internal Bone Remodeling
In this section, the constitutive model presented in Hazelwood et al. [1] for
bone remodeling, which includes a number of relevant mechanical and
biological processes, is described. The model can be used to identify differ-
ences in modeling behavior as a volume element of bone is placed in disuse
or overload.
