Biomedical Engineering Reference
In-Depth Information
with time when
P
= 1.6 kN. In other cases, the remodeling rate decreases
with the remodeling process. Nevertheless, the remodeling rate of surface
remodeling increases during the remodeling process. The surface remod-
eling finally reaches an equilibrium state at the same time as the internal
remodeling, but the time at which the surface remodeling process begins
is different. The greater the axial loading is, the earlier the onset of surface
remodeling will be.
These results can be illustrated as follows. We can see that surface bone
remodeling is not triggered because of a comparative insufficiency of the
growth factors that are generated by the body when
P
= 1.6 kN. Therefore,
the internal bone remodeling rate remains unchanged. However, as the envi-
ronmental stimulus increases, more growth factors are secreted and finally
exceed the consumption capacity of the internal bone remodeling. Some
of the growth factors are then transported to the bone surface, where new
bone material is deposited and surface remodeling is triggered, resulting in
a decrease of the rate of internal bone remodeling. As the porosity of bone
structure decreases, more and more growth factors are transported to the
bone surface. Hence, the internal bone remodeling rate decreases as the sur-
face rate increases, and they finally cease at the same time when the remod-
eling process reaches equilibrium.
This is different from the following cases because of the inhomogeneity
of the environmental stimulus. Moreover, if the environmental stimulus
increases, more growth factors are released to the bone fluid. It is obvi-
ous that saturation can then more easily be reached and surface remod-
eling triggered earlier. Coupled remodeling is obviously more effective
than separate remodeling. The final porosity is then increased as the envi-
ronmental stimulus increases. Bone tissue seems to have the capacity to
prevent the bone structure from becoming so dense that the bone fluid
cannot be transported effectively. Determination of the exact threshold is
still an open question; further theoretical and experimental investigation
is needed.
5.5.2.2 Effect of Transverse Pressure on Bone Remodeling Process
In this section we consider the influence of transverse loads on the bone
remodeling process. The results are shown in Figures 5.12 and 5.13. The
applied loadings are
P
= 1.2 kN,
p
t
= 0.1, 0.2, 0.3, 0.4, 0.5 Mpa; no other load-
ings are applied.
It can be seen from Figures 5.12 and 5.13 that transverse pressures have
some effects on the bone remodeling process that are similar to those of axial
loadings. For example, transverse pressure can also make the bone structure
stronger and denser. It can increase the cross-sectional area of a bone and
make it thicker. Moreover, the change rate increases as the magnitude of the
loading increases. For surface remodeling, a difference in trigger time also
exists. However, as noted, other differences are evident. First, for internal
