Biomedical Engineering Reference
In-Depth Information
11.5.2.4
Mechanical-Stimuli-Mediated Aggrecan
Biosynthesis
On the basis of experimental observations, cellular response to mechanical
stimuli exhibits a threshold behavior, with aggrecan production only trig-
gered when the interstitial fluid velocity (i.e., the Darcy velocity
v
d
) exceeds
a certain threshold (i.e.,
v
0
=0
.
25
m
/
s). Further increases in the fluid veloc-
ity were not observed to increase the aggrecan production rate [32]. These
observations can be described mathematically as
s
pm
=
λ,
µ
(
|
v
d
|≥
v
0
)
(11.80)
0
,
(
|
v
d
|
<
v
0
)
Recall that the maximum Darcy velocity, in the unconfined compression
tests we have focused on previously, occurs near the periphery of the cartilage
discs. It is expected then that the mechanical-loading-induced aggrecan pro-
duction will be mainly confined to the outer edge of the cartilage disc and for
the larger strain and loading frequencies cases. This expectation is borne out
in Figure 11.16, in which typical variations in the Darcy velocity profiles as a
function of radial distance from center of the cartilage under various loading
conditions (i.e., 0.1 Hz at 10% strain, 0.01 Hz at 10% strain, and 0.001Hz at
10% strain) are shown. It can be seen that a loading regime of high frequency
5.0
4.0
0.1 Hz at 10% Strain
3.0
2.0
0.01 Hz at 10% Strain
1.0
0.001 Hz at 10% Strain
0.25
m
m/s
0.0
0
0.3 0.6
Radial distance from the centre of cartilage (mm)
0.9
1.2
1.5
FIGURE 11.16
Profiles of Darcy velocity as a function of radial distance from center of the
cartilage under various loading conditions. The aggrecan synthesis is stimu-
lated in the region where the Darcy velocity
v
d
≥
v
0
(i.e., 0.25 [32]) and is
indicated by the dotted line.
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