Biomedical Engineering Reference
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ences for the untreated and stented aneurysmal artery based on time frame indices
t/T
= 0.1, 0.3, 0.6 and 1.0.
7.4.5
Parametric Study for Design of Stent in Aneurysm
Pressure gradient and shear strain rate maps were analysed for the start and end of sys-
tole as well as the diastolic phase of one cardiac cycle which correspond to t/T = 0.1,
0.3 and 0.6 respectively. At the monitor point in the centre of the aneurysmal bulge,
these fluid mechanical values were extracted for comparison. Pressure gradients
were almost similar at the start of the time cycle (
t/T
= 0.1) at Δ
P
= 31.61 kgm
−2
s
−2
for the non-stented flow and Δ
P
= 31.61, 37.10, and 39.70 kgm
−2
s
−2
for stented
aneurysmal flows with 3, 4 and 7 struts present at the aneurysmal neck. However,
Δ
P
decreases with time. After stenting with 3 struts, Δ
P
decreases. With stenting us-
ing 4 struts, Δ
P
decreases further. Based on stenting with 7 struts, the pressure gra-
dient drops steeply to 0.392 kgm
−2
s
−2
at
t/T
= 0.3, Δ
P
= 0.0346 kgm
−2
s
−2
at
t/T
= 0.6,
and Δ
P
= 0.00843 kgm
−2
s
−2
at
t/T
= 1.0, which shows that stent is over-designed such
that the drop in pressure gradient becomes too excessive.
The same variation exists for the shear strain rate which is almost the same for
the non-stented case as the stented case initially at
t/T
= 0.1 (with
γ
= 1.53, 2.11 s
−1
,
1.72 s
−1
and 2.14 s
−1
(for untreated and stented cases with 3, 4 and 7 struts re-
spectively). However, g˙ decreases to 3.073 (for 3 struts), 1.66 (for 4 struts) and
0.359 s
−1
(for 7 struts) at
t/T
= 0.3, and to 1.69 (for 3 struts), 1.06 (for 4 struts) and
0.0579 s
−1
(for 7 struts) at
t/T
= 0.6, which shows gentle reduction compared to the
pressure gradient. After stenting with 7 struts,
γ
is reduced drastically. This denotes
too much blockage of flow into the aneurysm such that the shear strain decreases
excessively.
Based on the numerical simulation and flow analysis, flow fields on untreated
and stented aneurysmal arteries were characterized. The streamline tracings enable
the visualization of a large-scale vortex in an aneurysmal sac. The results demon-
strate that the stenting causes a reduction of pressure, velocity, vorticity shear rate.
Reduced pressure exerted by blood on the aneurysmal sac will decrease the risk of
rupture. However, lower volume of flow into the sac increases the viscosity of blood
in the aneurysm (Kim et al. 2010b). Reduced vorticity in the sac also corresponds
to a lower fluid shear stress and shear strain rate. Note that high shear stress is nec-
essary for preventing platelet-dependent thrombosis (Sukavaneshvar et al. 2000a).
Moreover, reduced blood into the aneurysm also means flow stagnancy and the
induction of thrombosis increases. All these undesirable conditions will aggravate
aneurysm rupture (Liou and Liou 2004a). Therefore, the type of stents deployed has
to be of sufficient porosity to minimise aneurysmal rupture (Kim et al. 2010b) but
prevent platelet aggregation. The study of fluid mechanical properties in non-stent-
ed and stented aneurysmal flow can enable medical experts to evaluate the effective-
ness of stent designs and their corresponding porosities in prevention of aneurysm
dilation leading to rupture.
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