ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

and the plaque inner surface, the device was subjected to cycles of axial shortening of
about 5%.
4. RESULTS AND DISCUSSION
Comparison between cyclic pressure and axial compression load in case of concentric
plaque. The effects of cyclic pressure and axial compression loading in terms of mean
and alternating strains, are reported in Figure 5, for the concentric plaque model.
Figure 5. Stent after the placement in the atherosclerotic SFA model. In the black boxes are proposed the
distribution map of mean and alternating ε 1 principal in the most stressed zones, in case of cyclic pressure (a)
and in case of axial compression loading (b).
The comparison between the two different loading conditions shows that εm values are
comparable as determined by the oversizing ratio, that is essentially the same in the two
cases analyzed: the maximum values in both cases corresponds to the plaque peak,
where the oversizing is higher. On the contrary, as regards to the εa values, it is evident
that the strain induced by cyclic pressure is one order lower than the strain induced by
axial compression (0.031% vs. 0.16%). In both cases, the maximum value of εa is
located at the ends of the plaque, where the plaque has a less thickness: this is due to the
non-uniform stiffness of the vessel-plaque system related to the different thickness of
the plaque along the longitudinal and radial directions. As a consequence, the stent
undergoes a higher loading in relation to the low radial and axial stiffness of the vesselplaque
wall. Moreover, the comparison results are presented in terms of constant-life
diagram. Figure 6 shows a representation of the pairs of values (εm; εa) resulting from
cyclic pressure and axial compression loading on the stent deployed in the SFA model;
they are compared with the fatigue limit for 10 7 cycles, determined by Pelton and
colleagues 7 . Considering that the material parameters of the Nitinol tested by Pelton et
al. are not known and hence they could not correspond to the ones used in the finite
element simulations, in Figure 6 the limit curve obtained from literature (continuous
line) was shifted up and down (dot lines) in order to represent lower or greater risk of
fatigue failure for the material. This representation was made to take into account a
possible variability in the material fatigue properties.
Based on these results, it can be concluded that the loading due to arterial blood
pressure does not represent the predominant cause for the device fatigue failure. In both
loading conditions, it can be noted that all the most stressed point are below the curve
that defined the fatigue limit of the material: however, it must keep in mind that this is
only a qualitative comparison, because the fatigue characterization of the stent material
is not available.