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

4. RESULTS AND DISCUSSION
Results indicated that a proper way to apply global 5% compression on the stents was
used in simulations. In fact, after the compression, stents show a shortening about 4.7%
of the total length (45.8 mm) in all the 3 models.
Figure 2 shows the distribution of mean and alternating first principal strain through the
stent in the case of single stent without stiffening. A maximum value of 4.53% and
0.168% respectively was reached and the strain distributions are uniform along the
whole stent length.
Figure 2. First princiapl strain mean (on left) and amplitude (on rigth) distribution through the 5% axially
compressed stent in the single stent configuration without stiffening after deployment in a silicone tube.
In the magnification areas higher strain regions are depicted.
Concerning the double stents model, while there is no relevant difference in the
maximum value of mean strain, higher alternating strain is reached (0.2%). Moreover,
the alternating strain field (Figure 3) show a non-uniform distribution with higher strain
values concentrated in the links close to the overlapping portion. Here the passage from
two stents to a single stent caused a relevant change in the axial stiffness. Hence, the
regions next to overlapped portion were subjected to higher cyclic compression while in
the overlapping region lower value of ε I a ( 0.15%) is reached due to a higher stiffness.
The comparison on the constant-life diagram (Figure 3) of stent ε I m and ε I a strains in the
two above conditions, as expected, shows that the overlapped condition is associated
with a higher risk of fatigue failure. Indeed as the mean values are quite the same the
strain amplitude is higher (0.168% vs 0.2%). For the double stent model all the
alternating strain values higher than 0.168% are located in the link of the single stent
portion next to the overlapped region. These finding may explain the higher rate of stent
fracture at this location reported in literature.
The single stent model with stiffening of the central region shows interesting results.
The alternating strain distribution through the stent (Figure 4) is very similar to that
obtained for the double stents model. Higher strain values are concentrated in the link
near the overlapped region due to the stiffness change effect while the central portion is
subjected to lower cyclic compression. The comparison on the Constant-life diagram
shows very similar results for the two considered overlapped stent models confirming
that the simplify model is consistent in simulating the overlapping condition.
Moreover, the single stent model implies lower computational cost as the complexity of
the model is reduced due to the number of implemented contact required by the two
approaches (2 vs 5). The total computational time on an 8-cpus cluster node is
significantly reduced (about 3 vs 5 days).