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

Figure 1. Scheme of the two approaches used to simulate stent overlapping. On left, two distinct stents
was used, with an overlapping region of 11 mm (corresponding to four strut rings). On right, a single stent
model was considered, for which to the four central strut rings a twice elastic modulus was assigned.
Standard rigid-flexible contact pairs were created between the rigid body and the outer
surface of the stent. No separation flexible-felxible contact pairs were defined between
the outer surface of the stent and inner surface of the artery. For both type of contact a
friction coefficient of 0.2 was assumed. In the double stent models approach another
contact pairs have to be defined between the two overlapped stents. Based on the
hypothesis that in the overlapping region relative movement of the two stents is nearly
zero a bonded flexible-flexible contact pairs were defined between the inner surface of
the first deployed stent and the outer surface of the second deployed stent. According to
literature [11], for all the 3 simulations considered in this study (see Tab. 1), cyclic
loading compression of 5% was applied by means of displacement control on the ends
of the artery model.
Proper boundary conditions were applied during the various step of the analysis. In
Table 1 the steps required for the analysis were summarized for both double stents and
single stent simulations.
Table 1. Summary of the simulation steps for the three considered models.
step Double stents model
Single stent model
(with stiffening)
Single stent model
(without stiffening)
1 Crimping of the two stents Crimping of the stent Crimping of the stent
2 Deployment of the first stent Deployment of the stent Deployment of the stent
3
Deployment of the second
stent
Axial compression (5%) Axial compression (5%)
4 Axial compression (5%) Axial release Axial release
5 Axial release - -
As reported in literature [12,13] the risk of fatigue fracture was assessed on the basis of
amplitude and mean values of the cyclic first principal strain through the stent.
Appropriate user-define script written in APDL language were developed to compute
mean (ε I m) and alternating (ε I a) first principal strain values that occur in each stent
element in response to the application of a load cycle. The results thus calculated are
shown in the form of contour maps and plotted on a constant-life diagram as mean
strain vs. strain amplitude, on which they are qualitatively compared to the 10 7 fatigue
strain limit of Nitinol for stents derived from Pelton et al. [13].