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

typically occurs in cases of very long lesions, which often require the use of two or
more overlapping stents. Moreover, several authors [3,4,5,6] reported that in case of
overlapped stents fracture often occur in region next to the overlapped portion. As
hypothesized by Smouse et al. [7] this is due to the fact that the devices overlapping
portion has a greater stiffness than that of the individual stents, and as such tends to
deform less in relation of the biomechanical forces acting in the femoropopliteal district.
It is assumed therefore that in the stent portions adjacent to the overlapping region is
created a stress concentration which can lead to failure of the devices. Based on these
evidences, in this study, finite element method was used to investigate the cyclic axial
compression fatigue behavior of overlapped Nitinol stents after their deployment in a
simplified artery model. Two different approaches were adopted: i) simulating the
deployment of a stent into another stent and ii) using a single stent with double stiffness
in the overlapping region.
3. MATERIAL AND METHOD
To investigate the fatigue behavior of overlapped self-expandable Nitinol stents finite
element models were created using the commercial code ANSYS (Ansys Inc.,
Canonsburg, PA, USA). For this purpose stent models, resembling the geometry of the
Maris Plus self-expandable pheripheral stent (Medtronic Invatec, Roncadelle, BS, Italy)
with an outer diameter of 8 mm, were reconstructed. The Nitinol pseudoelastic behavior
was described through the Shape Memory Alloy material model implemented in
ANSYS [8], using typical material parameters.
To simulate cyclic axial compression of two overlapped stents after their deployment in
a simplified artery model two different approaches were adopted. In a first model
(double stents model) the subsequent deployment of two different stents with a common
overlapped region were simulated. In a second simplified way (single stent model) in
order to resemble the overlapped condition only a single stent with increased stiffness in
the central overlapping portion was considered. For the first approach two distinct stent
models were used each with a length of 28.4 mm. Based on literature [9,10], the
overlapping region was assumed to be 11 mm. Hence after stents deployment in the
artery model the two stents cover a total length of 45.8 mm. For the simplified approach
a single stent with a whole length of 45.8 mm was considered with a central region of
11 mm in which the material properties were modified increasing double the material
stiffness (Figure 1).
To better understand the stent fatigue behavior in the case of overlapping also a finite
element model with a single stent (total length of 45.8mm) without any stiffening in the
central region was simulated.
As the presence of the artery was considered only to simulate the oversizing to witch
stents are subjected in vivo, for both approaches a silicon tube with elastic behavior was
used. Accordingly with the in vitro study of Nikanarov [11] an oversizing ratio (outer
stent diameter/inner tube diameter) of 1.4 was assumed that corresponds to an inner tube
diameter of 5.7 mm. Both stent and artery models were meshed using completeintegration
cubic eight-node elements (SOLID 185).
Stent deployment in the artery requires the simulation of stent crimping within the tube
followed by a step in which the stent is left free to self-expand and comes into contact
with the artery inner wall. Both phases were simulated by imposing a uniform radial
displacement to a rigid cylindrical surface which is in contact with the stent outer
surface.