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

NUMERICAL SIMULATIONS OF FATIGUE FOR
TWO OVERLAPPED STENTS IN PERIPHERAL ARTERIES
1. ABSTRACT
A. Meoli 1,2 , E. Dordoni 1,2 , L. Petrini 1 ,
W. Wu 1 , G. Dubini 1 , G. Pennati 1 , F. Migliavacca 1
A typical endovascular procedure to restore blood flow perfusion in stenotic peripheral
arteries consists in balloon angioplasty and deployment of self-expanding Nitinol stents.
However, movements during gait produce large and cyclic deformations of the
superficial femoral artery, increasing the risk of fatigue fracture of implanted stents.
Fatigue failure typically occurs in cases of very extended lesions, which often require
the use of two or more overlapping stents. In this study, finite element models were
used to study the fatigue behavior of overlapped Nitinol stents. 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. Simulations of
cyclic axial compression of stents deployed in a simplified cylindrical model of a
peripheral artery were then performed (Ansys Inc., Canonsburg, PA, USA). Fatigue risk
was assessed on the basis of amplitudes (ε I a) and mean values (ε I m) of the cyclic first
principal strain through the stent. Similar results were found in both approaches: strain
concentrates in the links close to the overlapping portion where the abrupt change in
stiffness causes higher cyclic compression (ε I a 0.19%), while in the overlapping region
the value of ε I a is lower ( 0.15%), due to a higher stiffness. These findings help to
explain the high incidence of stent fracture observed in various clinical trials located
close to the overlapping portion. Moreover, this study demonstrates the possibility of
using a simplified model with a region with double stiffness, reducing significantly the
computational time.
2. INTRODUCTION
Among different interventional techniques for treatment of stenotic peripheral arteries,
the most interesting endovascular procedure used to restore blood flow perfusion
consists in the placement of self-expanding Nitinol stents in the pathological vessels
preceded by a lesion pre-dilatation with an angioplasty balloon. Several clinical studies
[1,2] reported high rates of technical success of the stenting procedure and higher vessel
patency rates in the medium to long-term follow-up than that obtained with balloon
angioplasty alone. Although Nitinol femoropopliteal stent implantation has proved to be
a valuable therapeutic strategy in restoring blood flow, to date its long-term
effectiveness is undermined by complications related to fatigue failure of these devices.
Numerous studies [3,4,5] reported high fracture rates of stents implanted in the
femoropopliteal segment, often associated with in-stent restenosis, with a significant
correlation between the incidences of stent fracture, the lesion length and the number of
implanted devices to treat a single long lesion. In this district hip and knee joint
movements during gait produce quite large and cyclic deformations of the superficial
femoral artery, increasing the risk of fatigue fracture of implanted stents. Fatigue failure
1
Laboratory of Biological Structure Mechanics, Structural Engineering Department, Politecnico di
Milano, Piazza Leonardo da Vinci, 32, 20133 Milan, Italy
2
Department of Bioengineering, Politecnico di Milano, Piazza Leonardo da Vinci, 32, 20133 Milan, Italy