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

FINITE ELEMENT ANALYSES OF IN VIVO FATIGUE BEHAVIOR OF
PERIPHERAL STENTS: EFFECT OF PLAQUE FEATURES
1. ABSTRACT
E. Dordoni 1,2 , A. Meoli 1,2 , W. Wu 1 ,
F. Migliavacca 1 , G. Dubini 1 , G. Pennati 1 , L. Petrini 1
Fatigue resistance of Nitinol peripheral stents implanted into femoropopliteal arteries is
a critical issue due to the particular biomechanical environment of this district. Hip and
knee joint movements associated with patient’ daily activities expose the superficial
femoral artery, and therefore the implanted stents, to large and cyclic deformations.
These loadings, combined with the cyclic loading due to the arterial blood pressure,
may cause fatigue fracture of stents. The stress/strain field throughout the stent is likely
affected by plaque features. In this study, finite element simulations of angioplasty,
stenting and subsequent in vivo loading conditions (cyclic pressure and axial
compression) have been developed in different stenotic vessel models. A model of a
stent, resembling the geometry of a commercial peripheral stent, has been reconstructed.
A parametric model of stenotic artery has been developed, described by vessel inner
diameter, obstruction ratio, length, plaque asymmetry and sharpness. The results,
analyzed in terms of amplitudes (ε 1a ) and mean values (ε 1m ) of the first principal strain
through the stent, showed that: i) cyclic pressure effects are negligible if compared with
axial compression ones; ii) replicating a realistic vessel morphology is fundamental,
since plaque features affect fatigue resistance of the stent with the same loading
conditions.
2. INTRODUCTION
Stenting in peripheral arteries has proved problematic, with success rates hindered by
in-stent restenosis and stent fracture. Initially, balloon-expandable stainless steel stents
were used, but the high initial technical success rates diminished over time, until the
cumulative patency rates were the same for angioplasty alone and stenting after 1 and 2
years 1 . The problems associated with the procedure lead to the development of more
flexible, self expandable Nitinol stents. While these stents decreased the rates of
restenosis compared with angioplasty alone 2 , restenosis rates are still high. Moreover,
the mechanical environment of the peripheral arteries, caused by joint movements, may
contributes to increase the risk of fracture associated to these devices 3 . Today’s device
manufacturers use a variety of experimental tests to ensure that stents have accurate
properties. However, the small and complex geometry of the stents often does not allow
to carry out all the test usually required for a standard medical device. In this regards
computational analyses, based for example on the Finite Element Method (FEM) can be
adopted. While in the literature there are many computational studies on the
optimisation of stent mechanical properties, which can surely lead to a better long-term
efficacy of the device itself, most studies do not consider how stenotic vessel features
and in vivo loading conditions influence stents fatigue resistance. The aim of this study
is to investigate how the plaque features and in vivo loading conditions influence the
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