On the effects of different strategies in modelling balloon-expandable stenting by means of finite element method
Introduction
Endovascular stents are medical devices inserted percutaneously into a stenotic artery to allow blood perfusion to the downstream tissues. Despite the recent advantages brought by the new generation devices such as drug-eluting stents, some critical issues still remain regarding the efficacy of such devices (Serruys and Kukreja, 2006). Restenosis, i.e. the re-narrowing of the artery after its treatment with angioplasty or stenting, is the most common occurrence after these procedures, which usually occurs a few months after the initial procedure (Serruys et al., 1998). Many factors have been found to influence the degree of restenosis, such as the degree of damaged endothelial cells and the depth of the injury (Schwartz et al., 1992; Kornowski et al., 1998), the plaque composition and shape, the type of stent expansion (self- or balloon expandable; Harnek et al., 2002; Morton et al., 2004), the design of the stent (Rogers and Edelman, 1995; Edelman and Rogers, 1998) and the local fluid dynamics (Wentzel et al., 2001). As the stent deployment inside an artery generates anomalous stresses and deformations in arterial walls, it is easy to imagine that it might have some influence on the later progression of in-stent restenosis. In recent years, computational structural analyses have emerged as an important tool to investigate the mechanical response to angioplasty and stent placement inside arterial walls (Lee et al., 1993; Oh et al., 1994; Rogers et al., 1999; Gourisankaran and Sharma, 2000; Auricchio et al., 2001; Holzapfel et al., 2002, Holzapfel et al., 2005a; Prendergast et al., 2003; Migliavacca et al., 2004, Migliavacca et al., 2005, Migliavacca et al., 2007; Lally et al., 2005; Liang et al., 2005; De Beule et al., 2006; Bedoya et al., 2006; Hall and Kasper, 2006; Ballyk, 2006; Wang et al., 2006; Takashima et al., 2007; Wu et al., 2007).
Most of the available stents use a folded polymeric balloon that is inflated on insertion. The effect of balloon inflation on arterial walls is not insignificant, since as the balloon starts to open from its heads, it stresses the surrounding vascular tissue. In the recent literature, the stent has been expanded by imposing pressure to the internal stent struts (Dumoulin and Cochelin, 2000; Migliavacca et al., 2002, Migliavacca et al., 2005; Holzapfel et al., 2005a; De Beule et al., 2006) or by enlarging a rigid cylinder inside the stent in displacement control (Hall and Kasper, 2006; Takashima et al., 2007; Wu et al., 2007). In other studies, the balloon is modelled as a silicon cylinder using either a linear elastic (Wang et al., 2006), a bilinear hyperelastic material capable of modelling the unfolding phase of the balloon (Liang et al., 2005; Raamachandran and Jayavenkateshwaran, 2007) or a trifolded elastic cylinder (De Beule et al., 2007). Moreover, based on our knowledge of balloon modelling, a balloon accounting for the presence of heads has been proposed only recently by Gasser and Holzapfel (2007), and Ju et al. (2008). The former showed a detailed stress analysis of the arterial tissue based on a balloon/artery contact and proposed a finite element simulation of a Gruentzig-type balloon catheter; the latter limited their analyses to the interaction of the stent and the balloon. However, the effects of different expansion procedures on the stent and on the arterial wall have not been carefully analysed.
The objective of this study is to present three different modelling solutions (namely the load, the displacement control conditions and the modelling of balloon expansion) that could be adopted to study stent-free expansion and stent expansion inside an artery. Indeed, it is important to understand not only the mechanical response of the stent in the design phase, but also the arterial stress status caused by the device. Advantages and disadvantages of the different modelling strategies are presented and discussed.
Section snippets
Materials and methods
Three finite element simulations of stent expansion were performed with the following loading conditions: (i) a uniform pressure imposed on its internal surface, (ii) a rigid cylindrical surface expanded with displacement control and (iii) a polymeric deformable balloon previously deflated. Two sets of simulations were run; in one of them, the stent was freely expanded; in the other, the stent expansion was confined into a model of artery to evaluate the influence of the modelling technique on
Free expansion
The deformed configurations of the stent at the beginning, the middle and the end of the stent expansion process are shown in Fig. 2 for the three models investigated. The stent expanded under load control conditions (free-LOAD model) presented a barrel-shape during all the expansion phases, while the stent opened by the cylinder (free-CYLINDER model) maintained its cylindrical shape according to the linear displacement imposed to the inner cylinder. Conversely, the free-BALLOON model started
Discussion
In this study, the effect of different stent expansion modelling techniques has been investigated by means of the finite element method. A model of a balloon commonly used to expand stents has also been developed to evaluate whether the balloon presence can be neglected in terms of effects on the stent and wall stresses. In particular, the threefolded balloon was simulated using a multi-folded model, including closed heads virtually linked to a catheter.
The developed models allowed analysis of
Conflict of interest
There are no conflicts of interest to declare by the authors.
Acknowledgements
This work has been partially supported by the Italian Institute of Technology (IIT), within the project “Models and Methods for local drug delivery from Nano/Micro-structured materials”. The authors gratefully acknowledge the support of Dr. J. Simmonds from Great Ormond Street Hospital for Children, London, UK.
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