Assessment of tissue prolapse after balloon-expandable stenting: Influence of stent cell geometry

https://doi.org/10.1016/j.medengphy.2008.11.002Get rights and content

Abstract

Restenosis is a re-narrowing or blockage of an artery at the same site where treatment, such as a balloon angioplasty or stent procedure, has already taken place. Several clinical trials have shown a significant reduction in the restenosis rates with endovascular stenting. The purpose of stenting is to maintain the arterial lumen open by a scaffolding action that provides radial support. However, stenting can cause a vascular injury during the deployment. Indeed, in-stent restenosis remains a major problem in percutaneous coronary intervention, requiring patients to undergo repeated procedures and surgery. The loading imposed by the deployment of the stent on the artery is involved in the restenosis process. Furthermore, it is well known that the stent design plays a role in the outcome of the stenting interventional procedure. This study compares the mechanical effects of the expansion of five different designs of balloon-expandable stents in a coronary artery by means of numerical models based on the finite element method. An index for the evaluation of the tissue prolapse based on the expanded configuration reached by the stent cells is proposed. The effects of the balloon inflation and deflation are included in the present study. Wall stresses and tissue prolapse of the vessel wall within the stent cells are evaluated and compared among the different stent designs. Results show that the printed area does not predict prolapse, and that the proposed index (PI) does correlate with tissue prolapse.

Introduction

Coronary stents are endovascular devices used to restore blood perfusion in a stenosed artery. Nowadays, stent implantation is a common and well-established interventional procedure with a high rate of success when compared with angioplasty alone [1], [2]. Although the new generation of devices such as drug eluting stents have brought recent advantages, some limitations are still present and the major ones are those associated with the degree of ‘in-stent restenosis’ or ‘late stent thrombosis’ processes [3], [4]. Restenosis occurs when the treated vessel becomes blocked again. It usually occurs within 6 months after the initial procedure [5]. Different stages are involved in this process and they can be summarized, as reported by Edelman and Rogers [6], to be thrombosis, inflammation, proliferation and remodelling. Factors influencing the restenosis process include the degree of damaged endothelial cells and the depth of the injury [7], [8], the plaque composition and shape [9], the design of the stent [10], the type of stent expansion (self or balloon-expandable) [11], [12] and the local fluid dynamics [13]. Most of the available stents use a folded polymeric balloon that is inflated upon insertion. The effect of the balloon inflation on the arterial wall is not insignificant, since the balloon starts to open from its tapered ends, locally stressing the vascular tissue. Mathematical models of the stenting procedure have emerged in recent years as an effective tool to investigate the mechanical response to angioplasty and stent placement in the arterial wall [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. The main modelling issues are related to the stent design, the expansion modalities, and the interaction between the stent and/or balloon and the artery.

Concerning the modelling, the ways to model expansion of a balloon-expandable stent include: displacements control, pressure control or explicitly modelling the balloon. In many numerical models, the modalities of stent expansion usually involve enlargement of a rigid cylinder inside the stent [24], [27], [28] or application of a pressure to the internal struts of the stent [17], [20], [22], [30], [31]. With regards to the balloon, it has been modelled as a silicon cylinder with either a linear-elastic [26] or a bilinear hyperelastic material capable of modelling the unfolding phase of the balloon [21], [32]. Recently, Kiousis et al. [29] proposed a cylindrical orthotropic model for the balloon to take into account the peculiar balloon behaviour. De Beule and colleagues [33] proposed a trifolded linear-elastic balloon model to expand the stent. Ju et al. [34] described the balloon with two parameter hyperelastic Mooney–Rivlin model. In this study a balloon accounting for the presence of tapered ends has been adopted as reported in a study from our laboratory [35] where the effects of different expansion procedures on the stent and on the arterial wall were carefully analysed. Indeed, our computational results showed that modelling the balloon is essential to estimate the level of stresses caused on arterial walls during stent expansion. From few of the above quoted studies, one of the common results is that the stent deployment inside an artery has implications on the stresses and deformations in the arterial wall and hence has an impact on the progression of in-stent restenosis.

The deformations of the arterial wall are related to the prolapse of the arterial tissue within the stent struts. This feature is important because it influences the fluid dynamics altering the fluid flow patterns, which have been found to contribute to the onset and progression of in-stent restenosis [36]. This study is focused on the evaluation of the arterial wall stress state and the estimation of the tissue prolapse caused by a stent insertion. Indeed, it aims at comparing the different stresses induced in the vascular wall of a coronary vessel by five different coronary stents. Furthermore, after the deflation of the balloon, the tissue prolapse within the stent cells is evaluated for the different stent designs.

Section snippets

Materials and methods

The expansion of stents inside an atherosclerotic coronary artery was simulated with quasi-static analyses by means of the commercial code ABAQUS/Explicit (Abaqus Inc., RI, USA) based on the finite element method. To simulate a quasi-static analysis, the ratio between kinetic and internal energy was kept below 5% throughout most of the process. This purpose was reached setting the step time of the simulations to 3 s. Five models were developed, each constituted by the same coronary artery and a

Results and discussion

The deformed configurations reached by the five-stent models are shown in Fig. 4 at three different instants: maximum dogboning effect (Fig. 4a), maximum expansion (Fig. 4b) and end of the deflation process which includes the arterial recoil (Fig. 4c). The deformed configurations at the maximum expansion have been superimposed with the equivalent plastic strain (PEEQ) areas. Model E shows the greatest areas of plastic deformation followed by models B, D, C and A. These findings have a

Conflict of interest statement

None of the authors disclose any financial and personal relationships with other people or organisations that could inappropriately influence (bias) this work.

Acknowledgements

The financial support of the IIT (Italian Institute of Technology) is acknowledged.

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