Mechanical behavior of coronary stents investigated through the finite element method

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Abstract

Intravascular stents are small tube-like structures expanded into stenotic arteries to restore blood flow perfusion to the downstream tissues. The stent is mounted on a balloon catheter and delivered to the site of blockage. When the balloon is inflated, the stent expands and is pressed against the inner wall of the coronary artery. After the balloon is deflated and removed, the stent remains in place, keeping the artery open. Hence, the stent expansion defines the effectiveness of the surgical procedure: it depends on the stent geometry, it includes large displacements and deformations and material non-linearity.

In this paper, the finite element method is applied (i) to understand the effects of different geometrical parameters (thickness, metal-to-artery surface ratio, longitudinal and radial cut lengths) of a typical diamond-shaped coronary stent on the device mechanical performance, (ii) to compare the response of different actual stent models when loaded by internal pressure and (iii) to collect suggestions for optimizing the device shape and performance.

The stent expansion and partial recoil under balloon inflation and deflation were simulated. Results showed the influence of the geometry on the stent behavior: a stent with a low metal-to-artery surface ratio has a higher radial and longitudinal recoil, but a lower dogboning. The thickness influences the stent performance in terms of foreshortening, longitudinal recoil and dogboning.

In conclusion, a finite element analysis similar to the one herewith proposed could help in designing new stents or analyzing actual stents to ensure ideal expansion and structural integrity, substituting in vitro experiments often difficult and unpractical.

Introduction

Intravascular stents are small tube-like structures placed into stenotic arteries to restore blood flow perfusion to the downstream tissues. The first implanted stent was described by Dotter (1969) to treat arterial shrinkage, but the clinical routine implantation began in the 1990s to improve the limitations of balloon angioplasty, such as restenosis and abrupt closure (Dangas and Fuster, 1996; Gottsauner-Wolf et al., 1996). If compared to angioplasty, higher efficiency of stents is supported by randomized trials and clinical studies (Fischman et al., 1994; Serruys et al., 1994; Versaci et al., 1997). Nevertheless, problems and difficulties remain, such as migrations, collapses, cloth formations or positioning difficulties (Bjarnason et al., 1993; Wong et al., 1996; Rosenfield et al., 1997).

Different typologies of stents are available on the market and the importance for the operator to know the different physical properties of the stent selected to treat a specific lesion is recognized. Up to now, apart from the manufacturer claims, available useful information come from some experimental comparative studies (Rieu et al., 1999; Dyet et al., 2000; Ormiston et al., 2000; Barragan et al., 2000). Only recently, numerical analyses with finite element method (FEM) have been proposed as an alternative approach to investigate mechanical properties of intravascular stents.

Although FEM is nowadays a methodology well known and widely used in many engineering fields (mechanical, structural, aeronautical, etc.), it is worthwhile remembering that the reliability of the results clearly depends on the assumptions and hypotheses adopted in the analysis. Indeed, the stent expansion includes geometric and material non-linearities, which are difficult to be properly simulated. As far as we know, apart from in vitro fluid dynamics studies on intravascular stent (Peacock et al., 1995; Fabregues et al., 1998), structural FEM analyses were used by Dumoulin and Cochelin (2000) to evaluate and characterize some mechanical properties of balloon-expandable stent, by Etave et al. (2001) to compare the performance of two different types of stent, by Auricchio et al. (2001) who realized a 3D study of the stent–artery interaction during stent deployment, by Rogers et al. (1999) who studied a 2D balloon–artery interaction, and by Oh et al. (1994) who exploited FEM to analyze the stress state of atherosclerotic artery during balloon angioplasty. Examples of the advantages in the use of FEM model in predicting the mechanical behavior of stents and balloons are reported also by Whitcher (1997).

Actually, although intravascular stents are nowadays routinely and successfully used, research and developments are still necessary, in particular to improve the design and to reduce the long-term failure.

The purpose of this work is to show how the FEM can be used in the optimization of the design of coronary intravascular stent. In particular, the FEM is herewith applied to investigate the effects of different geometrical features on the mechanical performance of a widely adopted stent and to compare the response of different stent models.

To reach these goals, the study was organized as described in the following:

  • A 3D FEM model of a typical diamond-shaped (DS) intravascular stent (Palmaz-Schatz)1 was developed to investigate the effects of different geometrical features (such as thickness, longitudinal and radial length of cuts) on the mechanical behavior of the stent. The stent performance was evaluated in terms of radial recoil, longitudinal recoil, foreshortening, and dogboning. On the basis of some preliminary results, a modification of this geometry is also proposed to minimize the dogboning effect.

  • Two additional types of stent similar in the strut design to those available on the market, and comparable in terms of dimension (length, diameter, thickness of the strut) to the typical DS stent were studied.

Section snippets

3D geometrical model

A 3D model of a typical DS intravascular stent, obtained, for example, from a cylinder worked with laser technology is depicted in Fig. 1 in its unexpanded configuration. It is assumed to be a tube with rectangular slots on its surface. The stent has a length L of 16 mm, an outer diameter D of 1.2 mm, a thickness s of 0.1 mm, five slots in the longitudinal direction and 12 slots in the circumferential direction with length l of 2.88 mm, and a metal/artery index of 0.3 (model DS). The metal/artery

DS stent

The first series of simulations evinced the strong influence of the metal-to-artery ratio (αP/αV) on the pressure increment necessary to expand the stent up to a radius of 2 mm in the central zone (Fig. 3 top). Indeed, in the case of αP/αV=0.2, a value of 0.12 MPa is sufficient to expand the stent; an increment of only 0.09 MPa produces a radius of 2 mm very steeply; in the case of αP/αV=1.46, 0.57 MPa is necessary to start the expansion and only after an additional increment of 0.34 MPa, the radius

Limitations

The modeling of a coronary stent from the implantation until the complete integration in the host artery is a challenging study. Indeed, the implantation involves contact between balloon, stent and diseased artery; the long-term behavior involves interaction between the stent, the artery and the blood flowing through this structure. This study addresses only the free stent expansion. The interaction between the balloon and the stent has not been modeled, even if the effects of the balloon

Conclusions

A finite element analysis similar to the one herewith proposed could help in designing new stents or analyzing actual stents to ensure ideal expansion and structural integrity, substituting in vitro experiments often difficult and unpractical.

These results are promising and useful in the study of the mechanical performances of the stent itself. In particular, they pointed out the possibility to optimize the radial expansion of the classical stent by varying the αP/αV ratio along the stent or

Acknowledgements

The partial support of the National Research Council (CNR) grant Agenzia2000 is duly acknowledged.

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