Analysis of wall shear stress in stented coronary artery using 3D computational fluid dynamics modeling

https://doi.org/10.1016/j.jmatprotec.2007.06.010Get rights and content

Abstract

In this study, an investigation on the effects of stent geometry on blood flow in a stented human coronary artery 2D and 3D computational fluid dynamics (CFD) models with different stent geometries is reported. Blood velocity profiles and shear stress values were computed in three different sites, including stented arterial segment, pre-stent, and post-stent regions. Blood flow was assumed as a fully developed incompressible Newtonian flow. Rigid boundary conditions were assumed for all models. The governing Navier–Stokes equations were solved using commercial software. The arterial wall shear stress distribution was investigated in three major regions and critical sites were located. It is concluded that shear stress is influenced by three stent design parameters, i.e., strut spacing, strut profile, and number of struts. To achieve the most secure shear stress value, the optimum stent geometry can be obtained with respect to the mentioned parameters for a specific arterial segment. Different stents may be used for different arteries and arterial branches due to the dependency of the shear stress value to the geometry of the artery. It is shown that analyses of wall shear stress profile between stent struts, and in pre-stent and post-stent regions are essential in stent design. Additionally, it is shown that by application of a flow divider, the wall shear stress in stented segment increases markedly.

Introduction

Atherosclerosis is a vascular disease that reduces arterial lumen size through plaque formation and arterial wall thickening (Kumar et al., 1992). Several risk factors have been identified that contribute to the progression of the disease. The most important factors include smoking, hypertension, diabetes, blood pressure, aging, increased levels of plasma cholesterol, and hemodynamics (Virmani et al., 2004, Hayashi et al., 1996, Moore and Ku, 1994). The systemic nature of these risk factors cannot explain the observation that atherosclerosis occurs predominantly at certain locations like bifurcations and side branches (Nerem, 1992, Glogov et al., 1998). As these sites are associated with deviations of the normal velocity field, it has been postulated that flow-induced shear stress, acting on the endothelial cells, plays an important role in plaque localization and plaque growth (Bluestein et al., 1996, Lei et al., 1995, Giddens and Zarins, 1993). Specifically, low mean shear stress, oscillating shear stress, high particle residence times, and non-laminar flow all have been shown to occur in locations where intimal thickening is greatest (Lee and Chiu, 1996, Mongrain and Rodés-Cabau, 2006, LaDisa et al., 2005a).

Advanced stages of atherosclerosis can produce severe arterial stenosis requiring clinical intervention. The primary forms of intervention include bypass grafting, balloon angioplasty and stent deployment. Intravascular stents, which are small tube-like structures, can be placed into stenotic arteries to restore blood flow perfusion to the downstream tissues. Approximately one million patients worldwide undergo a non-surgical coronary artery interventional procedure each year (Nicoud et al., 2005). Stent implantations are used in 60–80% of procedures. Intra-stent restenosis occurs in 20–30% of the cases following the procedures. The phenomenon of restenosis after endothelial damage following the stent implantation is an important parameter in stent design.

Neointimal proliferation (NIP), composed of smooth muscle cell proliferation and extra-cellular matrix, is one of the major mechanisms of the intra-stent restenosis. Three distinct phenomena have been known to cause NIP: (a) The expansion of the stent wires at the time of the implantation resulting in vascular trauma. Here, there is a possible correlation between vascular injury and neointimal hyperplasia. (b) Stent implantation might induce complex interactions between blood components and stent. The materials and the roughness of stent surface may affect the adsorption of plasma proteins. (c) The stimulation of endothelial cells by arterial wall shear stress (WSS) and flow pattern might be determinants in restenosis. The fluid–structure interaction between stent wires and blood flow may alter WSS values, particularly between stent struts. Variations in the failure rates associated with different stent designs have led researchers to investigate the role of near wall flow patterns (Danenberg et al., 2002, Farb et al., 2002).

Some studies in the literature have suggested a link between stent design and restenosis (Kastrati et al., 2001, LaDisa et al., 2004, LaDisa et al., 2005b, Murata et al., 2002, Wentzel et al., 2000). It is also clear that local blood flow patterns are affected by stent design; hence, the relationships between blood flow patterns, stent design, and the process of restenosis should be investigated thoroughly. Although it is difficult to verify conclusively such relationships without suitable in vivo studies, CFD can provide an excellent research tool to help understand these underlying issues. The in vivo studies have confirmed that an increase of the local shear stress reduces the neointimal formation (Wentzel et al., 2001, Wentzel et al., 2003, Carlier et al., 2003).

CFD is being employed by several researchers to explore further the nature of flow stagnation patterns on stent strut shape and spacing (Berry et al., 2000, Berry et al., 2002, Henry, 2001, Barakat, 2001). These studies have confirmed that the flow stagnation patterns between stent struts depend most strongly on stent strut spacing and have demonstrated that vessel wall compliance has little effect on these flow patterns. Most previous stent CFD research studies have not included the complex real geometries that are present during actual clinical interventional procedures. Instead, most studies consider simplified and idealized geometry. Although this can provide a great deal of information, fully three-dimensional studies need to be considered.

This investigation, for the first time, presents comprehensive 2D and 3D computational fluid dynamics models to analyze blood flow and WSS values in models of stented human coronary artery with different geometrical parameters.

Section snippets

Materials and methods

CFD modeling was applied to investigate the steady flow field in the vicinity of stents placed within arterial segments in a coronary artery model. Since 3D modeling of stent geometry is difficult, we used a 2D modeling for investigating the effects of the stent strut profile and stent strut spacing on the flow pattern. Then, we used the results of the 2D modeling to optimize these parameters in a 3D stent modeling. In 3D modeling, the effects of stent geometry, curvature, and flow divider on

Effect of the stent strut spacing and shape

In 2D CFD simulations, the results have shown that wall shear stress distribution between stent struts was sensitive to stent strut spacing. These results show that depending on the ratio of strut width (W) to strut height (H), the flow separation zone, and consequently, the WSS value changed markedly. Fig. 3 illustrates the effects of stent strut spacing on blood flow pattern for stents with different W/H. To investigate the variation of WSS value with stent parameters, the ratio of shear

Conclusions

The current results confirm and extend our previous observations and further demonstrate the importance of stent geometry on intravascular fluid dynamics. The 2D computational fluid dynamics modeling results demonstrate that mechanical parameters which might contribute to restenosis are highly sensitive to stent design. The blood flow pattern and WSS values are markedly influenced by the stent strut spacing, the shape of strut, and the number of rings. Lower WSS was observed within the stented

Acknowledgements

The authors wish to thank Dr. N. Fatouraee and Dr. F. Firouzi (Department of Biomechanics, Faculty of Biomedical Engineering, Amirkabir University of Technology) for their valuable advice on the technical and computational subjects.

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