Coronary stent implantation changes 3-D vessel geometry and 3-D shear stress distribution

Abstract

Mechanisms of in-stent restenosis are not fully understood. Shear stress is known to play a role in plaque and thrombus formation and is sensitive to changes in regional vessel geometry. Hence, we evaluated the regional changes in 3-D geometry and shear stress induced by stent placement in coronary arteries of pigs.

Methods. 3-D reconstruction was performed, applying a combined angiographic and IVUS technique (ANGUS), from seven Wallstents (diameter 3.5 (n=3) and 5 mm (n=4)), which were implanted in seven coronary arteries of five pigs. This 3-D geometry was used to calculate locally the curvature, while the shear stress distribution was obtained by computational fluid dynamics. Local changes in shear stress were obtained at the entrance and exit of the stent for baseline (0.65±0.22 ml/s) and hyperemic flow (2.60±0.86 ml/s) conditions.

Results. After stent implantation, the curvature increased by 121% at the entrance and by 100% at the exit of the stent, resulting in local changes in shear stress. In general, at the entrance of the stent local maxima in shear stress were generated, while at the exit both local maxima and minima in shear stress were observed (p<0.05). Additionally, the shear stress at the entrance and exit of the stent were correlated with the local curvature (r: 0.30–0.84).

Conclusion. Stent implantation changes 3-D vessel geometry in such a way that regions with decreased and increased shear stress occur close to the stent edges. These changes might be related to the asymmetric patterns of in-stent restenosis.

Introduction

Interventional techniques, like balloon angioplasty, have shown to relieve hemodynamic relevant arterial stenosis caused by atherosclerosis. However, in 30–50% of the treated cases restenosis develops, which is for 60–70% caused by arterial shrinkage (`arterial remodeling') and for 30–40% by neointimal hyperplasia (Mintz et al., 1996). While stent implantation prevents arterial remodeling the rate of restenosis is still 20–35%, largely due to neointimal formation (Fischman et al., 1994; Serruys et al., 1994).

Despite the importance of the problem, an appropriate therapy is still lacking. (Leon et al., 1996). A reason might be that shear stress has not been recognized as factor in explaining in-stent restenosis yet. Arguments in favor of shear stress is that it is known to be involved in a variety of processes related to cellular growth and thrombosis, probably through the activation of several genes (Malek and Izumo, 1995; Nikol et al., 1996; Strony et al., 1993). In addition, shear stress has been shown to be changed regionally after stent placement and might therefore explain the fact that in-stent restenosis is not distributed homogeneously in stents (Fontaine et al., 1994; Peacock et al., 1995).

The aim of the present study is to evaluate regional changes in 3-D geometry and shear stress distribution after stent placement in curved coronary arteries, applying a new technique that enabled us to 3-D reconstruct coronary arteries in three dimensions (3-D) (`ANGUS', Slager et al., 1997) and to calculate regional shear stress in vivo.