Hemodynamic performance of open- and closed-cells coronary stents: a computational fluid dynamics idealized study

Coronary arteries can undergo progressive stiffening with loss of elasticity due to atherosclerosis. This pathological process consists in the accumulation of lipids at the arterial wall with the formation of an atherosclerotic plaque that may grow and narrow the arterial lumen, restricting blood flow to the myocardium. Nowadays, percutaneous coronary intervention (i.e., angioplasty with stent implantation) is the most common non-surgical treatment of narrowed coronary arteries. A stent is an expandable tubular structure made up of a mesh network that provides mechanical support to the vessel wall, restoring physiological conditions and ensuring proper blood flow to the heart. Several types of stents are commercially available and, based on composition, structure, and properties, are classified in three main families, namely (i) Bare Metal Stents, (ii) Drug Eluting Stents, and (iii) Bioabsorbable Stents. Despite stent implantation has become increasingly popular in recent decades due to the effectiveness in preventing and treating cardiovascular disease, it alters the vessel geometry and generates a new local fluid dynamic environment, which may lead to future complications, such as in-stent restenosis or late stent thrombosis. The association between these adverse events and the presence of altered local hemodynamics has been demonstrated, with the wall shear stress (WSS) emerging as an important factor involved in the pathological mechanism leading to stent failure. The canonical hemodynamic indexes use information based on WSS magnitude and/or direction, although they are found to be weak predictors of stent failure. In this context, the analysis of the WSS topological skeleton (TS) has gained more and more interest by extending the current understanding of the association between local hemodynamics and vascular diseases. In particular, very recent findings have suggested the capability of WSS TS features to (i) identify the presence of stent ring and (ii) capture the presence of malapposed struts, and thus candidate WSS TS as an additional tool for understanding the hemodynamic-related processes leading to stent failure. This thesis work investigates the near-wall hemodynamics impact of three commercially available coronary stent designs in terms of canonical WSS-based descriptors and WSS TS features in idealized computational fluid dynamics models of stented coronary arteries. In detail, one closed- (Cypher Stent, Cordis) and two open-cells (Coroflex Stent, B.Braun and Xience Prime Stent, Abbott) stent designs were analyzed. The results show an overall agreement among the three investigated stent designs in terms of luminal distributions of canonical WSS-based descriptors and WSS TS features. In particular, repetitive patterns of WSS divergence were observed at the luminal surface, highlighting a WSS contraction action exerted proximal to the stent struts and a WSS expansion action distal to the stent struts. The latter was independent from the stent design. Lower extension of the luminal surface areas exposed to deranged hemodynamics and marked variability of WSS expansion/contraction action along the cardiac cycle emerged for Cypher Stent with respect to Coroflex and Xience Prime Stent. In conclusion, these findings (i) suggest a minor impact of closed-cells stent design (with respect to open-cells ones) on local coronary hemodynamics and (ii) support the use of WSS TS for a deeper understanding of the hemodynamic-driven processes underlying stent failure.