An idealized computational fluid dynamics study comparing the hemodynamic performance of closed- and open-cell coronary stent designs

Coronary atherosclerosis is a widely prevalent and life-threatening cardiovascular disease caused by the accumulation of lipids in the coronary artery walls. This disease results in a progressive reduction of the luminal cross-sectional area available to the passage of blood flow, thus increasing the risk of myocardial infarction. There are several approaches to treat the disease and its consequences. Today percutaneous coronary intervention (PCI), which consists in the implantation of a stent within the artery, is the most common one. Coronary stents have revolutionized the treatment of coronary artery disease by providing a less invasive approach than open-heart surgery. Stents are expandable tube-shaped structures, generally thin and mesh-like. After their expansion, they provide mechanical support to the vessel wall, by restoring its physiological condition. Several types of stents are available in the market, classified based on fabrication materials, structure, and properties. These are primarily categorized as bare metal stents, drug eluting stents, and bioabsorbable scaffolds. Most patients who undergo coronary stent procedures experience positive outcomes, with significant improvements in coronary blood flow and a reduced risk of stroke. However, stent implantation might disrupt normal hemodynamics, leading to potential complications, including restenosis and late thrombosis. The association between PCI failure and the presence of altered hemodynamics has been demonstrated, in part, with the quantification of hemodynamic descriptors such as the wall shear stress (WSS). Canonical hemodynamic descriptors (e.g., time-averaged WSS – TAWSS, oscillatory shear index – OSI, relative residence time – RRT), based on WSS, are used to predict adverse events following PCI. With the same objective, the analysis of the WSS topological skeleton (TS) has recently gained interest due to its ability to (i) identify the presence of stent rings and (ii) capture the presence of malapposed struts. This thesis work aims to investigate the hemodynamic impact of three commercial stent designs, in terms of canonical WSS-based descriptors and WSS TS features, in idealized computational models of stented coronary artery. The stent models under analysis included the (i) Skylor (Invatec), (ii) Ultimaster (Terumo) and (iii) BioFreedom (Biosensors Int.) stents. The Skylor stent features a closed-cell design, while the others have an open-cell design. The findings of the study revealed a common trend in the luminal distribution of both WSS-based canonical descriptors (TAWSS, OSI, RRT) and WSS TS features. In particular, independently of the stent design, repetitive patterns of WSS divergence were observed at the stented luminal surface, highlighting a WSS contraction action exerted proximal to the stent struts and a WSS expansion action distal to the stent struts. The Ultimaster and BioFreedom stents exhibited less extension of areas exposed to altered hemodynamics and high variability in the WSS vector expansion/contraction compared to the Skylor stent. In conclusion, this analysis of three different stent designs, including one closed-cell and two open-cell stents, suggests that open-cell stents have a lesser impact on local hemodynamics compared to closed-cell stents. Furthermore, the study underscores the importance of utilizing information from the analysis of the topological skeleton of WSS for a more profound understanding of post-implantation hemodynamic-driven processes underlying stent failure.