Impact of commercial coronary stent cell shape and strut thickness on the hemodynamic-related risk of in-stent restenosis: a computational study

Coronary atherosclerosis is a globally widespread cardiovascular disease, consisting in a stiffening of the coronary wall with the formation of plaques, narrowing the lumen, and potentially causing insufficient oxygen supply to myocardium or completely occlusion of the coronary artery with subsequent myocardial infarction. The most common treatment for coronary artery revascularization is percutaneous coronary intervention with stent implantation. Stents are expandable tubular mesh structures that are inserted into the atherosclerotic arteries to provide mechanical support to the vessel and restore normal blood flow. Coronary stents are classified based on material, structure and properties, into three main categories: bare metal stents (BMS), drug-eluting stent (DES), and bioresorbable stents (currently under investigation). However, stent placement may lead to adverse outcomes such as in-stent restenosis (ISR). In detail, previous studies have demonstrated a relationship between local hemodynamic alteration and adverse coronary events, identifying the wall shear stress (WSS) as the main biomechanical marker/predictor of stent failure risk. In this context, this thesis aims to investigate the hemodynamic impact of cell shape and strut thickness of various coronary stents available on the market at early post-implantation (PI) and follow-up (FU) stages. To achieve this, the 3D geometries of 12 coronary stents (2 BMS, 10 DES) were reconstructed. Two stents presented a closed-cell design. To focus the analysis on stent shape, a uniform thickness of 80 μm was considered for all models. Additionally, 12 models with real backbone thickness were created. Finally, PI and FU stent geometries were reconstructed. Forty-four unsteady-state CFD simulations were performed in idealized stented coronary models. The hemodynamic-related risk of ISR was evaluated in terms of canonical WSS-based quantities (i.e., TAWSS, OSI, and RRT). Based on recent evidence, the variation in WSS contraction/expansion action on the endothelium along the cardiac cycle was also quantified by the topological shear variation index (TSVI). Luminal surface areas (SAs) exposed to deranged hemodynamics were identified. Overall, repetitive patterns of WSS divergence were observed at the luminal stented surface, with a WSS contraction action proximal to each stent struts and a WSS expansion action distally. These patterns were common to all the designs. Independent of stent design, disturbed hemodynamics SAs were mainly located nearby stent struts and links, with design Taxus Libertè characterized by the largest one (>36%). The luminal distributions of hemodynamic quantities showed that, independent of stent shape and thickness, design Taxus Libertè exhibited the worst hemodynamic performance (lowest TAWSS and highest TSVI values). Conversely, the Orsiro design exhibited the best hemodynamic performances. Overall, stent hemodynamic performance was inversely related to strut thickness, as emerged from PI and FU models comparison. In conclusion, in this thesis work the hemodynamic-related risk of ISR for 12 coronary stent designs available on the market was investigated using idealized CFD models. A major impact emerged for stent shape, unexpectedly independent of closed- or open-cell configuration. A marked effect of stent thickness emerged only on OSI. In the future, the presented stent dataset could be useful for benchmarking purposes in the development of innovative stent designs and assessing patient-specific risk of ISR.