Hemodynamic performance of current drug-eluting stents: a computational fluid dynamics study

Coronary arteries are particularly prone to the formation of atherosclerotic plaques, which may cause the narrowing of arterial lumen, and result in insufficient oxygen supply to the myocardium, or total occlusion of the vessel, with subsequent myocardial infarction. Nowadays, the most common non-surgical treatment for stenotic coronary arteries is percutaneous transluminal angioplasty with stent implantation. Stents are wide-mesh tubular metal nets that are inserted into the artery and expanded near the lesion to restore baseline blood flow. Although the use of stents is a well-established treatment for stenotic coronary arteries, its implantation modifies arterial geometry and local fluid dynamics, potentially leading to complications such as intra-stent restenosis or late stent thrombosis. A large body of literature has demonstrated the association between these adverse events and the presence of altered local hemodynamics. In particular, wall shear stress (WSS) has been identified as an important factor involved in the pathological mechanism leading to stent failure. Several indexes mostly based on WSS magnitude and/or direction has been proposed as markers/predictors of stent failure risk, although they were found to have only a weak predictive potential. In this context, a growing interest has recently emerged on the topological skeleton (TS) of the WSS, due to its capability to better elucidate the complexity of local coronary hemodynamics, highlighting areas with adverse phenomena, such as flow stagnation, separation and recirculation. In particular, very recent findings have suggested the capability of WSS TS features to (i) detect the presence of a stent ring and (ii) identify malapposed struts. Therefore, the WSS TS could be used as an additional tool to understand hemodynamic processes that lead to stent failure. By using computational fluid dynamics, this thesis project analyses the impact of three different designs of drug-eluting stents (DES) on near-wall hemodynamics of idealized coronary models. In detail, the Synergy stent from Boston Scientific, the Nobori stent from Terumo, and the Orsiro stent from Biotronik were investigated. The ultimate goal is to perform a fluid dynamic comparison, based on the canonical WSS-based descriptors of disturbed flow and on WSS TS features. Overall, a good agreement of luminal distributions of the hemodynamic quantities among the three stents designs emerges from the results. In particular, 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. This WSS action pattern was independent from the stent design. Lower extension of the luminal surface areas exposed to low and/or oscillatory WSS emerged for Orsiro stent with respect to Synergy and Nobori stents. Conversely, a larger luminal surface area exposed to marked variability of WSS expansion/contraction action along the cardiac cycle resulted for Orsiro Stent compared to the others. The latter may be ascribed to a marked irregularity of Orsiro stent links with respect to Synergy and Nobori stents, reflecting only on WSS TS features. In conclusion, these findings indicate that (i) the three investigated DES stent designs have a similar impact on local coronary hemodynamics and (ii) support the use of WSS TS for a deeper understanding of the hemodynamic-driven processes underlying stent failure.