Fluid dynamic performance of commercially available coronary stent designs: a computational study

Atherosclerosis is the major cause of coronary artery disease and a leading cause of death globally. It consists in the accumulation over time of fatty deposits and inflammatory cells, forming plaques narrowing the arteries and impeding blood flow. The most common non-surgical treatment for stenotic coronary arteries is percutaneous transluminal angioplasty with stent implantation. Stents are thin, mesh-like, inflatable tube-shaped structures, offering mechanical support to the vessel wall after expansion and restoring its physiological state. There are several types of coronary stents on the market, classified according to both their material and main function in: metallic stents, drug-eluting stents, and bioresorbable stents. The wide variety of geometries and drugs allows for a wide range of patients to be treated. Although stents can provide significant mechanical support, their long-term presence in the body has noticeable impact on hemodynamics, which can lead to post-implant complications or even implant failure. Literature has demonstrated the association between altered local hemodynamics and these adverse events. Wall shear stress (WSS) has been identified as an instrumental predictor of stent post-implantation complications. Several indexes mostly based on WSS magnitude and/or direction (i.e., time-averaged WSS - TAWSS, oscillatory shear index - OSI, relative residence time - RRT) have been proposed as markers/predictors of stent failure risk, although they were found to have only a weak predictive potential. A growing interest has emerged on the WSS topological skeleton (TS), used as an additional quantity to understand hemodynamic processes leading to stent failure. The latter is motivated by its capability to (i) elucidate the complexity of local coronary hemodynamics, highlighting adverse phenomena, such as flow stagnation, separation and recirculation, (ii) detect the presence of a stent ring and (iii) identify malapposed struts. By using computational fluid dynamics, the aim of this thesis is to analyse the hemodynamic impact of two different open-cell coronary stent designs currently available on the market, namely the Promus Premier (Boston Scientific) and Taxus Libert¿ (Boston Scientific) stents. 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 the hemodynamic descriptors luminal distributions between the two stents designs emerged from the results: repetitive patterns of WSS divergence were observed at the stented luminal surface, highlighting a WSS contraction action proximally to the stent struts and a WSS expansion action distally to the stent struts. These WSS action patterns were independent from the stent design. Lower extension of the luminal region exposed to low and/or oscillatory WSS emerged for Promus Premier stent with respect to Taxus Libert¿ stent. Accordingly, smaller luminal surface area exposed to high variability in WSS contraction/expansion action during the cardia cycle resulted for Promus Premier stent. In conclusion, this thesis work allowed characterizing the main differences in coronary hemodynamics post-stent implantation between two open-cell stent design. Findings from this study indicate that (i) the two investigated stent designs have a similar impact on the local coronary hemodynamics and (ii) support the use of WSS TS for a deeper understanding of the hemodynamic-driven processes underlying stent failure.