Exploring the link between geometry and hemodynamics in syntethic abdominal aortic aneurysm models

Abdominal aortic aneurysm (AAA) is a pathological dilatation of the arterial wall, thus leading to its weakening and in many cases rupture, an event correlated with very high mortality rates. For this reason, the study of the pathogenesis, mechanisms of progression and subsequent rupture of AAAs is a very current topic of great clinical relevance. The biggest issue lies in the lack of a reliable parameter that can predict the risk of rupture of AAAs. Currently, the maximum diameter value is used as a threshold to define a AAA at high risk of rupture. Recent literature, however, has shown that this parameter alone cannot be considered a reliable predictor of rupture risk; in fact, some AAAs, despite reaching very large sizes, never rupture, while others with a small diameter, undergo premature rupture. It is currently clear that hemodynamics plays a critical role in the processes involved in the genesis and progression of AAAs, being one of the causes of wall degradation, particularly in the presence of disturbed flow and non-physiological values of hemodynamic quantities. According to the "geometry shapes the flow" paradigm, the geometric characteristics of the vessels have an impact on the hemodynamic patterns generated in vascular districts. Indeed, complex geometries can contribute to the occurrence of disturbed flow conditions, such as recirculation and separation zones which are known to trigger the activation of dysfunctional biochemical pathways. The above observations strongly motivate the study of fluid dynamics within AAAs’ different geometries. The goal of the present thesis is to elucidate the link between hemodynamics and vascular geometry in the AAAs, performing a Computational Fluid Dynamics (CFD) study. Pulsatile blood flow was simulated in 26 synthetic models of AAAs, previously generated performing a statistical shape analysis on real patient-specific geometries, in order to obtain larger reliable datasets heterogeneous in terms of size and geometric characteristics. After performing unsteady-state CFD simulations, both near-wall and intravascular hemodynamics were analysed. For the former, classical descriptors based on the wall shear stress (WSS), and recently-introduced quantities based on WSS topological skeleton were computed, whereas intravascular flows were characterized in terms of vortical and helical flow structures. The geometry of AAAs was then assessed, and a statistical analysis was performed to correlate hemodynamic with geometric parameters. Among the main results, it emerged that the WSS, as expected, is found to have lower than normal values on the wall of the aneurysm, with an oscillating pattern throughout the cardiac cycle. Moreover, the contraction/expansion action exerted by WSS on the endothelium along the cardiac cycle, seemed to be pronounced in the aneurysmatic sac. Vortical structures were also observed in the AAA models and were found to persist throughout the cardiac cycle. The statistical analyses allowed to assess and quantify the effect of the investigated AAA geometric features on the development and organization of both near-wall and intravascular large-scale flow structures. In conclusion, findings from this study provide insight into the hemodynamics characterizing the AAA disease and its link with vascular geometry, contributing, in a wider perspective, to the translation of computational methods in real-world clinical settings.