Biomechanical Aspects of Abdominal Aortic Aneurysm (AAA) and its Risk of Rupture: Fluid Structure Interaction (FSI) Studies

A series of numerical studies to elucidate the effect of various biomechanical parameters on AAA disease progression and its risk of rupture is presented, highlighting a methodology based on biomechanical considerations for a reliable patient specific prediction of AAA risk of rupture. Fluid structure interaction simulations of normal aortas, non-ruptured, and contained ruptured AAA (rAAA) were conducted in patient specific geometries reconstructed from CT scans. The models included the arterial wall, intraluminal thrombus (ILT), and calcifications. Parametric studies of the effects of iliac bifurcation and neck angulation, and of hypertension were also conducted. Advanced constitutive material models included wall anisotropy and structural strength of the aorta. Our studies demonstrate that the anisotropic simulations showed higher peak wall stresses as compared to isotropic ones, indicating that the latter may underestimate the AAA risk of rupture. The ILT appeared to provide a cushioning effect reducing the stresses, while small calcifications appeared to weaken the wall and contribute to the rupture risk. The location of the maximal wall stresses and rupture potential index (RPI) in ruptured AAA simulations overlapped the actual rupture region, demonstrating the predictive capabilities of the methodology. Parametric studies of the AAA iliac bifurcation and neck angulation in idealized streamlined AAA geometries indicated a trend of increasing peak wall stresses with increasing iliacs and neck angulation, with the appearance of two local minima in the iliac bifurcation angle peak stresses pointing to the possibility of an adaptation response in an attempt to achieve a temporary reduction of the peak wall stresses during the disease progression. Such geometric parametric dependence can be further used to augment AAA diagnostics. This may help clinicians estimate the AAA risk of rupture to establish whether a risky surgical intervention is warranted.