Fluid-Structure interaction modeling of healthy carotid bifurcation: impact of the use of full invariants in the anisotropic terms of the Holzapfel-Gasser-Ogden material model

The carotid artery represents a crucial component of the cardiocirculatory system, playing a fundamental role in the delivery of oxygenated blood to the brain. The typical distensibility of the artery wall is altered by aging and by the presence of cardiovascular diseases (CVD). The vascular tissue shows nonlinear and anisotropic mechanical behavior, due to the action of collagen fibers constituting it. Understanding the pathophysiology of the carotid artery is crucial for CVD prevention, early diagnosis, and treatment. Within the field of numerical modeling, computational fluid dynamics (CFD) represents a powerful and reliable tool used to study vascular hemodynamics and its relationship with the onset and progression of atherosclerosis (AS). However, CFD studies usually rely on the assumption of rigid walls. To overcome this assumption, distensibility is introduced by the implementation of fluid-structure interaction (FSI) modeling to assess the influence of the mechanical behavior of arterial walls on local hemodynamics. This work focused on both rigid and FSI analysis of healthy carotid bifurcation models, reconstructed from magnetic resonance images to evaluate the mechanical behavior of the carotid wall and its influence on hemodynamics. To describe the mechanical properties of the carotid wall, both the standard Holzapfel-Gasser-Ogden (HGO) model in the decoupled form, where all the anisotropic terms use isochoric invariants, and its modified formulation, the HGO-ma model, where all the anisotropic terms use full invariants, were implemented. Two-way fully coupled FSI simulations were performed, using the arbitrary Lagrangian-Eularian (ALE) method, and accounting for prestress of the arterial wall and surrounding tissues support. The distensibility of the arterial wall was quantitatively assessed, yielding consistent results with in vivo measurements derived from the literature. The HGO-ma model demonstrated slightly higher stiffness compared to the standard HGO model, but no significant differences were observed between the two materials. Hemodynamic descriptors based on wall shear stress (WSS) and intravascular flow features were calculated. The results were compared with rigid-wall CFD, revealing a moderate influence of vessel distensibility on the explored hemodynamic features. Structural principal stresses and deformations were found to be concentrated predominantly at the bifurcation apex, while hemodynamic indices were predominantly localized in the bulb region. An appreciable co-localization in the bifurcation region interface was observed between areas simultaneously exposed to disturbed blood flow, recognized as a predictive marker for the onset of AS, and high stresses and deformations. According to the findings of this thesis work, the rigid wall assumption seems to remain a valid hypothesis when investigating the correlations between hemodynamics and the insurgence of AS at the carotid bifurcation. Nevertheless, the FSI approach represents a valuable tool for a better understanding of the biomechanics of the carotid artery and enables the study of the combined effects of hemodynamic stresses and structural stress, on the inner vessel wall, in AS initiation and progression.