Introduction: Cerebral arterial bifurcations represent preferred sites for aneurysm formation, especially when associated with variations in divider geometry, notably wider bifurcation angles. We hypothesized that higher bifurcation angles may be correlated to stronger flow recirculation and shear gradients in the bifurcation region migrating towards daughter branches.
Methods: 3D rotational angiograms of 13 MCA bifurcations (6 controls) and parametric models with increasing bifurcation angles (45° to 240°) were analyzed using computational fluid dynamic (CFD) simulation. Wall shear stress (WSS) vectors along cross-sectional planes distal to the bifurcation apex were decomposed as rotational WSS (in plane vectors), and normal WSS (orthogonal to plane vectors). Rotational WSS (RWSS) and RWSS gradients (RWSSG) were evaluated immediately distal to the apex.
Results: Increased bifurcation angles led to increased RWSS and increased positive RWSSG. RWSS decreased distally from the apex in all models, but at a slower rate in wider bifurcations. In asymmetric bifurcations, RWSS was larger on the daughter corresponding to the larger angle. Aneurysmal MCA bifurcations were characterized by a wider acceleration area (high WSS, high positive WSSG) compared to non-aneurysmal bifurcations. In addition, aneurysmal bifurcations had significantly higher RWSS (p=.01) and RWSSG (p=.03) compared to control non-aneurysmal bifurcations.
Conclusions: Aneurysm presence was correlated with higher RWSS and RWSSG at the apical region. RWSS was at maximum closer to the apex, fading distally from the point of flow separation, with slower flow convenience at wider angles. The higher RWSS at large angles adds a new dimension to the hemodynamic insult at the apical area by quantifying significant forces acting in multiple directions in the close proximity of the apex, distal into the daughter vessels, and outside of the medial pad protection. These previously undescribed hemodynamic shear stresses may trigger the destructive remodeling, previously associated with increasing WSS and positive WSSG, needed for aneurysm initiation.
Patient Care: The purpose of this study is to explore the effect of the widening of the bifurcation angle on the hemodynamic environment downstream from the bifurcation apex. The results will result in a better understanding of the underlying mechanisms responsible for the initiation, development and progress of intracranial aneurysms. We hope it will lead to better methodologies for clinical evaluation, rupture risk stratification, and treatment planning of intracranial aneurysms.
Learning Objectives: By the conclusion of this session participants should be able to (1) describe the correlation between higher bifurcation angles and stronger flow recirculation as well as shear gradients in the bifurcation region migrating towards daughter branches, and to (2) discuss the implication of higher hemodynamic stresses distal from the bifurcation and outside of the medial pad, potentially leading to destructive remodeling and aneurysm initiation.