Anatomical Variations in Circle of Willis and Intracranial Aneurysm Formation

Abstract

Background: Intracranial aneurysm (IA) can be commonly found in the circle of Willis (CoW), and a higher morbidity of IA is found to be associated with a higher percentage of an incomplete CoW. Hemodynamic factors are believed to play an important role in aneurysm formation. However, how the anatomical variations in CoW lead to hemodynamic difference and what hemodynamic parameters play important roles in aneurysm formation have not been quantified and assessed. Methods and Results: Thirty patients were included and based one computed tomography angiography, they were divided into three groups (10 patients per group): a normal group (normal CoW and without aneurysms), an absence group (lacking the anterior cerebral artery and with aneurysms), and a hypoplasia group (hypoplasia of the anterior cerebral artery and with aneurysms). Patient-specific 3D models of the CoW were reconstructed by removing all the aneurysms and CFD simulations were performed on the 30 patient-specific models. Six hemodynamic parameters (blood pressure, average wall shear stress (AWSS), AWSS gradient (AWSSG), oscillatory shear index (OSI), aneurysm formation indicator (AFI), and gradient oscillatory number (GON)) were analyzed and compared between the three groups. The hypoplasia group was found to be significantly higher in AWSS (7.41±1.70vs5.44±1.31, p=0.029) and AFI (0.9959±0.0012vs0.9945±0.0011, p=0.037) than those of the normal group. Conclusions: Anatomical variation in the anterior circulation of CoW resulted in abnormal hemodynamic environment of cerebral arteries, and this may lead to formation of intracranial aneurysm. AWSS and changes in the direction of WSS may be the important hemodynamic parameters leading to aneurysm formation.