Identification of Critical Hemodynamic Parameters that Correlate to Bifurcation Sites with High Rates of Intracranial Aneurysm Occurrence
Alfano, Jaclyn M.
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The objective of this study was determine the site distribution of intracranial aneurysms (IA) within and around the Circle of Willis, and to identify the critical hemodynamic factors that correlate to cerebral bifurcations with high rates of IA occurrence. The first part of this study (Part I) was a systematic review aimed at determining the occurrence rate of intracranial aneurysms at ten bifurcation sites. The occurrence rate of IA at intracranial bifurcation sites varies widely and has not been previously reviewed. In our review, we did a systematic analysis of the literature to calculate the percent occurrence of ruptured intracranial aneurysms (RIA), unruptured intracranial aneurysms (UIA), and all IA (ruptured + unruptured) at ten cerebral bifurcations, and determine their site distribution. Nineteen studies (28,579 aneurysms, 18 countries, US/Canada/Europe/Australia/Japan) were included into an overall result of percent occurrence for RIA, UIA, and all IA. ACOM and MCA Bifurcations have the highest overall IA occurrence rates (both 25%), followed by ICA-PCOM (18%). For RIA, the sites with the most frequent occurrences are ACOM, ICA-PCOM, and MCA Bifurcations, in descending order (35%, 21%, and 20%). In all three categories (all IA, RIA, and UIA), Vertebral-PICA, the Vertebrobasilar junction, and BA-SCA have the lowest occurrence rates (2% or less). Although it is evident from these results that certain intracranial bifurcations have a predisposition for IA, and that certain sites have an increased occurrence of ruptured aneurysms, the contributing factors are unknown. However, it has been speculated that hemodynamic factors play a key role in aneurysm localization. In the second part of this study (Part II), we aimed to identify the critical hemodynamic factors that correlate to bifurcation sites with high rates of IA occurrence. IA occurrence rates were based on results of the overall IA group in part I. Specifically, we aimed to correlate hemodynamic factors within the flow acceleration zone to IA occurrence rates, since destructive aneurysmal remodeling has been shown to localize within this zone in animal models. The 3-D geometries of 114 human intracranial arterial bifurcations, distributed amongst ten cerebral bifurcations within and around the Circle of Willis, were obtained by 3-D rotational angiography. On the segmented 3-D models we performed computational fluid dynamics (CFD) simulations and calculated the distributions of the wall shear stress ( WSS ) and its spatial gradients ( WSSG ) within the flow acceleration zone. To identify which hemodynamic factors correlate to rate of IA occurrence, we performed mixed linear regression analysis. Mixed linear regression analysis revealed that MAX WSS, AVG WSS, MAX WSSG, AVG WSSG , and flow acceleration zone area ( AREA AZ ) each show significant independent positive correlation with bifurcation IA occurrence rate. Of the hemodynamic parameters, MAX and AVG WSS show the strongest independent correlation, followed by MAX and AVG WSSG , and then AREA AZ . Additionally, the interaction of AVG WSS x AVG WSSG correlates to IA occurrence rate. Therefore, bifurcation sites with higher rates of IA occurrence have significantly larger flow acceleration zone areas with significantly higher WSS and WSSG , leaving them more susceptible to the destructive aneurysmal remodeling that has been localized to the flow acceleration zone in animal models.