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  • High Inclination Angles Lead to Flow Recirculation and Aneurysmogenic High Positive Wall Shear Stress Gradients in Cerebral Bifurcations

    Final Number:

    Adel M. Malek MD PhD; James Hippelheuser; Alexandra Lauric

    Study Design:
    Laboratory Investigation

    Subject Category:

    Meeting: Congress of Neurological Surgeons 2018 Annual Meeting

    Introduction: Arterial bifurcation geometry determines the local hemodynamic environment and has been previously identified as a predisposing factor for aneurysmal presence. Bifurcations harboring aneurisms have wider angles and more pronounced inclination compared to controls. We sought to determine the effect of the bifurcation inclination angle on the vessel hemodynamics and analyze its role in aneurysm formation.

    Methods: Bifurcation parametric models were created at increasing bifurcation angles (60°-120°) and degrees of inclinations (30°-90°) between parent and daughter vessels. 3D rotational angiography of 4 MCA bifurcations (3 aneurysms, 1 controls) were available for computational fluid dynamic (CFD) simulations at and around the bifurcation apex. Wall shear stress (WSS) and WSS gradients (WSSG) were analyzed at the apex in all models.

    Results: Higher inclination angles induce flow recirculation at the inclination elbow, starting from an angle of 45°. At inclinations angles larger than 60°, flow reversal is revealed in time-average WSS. Increasing the inclination angles results in a widening of the reversal area and the formation of recirculation focal points at the apex leading to lower WSS, but increasing positive WSSG. Wider bifurcation angles magnifies this phenomenon and leads to well-developed vortical structures at bifurcation angles wider than 120°. High inclination angles in patient-derived data show that bifurcation harboring aneurysms are characterized by strong flow reversal, in contrast to the control model.

    Conclusions: High degree of inclination between parent and daughter vessels at cerebral bifurcations triggers a particular blood flow pattern characterized by flow reversal and formations of recirculation focal points. This phenomenon leads to the development of aneurysmogenic positive WSSG as a function of angular geometry, and provides a mechanotransductive link to aneurysm development. This suggests therapeutic strategies aimed at altering underlying unfavorable geometry and deciphering the molecular endothelial response to shear gradients in a bid to disrupt the associated aneurysmal degeneration.

    Patient Care: Identifying geometrical features of patients' cerebral vasculature that places them at greater risk of developing aneurysms provides an opportunity to perform pro-active surveillance for early aneurysm detection. These findings may also help explain the mode of operation of various endovascular devices that cause vessel straightening thus decreasing the inclination angle, and decreasing aneurysmogenic hemodynamic conditions.

    Learning Objectives: By the conclusion of the presentation, the participants should be able to 1)Understand concepts of bifurcation geometry and their effects on hemodynamics,2)Become familiar with patient-based modeling, 3)Gain familiarity with vessel wall and cellular mechano-transduction and their effects on vessel remodeling and disease

    References: 1. Tütüncü F, Schimansky S, Baharoglu MI, Gao B, Calnan D, Hippelheuser J, Safain MG, Lauric A, Malek AM. Widening of the basilar bifurcation angle: association with presence of intracranial aneurysm, age, and female sex. Journal of neurosurgery. 2014 Dec;121(6):1401-10. 2-Baharoglu MI, Lauric A, Wu C, Hippelheuser J, Malek AM. Deviation from optimal vascular caliber control at middle cerebral artery bifurcations harboring aneurysms. Journal of biomechanics. 2014 Oct 17;47(13):3318-24. 3-Baharoglu MI, Lauric A, Safain MG, Hippelheuser J, Wu C, Malek AM. Widening and high inclination of the middle cerebral artery bifurcation are associated with presence of aneurysms. Stroke. 2014 Sep 1;45(9):2649-55.

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