Orientation relative to gravity is rarely considered when assessing hemodynamics in intracranial aneurysms (IAs). Because head posture is readily adjustable during daily life and in clinical care yet is typically overlooked in studies, understanding gravitational effects bears direct translational significance. We investigated the effect of gravity on IA hemodynamics using an in vitro study. Flow through two patient-specific aneurysm phantoms, a basilar tip aneurysm (BTA) and an internal carotid artery (ICA) aneurysm, were measured using volumetric particle tracking velocimetry (PTV); velocity fields with each IA in a vertical orientation (i.e., patient upright) and horizontal orientation (i.e., patient laying down) were captured. The same pulsatile inflow flowrate was maintained across both orientations. Subsequently, turbulent kinetic energy (TKE), time-averaged wall shear stress (TAWSS), oscillatory shear index (OSI), and velocity-gravity alignment probability-density functions were computed. Our findings revealed that geometry dictated the gravitational response. In the coaxial BTA, the mean centerline velocity was 21% higher in the vertical orientation. Further, for the BTA, the vertical orientation produced a Bland-Altman bias of $+0.019\mathrm{m}{\mathrm{s}}^{-1}$ (95% limits –0.033 to 0.070 m ${\mathrm{s}}^{–1}$), nearly twice as large TAWSS (1.08 vs 0.70 Pa) and reduced OSI (0.135 vs 0.158), shifting the sac from a thrombus-prone low-shear/high-oscillation regime to a high-shear state associated with acute wall erosion. Horizontal rotation reversed these trends and yielded elevated TKE, confirming a gravitational toggle between inertia- and buoyancy-dominated flow. The helical-neck ICA showed minimal orientation sensitivity ($<0.11$ Pa change in TAWSS; $<0.05$ in OSI), indicating that intrinsic curvature overwhelms gravitational forcing. These findings demonstrate that gravity is an active, morphology-dependent load: posture can drive clinically relevant shear-polarity switches in gravity-aligned bifurcation aneurysms yet is secondary in curvature-dominated side-wall lesions. Incorporating sac-axis orientation into imaging and computational pipelines may refine patient-specific risk stratification and inform gravity-aware therapeutic strategies.

Close

PrevNext

View figure in article

• Biological fluid dynamics
• Flow-vessel interactions