Flow Simulations to Establish the Relationship Between the Inflow Zone in the Neck of a Cerebral Aneurysm and the Positions of Struts
Authors: Watanabe Kazuhiro , Anzai Hitomi , Ohta Makoto 
Published: February 2017

Flow-diverter (FD) stent implantation is an attractive treatment for cerebral aneurysms because of its low level of invasiveness. FD stent has a fine mesh structure, and the aim of FD implantation is to reduce the blood flow in an aneurysm by covering the aneurysm orifice. However, the fine mesh of the implant poses the risk of parent artery occlusion.
One approach for avoiding this risk is to use a stent with a higher porosity. Previous studies have shown that placing a strut to disturb the inflow entering an aneurysm can promote a higher reduction in aneurysm flow. However, Hirabayashi et al. reported that a high-porosity stent can be sensitive to misdeployment in flow reduction. We hypothesized that a positioning error in flow reduction was sensitive to the relative position of the strut to the inflow configuration. In this study, we performed flow simulation to investigate the relationship between the inflow zone of the aneurysm neck and the positions of struts.
Lattice Boltzmann (LB) flow simulation was performed to allow a comprehensive study of strut positions. Two rectangular solids were used as the strut model. Steady flow simulation was applied to models based on ideal and realistic three-dimensional (3D) aneurysm geometry, changing two strut positions along the neck plane. For both models, velocity boundaries were imposed on the inlet and a constant pressure boundary was imposed on the outlet.
Average flow velocity in an aneurysm was calculated to evaluate the dependency of the flow reduction effect on the deployment position. We analyzed aneurysm flow using the following three strategies to observe the relationship between flow configuration, strut configuration, and flow reduction.
Analysis A: Flow reduction rate (Rf) with one strut. A strut was moved from the proximal to the distal neck (perpendicular deployment) or from outside to inside (parallel deployment).
Analysis B: Rf with two struts. One strut (strut A) was fixed in a specific position on the neck plane. The other (strut B) was moved along the neck plane in parallel to strut A.
Analysis C: Rf with two struts. Strut B was located on the distal or inner side of strut A. The distance between the two struts was changed, and the two struts were moved along the neck plane while maintaining that distance.
From the results of Analyses A and B, we confirmed a critical area in the inflow zone that maintained a high flow reduction regardless of the position of the second strut.
The results of Analysis C confirmed that there were several distances between the struts at which flow reduction was almost constant. This constant reduction was maintained when one of the struts was located in the critical area, whereas the reduction was disturbed if both struts were located outside the area.
These results suggest that the influence of positioning errors can be reduced by constantly placing at least one strut in a critical area, resulting in a high flow reduction. This may lead to optimal stent porosity for flow reduction and robustness of deployment.