Yasuhiro Shobayashi

Mechanical design of an intracranial stent for treating cerebral aneurysms

Endovascular treatment of cerebral aneurysms using stents has advanced markedly in recent years. Mechanically, a cerebrovascular stent must be very flexible longitudinally and have low radial stiffness. However, no study has examined the stress distribution and deformation of cerebrovascular stents using the finite element method (FEM) and experiments. Stents can have open- and closed-cell structures, and open-cell stents are used clinically in the cerebrovasculature because of their high flexibility. However, the open-cell structure confers a risk of in-stent stenosis due to protrusion of stent struts into the normal parent artery. Therefore, a flexible stent with a closed-cell structure is required. To design a clinically useful, highly flexible, closed-cell stent, one must examine the mechanical properties of the closed-cell structure. In this study, we investigated the relationship between mesh patterns and the mechanical properties of closed-cell stents. Several mesh patterns were designed and their characteristics were studied using numerical simulation. The results showed that the bending stiffness of a closed-cell stent depends on the geometric configuration of the stent cell. It decreases when the stent cell is stretched in the circumferential direction. Mechanical flexibility equal to an open-cell structure was obtained in a closed-cell structure by varying the geometric configuration of the stent cell.

Structural Analysis for Wingspan Stent in a Perforator Model

Perforator infarction represents a critical problem after intracranial Wingspan stent. To explore the mechanism of perforator infarction, we simulated the stent-artery interaction at an atheromatous plaque with perforator. Structural deformation and biomechanical stress distribution after stenting were analyzed. High radial stress values were located along the stent struts, which surrounded the area with high circumferential stress. Stretched perforator orifice in a circumferential direction after stenting was simulated. These results show that structural deformation could play a role in the mechanism of perforator occlusion after Wingspan stenting.

Simulated biomechanical responses at a curved arterial segment after Wingspan Stent deployment in swine.

Abstract Objectives: Endovascular treatment with the Wingspan Stent is frequently associated with in-stent restenosis at the curved portion, leading to late-phase stroke. To explore the cause of stroke complications after treatment with the Wingspan Stent, we simulated the biomechanical responses at a curved arterial segment using the finite element method. Methods: A Wingspan stent was deployed at a slightly curved ascending pharyngeal artery (APA) in swine. Several stress distributions modeling solid mechanics were analyzed with structural deformation. Histopathological analysis of the selected APA was assessed at 28 days after stenting. Results: Arterial straightening was simulated in this study. Both radial stress (RS) and circumferential stress (CS) concentrations increased at both stent ends. Marked lower axial stress (AS) concentration was observed at the outer wall of an arterial curvature. The proximal stent segment, ending in the curved portion, significantly impacted the solid mechanical environment. Eccentric neointimal hyperplasia was observed at the curved segment. Discussion: These results show that the Wingspan stent exaggerated the non-uniform stress distributions in a curved artery. The understanding of stent–arterial wall interactions is of value to identify the current limitations of intracranial stenting, and will help to improve this treatment methodology and future devices.

Comparison of simulated structural deformation with experimental results after Wingspan stenting

Abstract Objectives: Biomechanical stress distribution correlates with the biological responses after stenting. Computational analyses have contributed to the optimization of stent geometry. In particular, structural analysis based on pre-operative angiography can be used to predict the stent–artery interaction before endovascular treatments. However, the simulated results need to be validated. In this report, we compared the simulated arterial structure with post-operative images after an intracranial Wingspan stent. Methods: A Wingspan stent was deployed at a slightly curved ascending pharyngeal artery (APA) in the swine. Using a finite element method (FEM), the configuration after stenting was simulated and quantitatively compared with post-procedural 3D angiography. Results: The finite element analysis demonstrated arterial straightening after stenting. The simulated images were similar to the experimental results with respect to the curvature index of the center line and the cross-sectional areas. Conclusion: We assessed the simulated structural deformation after Wingspan stenting, by comparison with experimental results.

O-017 Effect of biomechanical environment in vessel wall on stent restenosis

Introduction Stenting has become a standard procedure to treat atherosclerotic vessels. However, restenosis is reported in approximately one-third of patients treated with intracranial stenting, and its avoidance is drawing attention. The cause of restenosis includes vessel injury due to pressure from stent expansion and neointimal thickening due to decrease in vessel wall shear stress (WSS). Although the severity of vessel injury is proportional to neointimal thickness and restenosis, they are discussed separately. The object of this research is to examine the stress concentration and wall shear stress by a numerical analysis and reveal their configuration dependent relationship and effect to stent structure. Materials and methods A commercial open cell coronary stent model was used in this research to make a comparison with other research. Firstly, stent expansion in an intact vessel was simulated by the finite element method and the stress distribution of stented vessel was analyzed. Next, computational fluid dynamics was conducted to examine blood flow and WSS distribution. Furthermore, stent structure was modified to improve stress concentration and WSS. Stress concentration was avoided by connecting adjacent stent struts and making a closed cell model. This model was named model 1. In contrast, low WSS was avoided by reducing the number of stent struts in the longitudinal direction, named model 2. Vessel stress and WSS distribution was examined to look into the effect of stent structure on vessel stress and WSS. Results and discussion Von Mises stress and WSS distribution of stented arterial lumen is shown in Abstract O-017 figure 1. Localized radial stress and low WSS, below 1 Pa, was observed around intersections of stent struts after stent expansion. Since endothelial cells and internal elastic lamina within the vessel is deformed in the same area, vessel injury is implied. Moreover, decrease in WSS and hoop stress was observed throughout the stented vessel lumen. Like WSS, hoop stress promotes neointimal thickening. Therefore, restenosis is assumed due to the interaction of vessel injury and neointimal thickening. Also, the modified model 1 moderated stress concentration but induced low WSS. On the other hand, model 2 improved low WSS in the vessel lumen but stress concentration was observed. It is presumed that stress concentration improves when the contact area of the stent strut increases whereas the area subjected to low WSS increases with the addition of contact area of stent struts. Therefore, a stent design with connected and reduced stent struts is feasible to avoid restenosis.Abstract O-017 Figure 1

008 Biomechanical design of the intracranial stent for the cerebral aneurysm treatment

Introduction: The mechanical characteristic required for an aneurysm stent is high longitudinal flexibility and low radial stiffness. Longitudinal flexibility is necessary to navigate the tortuous shape inherent to the intracranial circulation to reach lesions beyond the carotid siphon. Low radial stiffness is necessary to reduce the risk of vascular damage. However, mechanical behavior such as stress distribution and deformation by finite element method (FEM) and experiments of cerebrovascular stent has not yet been studied. The aim of this study is to …