Tetsuya Tanoue

Wall Shear Stress Distribution Inside Growing Cerebral Aneurysm

BACKGROUND AND PURPOSE: Hemodynamic stimulation has been suggested to affect the growth of cerebral aneurysms. The present study examined the effects of intra-aneurysmal hemodynamics on aneurysm growth. MATERIALS AND METHODS: Velocity profiles were measured for 2 cases of AcomA aneurysms. Realistically shaped models of these aneurysms were constructed, based on CT angiograms. Flow fields and WSS in the models were measured by using particle image velocimetry and LDV. In 1 case, hemodynamic changes were observed in 4 stages of growth over a 27-month period, whereas no development was observed in the other case. RESULTS: The growing model had a smaller and more stagnant recirculation area than that in the nongrowth model. The WSS was markedly reduced in the enlarging region in the growing models, whereas extremely low WSS was not found in the nongrowth model. In addition, a higher WSSG was consistently observed adjacent to the enlarging region during aneurysm growth. CONCLUSIONS: The results indicated that the flow structure of recirculation itself does not necessarily lead to high likelihood of cerebral aneurysm. However, WSSG and WSS were distinctly different between the 2 cases. Higher WSSG was found surrounding the growing region, and extremely low WSS was found at the growing region of the growing cerebral aneurysm.

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.

Wall shear stress distribution inside induced cerebral aneurysm on rabbit

Introduction: Cerebral aneurysm is a cerebrovascular disease resulting to subarachnoid hemorrhage. The generation, development and finally rupture of cerebral aneurysm could be related to hemodynamic stimulate, especially wall shear stress (WSS). WSS have been considered to play a role on the expression of such enzymes as iNOS, MMP-2 and MMP-9 leading to weakening of the arterial wall. However, detailed WSS distribution contributing to the enzyme expression has not been clarified inside cerebral artery. The purpose of our study is to clarify the WSS distribution corresponding to the enzyme expressions. Materials and Methods: We tried to induce cerebral aneurysm on rabbits, and created the realistic silicone model of the induced aneurysm. The flow field and WSS were measured using PIV (particle image velocimetry) and LDV (laser Doppler velocimeter) inside the realistic model. Furthermore, histological structure and enzyme expression were evaluated on the image of elastica van Gieson stain and on the immunostaining images. Results: Induced cerebral aneurysms were found in posterior circulation; the arterial bifurcation at basilar artery or posterior communicating artery. The induced cerebral aneurysm had an arterial wall structure with thinned media and lacked IEL. The thinned media was observed at the bulge top area wth low WSS distribution. On the other hand, lacked IEL was observed near the aneurysm neck area. Although WSS was not so low at the area of lacked IEL, the WSS spatial gradient appeared with high value at this area. The lack of occurs before the thinning of media on the process of aneurysm formation. In addition, MMP-2 and MMP-9 showed similar trend as the WSS spatial gradient. From these findings, high WSS gradient seems to have an effect on the early stage of cerebral aneurysm generation, stimulating the enzymes expression which leads to the destruction of the IEL.