Fluid-structure Interaction Research On Hemodynamics In Vessel-stent Coupling Systems Based On Bulged Inner-wall Model

With the improvement of the quality of people's life and the changes of lifestyle, cardiovascular disease, especially the coronary heart disease (CHD), has become one of the common diseases which does great harm to the health of human beings. Nowadays, the most effective therapy to CHD is stent implantation. Stent implantation has been an important method to treat CHD because of its advantages of little trauma and remarkable effect. However, in-stent restenosis (ISR) is one of the non-ignorable potential threats to stent implantation. Hemodynamics condition is closely related to ISR. So it is significant to study the hemodynamics condition after stent implantation. However, it is difficult to conduct an in vivo/vitro experimental study. Indeed, the curvature effects related to the complex cylindrical model as well as the presence of stent lead to diffraction effects and shade area. Therefore, numerical simulation technology plays an important role in hemodynamics study.Present numerical simulations are based on the models whose cross sections are simplified to circular sections after implanting stent, which are called cylindrical inner-wall models. And most of these simulations are conducted with the method of CFD. In these simulations, the vessel wall is supposed to be rigid wall. As a matter of fact, stent expansion would change the inner surface of blood vessel. After expansion, the cross sections are no longer circular sections according to the animal experiments and angiographies. Meanwhile there is an interaction between the vessel wall and blood flow. Therefore, reconstructing the deformed coupling model (which is called bulged inner-wall model) after stent expansion is the crucial work in this study. In order to research the influence of hemodynamics condition to ISR and stent optimization, this model is analyzed with the method of fluid-structure interaction (FSI). The main achievements of this study are listed as follows:(1) The basic theories and methods of finite element numerical simulation of stent-vessel coupling expansion were studied. Highly nonlinear problems were elaborated from geometric nonlinearity, material nonlinearity and nonlinear boundary condition. The large deformation theory was deduced in this work to support the numerical simulation of stent-vessel coupling expansion. Then the result of simulation was analyzed. The analysis mainly includes the rate of stent expansion, rate of stent shortening, stress and strain of different parts of the model. The result indicates that the stress after stent expansion is between the yield strength and tensile strength, which means that when the expansion completes, the plastic deformation has occurred on the stent, but the interior stress is not large enough to make the stent fracture. The result also shows that inner wall of blood vessel is bulged in the stent area. This is the foundation of reconstructing the deformed stent-vessel model.(2) The method of reconstructing deformed coupling model after stent expansion was studied. The model reconstruction is based on the finite element model which is obtained by the numerical simulation of stent-vessel coupling expansion. The information of key nodes of the finite element model was measured in the post-processing software. Then deformed stent-vessel coupling entity model was reconstructed in the modeling software using the information of key nodes. And the reconstructed model was aimed to be used in the FSI analysis.(3) The key technologies of FSI on hemodynamics in vessel-stent coupling system were studied. The interaction mechanism of stent and vessel and the reason of ISR were comprehensively studied. Hemodynamics in the bulged inner-wall model was first analyzed. The result shows that low wall shear stress mainly occurs adjacent to the stent strut, and pulsatile flow has an obvious impact on vessel wall. Secondly, the FSI analysis results of cylindrical inner-wall model and bulged inner-wall model were compared. It indicates that the cylindrical inner-wall model weakens the low wall shear stress area, and the impact of pulsatile flow to bulged inner-wall model is more obvious. Compared to the cylindrical inner-wall model, the bulged inner-wall model can truly reflect the condition of ISR, and it is beneficial to the optimization of stent structure. Finally, three bulged inner-wall models with different stent thicknesses were respectively analyzed with the method of FSI. The result shows that the stent thickness has no significant influence to the overall distribution of wall shear stress, but it has a relatively large impact to the distribution of low wall shear stress. As a conclusion, stent implantation will inevitably lead to flow disturbance of near wall, and bulged vessel wall occurs periodic stress and displacement, which will enhance the possibility of intimal hyperplasia. So on the premise of meeting other mechanical property requirement, a stent should be with a thinner thickness to reduce the probability of ISR.