【作者】 刘威；

【导师】 程洁；

【作者基本信息】 东南大学 ， 机械制造及其自动化， 2016， 硕士

【摘要】 随着人民生活质量的提高和生活方式的变化等,心血管疾病日益成为危害人类身体健康的常见病症之一,其中又以冠心病危害最甚。支架植入术以其创口微小、效果显著等优势成为冠心病的重要治疗手段。但是支架内再狭窄是支架植入术不可忽视的潜在威胁之一,而血流动力学状况与支架内再狭窄密切相关,因此对植入支架后血管内血流动力学的研究显得尤为重要。由于血管形状引起的曲率效应和冠脉支架存在而导致的衍射现象及阴影区域,使得很难进行体外实验研究,因此数值仿真技术在研究血流动力学方面发挥了至关重要的作用。目前大多数仿真研究是将支架处血管横截面简化为圆面的模型(称之为圆柱型内壁模型)来进行的,且多为假定血管壁是刚性壁的纯流体分析。而实际上,根据动物实验和血管造影研究发现,支架扩张会改变血管横截面形状,形成非圆截面：同时血管壁和血流之间存在相互作用。因此,本文以建立扩张后变形支架血管耦合模型(称之为凸入型内壁模型)为切入点,进行流固耦合分析,以研究冠脉内血流动力学情况对支架内再狭窄的影响,并对支架结构优化提出建议。本文的主要研究成果如下：(1)研究了支架血管共同扩张数值模拟的基础理论和实施方法。通过几何非线性、材料非线性和边界条件非线性三个方面阐述了高度结构非线性问题,并推导了大变形基本理论。进行了针对支架血管共同扩张的数值模拟,得到了扩张后血管、支架的变形结果,分析了支架的扩张率和缩短率,以及模型各个部件的应力应变等模拟数据。结果表明,支架扩张后内应力在材料的屈服强度和抗拉强度之间,说明支架扩张完成后已经产生塑性变形,但是并没有出现超过抗拉强度的应力而导致的支架断裂现象。研究还发现,在支架网孔内,血管壁出现内凸现象,这一现象是构建更为准确的血管支架流固耦合模型的基础。(2)研究了构建变形后耦合模型的方法。本文中变形模型的重建主要依靠成果(1)中共同扩张模拟后得到的变形的有限元模型。在后处理软件中测量得到若干变形后有限元模型关键节点信息,利用得到的节点位置信息,在三维建模软件中重建了变形后的支架、血管耦合实体模型。该模型能够准确地反映出支架扩张后血管内壁真实的变形情况。该模型将被应用到后续的流固耦合分析当中。(3)研究了血管一支架流固耦合模型血流动力学数值模拟的关键技术,深入全面地研究了支架植入病变部位后血流、血管相互作用机理,以及支架内再狭窄成因。首先对凸入型内壁模型进行血流动力学分析,发现低壁面剪应力主要存在于支架杆和支架连接筋附近,脉动流对血管壁冲击作用明显。其次对圆柱型内壁模型和凸入型内壁模型的模拟结果进行对比。对比发现,圆柱型内壁模型弱化了低壁面剪应力区域,凸入型内壁模型在支架区域受脉动流冲击导致的变形更为显著。相比于圆柱型内壁模型,凸入型内壁模型更能真实地反映出支架内再狭窄发生的情况,有利于后续的支架结构的优化。最后对三种不同支架厚度的模型进行了流固耦合分析,对比发现,虽然支架厚度对冠脉内壁面剪应力的整体分布影响不大,但是对低壁面剪应力区域的分布影响较大。数值模拟结果表明,支架植入必然会导致近壁面血流发生扰动,凸入的血管壁发生周期性应力和位移,从而增大内膜增生的可能性；在保证支架强度的同时,应尽可能的减小支架厚度,从而降低支架内再狭窄发生几率。更多还原

【Abstract】 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.更多还原