【作者】 李宁；

【导师】 顾元宪； 张洪武；

【作者基本信息】 大连理工大学 ， 力学， 2011， 博士

【摘要】 心血管病是威胁居民健康的重大疾病。据统计,国内每年死于心血管病的人数约为三百万,占总死亡人数的45%左右,每年用于心血管病的医疗费用超过1300亿元人民币。冠状动脉内支架植入术是治疗冠心病的一种有效手段,近年来越来越多地得到了广泛应用。冠脉支架的力学性能,是关系到支架术效果的关键因素之一。许多医务工作者和工程界的学者都在努力研究这一课题。本文利用有限元方法来研究冠脉支架的力学性能。论文的主要工作包括：针对冠脉裸支架结构,研究了有限元模型所采用的单元类型(高阶实体单元、低阶实体单元、壳单元、梁单元)和网格剖分密度对计算效率和精度的影响,模拟了冠脉裸支架全寿命内的变形过程所涉及到的力学性能。通过模拟支架生产过程中的压握变形、手术过程中的膨胀变形以及在冠状动脉内长期工作时的压缩变形,本文发现：压握过程对膨胀性能的影响较小,而膨胀程度对支架在工作状态下抵抗压缩载荷的径向支撑力具有较大的影响。另外,根据支架的膨胀性能曲线进行深入分析后得到了膨胀速度曲线,发现了膨胀速度曲线与支架的最佳膨胀范围的内在联系。本文对冠脉支架与球囊的接触问题进行了模拟,得到了比较准确的变形结果。在此基础上,提出了利用应变能来计算真实接触压力的方法。一般来说,通过接触分析所得到的球囊与支架间的接触压力,与实际情况下均匀一致的压力相比,误差往往比较大,这是由于支架金属丝比较细而只能在接触面上采用较粗的网格单元所导致的。根据功能原理,并且考虑到接触压力是驱动支架膨胀而产生应变能的唯一直接原因,本文利用支架应变能占全部应变能的百分比对膨胀载荷进行了修正,得到了支架与球囊之间真实而且均匀的接触压力,其结果与裸支架的膨胀曲线相互一致。这个方法可以作为验证接触分析结果正确性的依据。针对冠脉支架的纵向柔顺性,建立的悬臂梁弯曲模型,克服了三点载荷法和简支梁模型的缺陷,能够更准确、更全面地描述支架整体弯曲性能。以该模型为基础,本文详细研究了目标支架在较大弯曲曲率范围内的瞬时抗弯刚度情况,结果表明：在弯曲曲率较小的情况下,支架瞬时抗弯刚度一般表现为各向同性；而当弯曲曲率较大时,则出现非常明显的各向异性。此外,利用悬臂梁模型还发现,支架弯曲时还伴随发生了在弯曲平面外的侧向偏转以及绕支架中心轴的扭转变形。这两种附带变形是由支架弯曲变形而引起的,在一定程度上改变了支架的实际弯曲方向和曲率。这两种附带变形也正是瞬时弯曲刚度存在各向异性的证据。在优化设计工作中,利用ANSYS的APDL语言开发了适用于冠脉血管支架有限元分析的计算程序,该程序具有自动化运行、参数化建模、仝过程分析、鲁棒性计算及优化等特点,将此程序进一步加以完善后,即可作为专门用于血管支架力学性能模拟的软件。以此程序为基础,本文分别建立了描述支架膨胀综合性能和柔顺性能的数学模型,对目标支架的几何形状及尺寸进行了优化,改善了目标支架各项力学性能。以往在血管支架设计领域里采用的传统模式是对有限数量的设计方案分别进行数值模拟或实验分析,然后比较支架的力学性能结果并进行人工选优。本文的优化工作则将最优化理论拓展到血管支架的设计工作,有效地提高了血管支架优化设计工作的效率。更多还原

【Abstract】 Coronary artery heart disease (CHD) has become the most dangerous reason which threatens human health. It is reported by national authority that three million people die of CHD every year and the number reaches 45% of total death. Annually, it costs more than 130 billion RMB in this field. Coronary artery stent implantation is the latest effective therapy. As stent technology develops rapidly, stent is used in more heart surgery instead of CABG (coronary artery bypass graft surgery). The mechanical properties of stent are important. Cardiologists from hospitals, engineers who design the products and scientists of mechanics community share the same interest on this subject. This dissertation studies the mechanical properties of stent utilizing finite element method.At the beginning of this dissertation, a bare stent is studied. Firstly, attention is paid to how element type and mesh density influence the simulation results of deformation. Secondly, the expansion speed curve is derived from the expansion curve, and an instinct relationship between the expansion speed and the recommended expansion range is obtained. Thirdly, the deformation during crimp stage to assemble stent and balloon together is simulated and the results show that it does not affect expansion and compression. However, the expanding process will change the resistance to external pressure of stent greatly. At last, beam element is employed to construct the finite element model. In terms of computational accuracy and effectiveness, beam element model is compared with those of shell and solid element.Contact analysis involving stent and balloon structures is accomplished. The results are in accordance with the actual deformation. The contact pressure obtained from contact elements, which should have been average, varies greatly due to the coarse meshes. In order to validate the simulation, strain energies of all elements are collected from results when the non-linear iteration converges every time. By re-calculating the expanding load according to strain energy distribution, real contact pressure is obtained and the results match the expanding force used for bare stent very well.A cantilever model is proposed to mimic stent’s flexibility, which has some advantages over 3-point load method and the simple beam model. The IBS (Instant Bending Stiffness) of stent is studied and it is found that IBS maybe anisotropic when curvature grows large enough. In addition, a deflection and a rotation angle occur to the stent because of the anisotropic IBS.Design optimization is the most important content of this research. Optimization theory is applied to improve the mechanical properties of the stent. A program which is automatic, parametric, integrated and robust is developed using ANSYS parametric design language. According to the findings that obtained respectively from the simulations about expansion and flexibility of chapter 2 and 4, objective functions to improve the expansion properties and flexibility are proposed. And then, the proper design variables are chosen and the entire optimization models are built in mathematical formulation. Finally, the optimization tasks are performed and the optimal designs are obtained successfully. This research introduces the optimization theory to the design of coronary stent, which improves the design efficiency in this field.更多还原