冠脉血管支架介入耦合系统力学行为数值模拟研究

针对冠脉支架植入术后引起的血管内再狭窄问题,开展了冠脉支架介入耦合系统力学行为的数值模拟研究。基于Ogden非线性弹性理论,构建了冠脉血管和动脉粥样硬化斑块的超弹性本构模型。通过非线性有限元法,建立了冠脉支架与狭窄血管的耦合作用模型,研究了冠脉支架在经历压握收缩、压握卸载、球囊扩张与球囊收缩等介入过程后的体内扩张性能,分析了冠脉支架的介入对狭窄血管损伤及再狭窄的力学影响因素。对比分析了S型支架和N型支架介入后狭窄冠脉血管的生物力学响应,数值计算结果表明:狭窄冠脉血管在支架支撑体波峰处存在较高的应力梯度,而且由于2种支架联接筋结构的类似性,血管内膜与斑块的应力分布规律一致。但是,N型支架的径向回弹率与轴向短缩率均小于S型支架,导致了更高的狭窄血管壁面峰值应力和应力梯度,更易于引起冠脉血管损伤造成血管内再狭窄。综上,该文提出的冠脉支架介入耦合系统力学模型,对于优化支架结构、抑制冠脉血管再狭窄问题,提供了重要的理论依据和临床参考。     

冠脉支架对弯曲血管损伤机理的非线性有限元分析

针对支架植入术后的冠脉血管内支架再狭窄问题,通过非线性有限元法建立弯曲冠脉-支架介入系统生物力学模型及冠脉支架纵向柔顺性计算模型,开展了冠脉支架结构设计对血管壁面损伤与再狭窄的影响机理研究。数值分析结果表明,弯曲血管在支架端部存在高应力梯度,所产生的最大应力是直冠脉的4~6倍,易于损伤血管从而形成血管内支架再狭窄问题。冠脉支架结构的纵向柔顺性对弯曲血管的整体应力水平具有显著的影响,C型支架的纵向柔顺性远低于S型支架和N型支架,植入后所引起的血管整体应力水平最高;S型支架具有最好的纵向柔顺性,植入后所引起的血管整体应力水平最低。从支架与血管的相互作用以及支架纵向柔顺性的角度解释了临床中弯曲血管内支架再狭窄率较高的力学原因,为支架的临床选择及优化设计提供了重要指导作用。     

球囊扩张式冠脉支架扩张变形机理的数值方法研究

通过非线性有限元方法,建立了球囊扩张式冠脉支架自由扩张和狭窄血管体内扩张的数值计算模型。针对两种不同联接筋结构的冠脉支架,分析了S型和N型支架的自由扩张性能和介入狭窄血管后的体内扩张性能,研究了支架结构对血管组织机械损伤的影响机理。数值模拟结果表明,在自由扩张阶段,S型和N型支架变形均匀,两种支架的径向回弹率和轴向短缩率趋于一致,并且具有良好的扩张均匀性与一致性,有利于支架扩张狭窄血管和在血管内的精确定位。在体内扩张阶段,由于狭窄血管的约束作用,S型和N型支架的径向回弹率均高于自由扩张阶段,轴向短缩率均低于自由扩张阶段;但是S型支架介入狭窄血管后的径向回弹率和轴向短缩率略高于N型支架,引起了更低的血管应力水平和应力梯度,减少了血管组织的机械损伤,降低了支架介入术后的血管内再狭窄率。综上,提出的计算模型对于冠脉支架的生物力学性能评估、结构优化设计与介入术后的血管损伤评价提供了重要的理论依据。     

狭窄血管处管壁的变形与应力分析

该文根据血液流动连续性方程、运动方程及血管壁弹性力学方程,通过边界条件的耦合,在给定了血压波形函数的基础上,假设堵塞斑块与血管壁弹性性质相同时,求解了狭窄血管管壁的径向位移及环向应力;分析了不同狭窄程度对血管壁变形及应力的影响;给出了不同狭窄状态下,血管植入支架所需的支撑力,从而计算出植入支架后血管壁的径向位移及应力状态。该文的研究结果可为临床上对狭窄血管植入支架的变形与受力分析提供参考。     

狭窄血管内支架变形行为及力学性能模拟研究

狭窄率反映血管的堵塞程度,不同狭窄率对血管内支架力学性能的影响不同。利用有限元法对4种支架分别在30%,40%,50%的狭窄血管内进行力学性能模拟分析,研究狭窄率对支架变形行为和力学性能的影响。结果显示,狭窄率对支架的等效应力影响较小,而支架结构对血管内应力分布具有明显的影响,其中开环支架对血管内应力影响大于闭环支架;随着血管内狭窄率的升高,支架轴向缩短率呈减小的趋势、径向回弹率与扩张不均匀性呈增大的趋势,血管内等效应力具有增加的趋势;狭窄率对闭环支架径向回弹率与扩张不均匀性的影响比开环支架更显著。通过对狭窄率影响的分析,再现了不同堵塞程度血管对不同支架的影响,为支架的设计与植入提供理论依据。     

血管内支架的疲劳强度的有限元模拟分析

为了揭示支架支撑体长度、宽度及圆弧曲率半径对其疲劳强度的影响规律,利用有限元法对不同结构参数的9种血管内支架进行动静态模拟分析,并用Goodman线图评价了支架的疲劳强度。结果显示,静态扩张过程中,随着支撑体长度或圆弧曲率半径的增加,支架所需的最大等效应力有减少的趋势,而随着支撑体宽度的增加,支架所需的最大等效应力有增加的趋势。动态加载过程中,支撑体长度、支撑体宽度及圆弧曲率半径均与疲劳安全系数成反比,其中支撑体宽度对疲劳安全系数的影响要比圆弧曲率半径和支撑体长度的影响较明显。关于支架疲劳强度的有限元分析结果,为支架的优化设计提供科学的理论依据。     

CoCr合金冠脉支架扩张变形的生物力学性能研究

通过非线性有限元方法研究了冠脉支架扩张过程的变形机理和生物力学性能。在不考虑血管壁和斑块的作用下,研究支架压握收缩、自由扩张和卸载过程中的有限元模型的网格划分、材料本构模型建立、边界条件、载荷定义和接触处理等关键技术。根据有限元计算结果得到了3种支架的轴向缩短率、扩张均匀性、径向反弹率、径向支撑力和柔顺性等生物力学性能参数,并与实验测试数据进行了分析比较,数值结果与扩张实验数据吻合较好。模拟方法为冠脉支架的优化设计及性能评估提供了科学的依据。     

全降解冠脉支架研究进展

血管支架是指在管腔球囊扩张成形或自身膨胀的基础上,在病变段置入内支架以达到支撑狭窄闭塞段血管,减少血管弹性回缩及再塑形,保持管腔血流通畅的目的。自1987年第一次被用于治疗心血管疾病,心血管支架经历了巨大的发展,至今为止已经有40种不同类型的商业支架,还有更多的在研究中。心血管支架的迅速发展主要在于它们成功地扩张了血管官腔,减少了再狭窄率,比单纯的血管成形术减少了局部缺血引起的并发症[1]。     

具有周期结构的血管支架有限元分析

用于治疗血管狭窄的血管支架是一个具有周期微结构的圆管型结构.该文分析的是管型气囊扩张式的支架,在植入血管的过程中,支架随着气囊的受压膨胀而受到内压继而发生形式为均匀膨胀的弹塑性变形.该文自行设计了一种支架,并选择适当的周期微结构即代表性单胞作为数值仿真的模型,构造了相应的周期边界条件,对上述变形过程进行了有限元分析.最后通过后处理程序得到完整支架的分析结果.结果主要包含两个方面:一是对应力和变形的预测.这对血管支架的设计以及长期服役的效果分析是至关重要的;另一个结果是给出了内压与支架直径之间的关系曲线.可为医生的植入手术提供重要参考.分析采用ABAQUS/Explicit分析模块.因为只分析一个代表性单胞就可以代替对整个支架结构的分析,所以可大大节约计算成本.     

新型钛合金血管内支架研究

钛合金血管内支架由钛合金管经激光雕刻而成，此类支架血管壁之间是面接触，能够提供较高的径向支撑强度．对新型钛合金血管内支架的加工、表面处理、性能测试、生物学评价等4个方面做了系统研究．激光雕刻后的钛合金血管支架通过机械、电化学等方法进行表面处理后，经测试，得出：钛合金血管内支架柔韧性好、支撑力强，回缩率低，整体性能达标，且具有优良的生物相容性，再狭窄率低，符合现有国际支架的标准。     

Analysis of the whole implementation process and optimization of a Nitinol superelastic stent

Nitinol superelastic stents have been widely used to treat the vascular stenosis due to its excellent mechanical behavior and biocompatibility. However, there exist conflicts between the functional properties and mechanical properties of the stent. An optimization method has been employed to deal with the conflictions with the consideration of the whole implementation process of the stent in this paper. A straight vascular with tumor inside is considered. A commonly used NiTinol superleastic stent with diamond shape strut is employed. The vascular wall tissue and stenotic plaque are also treated as hyperelastic materials. Softwares Isight, ABAQUS and Solidworks are utilized to perform the optimization job. It can be seen that the stresses are high at the areas around the fillets of the stent due to the stress concentration from a primary analysis. Therefore, the two fillets radius, thickness and radius of the stent are chosen as four optimization variables. The optimization object is to decrease the maximum stress of stent and increase the displacement of the plaque. After the optimization, the maximum stress can be decreased by 8.2 %, which implies that the stent's work life can be increased. The stenosis of the blood vessel can be decreased from 56 % to 40.0 %.     

Intimal hyperplasia following implantation of helical-centreline and straight-centreline stents in common carotid arteries in healthy pigs: influence of intraluminal flow?

Intimal hyperplasia (IH) is a leading cause of obstruction of vascular interventions, including arterial stents, bypass grafts and arteriovenous grafts and fistulae. Proposals to account for arterial stent-associated IH include wall damage, low wall shear stress (WSS), disturbed flow and, although not widely recognized, wall hypoxia. The common non-planarity of arterial geometry and flow, led us to develop a bare-metal, nitinol, self-expanding stent with three-dimensional helical-centreline geometry. This was deployed in one common carotid artery of healthy pigs, with a straight-centreline, but otherwise identical (conventional) stent deployed contralaterally. Both stent types deformed the arteries, but the helical-centreline device additionally deformed them helically and caused swirling of intraluminal flow. At sacrifice, one month post stent deployment, histology revealed significantly less IH in the helical-centreline than straight-centreline stented vessels. Medial cross-sectional area was not significantly different in helical-centreline than straight-centreline stented vessels. By contrast, luminal cross-sectional area was significantly larger in helical-centreline than straight-centreline stented vessels. Mechanisms considered to account for those results include enhanced intraluminal WSS and enhanced intraluminal blood–vessel wall mass transport, including of oxygen, in the helical-centreline stented vessels. Consistent with the latter proposal, adventitial microvessel density was lower in the helical-centreline stented than straight-centreline stented vessels.     

Computational fluid dynamic simulations of image-based stented coronary bifurcation models

One of the relevant phenomenon associated with in-stent restenosis in coronary arteries is an altered haemodynamics in the stented region. Computational fluid dynamics (CFD) offers the possibility to investigate the haemodynamics at a level of detail not always accessible within experimental techniques. CFD can quantify and correlate the local haemodynamics structures which might lead to in-stent restenosis. The aim of this work is to study the fluid dynamics of realistic stented coronary artery models which replicate the complete clinical procedure of stent implantation. Two cases of pathologic left anterior descending coronary arteries with their bifurcations are reconstructed from computed tomography angiography and conventional coronary angiography images. Results of wall shear stress and relative residence time show that the wall regions more prone to the risk of restenosis are located next to stent struts, to the bifurcations and to the stent overlapping zone for both investigated cases. Considering a bulk flow analysis, helical flow structures are generated by the curvature of the zone upstream from the stent and by the bifurcation regions. Helical recirculating microstructures are also visible downstream from the stent struts. This study demonstrates the feasibility to virtually investigate the haemodynamics of patient-specific coronary bifurcation geometries.     

Mechanical behavior of peripheral stents and stent-vessel interaction: A computational study

In this paper stents employed to treat peripheral artery disease are analyzed through a three-dimensional finite-element approach, based on a large-strain and large-displacement formulation. Aiming to evaluate the influence of some stent design parameters on stent mechanics and on the biomechanical interaction between stent and arterial wall, quasi-static and dynamic numerical analyses are carried out by referring to computational models of commercially and noncommercially available versions of both braided self-expandable stents and balloon-expandable stents. Addressing isolated device models, opening mechanisms and flexibility of both opened and closed stent configurations are numerically experienced. Moreover, stent deployment into a stenotic peripheral artery and possible postdilatation angioplasty (the latter for the self-expandable device only) are simulated by considering different idealized vessel geometries and accounting for the presence of a stenotic plaque. Proposed results highlight important differences in the mechanical response of the two types of stents, as well as a significant influence of the vessel shape on the stress distributions arising upon the artery-plaque system. Finally, computational results are used to assess both the stent mechanical performance and the effectiveness of the stenting treatment, allowing also to identify possible critical conditions affecting the risk of stent fracture, tissue damage, and/or pathological tissue response.     

A review on biodegradable materials for cardiovascular stent application

A stent is a medical device designed to serve as a temporary or permanent internal scaffold to maintain or increase the lumen of a body conduit. The researchers and engineers diverted to investigate biodegradable materials due to the limitation of metallic materials in stent application such as stent restenosis which requires prolonged anti platelet therapy, often result in smaller lumen after implantation and obstruct re-stenting treatments. Biomedical implants with temporary function for the vascular intervention are extensively studied in recent years. The rationale for biodegradable stent is to provide the support for the vessel in predicted period of time and then degrading into biocompatible constituent. The degradation of stent makes the re-stenting possible after several months and also ameliorates the vessel wall quality. The present article focuses on the biodegradable materials for the cardiovascular stent. The objective of this review is to describe the possible biodegradable materials for stent and their properties such as design criteria, degradation behavior, drawbacks and advantages with their recent clinical and preclinical trials.     

In vitro host response assessment of biomaterials for cardiovascular stent manufacture

The deployment of a vascular stent during angioplasty has greatly reduced the risks of restenosis. However, the presence of the device still induces a host response as well as a mechanical action on the blood vessel wall and an alteration of the haemodynamics. Platelet and inflammatory cells can adhere on the stent surface and be activated to produce biochemical signals able to stimulate an excessive proliferation of the smooth muscle cells with the consequent obstruction of the vessel lumen. For these reasons, the host response to two of the materials used in stent manufacture, stainless steel and diamond-like carbon, was investigated in vitro. The data showed that stainless steel induced a higher level of host response both in terms of platelet aggregation and macrophage activation. However, the spreading of inflammatory cells was more accentuated on diamond-like carbon. The inflammatory cells produced levels of platelet-derived growth factor, a key signal in smooth muscle cell proliferation, similar to stainless steel thus suggesting that carbon coatings may not be able to prevent restenosis.     

Analysis of the compression process and dimensional optimization of self‐expanding stent of a nickel titanium alloy

The vascular stents are important devices introduced into the vessel to protect the lumen from unfriendly stenosis so that the blood can flow naturally. During its implantation to the vessel, the stent should be compressed to a delivery system with very small dimension to accomplish the minimal invasive operation. At this stage, the stent will withstand very large stresses which will cause large damages in the structure. In this paper, the compression process of the stent was analyzed by finite element analysis method with software ABAQUS. The stress, strain and martensite volume fraction distributions were investigated at the end compression. Results show that the stent fillets are the dangerous areas for the potential failure. Subsequently, the dimensional optimization was performed to decrease the maximum concentration stress. After the optimization, the maximum stress can be decreased by 14.2%, which means the stent's work life can be increased.     

钽离子注入对镍钛合金表面生物学性能的影响

镍钛合金血管支架植入后可引发血栓和支架再狭窄, 且对损伤的血管内壁无修复作用, 需进行表面改性赋予其抗凝血和促内皮化生物学功能。本研究采用等离子体浸没离子注入与沉积(PIII&D)技术将钽(Ta)注入至镍钛合金, 研究Ta离子注入对镍钛表面理化特性及生物学性能的影响规律。结果表明, 调控Ta离子注入时间, 可在镍钛表面分别构建含Ta、Ta/Ta₂O₅、Ta/Ta₂O_5-x/Ta₂O₅三种不同组分的改性层。各种改性样品中, 含Ta/Ta₂O_5-x/Ta₂O₅的改性镍钛表面亲水性均更好, 可提供更多细胞附着位点, 促进人脐静脉内皮细胞早期粘附和铺展, 并提高其增殖能力。相比仅含单质Ta的改性镍钛表面, 含Ta/Ta₂O_5-x/Ta₂O₅改性镍钛表面的血液相容性更高, 血小板粘附数量显著减少, 且基本保持为未被激活的球形状态; 各组改性表面的溶血率远低于5%阈值, 均未发生明显溶血现象。上述结果说明, Ta离子注入改性镍钛血管支架在降低血栓形成、加速内皮化方面具有潜在应用。     

Development of a polymer stent with shape memory effect as a drug delivery system

The article presents a new concept for vascular endoprothesis (stent). Almost all commercially available stents are made of metallic materials. A common after effect of stent implantation is restenosis. Several studies on metal stents coated with drug show, that the use of a drug delivery system may reduce restenosis. The purpose of this work is to develop a new stent for the drug delivery application. The shape memory properties of thermoplastic polyurethane allow to design a new fully polymeric self-expandable stent. The possibility to use the stent as a drug delivery system is described.