Computational fluid dynamics analysis of balloon-expandable coronary stents: Influence of stent and vessel deformation

In many computational fluid dynamics (CFD) studies of stented vessel haemodynamics, the geometry of the stented vessel is described using non-deformed (NDF) geometrical models. These NDF models neglect complex physical features, such as stent and vessel deformation, which may have a major impact on the haemodynamic environment in stented coronary arteries. In this study, CFD analyses were carried out to simulate pulsatile flow conditions in both NDF and realistically-deformed (RDF) models of three stented coronary arteries. While the NDF models were completely idealised, the RDF models were obtained from nonlinear structural analyses and accounted for both stent and vessel deformation. Following the completion of the CFD analyses, major differences were observed in the time-averaged wall shear stress (TAWSS), time-averaged wall shear stress gradient (TAWSSG) and oscillatory shear index (OSI) distributions predicted on the luminal surface of the artery for the NDF and RDF models. Specifically, the inclusion of stent and vessel deformation in the CFD analyses resulted in a 32%, 30% and 31% increase in the area-weighted mean TAWSS, a 3%, 7% and 16% increase in the area-weighted mean TAWSSG and a 21%, 13% and 21% decrease in the area-weighted mean OSI for Stents A, B and C, respectively. These results suggest that stent and vessel deformation are likely to have a major impact on the haemodynamic environment in stented coronary arteries. In light of this observation, it is recommended that these features are considered in future CFD studies of stented vessel haemodynamics.     

The Influence of Strut-Connectors in Stented Vessels: A Comparison of Pulsatile Flow Through Five Coronary Stents

The design of coronary stents has evolved significantly over the past two decades. However, they still face the problem of in-stent restenosis, formation of neointima within 12 months of the implant. The biological response after stent implantation depends on various factors including the stent geometry which alters the hemodynamics. This study takes five different coronary stent designs, used in clinical practice, and explores the hemodynamic differences arising due to the difference in their design. Of particular interest is the design of the segments (connectors) that connect two struts. Pulsatile blood flow analysis is performed for each stent, using 3-D computational fluid dynamics (CFD), and various flow features viz. recirculation zones, velocity profiles, wall shear stress (WSS) patterns, and oscillatory shear indices are extracted for comparison. Vessel wall regions with abnormal flow features, particularly low, reverse, and oscillating WSS, are usually more susceptible to restenosis. Unlike previous studies, which have tried to study the effect of design parameters such as strut thickness and strut spacing on hemodynamics, this work investigates the differences in the flow arising purely due to differences in stent-shape, other parameters being similar. Two factors, the length of the connectors in the cross-flow direction and their alignment with the main flow, are found to affect the hemodynamic performance. This study also formulates a design index (varying from 18.81% to 24.91% for stents used in this study) that quantifies the flow features that could affect restenosis rates and which, in future, could be used for optimization studies.     

Three-dimensional numerical simulations of physiological flows in a stented coronary bifurcation

When atherosclerotic lesions are found within a coronary bifurcation, a double stent implantation is sometimes required to treat the disease of each branch. The clinical procedure can result in the positioning of several stents in the bifurcation. In the study, physiological flows in typical configurations of such stented coronary bifurcations were numerically modelled using the finite volumes method. Two deployed Palmaz stents were inserted in a 90° coronary bifurcation, simulating a double stent implantation. As the geometric position of the metallic stent cells can vary, several models of broken cells were proposed and compared to characterise the influence of the stent struts protruding into the collateral branch. Flow features in the bifurcation surroundings changed from one model to another. These changes could lead to the occurrence of flow stasis and also of recirculation areas downstream from the struts, depending on the way the strut was opened. The stent struts protruding into the lumen of the collateral branch induced high values of shear stress at the stent wall of about 20 Nm⁻², which could stimulate platelet activation. In addition, these areas of high shear stress values were concomitant with areas of low shear stress values of about 0.5 Nm⁻². These regions could be prone to platelet adhesion and so to thrombo-embolic complications. The analysis of the flow field indicated that it would be judicious to use dedicated bifurcated stents to treat bifurcation lesions.     

Determination of the influence of stent strut thickness using the finite element method: implications for vascular injury and in-stent restenosis

Many clinical studies, including the ISAR-STEREO trial, have identified stent strut thickness as an independent predictor of in-stent restenosis where thinner struts result in lower restenosis than thicker struts. The aim of this study was to more conclusively identify the mechanical stimulus for in-stent restenosis using results from such clinical trials as the ISAR-STEREO trial. The mechanical environment in arteries stented with thin and thicker strut stents was investigated using numerical modelling techniques. Finite element models of the stents used in the ISAR-STEREO clinical trial were developed and the stents were deployed in idealised stenosed vessel geometries in order to compare the mechanical environment of the vessel for each stent. The stresses induced within the stented vessels by these stents were compared to determine the level of vascular injury caused to the artery by the stents with different strut thickness. The study found that when both stents were expanded to achieve the same initial maximum stent diameter that the thinner strut stent recoiled to a greater extent resulting in lower luminal gain but also lower stresses in the vessel wall, which is hypothesised to be responsible for the lower restenosis outcome. This study supports the hypothesis that arteries develop restenosis in response to injury, where high vessel stresses are a good measure of that injury. This study points to a critical stress level in arteries, above which an aggressive healing response leads to in-stent restenosis in stented vessels. Stents can be designed to reduce stresses in this range in arteries using preclinical tools such as numerical modelling.     

Comparison of slotted tube versus coil stent implantation for ostial left anterior descending coronary artery stenosis: initial and late clinical outcomes.

Balloon angioplasty of ostial left anterior descending coronary artery (LAD) lesions has been associated with a high rate of acute complications and late restenosis. Recently, coronary stenting has been proposed as an effective treatment modality for ostial LAD lesions. To evaluate the effects of stent design on the development of late restenosis, we retrospectively analyzed the efficacy of slotted-tube stent implantation (40 patients, Palmaz-Schatz stent) and coil stent implantation (15 patients, tantalum Cordis stent) of ostial LAD stenosis. Six-month angiographic follow-up data were obtained in 31 patients (82%) with slotted-tube stent implantation and 12 patients (86%) with coil stent implantation. Angiographic restenosis was defined as > or =50% diameter stenosis. The angiographic restenosis rate was significantly lower in slotted-tube stent implantation (32%) than in coil stent implantation (67%) (p<0.05). Target lesion revascularization rate of slotted tube stent implantation was significantly lower (26%) than that of coil stent implantation (57%) (p<0.05). Coil stent implantation of ostial left anterior descending artery lesions was associated with higher late restenosis compared with slotted tube stent implantation. In conclusion, slotted-tube stent implantation might be considered to improve late clinical outcomes of ostial LAD lesions.     

Computational hemodynamics of an implanted coronary stent based on three-dimensional cine angiography reconstruction

Coronary stents are supportive wire meshes that keep narrow coronary arteries patent, reducing the risk of restenosis. Despite the common use of coronary stents, approximately 20-35% of them fail due to restenosis. Flow phenomena adjacent to the stent may contribute to restenosis. Three-dimensional computational fluid dynamics (CFD) and reconstruction based on biplane cine angiography were used to assess coronary geometry and volumetric blood flows. A patient-specific left anterior descending (LAD) artery was reconstructed from single-plane x-ray imaging. With corresponding electrocardiographic signals, images from the same time phase were selected from the angiograms for dynamic three-dimensional reconstruction. The resultant three-dimensional LAD artery at end-diastole was adopted for detailed analysis. Both the geometries and flow fields, based on a computational model from CAE software (ANSYS and CATIA) and full three-dimensional Navier-Stroke equations in the CFD-ACE+ software, respectively, changed dramatically after stent placement. Flow fields showed a complex three-dimensional spiral motion due to arterial tortuosity. The corresponding wall shear stresses, pressure gradient, and flow field all varied significantly after stent placement. Combined angiography and CFD techniques allow more detailed investigation of flow patterns in various segments. The implanted stent(s) may be quantitatively studied from the proposed hemodynamic modeling approach.     

药物洗脱支架植入病灶部位后的血流特性

目的 研究冠脉内植入药物释放支架后的血液流动特性,为临床应用及改善药物洗脱支架的设计提供理论指导。方法 建立病灶部位植入药物洗脱支架后的数值模型,利用计算流体动力学方法进行药物浓度分布及壁面剪切力分布的数值分析研究,并将计算结果与裸金属支架植入后的计算结果进行比较。结果 药物洗脱支架在血流过程中产生的低壁面剪切力区域几乎都伴随着较高的药物浓度分布,而低药物浓度区域则伴随的是较高的壁面剪切力分布,显著地减少了低壁面剪切力区域或低药物浓度区域独立存在的区域。理论上,药物释放支架在药物释放阶段比植入裸金属支架具有优越性。结论 药物洗脱支架能够显著降低支架内的再狭窄率。详细了解药物洗脱支架的流场分布规律将有利于改善药物洗脱支架的设计,进一步提高支架的整体性能,为临床应用提供理论依据。     

The Stress–Strain Behavior of Coronary Stent Struts is Size Dependent

Coronary stents are used to re-establish the vascular lumen and flow conditions within the coronary arteries; the typical thickness of a stent strut is 100 m, and average grain sizes of approximately 25 m exist in stainless steel stents. The purpose of this study is to investigate the effect of strut size on the stress strain behavior of 316 L stainless steel. Other materials have shown a size dependence at the micron size scale; however, at present there are no studies that show a material property size dependence in coronary stents. Electropolished stainless steel stent struts within the size range of 60–500 m were tensile tested. The results showed that within the size range of coronary stent struts a size dependent stress–strain relationship is required to describe the material. Finite element models of the final phase of fracture, i.e., void growth models, explained partially the reason for this size effect. This study demonstrated that a size based stress–strain relationship must be used to describe the tensile behavior material of 316 L stainless steel at the size scale of coronary stent struts. © 2003 Biomedical Engineering Society. PAC2003: 8719Rr, 8780Rb, 8719Uv     

支架内再狭窄的生物力学研究进展

支架介入已成为治疗心脑血管狭窄最有效的方式之一,但术后发生支架内再狭窄的问题却不可忽视。支架内再狭窄的发生,不仅与支架-血管间柔顺性失配以及支架对血管的机械性支撑引起的应力有关,而且与支架置入后血管内血流动力学环境变化引起的内膜增生有关。回顾了狭窄血管置入支架的生物力学研究历史和现状;特别从固体力学和血流动力学的角度,阐述了近年来学者对狭窄血管置入支架的生物力学数值模拟研究进展;并分析总结了支架内再狭窄的生物力学因素。从支架介入治疗过程来看,数值模拟技术可以为研究支架置入术和支架内再狭窄之间的关系提供很好的技术手段,对支架的设计以及介入治疗提供更加科学的指导。     

Numerical Study of Blood Clots Influence on the Flow Pattern and Platelet Activation on a Stented Bifurcation Model

Stent implantation is a common procedure followed in arteries affected by atherosclerosis. This procedure can lead to other stenting-related problems. One of these is the deposition and accumulation of blood clots over stent struts. This process can have further consequences, in so far as it can introduce modifications to the flow pattern. This problem is especially critical in stented bifurcations, where resulting stent geometry is more complex. In this regard, a numerical study is presented of the effect on the flow pattern and platelet activation of blood clot depositions on the stent struts of a stented coronary bifurcation. The numerical model is first validated with experimental measurements performed for this purpose. Experiments considered a flow with suspended artificial thrombi, which naturally deposited on stent struts. The location and shape observed were used to create numerical thrombi. Following this, numerical simulations were performed to analyze the influence of the presence of thrombi depositions on parameters such as Time Averaged Wall Shear Stress, Oscillatory Shear Index or Relative Residence Time. Finally, a study was also carried out of the effect of different geometrical configurations, from a straight tube to a stented bifurcation model with thrombus depositions, on platelet activation.     

冠脉支架内再狭窄的血流动力学研究进展

冠脉支架术后再狭窄是严重且高发的医学事件。局部血流动力学因素,特别是壁面剪切应力(WSS),对冠脉粥样硬化斑块的形成、发展和不均匀性有着重要的影响。最近的基础和临床在体研究表明,WSS也可能与支架内再狭窄的发生有关。从支架内再狭窄的形成机制出发,分析冠脉支架后的力学环境对再狭窄的作用机制,详细阐述近年基于计算流体力学（CFD）方法的冠脉支架内再狭窄的血流动力学研究进展。     

Changes of flow characteristics by stenting in aneurysm models: influence of aneurysm geometry and stent porosity

An endovascular technique using a stent has been developed and successfully applied in the treatment of wide neck aneurysms. A stent can facilitate thrombosis in the aneurysm pouch while maintaining biocompatible passage of the parent artery. Insertion of the stent changes the flow characteristics inside the aneurysm pouch, which can affect the intra-aneurysmal embolization process. The purpose of this study is to clarify the velocity and wall shear stress changes that are caused by stenting in fusiform and lateral aneurysm models. We used a flow visualization technique that incorporated a photochromic dye in order to observe the flow fields and measure the wall shear rates. The intra-aneurysmal flow motion was significantly reduced in the stented aneurysm models. Coherent inflow along the distal wall of the aneurysm was diminished and inflow was distributed along the pores of the stent wall in the stented models. Also, sluggish intra-aneurysmal vortex motion was well maintained in the stented aneurysm models during the deceleration phase. A less porous stent generally reduced the intraneurysmal fluid motion further, but the porosity effect was not significant. The magnitude and pulsatility of the wall shear rate were reduced by stenting, and the reductions were more significant in the lateral aneurysm models compared to the fusiform aneurysm models. The hemodynamic changes that were observed in our study can help explain the efficacy of in vivo thrombus formation caused by stenting. © 2002 Biomedical Engineering Society. PAC2002: 8719Uv, 8780-y, 8719Xx     

支架植入后趋直现象对血管壁局部生物力学环境的影响

目的研究支架植入后趋直现象对病变处血管壁力学环境变化的影响,探索支架植入后再狭窄的生物力学成因及支架优化方法。方法基于患者CT扫描图像,使用Mimics、Geomagic和Pro/E软件完成动脉、球囊以及理想与趋直两种支架模型的构建,应用ABAQUS软件建立球囊-支架-冠状动脉左前降至血管之间的力学模型,根据数值模拟计算结果,分析趋直对病变处血管壁力学环境的影响。结果在1.013 MPa充气压力下撑开支架后,与理想支架模型相比较,趋直支架模型的血管壁内表面出现Von Mises应力增大和明显的应力集中现象,尤其是在近心端、远心端及沿心肌表面区域。理想支架模型血管内壁平均和最大Von Mises应力分别0.39、5.12 MPa;趋直支架模型血管内壁平均和最大Von Mises应力分别为0.45、7.43 MPa。结论支架植入后发生的趋直现象将引起应力分布不均匀,出现应力集中,增大对血管壁造成的伤害,可能会引起血管的机械损伤及管壁重建,诱发支架内再狭窄的发生。研究结果可为术后支架内再狭窄的机理分析提供帮助,也为血管介入手术方案及支架优化设计提供参考。     

New biocompatible polymer surface coating for stents results in a low neointimal response

Clinical studies indicate a more pronounced endothelial response after stent implantation than after balloon inflation. This might be related to the metal surface of the stent, and therefore it is speculated that coating of the stent might partially prevent hyperplasia. One coated and one noncoated Palmaz-Schatz stent were implanted in two separate coronary arteries of seven pigs. The coating was composed of methylmethacrylate (MMA) (hydrophobic, 70 mol %) and 2-hydroxyethyl methacrylate (HEMA) (hydrophilic 30 mol %). After sacrifice (3 weeks), cross sections were made of the stented areas. Vessel wall reaction was calculated both independently and dependently of local vessel wall injury due to the stent struts. Overall, vessel wall reaction of the coated stents was lower than that of the noncoated stents. The degree of hyperplasia was linearly related to the degree of stent-induced vessel wall injury. Analyses of all the struts showed that significantly less hyperplasia occurred in the coated versus noncoated stents. In this porcine coronary artery model, the MMA/HEMA stent coating resulted in significantly reduced vessel wall response. However, it remains to be determined whether this favorable outcome will also be present in humans.     

Reproducibility of Image-Based Computational Fluid Dynamics Models of the Human Carotid Bifurcation

Recent studies have demonstrated the ability of magnetic resonance imaging (MRI) to provide anatomically realistic boundary conditions for computational fluid dynamics (CFD) simulations of arterial hemodynamics. To date, however, little is known about the overall reproducibility of such image-based CFD techniques. Towards this end we used serial black blood and cine phase contrast MRI to reconstruct CFD models of the carotid bifurcations of three subjects with early atherosclerosis, each imaged three times at weekly intervals. The lumen geometry was found to be precise on average to within 0.15 mm or 5%, while measured flow and heart rates varied by less than 10%. Spatial patterns of a variety of wall shear stress (WSS) indices were largely preserved among the three repeat models. Time-averaged WSS was reproduced best, on average to within 5 dyn/cm² or 37%, followed by WSS spatial gradients, angle gradients, and oscillatory shear index. The intrasubject flow rate variations were found to contribute little to the overall WSS variability. Instead, reproducibility was determined largely by the precision of the lumen boundary extraction from the individual MR images, itself shown to be a function of the image quality and proximity to the geometrically complex bifurcation region. © 2003 Biomedical Engineering Society. PAC2003: 8761Lh, 8757Nk, 8719Hh, 8719Rr, 8719Uv     

Effects of Cardiac Motion on Right Coronary Artery Hemodynamics

The purpose of this work was to investigate the effects of physiologically realistic cardiac-induced motion on hemodynamics in human right coronary arteries. The blood flow patterns were numerically simulated in a modeled right coronary artery (RCA) having a uniform circular cross section of 2.48 mm diam. Arterial motion was specified based on biplane cineangiograms, and incorporated physiologically realistic bending and torsion. Simulations were carried out with steady and pulsatile inflow conditions (mean Re_D=233, =1.82) in both fixed and moving RCA models, to evaluate the relative importance of RCA motion, flow pulsation, and the interaction between motion and flow pulsation. RCA motion with a steady inlet flow rate caused variations in wall shear stress (WSS) magnitude up to 150% of the inlet Poiseuille value. There was significant spatial variability in the magnitude of this motion-induced WSS variation. However, the time-averaged WSS distribution was similar to that predicted in a static model representing the time-averaged geometry. Furthermore, the effects of flow pulsatility dominated RCA motion-induced effects; specifically, there were only modest differences in the WSS history between simulations conducted in fixed and moving RCA models with pulsatile inflow. RCA motion has little effect on time-averaged WSS patterns. It has a larger effect on the temporal variation of WSS, but even this effect is overshadowed by the variations in WSS due to flow pulsation. The hemodynamic effects of RCA motion can, therefore, be ignored as a first approximation in modeling studies. © 2003 Biomedical Engineering Society. PAC2003: 8719Uv, 8719Hh, 8719St, 8719Rr     

A Novel Simulation Strategy for Stent Insertion and Deployment in Curved Coronary Bifurcations: Comparison of Three Drug-Eluting Stents

The introduction of drug-eluting stents (DES) has reduced the occurrence of restenosis in coronary arteries. However, restenosis remains a problem in stented coronary bifurcations. This study investigates and compares three different second generation DESs when being implanted in the curved main branch of a coronary bifurcation with the aim of providing better insights into the related changes of the mechanical environment. The 3D bifurcation model is based on patient-specific angiographic data that accurately reproduce the in vivo curvatures of the vessel segments. The layered structure of the arterial wall and its anisotropic mechanical behavior are taken into account by applying a novel algorithm to define the fiber orientations. An innovative simulation strategy considering the insertion of a folded balloon catheter over a guide wire is proposed in order to position the stents within the curved vessel. Straightening occurs after implantation of all stents investigated. The resulting distributions of the wall stresses are strongly dependent on the stent design. Using a parametric modeling approach, two design modifications, which reduce the predicted maximum values of the wall stress, are proposed and analyzed.     

Effect of a flow-streamlining implant at the distal anastomosis of a coronary artery bypass graft

Intimal thickening in the coronary artery bypass graft (CABG) distal anastomosis has been implicated as the major cause of restenosis and long-term graft failure. Several studies point to the interplay between nonuniform hemodynamics including disturbed flows and recirculation zones, wall shear stress, and long particle residence time as possible etiologies. The hemodynamic features of two anatomic models of saphenous-vein CABGs were studied and compared. One simulated an anastomosis with both diameter and compliance mismatch and a curvature at the connection, analogous to the geometry observed in a conventional cardiothoracic procedure. The other, simulated an anastomosis with a flow stabilizing anastomotic implant connector which improves current cardiothoracic procedures by eliminating the distal vein bulging and curvature. Physiologic flow conditions were imposed on both models and qualitative analysis of the flow was performed with dye injection and a digital camera. Quantitative analysis was performed with laser Doppler velocimetry. Results showed that the presence of the bulge at the veno-arterial junction, contributed to the formation of accentuated secondary structures (helices), which progress into the flow divider and significantly affect radial velocity components at the host vessel up to four diameters downstream of the junction. The model with the implant, achieved more hemodynamically efficient conditions on the host vessel with higher mean and maximum axial velocities and lower radial velocities than the conventional model. The presence of the sinus may also affect the magnitude and shape of the shear stress at locations where intimal thickening occurs. Thus, the presence of the implant creates a more streamlined environment with more primary and less secondary flow components which may then inhibit the development of intimal thickening, restenosis, and ultimate failure of the saphenous vein graft. © 2002 Biomedical Engineering Society. PAC2002: 8780Rb, 8719Rr, 8719Uv, 8763Lk, 8719Xx, 4262Be, 4780+v, 8710+e     

Enhancement of Stent-Induced Thromboembolism by Residual Stenoses: Contribution of Hemodynamics

In vitro stent-induced thromboembolism was altered by the presence of residual stenoses placed upstream or placed upstream and downstream of the stent. Heparinized (3 /ml) bovine blood was gravity fed through a conduit with a deployed coronary stent. Embolism was continuously monitored using a light-scattering microemboli detector, and the thrombus accumulated on the stent at the conclusion of the experiment was assessed gravimetrically. Gaussian stenoses (75% reduction in the cross-sectional area) were placed upstream or upstream and downstream of the stent to alter flow characteristics in the stent region. The presence of stenoses enhanced embolization from the stent in all cases, while end-point thrombus accumulation on the stent decreased with only an upstream stenosis present, and increased when upstream and downstream stenoses were present. Computational fluid dynamics with and without hypothetical model thrombi were used to ascertain the alterations in the flow environment caused by the stenoses and thrombi. Combining the computed hemodynamic parameters with experimental results indicated that increased radial transport of blood components and low wall shear stress provided by the stenoses and thrombi may explain the enhancement of end-point thrombus accumulation. Analysis further showed that thrombi growing at the stenosis-induced reattachment and separation points will be subjected to high shear forces which may explain the increased embolism when stenoses are present. © 2000 Biomedical Engineering Society. PAC00: 8719Uv, 8719Xx, 8780-y     

一种血管内支架的有限元模型及计算流体动力学分析

支架植入所造成的血栓、血管损伤及其对血流动力学的影响是造成支架内再狭窄的主要原因。我们利用有限元模型与计算流体动力学的方法,分析了一种支架在植入过程中与斑块、血管的相互作用及其对血流情况的影响。结果发现：支架植入后端部发生翘起,这容易损伤血管壁;支架植入模型所对应的即刻回缩率明显高于支架自身的回缩率,其结果分别为12.3%、3.1%;支架壁厚与连接筋设计能够引起血管壁面剪应力的明显变化。这对于血管内支架的设计具有一定的指导意义。