Stresses in peripheral arteries following stent placement: a finite element analysis

The success of stents to restore blood flow in atherosclerotic peripheral arteries is low relative to coronary arteries. It has been shown that joint flexion induces a mechanical environment that makes stent placement in these arteries highly incompatible, and damage and destruction of stents has been recorded. However, the effect of this environment on the stresses in the arteries is unknown. It is hypothesised that the stresses induced in arteries as a result of this mechanical environment could be sufficient to explain the relatively low success rates. To investigate this hypothesis, a finite element model of the stent-artery interaction was developed. Following stent expansion, bending was simulated by applying a displacement boundary condition to the artery. It is found that high stresses occur at the proximal/distal ends of the stent. As high stress and vascular injury are hypothesised to cause restenosis, the results presented here suggest that the mechanical environment of peripheral arteries could be the predominant cause of high restenosis rates.     

Cardiovascular stent design and vessel stresses: a finite element analysis

Intravascular stents of various designs are currently in use to restore patency in atherosclerotic coronary arteries and it has been found that different stents have different in-stent restenosis rates. It has been hypothesized that the level of vascular injury caused to a vessel by a stent determines the level of restenosis. Computational studies may be used to investigate the mechanical behaviour of stents and to determine the biomechanical interaction between the stent and the artery in a stenting procedure. In this paper, we test the hypothesis that two different stent designs will provoke different levels of stress within an atherosclerotic artery and hence cause different levels of vascular injury. The stents analysed using the finite-element method were the S7 (Medtronic AVE) and the NIR (Boston Scientific) stent designs. An analysis of the arterial wall stresses in the stented arteries indicates that the modular S7 stent design causes lower stress to an atherosclerotic vessel with a localized stenotic lesion compared to the slotted tube NIR design. These results correlate with observed clinical restenosis rates, which have found higher restenosis rates in the NIR compared with the S7 stent design. Therefore, the testing methodology outlined here is proposed as a pre-clinical testing tool, which could be used to compare and contrast existing stent designs and to develop novel stent designs.     

Gene delivery from a DNA controlled-release stent in porcine coronary arteries

Expandable intra-arterial stents are widely used for treating coronary disease. We hypothesized that local gene delivery could be achieved with the controlled release of DNA from a polymer coating on an expandable stent. Our paper reports the first successful transfection in vivo using a DNA controlled-release stent. Green fluorescent protein (GFP) plasmid DNA within emulsion-coated stents was efficiently expressed in cell cultures (7.9% +/- 0.7% vs. 0.6% +/- 0.2% control, p < 0.001) of rat aortic smooth muscle cells. In a series of pig stent-angioplasty studies, GFP expression was observed in all coronary arteries (normal, nondiseased) in the DNA-treated group, but not in control arteries. GFP plasmid DNA in the arterial wall was confirmed by PCR, and GFP presence in the pig coronaries was confirmed by immunohistochemistry. Thus, DNA-eluting stents are capable of arterial transfection, and could be useful as delivery systems for candidate vectors for gene therapy of cardiovascular diseases.     

Local methylprednisolone delivery using a BiodivYsio phosphorylcholine-coated drug-delivery stent reduces inflammation and neointimal hyperplasia in a porcine coronary stent model

Phosphorylcholine (PC)-coated stents have shown excellent blood and tissue biocompatibility in porcine coronary arteries. The purpose of this study was to determine the efficacy of local methylprednisolone (MP) delivery using PC-coated stents to inhibit inflammatory response and in-stent neointimal hyperplasia in an overstretched porcine coronary model. BiodivYsio (Biocompatibles, Farnham, Surrey, UK) PC-coated drug delivery (DD) stents and DD stents loaded with a high dose of MP (269 microg) were implanted in the coronary arteries of 20 pigs with a balloon/artery ratio of 1.2 : 1. At five days the peri-strut inflammatory response score and thrombus score of the MP-loaded DD stents were lower than in the control stents. The neointimal hyperplasia of MP-loaded DD stents was significantly reduced (0.80 +/- 0.10 versus 0.48 +/- 0.10 mm(2), p < 0.01). At four-week follow-up, the inflammatory response of MP-loaded stents was lower than the control stents, but without significant difference. The MP-loaded stents showed decreased peri-strut arterial injury and in-stent neointimal hyperplasia (2.42 +/- 0.87 versus 1.62 +/- 0.71 mm(2), p < 0.05). It is concluded that local vascular delivery of a high dose of MP from PC-coated DD stents could effectively decrease inflammatory response and thrombus formation after oversized stent deployment and result in a significant reduction of neointimal hyperplasia.     

Mechanical behavior of coronary stents investigated through the finite element method

Intravascular stents are small tube-like structures expanded into stenotic arteries to restore blood flow perfusion to the downstream tissues. The stent is mounted on a balloon catheter and delivered to the site of blockage. When the balloon is inflated, the stent expands and is pressed against the inner wall of the coronary artery. After the balloon is deflated and removed, the stent remains in place, keeping the artery open. Hence, the stent expansion defines the effectiveness of the surgical procedure: it depends on the stent geometry, it includes large displacements and deformations and material non-linearity.In this paper, the finite element method is applied (i) to understand the effects of different geometrical parameters (thickness, metal-to-artery surface ratio, longitudinal and radial cut lengths) of a typical diamond-shaped coronary stent on the device mechanical performance, (ii) to compare the response of different actual stent models when loaded by internal pressure and (iii) to collect suggestions for optimizing the device shape and performance.The stent expansion and partial recoil under balloon inflation and deflation were simulated. Results showed the influence of the geometry on the stent behavior: a stent with a low metal-to-artery surface ratio has a higher radial and longitudinal recoil, but a lower dogboning. The thickness influences the stent performance in terms of foreshortening, longitudinal recoil and dogboning.In conclusion, a finite element analysis similar to the one herewith proposed could help in designing new stents or analyzing actual stents to ensure ideal expansion and structural integrity, substituting in vitro experiments often difficult and unpractical.     

Mechanical and hydrodynamic effects of stent expansion in tapered coronary vessels

Percutaneous coronary intervention (PCI) has become the primary treatment for patients with coronary heart disease because of its minimally invasive nature and high efficiency. Anatomical studies have shown that most coronary vessels gradually shrink, and the vessels gradually become thinner from the proximal to the distal end. In this paper, the effects of different stent expansion methods on the mechanical and hemodynamic behaviors of coronary vessels and stents were studied. To perform a structural-mechanical analysis of stent implantation, the coronary vessels with branching vessels and the coronary vessels with large bending curvature are selected. The two characteristic structures are implanted in equal diameter expansion mode and conical expansion mode, and the stress and mechanical behaviors of the coronary vessels and stents are analyzed. The results of the structural-mechanical analysis showed that the mechanical behaviors and fatigue performance of the cobalt-chromium alloy stent were good, and the different expansion modes of the stent had little effect on the fatigue performance of the stent. However, the equal diameter expansion mode increased distal coronary artery stress and the risk of vascular injury. The computational fluid dynamics analysis results showed that different stent expansion methods had varied effects on coronary vessel hemodynamics and that the wall shear stress distribution of conical stent expansion is more uniform compared with equal diameter expansion. Additionally, the vortex phenomenon is not apparent, the blood flow velocity is slightly increased, the hydrodynamic environment is more reasonable, and the risk of coronary artery injury is reduced.

     

Delivery and release of nitinol stent in carotid artery and their interactions: a finite element analysis

Carotid angioplasty and stenting (CAS) has emerged as an effective alternative to carotid endarterectomy, and nitinol stents are commonly used in CAS. To evaluate biomechanical properties of nitinol carotid stents and their interactions with carotid arteries, a finite element method (FEM) model was built which is composed of a stenotic carotid tissue, a segmented-design nitinol stent and a sheath. Two different stents were considered to show the influence of stent design on the stent-vessel interactions. Results show that the superelastic stents were delivered into the stenotic vessel lumen through the sheath and self-expanded in the internal and common carotid artery. The stent with shorter struts may have better clinical results and the different stent designs can cause different carotid vessel geometry changes. This FEM can provide a convenient way to test and improve biomechanical properties of existing carotid stents and give clues for new nitinol carotid stent designs.     

Why and how to avoid stenting during brachytherapy

Intracoronary radiation is a promising therapy to reduce restenosis after percutaneous coronary intervention. It may be anticipated that radiation and intracoronary stents - the current standard coronary revascularization procedure - have a synergic antirestenosis effect. However, this potential benefit has not been proven in the clinical scenario. Indeed, this combined approach (stenting plus brachytherapy) may even be harmful. Delayed endothelialization and late stent malapposition are important drawbacks of implanting a metallic prosthesis in the setting of radiation therapy. Owing to the relatively high frequency of late thrombosis after stenting irradiated coronary arteries, the Food and Drug Administration required that the labeling of both gamma- and beta-radiation devices recently approved for clinical use explicitly advise avoidance of the placement of new stents. The pathophysiologic aspects as well as the clinical implications of the implantation of a new stent in association with radiation delivered by radioactive stents or catheter-based systems are discussed in this paper.     

Coronary stent implantation without balloon predilatation: a single-center experience

OBJECTIVE: Assessment of safety and efficacy of coronary stent deployment, without balloon predilatation. BACKGROUND: With newer high-performance balloon-premounted stents it has become more common to attempt coronary stent deployment without balloon pre- or postdilatation. METHODS: During 1998 524 coronary angioplasties were performed in the authors' institute, of which 279 resulted in coronary stenting. Of these 101 (36.2%) were stents without balloon predilatation (SWBP). PTCA was performed according to standard technique using mostly 7 F. guiding catheters, and 'rapid exchange' balloons and pre-mounted stents. RESULTS: Seventy-two patients had acute coronary syndromes (41 acute MI or post-MI angina, 28 unstable angina, 10 rescue PTCA after failed thrombolysis). Mean age was 56.4 3 11.1 years, 84.5% were males. Sixty per cent of the lesions were ACC-AHA type B2 or C. Target arteries were LAD 57.6%, LCX 21.2%, RCA 14.1% and SVG 7.1%. Procedure time was 18.2 3 17.3 minutes. Mean heparin dose was 3850 3 1570 units. Twenty-two patients received abciximab prior to stent deployment. Seven stents were not deployed without previous balloon dilatation and were retrieved safely via the guiding catheters and deployed after balloon dilatation. There was no stent embolization, ectopic suboptimal or partial stent deployment. Immediate angiographic success was obtained in 95 patients (94.1%). Minimal lumin diameter (MLD) increased from 0.27 3 0.15 to 3.23 3 2.1 mm. There were two in-hospital deaths (1.9%) due to cardiogenic shock. An intra-aortic balloon pump was required in eight patients. Two patients (1.9%) experienced subacute stent thrombosis. CONCLUSION: SWBP in selective groups of patients and lesions is feasible and safe. Larger randomized comparative trials are needed to assess the benefits and cost saving of this approach.     

Mechanical modeling of self-expandable stent fabricated using braiding technology

The mechanical behavior of a stent is one of the important factors involved in ensuring its opening within arterial conduits. This study aimed to develop a mechanical model for designing self-expandable stents fabricated using braiding technology. For this purpose, a finite element model was constructed by developing a preprocessing program for the three-dimensional geometrical modeling of the braiding structure inside stents, and validated for various stents with different braiding structures. The constituent wires (Nitinol) in the braided stents were assumed to be superelastic material and their mechanical behavior was incorporated into the finite element software through a user material subroutine (VUMAT in ABAQUS) employing a one-dimensional superelastic model. For the verification of the model, several braided stents were manufactured using an automated braiding machine and characterized focusing on their compressive behavior. It was observed that the braided stents showed a hysteresis between their loading and unloading behavior when a compressive load was applied to the braided tube. Through the finite element analysis, it was concluded that the current mechanical model can appropriately predict the mechanical behavior of braided stents including such hysteretic behavior, and that the hysteresis was caused by the slippage between the constituent wires and their superelastic property.     

Hemodynamics in coronary arteries with overlapping stents

Coronary artery stenosis is commonly treated by stent placement via percutaneous intervention, at times requiring multiple stents that may overlap. Stent overlap is associated with increased risk of adverse clinical outcome. While changes in local blood flow are suspected to play a role therein, hemodynamics in arteries with overlapping stents remain poorly understood. In this study we analyzed six cases of partially overlapping stents, placed ex vivo in porcine left coronary arteries and compared them to five cases with two non-overlapping stents. The stented vessel geometries were obtained by micro-computed tomography of corrosion casts. Flow and shear stress distribution were calculated using computational fluid dynamics. We observed a significant increase in the relative area exposed to low wall shear stress (WSS<0.5 Pa) in the overlapping stent segments compared both to areas without overlap in the same samples, as well as to non-overlapping stents. We further observed that the configuration of the overlapping stent struts relative to each other influenced the size of the low WSS area: positioning of the struts in the same axial location led to larger areas of low WSS compared to alternating struts. Our results indicate that the overlap geometry is by itself sufficient to cause unfavorable flow conditions that may worsen clinical outcome. While stent overlap cannot always be avoided, improved deployment strategies or stent designs could reduce the low WSS burden.     

Numerical investigation of the intravascular coronary stent flexibility

Nowadays stent therapy is widely adopted to treat atherosclerotic vessel diseases. The high commercial value of these devices and the high prototypation costs require the use of finite element analyses, instead of classical trial and error technique, to design and verify new models. In this paper, we explore the advantages of the finite element method (FEM) in order to investigate new generation stent performance in terms of flexibility. Indeed, the ability of the stent to bend in order to accommodate curvatures and angles of vessels during delivery is one of the most significant prerequisites for optimal stent performance. Two different FEM models, resembling two new generation intravascular stents, were developed. The main model dimensions were obtained by means of a stereo microscope, analyzing one Cordis BX-Velocity and one Carbostent Sirius coronary stent. Bending tests under displacement control in the unexpanded and expanded configuration were carried out. The curvature index, defined as the ratio between the sum of rotation angles at the extremes and the length of the stent, yielded comparative information about the capability of the device to be delivered into tortuous vessels and to conform to their contours. Results, expressed in terms of the bending moment-curvature index, demonstrated a different response for the two models. In particular the Cordis model showed a higher flexibility. Lower flexibility in the expanded configurations for both models was detected. However this flexibility depends on how the contact takes place between the different parts of the struts.     

Numerical study of the uniformity of balloon-expandable stent deployment

Stents are small tubelike structures, implanted in coronary and peripheral arteries to reopen narrowed vessel sections. This endovascular intervention remains suboptimal, as the success rate is limited by restenosis. This renarrowing of a stented vessel is related to the arterial injury caused by stent-artery and balloon-artery interactions, and a local subsequent inflammatory process. Therefore, efforts to optimize the stent deployment remain very meaningful. Several authors have studied with finite element modeling the mechanical behavior of balloon-expandable stents, but none of the proposed models incorporates the folding pattern of the balloon. We developed a numerical model in which the CYPHER stent is combined with a realistic trifolded balloon. In this paper, the impact of several parameters such as balloon length, folding pattern, and relative position of the stent with respect to the balloon catheter on the free stent expansion has been investigated. Quantitative validation of the modeling strategy shows excellent agreement with data provided by the manufacturer and, therefore, the model serves as a solid basis for further investigations. The parametric analyses showed that both the balloon length and the folding pattern have a considerable influence on the uniformity and symmetry of the transient stent expansion. Consequently, this approach can be used to select the most appropriate balloon length and folding pattern for a particular stent design in order to optimize the stent deployment. Furthermore, it was demonstrated that small positioning inaccuracies may change the expansion behavior of a stent. Therefore, the placement of the stent on the balloon catheter should be accurately carried out, again in order to decrease the endothelial damage.     

Circumferential vascular deformation after stent implantation alters wall shear stress evaluated with time-dependent 3D computational fluid dynamics models.

The success of vascular stents in the restoration of blood flow is limited by restenosis. Recent data generated from computational fluid dynamics (CFD) models suggest that stent geometry may cause local alterations in wall shear stress (WSS) that have been associated with neointimal hyperplasia and subsequent restenosis. However, previous CFD studies have ignored histological evidence of vascular straightening between circumferential stent struts. We tested the hypothesis that consideration of stent-induced vascular deformation may more accurately predict alterations in indexes of WSS that may subsequently account for histological findings after stenting. We further tested the hypothesis that the severity of these alterations in WSS varies with the degree of vascular deformation after implantation. Steady-state and time-dependent simulations of three-dimensional CFD arteries based on canine coronary artery measurements of diameter and blood flow were conducted, and WSS and WSS gradients were calculated. Circumferential straightening introduced areas of high WSS between stent struts that were absent in stented vessels of circular cross section. The area of vessel exposed to low WSS was dependent on the degree of circumferential vascular deformation and axial location within the stent. Stents with four vs. eight struts increased the intrastrut area of low WSS in vessels, regardless of cross-sectional geometry. Elevated WSS gradients were also observed between struts in vessels with polygonal cross sections. The results obtained using three-dimensional CFD models suggest that changes in vascular geometry after stent implantation are important determinants of WSS distributions that may be associated with subsequent neointimal hyperplasia.     

Effects of bifurcation-specific and conventional stents on coronary bifurcation flow. An experimental and numerical study

Our paper builds on existing research into conventional bare metal stents in order to assess new devices specifically designed for coronary bifurcation angioplasty. The first aim is to validate the numerical model against data from in vitro experiments on stented coronary phantoms. A surface mesh was built in accordance with micro-computed tomography images obtained from coronary stents implanted in silicone models and used for numerical analysis. Computational simulations for steady and unsteady cases generally agreed with their experimental counterparts. A second objective is to compare the hemodynamic performance of one of these new devices (Stentys) to that of conventional devices and stenting techniques in a simplified coronary bifurcation model. Four different coronary bifurcation stenting techniques were analyzed. We have focused on factors contributing to restenosis, such as wall shear stress (WSS), oscillatory shear index (OSI), pressure loss, and local normalized helicity (LNH). It was found that bifurcation-specific stents implanted in the side branch led to increased malapposition. This effect has proved to be more important than stent specific design characteristics such as strut size (different for conventional and Stentys stent). This conclusion is confirmed by means of drop in pressure and mechanical energy loss rate calculation; for the latter, the increase ranged from 9% to 17%, depending on the stenting technique, when dedicated stents were implanted in the side branch. The behavior patterns presented in this study should be double-checked against those obtained in more realistic geometries.     

Computational fluid dynamics study of common stent models inside idealised curved coronary arteries

The haemodynamic behaviour of blood inside a coronary artery after stenting is greatly affected by individual stent features as well as complex geometrical properties of the artery including tortuosity and curvature. Regions at higher risk of restenosis, as measured by low wall shear stress (WSS < 0.5 Pa), have not yet been studied in detail in curved stented arteries. In this study, three-dimensional computational modelling and computational fluid dynamics methodologies were used to analyse the haemodynamic characteristics in curved stented arteries using several common stent models. Results in this study showed that stent strut thickness was one major factor influencing the distribution of WSS in curved arteries. Regions of low WSS were found behind struts, particularly those oriented at a large angle relative to the streamwise flow direction. These findings were similar to those obtained in studies of straight arteries. An uneven distribution of WSS at the inner and outer bends of curved arteries was observed where the WSS was lower at the inner bend. In this study, it was also shown that stents with a helical configuration generated an extra swirling component of the flow based on the helical direction; however, this extra swirl in the flow field did not cause significant changes on the distribution of WSS under the current setup.     

A single center experience with the newly designed R Stent in small coronary vessels

BACKGROUND: Over the past 10 years stents have been used more frequently for the treatment of de novo coronary artery stenosis. Initally these devices were used primarily in coronary arteries with diameters ranging from 3.0 to 4.0 mm. Traditionally, coronary arteries less than 3.0 mm in diameter were treated with only balloon angioplasty, due to the unavailablity of flexible, low profile, small diameter stents. In the past three years, many stents have been designed to be implanted in small coronary arteries. OBJECTIVE: The objective of this study was to evaluate the safety and feasiblity of the R Stent in patients with coronary lesions located in coronary arteries with a reference diameter 2.5-3.0 mm. METHODS AND RESULTS: Between November 1998 and September 1999, 32 patients with stable (37%) and unstable (63%) angina treated with the R Stent were included in this study. The treated lesions were in the right coronary artery (RCA) (n = 13), left cirumflex coronary artery (LCX) (n = 10), and left anterior descending coronary artery (LAD) (n = 9). Of these lesions thirteen were anatomically complex. Stent deployment was successful in 97% with one crossing failure in a patient with a vessel tortuosity of greater than 75 degrees of the circumflex artery. No post-procedual major adverse cardiac and cerebrovascular event (MACCE) occurred within 30 days of stent implantation. After the procedure, patients were scheduled for a two-week telephone follow-up and a one-month clinical evaluation. At 30 days, only one patient (3%) experienced the recurrence of angina Canadian cardiovascular society classification ((CCS) Class 2). All other patients were event and angina free. CONCLUSION: This first clinical experience in patients with small vessel disease shows that the use of the R stent is safe and feasible with low rates of acute stent thrombosis.     

Immediate and long-term outcomes of coronary stenting in relation to stent implantation pressure in patients with coronary heart disease

The paper evaluates the pressure impact of implantation of intracoronary stents of different designs on the immediate and long-term outcomes of stenting. A retrospective study included 192 patients. Matrix and wire stents were implanted in 97 (50.5%) and 95 (49.5%) patients, respectively. In one part of the patients, the coronary stent was implanted under nominal pressure, in the other, the stent after implantation was extended with a balloon by using high pressure. Good immediate angiographic and clinical results were obtained in all 192 patients. Thirty three (17.2%) patients were found to have coronary spasm at the site of a stent. There was no significant difference in the development of coronary spasm, which was associated with the type of an implanted (matrix or wire) stent and with the pattern of its implantation (under high or nominal pressure). Dissection along the stent edges developed in 17 of the 192 patients, which amounted to 8.8%. It significantly more frequently developed in patients from Subgroup IA than in those from Subgroup IB. There was no significant difference in the development of restenosis in patients after implantation of matrix or wire stents and in the relation to the stent implantation pressure.     

Impact of Coronary Artery Curvature on the Longitudinal Stent Foreshortening: Real-World Observations

Longitudinal stent foreshortening is a known phenomenon, however, the impact of coronary artery curvature on longitudinal stent foreshortening remains unclear. The aim of this study is to determine the impact of coronary artery curvature on the longitudinal stent foreshortening in the real-world scenarios. A total of 86 consecutive patients underwent coronary stent implantation were included in the present study. The degree of coronary artery curvature was defined as the length of the coronary artery curvature divided by the straight length. Longitudinal stent foreshortening was defined as the stent length after implantation divided by the stent length before implantation. The mean longitudinal foreshortening rate of coronary stents was about 94% in curved coronary arteries. Longitudinal stent foreshortening rate was positively correlated with the degree of coronary artery curvature (r = –0.86, P < 0.01). Coronary artery curvature is associated with significant longitudinal foreshortening of coronary stents, thus longitudinal foreshortening should be considered on deciding the stent length in curved coronary artery and a longer stent is usually needed in curved coronary artery.