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

Effect of stent treatment on hemodynamics in iliac vein compression syndrome with collateral vein

BACKGROUND

Iliac vein compression syndrome (IVCS) leads to blood flow obstruction in the lower extremities and is usually treated with stents, but stenting may worsen the hemodynamics and increase the risk of thrombosis in the iliac vein. The present work evaluates the advantages and disadvantages of the stent on IVCS with a collateral vein.

METHODS

The computational fluid dynamics method is adopted to analyze the preoperative and postoperative flow fields in a typical IVCS. The geometric models of the iliac vein are constructed from medical imaging data. The porous model is used to simulate the flow obstruction in IVCS.

RESULTS

The preoperative and postoperative hemodynamic characteristics in the iliac vein are obtained, e.g., the pressure gradient at two ends of the compressive region and the wall shear stress. It is found that the stenting restores the blood flow in the left iliac vein.

CONCLUSION

Impacts of the stent are classified into short-term and long-term effects. The short-term effects are beneficial in relieving IVCS, i.e., shortening the blood stasis and reducing the pressure gradient. The long-term effects increase the risk of thrombosis in the stent, i.e., enlarging wall shear stress due to a large corner and a diameter constriction in the distal vessel, and suggests the need to develop a venous stent for IVCS.

Evaluation of Different Meshing Techniques for the Case of a Stented Artery

The formation and progression of in-stent restenosis (ISR) in bifurcated vessels may vary depending on the technique used for stenting. This study evaluates the effect of a variety of mesh styles on the accuracy and reliability of computational fluid dynamics (CFD) models in predicting these regions, using an idealized stented nonbifurcated model. The wall shear stress (WSS) and the near-stent recirculating vortices are used as determinants. The meshes comprise unstructured tetrahedral and polyhedral elements. The effects of local refinement, as well as higher-order elements such as prismatic inflation layers and internal hexahedral core, have also been examined. The uncertainty associated with individual mesh style was assessed through verification of calculations using the grid convergence index (GCI) method. The results obtained show that the only condition which allows the reliable comparison of uncertainty estimation between different meshing styles is that the monotonic convergence of grid solutions is in the asymptotic range. Comparisons show the superiority of a flow-adaptive polyhedral mesh over the commonly used adaptive and nonadaptive tetrahedral meshes in terms of resolving the near-stent flow features, GCI value, and prediction of WSS. More accurate estimation of hemodynamic factors was obtained using higher-order elements, such as hexahedral or prismatic grids. Incorporating these higher-order elements, however, was shown to introduce some degrees of numerical diffusion at the transitional area between the two meshes, not necessarily translating into high GCI value. Our data also confirmed the key role of local refinement in improving the performance and accuracy of nonadaptive mesh in predicting flow parameters in models of stented artery. The results of this study can provide a guideline for modeling biofluid domain in complex bifurcated arteries stented in regards to various stenting techniques.

New Stenting Technique to Achieve Favorable Jailing Configuration on Side Branch Ostium: Bent Stent Technique

According to data from stent-enhanced three-dimensional optical coherence tomography, incomplete stent apposition after side branch dilation in coronary bifurcation stenting can be reduced by the free carina type (no links bridged from a carina) and by distal cell rewiring. This is the first report to describe a bent stent technique that was devised to achieve the free carina type (no links bridged from a carina), as a favorable jailing configuration.

Optimal kissing balloon inflation after single-stent deployment in a coronary bifurcation model

AIMS

To define the optimal kissing balloon inflation (KBI) after single-stent deployment in a coronary bifurcation model.

METHODS AND RESULTS

We deployed stents in main vessels (MV) followed by KBI in various conditions and compared the stent configurations. A) KBI at the operator's discretion vs. under the guidelines of minimal balloon overlapping (MBO). Various stent configurations were observed after the former option, whereas similar maximal dilation points were observed under the MBO guidelines. B) Long balloon overlapping (LBO) vs. MBO with proximal MV dilated by a large balloon. The proximal MV was dilated to an ideal round shape with MBO versus an oval shape with LBO. C) Two-link vs. 3-link stents. Although the 2-link stent was advantageous to open the side branch, it incurred a risk of overdilatation of the proximal struts, whereas the 3-link stent preserved its structure. Computed simulations of coronary flow were analysed in the following left main coronary models: circle with a diameter of 4 and 5.5 mm, ellipse with longitudinal direction and tilt position. They revealed that the overdilated side was exposed to low shear stress regardless of its shape.

CONCLUSIONS

Optimal KBI can be achieved with MBO and proximal dilatation by an optimally sized balloon.

Numerical analysis of coronary artery bypass grafts: an over view

Arterial bypass grafts tend to fail after some years due to the development of intimal thickening (restenosis). Non-uniform hemodynamics following a bypass operation contributes to restenosis and bypass failure can occur due to the focal development of anastomotic intimal hyperplasia. Additionally, surgical injury aggravated by compliance mismatch between the graft and artery has been suggested as an initiating factor for progress of wall thickening along the suture line Vascular grafts that are small in diameter tend to occlude rapidly. Computational fluid dynamics (CFD) methods have been effectively used to simulate the physical and geometrical parameters characterizing the hemodynamics of various arteries and bypass configurations. The effects of such changes on the pressure and flow characteristics as well as the wall shear stress during a cardiac cycle can be simulated. Recently, utilization of fluid and structure interactions have been used to determine fluid flow parameters and structure forces including stress and strains relationships under steady and transient conditions. In parallel to this, experimental diagnostics techniques such as Laser Doppler Anemometry, Particle Image Velocimetry, Doppler Guide wire and Magnetic Resonance Imaging have been used to provide essential information and to validate the numerical results. Moreover, clinical imaging techniques such as magnetic resonance or computed tomography have assisted considerably in gaining a detailed patient-specific picture of the blood flow and structure dynamics. This paper gives a review of recent numerical investigations of various configurations of coronary artery bypass grafts (CABG). In addition, the paper ends with a summary of the findings and the future directions.

Modeling stented coronary arteries: where we are, where to go

In the last two decades, numerical models have become well-recognized and widely adopted tools to investigate stenting procedures. Due to limited computational resources and modeling capabilities, early numerical studies only involved simplified cases and idealized stented arteries. Nowadays, increased computational power allows for numerical models to meet clinical needs and include more complex cases such as the implantation of multiple stents in bifurcations or curved vessels. Interesting progresses have been made in the numerical modeling of stenting procedures both from a structural and a fluid dynamics points of view. Moreover, in the drug eluting stents era, new insights on drug elution capabilities are becoming essential in the stent development. Lastly, image-based methods able to reconstruct realistic geometries from medical images have been proposed in the recent literature aiming to better describe the peculiar anatomical features of coronary vessels and increase the accuracy of the numerical models. In this light, this review provides a comprehensive analysis of the current state-of-the-art in this research area, discussing the main methodological advances and remarkable results drawn from a number of significant studies.

Fast virtual deployment of self-expandable stents: method and in vitro evaluation for intracranial aneurysmal stenting

INTRODUCTION

Minimally invasive treatment approaches, like the implantation of percutaneous stents, are becoming more popular every day for the treatment of intracranial aneurysms. The outcome of such treatments is related to factors like vessel and aneurysm geometry, hemodynamic conditions and device design. For this reason, having a tool for assessing stenting alternatives beforehand is crucial.

METHODOLOGY

The Fast Virtual Stenting (FVS) method, which provides an estimation of the configuration of intracranial stents when released in realistic geometries, is proposed in this paper. This method is based on constrained simplex deformable models. The constraints are used to account for the stent design. An algorithm for its computational implementation is also proposed. The performance of the proposed methodology was contrasted with real stents released in a silicone phantom.

RESULTS

In vitro experiments were performed on the phantom where a contrast injection was performed. Subsequently, corresponding Computational Fluid Dynamics (CFD) analyzes were carried out on a digital replica of the phantom with the virtually released stent. Virtual angiographies are used to compare in vitro experiments and CFD analysis. Contrast time-density curves for in vitro and CFD data were generated and used to compare them.

CONCLUSIONS

Results of both experiments resemble very well, especially when comparing the contrast density curves. The use of FVS methodology in the clinical environment could provide additional information to clinicians before the treatment to choose the therapy that best fits the patient.

Importance of kissing balloon inflation in bifurcation stenting

Percutaneous bifurcation intervention is usually sufficient with a single-stent strategy. When the double-stent strategy is employed, higher restenosis and target lesion revascularization (TLR) rates are observed, especially at the side-branch ostium. The results may be improved, however, with refinement in techniques, for example, final kissing balloon inflation and double kissing balloon inflation.

Impact of three-dimensional characteristics of the left main coronary artery bifurcation on outcome of crush stenting

BACKGROUND

Crush stenting with drug-eluting stents is used to treat left main coronary artery (LMCA) bifurcations. However, the rate of restenosis at the left circumflex (LCX) artery ostium is high. The impact of the three-dimensional (3D) structure of LMCA bifurcation on the outcome of crush stenting with respect to restenosis has not been described.

OBJECTIVES

This study examined the stent expansion, deformity, overlapping, and apposition after crush stenting of LMCA bifurcations.

METHODS

Bare metal stents were crushed at LMCA bifurcations in a 3D model that reproduced actual angles, such that the stent deployed from the LMCA to the left anterior descending (LAD) artery crushed the stent deployed from the LMCA to the LCX, followed by kissing balloon inflation. The stents were inspected under fluoroscopy and endoscopy. The effect of the bifurcation angle on stent expansion was also examined.

RESULTS

In the 3D model, one stent overlapped the other in the distal LMCA, in contrast to the nearly parallel position of the stents observed in a separate two-dimensional model. When the LAD stent overlapped the LCX stent, the latter was crushed on the myocardial side of the vessel, and an unstented segment was observed on the nonmyocardial side, at the LCX ostium. When the overlap was reversed, the LCX stent was crushed on the nonmyocardial side and an unstented segment was observed on the myocardial side. A narrow LMCA-LCX angle was associated with less expansion of the LCX stent at the ostium than more distally, and with a higher likelihood of incomplete stent apposition.

CONCLUSIONS

Overlap of the LAD stent over, as opposed to under, the LCX stent was associated with close apposition of the stent to the vessel on the myocardial side, at the ostium of the LCX artery, where atherosclerotic plaques are likely to be present. The spatial plaque burden and bifurcation angle should be closely examined before crush stenting, and segments should not be left unstented over large plaques.