Local blood flow patterns in stented coronary bifurcations: an experimental and numerical study.

PURPOSE: Despite the atheroprone environment of blood flow in coronary bifurcations, limited quantitative information is available on the hemodynamics occurring in these geometries, both before and after their treatment with endovascular stents. Previous studies have focused on computational fluid dynamics (CFD) analyses and have bypassed the challenges associated with experimentally representing the flow environment, providing no means for validation. This study analyzed steady flow conditions in 3 bifurcation angles and 4 different single- and double-stenting procedures, which are used clinically in coronary bifurcations. METHODS: The numerical aspect of this study utilized geometries derived from CAD models (nonstented cases) and finite element simulations (stented cases). Digital particle image velocimetry (DPIV) testing was conducted within compliant bifurcating models for which an uncertainty analysis was performed at each measurement location for CFD validation purposes. Results were analyzed in terms of velocity magnitude contour maps and axial velocity profiles at several locations in the bifurcated vessels. RESULTS AND CONCLUSIONS: Qualitatively, the 2 approaches showed agreement in the bulk flow patterns. However, the velocity computed with CFD was outside the DPIV uncertainty estimates, which can be attributed to the intrinsic difference and modeling assumptions of the 2 approaches. The findings reveal that wider bifurcation angles and double-stenting procedures are both characterized by increased areas of low flow and recirculation. Additionally, inferior performance in terms of viscous and wall shear stresses was observed in double-stented cases.

Stent deformation, physical stress, and drug elution obtained with provisional stenting, conventional culotte and Tryton-based culotte to treat bifurcations: a virtual simulation study.

AIMS: This study sought to investigate the possible influence of different bifurcation stenting techniques on stent deformation, physical stress, and drug elution using a virtual tool that includes structural, fluid dynamics and drug-eluting numerical models. METHODS AND RESULTS: A virtual bench test based on explicit dynamics modelling was used to simulate procedures on bifurcated coronary vessels performed according to three different stenting techniques: provisional side branch stenting, culotte, and Tryton-based culotte. Geometrical configurations obtained after virtual stenting simulations were used to perform fluid dynamics and drug elution analyses. The results showed that substantially different patterns of mechanical deformation, shear stress and theoretical drug elution were obtained using the different techniques. Compared with conventional culotte, the dedicated Tryton seems to facilitate the intervention in terms of improved access to the main branch and to lower its biomechanical influence on the coronary bifurcation in terms of mechanical and haemodynamic parameters. However, since the Tryton stent is a bare metal stent, the drug elution obtained is lower. CONCLUSIONS: Numerical models might successfully complement the information on stenting procedures obtained with traditional approaches such as in vitro bench testing or clinical trials. Devices dedicated to bifurcations may facilitate procedure completion and may result in specific patterns of mechanical stress, regional blood flow and drug elution.

Influence of plaque calcifications on coronary stent fracture: a numerical fatigue life analysis including cardiac wall movement.

Coronary stent fracture is still an unresolved issue in the field of minimally invasive cardiovascular interventions due to its high rate of incidence and uncertain clinical consequences. Recent studies, based on clinical data, proved that there are several factors which can be identified as independently responsible of coronary stent fracture. Among these, calcifications, which increase the local stiffness and heterogeneity of atherosclerotic plaques, seem to play a major role. From a mechanical point of view, stent fracture in coronary arteries is triggered by the cyclic loading of pulsatile blood pressure combined with the movement of cardiac wall. In this context, this study aims at simulating the stent expansion in a model of epicardial atherosclerotic coronary artery and correlating the effects of cyclic blood pressure and cardiac wall movement on the stent fatigue resistance. Two ideal cases of atherosclerotic plaques were modelled: the first one included a localised plaque calcification; the latter one did not include such calcification. Results of stress/strain and fatigue analyses confirmed the influence of the plaque calcification on potential fracture of the devices. In addition, the effects of cardiac wall movement were quantified as more dangerous causes of the stent fatigue fracture with respect to the internal blood pressure oscillations. In conclusion, this study demonstrates the increased risk of coronary stent fracture associated to the presence of localised plaque calcifications. This work also suggests the necessity of more realistic biomechanical models which takes into account the heterogeneity of atherosclerotic plaques in order to assess the mechanical performances of coronary stents.

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.

Patient-specific simulations of stenting procedures in coronary bifurcations: two clinical cases.

Computational simulations of stenting procedures in idealized geometries can only provide general guidelines and their use in the patient-specific planning of percutaneous treatments is inadequate. Conversely, image-based patient-specific tools that are able to realistically simulate different interventional options might facilitate clinical decision-making and provide useful insights on the treatment for each individual patient. The aim of this work is the implementation of a patient-specific model that uses image-based reconstructions of coronary bifurcations and is able to replicate real stenting procedures following clinical indications. Two clinical cases are investigated focusing the attention on the open problems of coronary bifurcations and their main treatment, the provisional side branch approach. Image-based reconstructions are created combining the information from conventional coronary angiography and computed tomography angiography while structural finite element models are implemented to replicate the real procedure performed in the patients. First, numerical results show the biomechanical influence of stents deployment in the coronary bifurcations during and after the procedures. In particular, the straightening of the arterial wall and the influence of two overlapping stents on stress fields are investigated here. Results show that a sensible decrease of the vessel tortuosity occurs after stent implantation and that overlapping devices result in an increased stress state of both the artery and the stents. Lastly, the comparison between numerical and image-based post-stenting configurations proved the reliability of such models while replicating stent deployment in coronary arteries.

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.

Drug delivery patterns for different stenting techniques in coronary bifurcations: a comparative computational study.

The treatment of coronary bifurcation lesions represents a challenge for the interventional cardiologists due to the lower rate of procedural success and the higher risk of restenosis. The advent of drug-eluting stents (DES) has dramatically reduced restenosis and consequently the request for re-intervention. The aim of the present work is to provide further insight about the effectiveness of DES by means of a computational study that combines virtual stent implantation, fluid dynamics and drug release for different stenting protocols currently used in the treatment of a coronary artery bifurcation. An explicit dynamic finite element model is developed in order to obtain realistic configurations of the implanted devices used to perform fluid dynamics analysis by means of a previously developed finite element method coupling the blood flow and the intramural plasma filtration in rigid arteries. To efficiently model the drug release, a multiscale strategy is adopted, ranging from lumped parameter model accounting for drug release to fully 3-D models for drug transport to the artery. Differences in drug delivery to the artery are evaluated with respect to local drug dosage. This model allowed to compare alternative stenting configurations (namely the Provisional Side Branch, the Culotte and the Inverted Culotte techniques), thus suggesting guidelines in the treatment of coronary bifurcation lesions and addressing clinical issues such as the effectiveness of drug delivery to lesions in the side branch, as well as the influence of incomplete strut apposition and overlapping stents.

Sequential structural and fluid dynamic numerical simulations of a stented bifurcated coronary artery.

Despite their success, stenting procedures are still associated to some clinical problems like sub-acute thrombosis and in-stent restenosis. Several clinical studies associate these phenomena to a combination of both structural and hemodynamic alterations caused by stent implantation. Recently, numerical models have been widely used in the literature to investigate stenting procedures but always from either a purely structural or fluid dynamic point of view. The aim of this work is the implementation of sequential structural and fluid dynamic numerical models to provide a better understanding of stenting procedures in coronary bifurcations. In particular, the realistic geometrical configurations obtained with structural simulations were used to create the fluid domains employed within transient fluid dynamic analyses. This sequential approach was applied to investigate the final kissing balloon (FKB) inflation during the provisional side branch technique. Mechanical stresses in the arterial wall and the stent as well as wall shear stresses along the arterial wall were examined before and after the FKB deployment. FKB provoked average mechanical stresses in the arterial wall almost 2.5 times higher with respect to those induced by inflation of the stent in the main branch only. Results also enlightened FKB benefits in terms of improved local blood flow pattern for the side branch access. As a drawback, the FKB generates a larger region of low wall shear stress. In particular, after FKB the percentage of area characterized by wall shear stresses lower than 0.5 Pa was 79.0%, while before the FKB it was 62.3%. For these reasons, a new tapered balloon dedicated to bifurcations was proposed. The inclusion of the modified balloon has reduced the mechanical stresses in the proximal arterial vessel to 40% and the low wall shear stress coverage area to 71.3%. In conclusion, these results show the relevance of the adopted sequential approach to study the wall mechanics and the hemodynamics created by stent deployment.

Trends in biomedical engineering: focus on Patient Specific Modeling and Life Support Systems.

Over the last twenty years major advancements have taken place in the design of medical devices and personalized therapies. They have paralleled the impressive evolution of three-dimensional, non invasive, medical imaging techniques and have been continuously fuelled by increasing computing power and the emergence of novel and sophisticated software tools. This paper aims to showcase a number of major contributions to the advancements of modeling of surgical and interventional procedures and to the design of life support systems. The selected examples will span from pediatric cardiac surgery procedures to valve and ventricle repair techniques, from stent design and endovascular procedures to life support systems and innovative ventilation techniques.

Hemodynamics and in-stent restenosis: micro-CT images, histology, and computer simulations.

A model that combines the results of in vivo experiment, 3D image data, and computer simulation has been developed. Twelve identical stents were implanted into six healthy pigs and explanted at a range of different post-recovery periods from 6 h to 28 days. The stented vessel segments were embedded in methacrylate resin for the preparation of transverse histological sections and imaged using ultra-high resolution micro-CT. The resulting CT data was used to reconstruct the 3D geometry of the stents and one case was used to inform a 3D computational fluid dynamic model. Derived hemodynamic parameters such as wall shear stress (WSS), axial WSS, and oscillatory shear index were correlated with the distribution of neointimal hyperplasia, assessed from histomorphometric analyses. The direct comparison of hemodynamic parameters and biological response supports the hypothesis that low and oscillatory WSS lead to a greater neointimal response within the stented region. Moreover, the realistic geometry obtained from micro-CT images, characterized by proximal overexpansion and asymmetric deployment of the stent, leads to a markedly non-uniform distribution of WSS values and correlates with asymmetric neo-intimal growth. This correlation cannot be appreciated from studies of idealized geometries.

Modelling of the provisional side-branch stenting approach for the treatment of atherosclerotic coronary bifurcations: effects of stent positioning.

The most common approach to treat atherosclerosis in coronary bifurcations is the provisional side-branch (PSB) stenting, which consists sequentially of the insertion of a stent in the main branch (MB) of the bifurcation and a dilatation of the side branch (SB) passing through the struts of the stent at the bifurcation. This approach can be followed by a redilatation of the MB only or by a Final Kissing Balloon (FKB) inflation, both strategies leading to a minor stent distortion in the MB. The positioning of the stent struts in the bifurcation and the stresses generated in the stent and vessel wall are worthy of investigation for a better understanding of the mechanobiology of the system. For this purpose, a computer model of an atherosclerotic coronary bifurcation based on the finite element method was developed; the effects of performing the final redilatation with the two strategies utilising one or two balloons and those created by a different stent strut positioning around the SB were investigated. Results correlate well with previous experimental tests regarding the deformation following balloon expansion. Furthermore, results confirm firstly that the re-establishment of an optimal spatial configuration of the stent after the PSB approach is achieved with both strategies; secondly, results show that case of stent positioning with one cell placed centrally (with regard to the SB) should be preferred, avoiding the presence of struts inside the vessel lumen, which may reduce hemodynamic disturbances. The central positioning also resulted in a better solution in terms of lower stresses in the stent struts and, more importantly, in the vascular tissues.