Influence of Fluid-Structure Interaction on Wall Shear Stress in a Stented Coronary Artery Model

Previous studies indicate that the likelihood of rate of restenosis following installation of a bare metal stent to treat coronary artery disease is related to the magnitude of the wall shear stress in the artery. The current study seeks to understand if including fluid-structure interaction (FSI) in a computational model of a stented coronary artery significantly influences the predicted wall shear stress on exposed patches of the artery. As a secondary result, it also determines influence of FSI on the magnitude of WSSon the surface of the stent. COMSOLMultiphysics was the computational tool selected for this study. It was carried out using rigid (no-FSI) and compliant wall (FSI)models comprising of a straight user-defined coronary artery, blood domain and a realistic stent. The arterial wall and stent were modeled as linear elastic materials while the blood was represented by an incompressible Newtonian fluid. Blood flow was assumed to be laminar and its boundary conditions were derived from published physiological waveforms. A periodic Womersley velocity profile was prescribed as the inflow boundary condition and a periodic pressure was prescribed as the outflow condition. Quasi-stationary analyses were carried out on both the rigid and compliant-wall models at different times. A mesh convergence study lead to a mesh-independent model. On comparing the FSIand no-FSImodels, it was concluded that the influence of FSIwas prominent on the stent surface and in the distal region of the geometric model. Although differences between model predictions of wall shear stress varied throughout the period of the waveform, the ranges of difference depend on the axial location along the artery: 10-20% in the proximal region, 17-55% in the distal region, 10-35% within the stent openings, and 16-58% on the stent surfaces

Haemodynamics analysis of carotid artery stenosis and carotid artery stenting

Carotid stenosis is a local narrowing of the carotid artery, and is usually found in the internal carotid artery. The presence of a high-degree stenosis in a carotid artery may provoke transition from laminar to turbulent flow during part of the cardiac cycle. Turbulence in blood flow can influence haemodynamic parameters such as velocity profiles, shear stress and pressure, which are important in wall remodelling. Patients with severe stenosis could be treated with a minimally invasive clinical procedure, carotid artery stenting (CAS). Although CAS has been widely adopted in clinical practice, the complication of in-stent restenosis (ISR) has been reported after CAS. The incidence of ISR is influenced by stent characteristics and vessel geometry, and correlates strongly with regions of neointimal hyperplasia (NH). Therefore, the main purpose of this study is to provide more insights into the haemodynamics in stenosed carotid artery and in post-CAS geometries via computational simulation. The first part of the thesis presents a computational study on flow features in a stenotic carotid artery bifurcation using two computational approaches, large eddy simulation (LES) and Reynolds-averaged Navier-Stokes (RANS) incorporating the Shear Stress Transport model with the γ-Reθ transition (SST-Tran) models. The computed flow patterns are compared with those measured with particle image velocimetry (PIV). The results show that both SST-Tran and LES can predict the PIV results reasonably well, but LES is more accurate especially at locations distal to the stenosis where flow is highly disturbed. The second part of the thesis is to determine how stent strut design may influence the development of ISR at the carotid artery bifurcation following CAS. Key parameters that can be indicative of ISR are obtained for different stent designs and compared; these include low and oscillating wall shear stress (WSS), high residence time, and wall stress. A computationally efficient methodology is employed to reproduce stent strut geometry. This method facilitates the accurate reconstruction of actual stent geometry and details of strut configuration and its inclusion in the fluid domain. Computational simulations for flow patterns and low-density lipoprotein (LDL) transport are carried out in order to investigate spatial and temporal variations of WSS and LDL accumulation in the stented carotid geometries. Furthermore, finite element (FE) analysis is performed to evaluate the wall stress distribution with different stent designs. The results reveal that the closed-cell stent design is more likely to create atheroprone and procoagulant flow conditions, causing larger area to be exposed to low wall shear stress (WSS), elevated oscillatory shear index, as well as to induce higher wall stress compared to the open-cell stent design. This study also demonstrates the suitability of SST-Tran and LES models in capturing the presence of complex flow patterns in post-stenotic region.Open Acces

INTEGRATED DESIGN APPROACH FOR CORONARY STENTS USING FLEXINOL SHAPE MEMORY ALLOY

This research seeks to develop and verify a model for control of the shape memory alloy (SMA) Flexinol and apply such findings to practical application of the material as a platform for bare metal stenting technologies. Utilizing experimental data and material properties, a mathematical model of the thermoelectric contraction behavior of Flexinol wire samples was developed. This model accounted for variable resistance due to the shape memory effect of the Flexinol wire as it experiences a crystalline phase change. It also accounted for the change in the cross-sectional area of the wire as the wire experienced thermal expansion and contraction. The resulting constitutive equations were verified via experimentation. This thesis further expanded upon these models and presented the practical application of the SMA Flexinol as a platform for coronary artery stenting technologies. The research presented includes computer-aided design (CAD) modeling and finite element analysis (FEA) simulation of the stress loads when working conditions are applied, which revealed the response behavior of the proposed stent design. With the FEA verification that the Flexinol stent design will be able to sustain normal working conditions once implanted into the human body, it was demonstrated that the proposed low stress design has the potential to reduce the rate of stent failure and restenosis in comparison to typical technologies available on the market

Sequential Structural and Fluid Dynamics Analysis of Balloon-Expandable Coronary Stents: A Multivariable Statistical Analysis

Several clinical studies have identified a strong correlation between neointimal hyperplasia following coronary stent deployment and both stent-induced arterial injury and altered vessel hemodynamics. As such, the sequential structural and fluid dynamics analysis of balloon-expandable stent deployment should provide a comprehensive indication of stent performance. Despite this observation, very few numerical studies of balloon-expandable coronary stents have considered both the mechanical and hemodynamic impact of stent deployment. Furthermore, in the few studies that have considered both phenomena, only a small number of stents have been considered. In this study, a sequential structural and fluid dynamics analysis methodology was employed to compare both the mechanical and hemodynamic impact of six balloon-expandable coronary stents. To investigate the relationship between stent design and performance, several common stent design properties were then identified and the dependence between these properties and both the mechanical and hemodynamic variables of interest was evaluated using statistical measures of correlation. Following the completion of the numerical analyses, stent strut thickness was identified as the only common design property that demonstrated a strong dependence with either the mean equivalent stress predicted in the artery wall or the mean relative residence time predicted on the luminal surface of the artery. These results corroborate the findings of the large-scale ISAR-STEREO clinical studies and highlight the crucial role of strut thickness in coronary stent design. The sequential structural and fluid dynamics analysis methodology and the multivariable statistical treatment of the results described in this study should prove useful in the design of future balloon-expandable coronary stents

Sequential Structural and Fluid Dynamics Analysis of Balloon-Expandable Coronary Stents.

As in-stent restenosis following coronary stent deployment has been strongly linked with stent-induced arterial injury and altered vessel hemodynamics, the sequential numerical analysis of the mechanical and hemodynamic impact of stent deployment within a coronary artery is likely to provide an excellent indication of coronary stent performance. Despite this observation, very few numerical studies have considered both the mechanical and hemodynamic impact of stent deployment. In light of this observation, the aim of this research is to develop a robust numerical methodology for investigating the performance of balloon-expandable coronary stents in terms of their mechanical and hemodynamic impact within a coronary artery. The proposed methodology is divided into two stages. In the first stage, a numerical model of the stent is generated and a computational structural analysis is carried out to simulate its deployment within a coronary artery. In the second stage, the results of the structural analysis are used to generate a realistic model of the stented coronary lumen and a computational fluid dynamics analysis is carried out to simulate pulsatile blood flow within a coronary artery. Following the completion of the analyses, the mechanical impact of the stent is evaluated in terms of the stress distribution predicted within the artery whilst the hemodynamic impact of the stent is evaluated in terms of the wall shear stress distribution predicted upon the luminal surface of the artery. In order to demonstrate its application, the proposed numerical methodology was applied to six generic stents. Comparing the predicted performance of the generic stents revealed that strut thickness is likely to have a significant influence upon both the mechanical and hemodynamic impact of coronary stent deployment. Additionally, comparing the predicted performance of three of the investigated stents to the clinical performance of three comparable commercial stents, as reported in two large-scale clinical trials, revealed that that the proposed numerical methodology successfully identified the stents that resulted in higher rates of angiographic in-stent restenosis, late lumen loss and target-vessel revascularisation at short-term follow-up. In light of the conflicting requirements of coronary stent design, the proposed numerical methodology should prove useful in the design and optimisation of future coronary stents

Finite element and mechanobiological modelling of vascular devices

There are two main surgical treatments for vascular diseases, (i) percutaneous stent deployment and (ii) replacement of an atherosclerotic artery with a vascular graft or tissue engineered blood vessel. The aim of this thesis was to develop computational models that could assist in the design of vascular stents and tissue engineered vascular grafts and scaffolds. In this context, finite element (FE) models of stent expansion in idealised and patient specific models of atherosclerotic arteries were developed. Different modelling strategies were investigated and an optimal modelling approach was identified which minimised computational cost without compromising accuracy. Numerical models of thin and thick strut stents were developed using this modelling approach to replicate the ISAR-STEREO clinical trial and the models identified arterial stresses as a suitable measure of stent induced vascular injury. In terms of evaluating vascular graft performance, mechanical characterisation experiments can be conducted in order to develop constitutive models that can be used in FE models of vascular grafts to predict their mechanical behaviour in-situ. In this context, bacterial cellulose (BC), a novel biomaterial, was mechanically characterised and a constitutive model was developed to describe its mechanical response. In addition, the interaction of smooth muscle cells with BC was studied using cell culture experiments. The constitutive model developed for BC was used as an input for a novel multi-scale mechanobiological modelling framework. The mechanobiological model was developed by coupling an FE model of a vascular scaffold and a lattice free agent based model of cell growth dynamics and remodelling in vascular scaffolds. By comparison with published in-vivo and in-vitro works, the model was found to successfully capture the key characteristics of vascular remodelling. It can therefore be used as a predictive tool for the growth and remodelling of vascular scaffolds and graft

Oxygen Transport in Carotid and Stented Coronary Arteries

Oxygen deficiency, known as hypoxia, in arterial walls has been linked to increased intimal hyperplasia, which is the main adverse biological process causing in-stent restenosis. Stent implantation can have significant effects on the oxygen transport into the arterial wall. Helical flow has been theorised to improve the local haemodynamics and the oxygen transport within stented arteries. In this study an advanced oxygen transport model was developed to assess different stent designs. This advanced oxygen transport model incorporates both the free and bound oxygen contained in blood and includes a shear-dependent dispersion coefficient for red blood cells. In two test cases undertaken the results predicted by the advanced oxygen transport model were compared those predicted by simpler models, and in vivo measurements. Two other test cases analysed the predicted oxygen transport in three different stent designs, and the effects of helical flow on the haemodynamics and oxygen transport in stented coronary arteries. The advanced model showed good agreement with experimental measurements within the mass-transfer boundary layer and at the luminal surface; however, more work is needed for predicting the oxygen transport within the arterial wall. Simplifying the oxygen transport model within the blood produces significant errors in predicting the oxygen transport in arteries. It was found that different stent designs can produce significantly different amounts of hypoxic regions within the stented region. Additionally, helical flow increases the amount of oxygen transferred into the arterial wall, but only in a helical ribbon through the stented region that also experiences high wall shear stress spatial gradients

Hemodynamics of Stent Implantation Procedures in Coronary Bifurcations: an in vitro study

Stent implantation in coronary bifurcations presents unique challenges and currently there is no universally accepted stent deployment approach. Despite clinical and computational studies, to date, the effect of each stent implantation method on the coronary artery hemodynamics is not well understood. In this study the hemodynamics of stented coronary bifurcations under pulsatile flow conditions were investigated experimentally. Three implantation methods, provisional side branch (PSB), culotte (CUL), and crush (CRU), were investigated using time-resolved particle image velocimetry (PIV) to measure the velocity fields. Subsequently, hemodynamic parameters including wall shear stress (WSS), oscillatory shear index (OSI), and relative residence time (RRT) were calculated and the pressure field through the vessel was non-invasively quantified. The effects of each stented case were evaluated and compared against an un-stented case. CRU provided the lowest compliance mismatch, but demonstrated detrimental stent interactions. PSB, the clinically preferred method, and CUL maintained many normal flow conditions. However, PSB provided about a 300% increase in both OSI and RRT. CUL yielded a 10% and 85% increase in OSI and RRT, respectively. The results of this study support the concept that different bifurcation stenting techniques result in hemodynamic environments that deviate from that of un-stented bifurcations, to varying degrees.Comment: 33 pages, 8 figures, 3 table