A Rapid and Computationally Inexpensive Method to Virtually Implant Current and Next-Generation Stents into Subject-Specific Computational Fluid Dynamics Models

Computational modeling is often used to quantify hemodynamic alterations induced by stenting, but frequently uses simplified device or vascular representations. Based on a series of Boolean operations, we developed an efficient and robust method for assessing the influence of current and next-generation stents on local hemodynamics and vascular biomechanics quantified by computational fluid dynamics. Stent designs were parameterized to allow easy control over design features including the number, width and circumferential or longitudinal spacing of struts, as well as the implantation diameter and overall length. The approach allowed stents to be automatically regenerated for rapid analysis of the contribution of design features to resulting hemodynamic alterations. The applicability of the method was demonstrated with patient-specific models of a stented coronary artery bifurcation and basilar trunk aneurysm constructed from medical imaging data. In the coronary bifurcation, we analyzed the hemodynamic difference between closed-cell and open-cell stent geometries. We investigated the impact of decreased strut size in stents with a constant porosity for increasing flow stasis within the stented basilar aneurysm model. These examples demonstrate the current method can be used to investigate differences in stent performance in complex vascular beds for a variety of stenting procedures and clinical scenarios

Identification of Hemodynamically Optimal Coronary Stent Designs Based on Vessel Caliber

Coronary stent design influences local patterns of wall shear stress (WSS) that are associated with neointimal growth, restenosis, and the endothelialization of stent struts. The number of circumferentially repeating crowns NC for a given stent de- sign is often modified depending on the target vessel caliber, but the hemodynamic implications of altering NC have not previously been studied. In this investigation, we analyzed the relationship between vessel diameter and the hemodynamically optimal NC using a derivative-free optimization algorithm coupled with computational fluid dynamics. The algorithm computed the optimal vessel diameter, defined as minimizing the area of stent-induced low WSS, for various configurations (i.e., NC) of a generic slotted-tube design and designs that resemble commercially available stents. Stents were modeled in idealized coronary arteries with a vessel diameter that was allowed to vary between 2 and 5 mm. The results indicate that the optimal vessel diameter increases for stent configurations with greater NC, and the designs of current commercial stents incorporate a greater NC than hemodynamically optimal stent designs. This finding suggests that reducing the NC of current stents may improve the hemodynamic environment within stented arteries and reduce the likelihood of excessive neointimal growth and thrombus formation

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

Modeling Stented Coronary Arteries: Where We are, Where to Go

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Transient Cardiovascular Hemodynamics In A Patient-Specific Arterial System

The ultimate goal of the present study is to aid in the development of tools to assist in the treatment of cardiovascular disease. Gaining an understanding of hemodynamic parameters for medical implants allow clinicians to have some patient-specific proposals for intervention planning. In the present study a full cardiovascular experimental phantom and digital phantom (CFD model) was fabricated to study: (1) the effects of local hemodynamics on global hemodynamics, (2) the effects of transition from bed-rest to upright position, and (3) transport of dye (drug delivery) in the arterial system. Computational three dimensional (3-D) models (designs A, B, and C) stents were also developed to study the effects of stent design on hemodynamic flow and the effects of drug deposition into the arterial wall. The experimental phantom used in the present study is the first system reported in literature to be used for hemodynamic assessment in static and orthostatic posture changes. Both the digital and experimental phantom proved to provide different magnitudes of wall shear and normal stresses in sections where previous studies have only analyzed single arteries. The dye mass concentration study for the digital and experimental cardiovascular phantom proved to be useful as a surrogate for medical drug dispersion. The dye mass concentration provided information such as transition time and drug trajectory paths. For the stent design CFD studies, hemodynamic results (wall shear stress (WSS), normal stress, and vorticity) were assessed to determine if simplified stented geometries can be used as a surrogate for patient-specific geometries and the role of stent design on flow. Substantial differences in hemodynamic parameters were found to exist which confirms the need for patient-specific modeling. For drug eluting stent studies, the total deposition time for the drug into the arterial wall was approximately 3.5 months

Computational modelling of stent deployment and mechanical performance inside human atherosclerotic arteries

Atherosclerosis is the obstruction of blood stream caused by the formation of fatty plaques (stenosis) within human blood vessels. It is one of the most common cardiovascular conditions and the primary cause of death in developed countries. Nowadays stenting is a standard treatment for this disease and has been undergoing a rapid technological development. The aim of this PhD is to simulate the deployment of stents within atherosclerotic arteries in order to understand the mechanical performance of these devices. To this purpose, specific objectives were identified to study: (i) the effects of stent design, material and coating on stent deployment; (ii) the influence of balloon type, arterial constraints and vessel constitutive models in stenting simulation; (iii) the importance of plaque thickness, stenosis asymmetry and vessel curvature during the process of stent deployment; (iv) the necessity of considering vessel anisotropy and post-deployment stresses to assess stents mechanical behaviour; (v) the performance of biodegradable polymeric stents in comparison with metallic stents. Finite element (FE) analyses were employed to model the deployment of balloon-expandable stents. The balloon-stent-artery system was generated and meshed using finite element package Abaqus. Individual arterial layer and stenosis were modelled using hyperelastic Ogden model, while elastic-plastic behaviour with nonlinear hardening was used to describe the material behaviour of stents. The expansion of the stent was obtained by application of pressure inside the balloon, with hard contacts defined between stent, balloon and artery. The FE model was evaluated by mesh sensitivity study and further validated by comparison with published work. Comparative study between different commercially available stents (i.e. Palmaz-Schatz, Cypher, Xience and Endeavor stents) showed that open-cell design tends to have easier expansion and higher recoiling than closed-cell design, with lower stress level on the plaque after deployment. Also, stents made of materials with lower yield stress and weaker strain hardening experience higher deformation and recoiling, but less post-deployment stresses. Folded balloon produces sustained stent expansion under a lower pressure when compared to rubber balloon, with also increased stress level on the stent and artery. Simulations with different arterial constraints showed that stress on the plaque-artery system is higher for a free artery as a result of more severe stretch. Study of arterial constitutive models showed that saturation of expansion could not be noticed for models that neglect the second stretch invariant in the strain energy potential. Stent expansion is highly affected by plaque thickness, and stresses and recoiling increased considerably with the increasing level of stenosis. Asymmetry of the plaque causes non-uniform stent expansion and high levels of vessel wall stresses are developed in the regions covered by thin layer of plaque. Also, a reduction in stent expansion is observed with the increase of artery curvature, accompanied by an elevation of stresses in the plaque and arterial layers. Vessel anisotropic behaviour reduces the system expansion at peak pressure, and also lowers recoiling effect significantly. The post-deployment stresses caused by stent expansion increase the system flexibility during in-plane bending and radial compression. Comparative study of a PLLA stent (Elixir) and a Co-Cr alloy stent (Xience) showed that polymeric stent has a lower expansion rate and a reduction in final expansion than metallic stent

Stented Artery Biomechanics: A Computational and In Vivo Analysis of Stent Design and Pathobiological Response

Vascular stents have become a standard for treating atherosclerosis due to distinct advantages in trauma and cost with other surgical techniques. Unfortunately, the therapy is hindered by the risk of a new blockage (termed restenosis) developing in the treated artery. Clinical studies have indicated that stent design is a major risk factor for restenosis, with failure rates varying from 20 to 40% for bare metal stents. Subsequently, there has been a significant effort devoted to reducing failure rates by covering stents in polymer coatings in which anti-proliferative drugs are embedded, however complications have arisen (e.g. incomplete endothelization, lack of success in peripheral arteries, lack of long-term follow-up studies) that have limited the success of this technology. It has been thought that restenosis is directly related to the mechanical conditions that vascular stents create. Moreover, it has been hypothesized that stents that induce higher non-physiologic stresses result in a more aggressive pathobiological response that can lead to restenosis development. In this study, a combination of computational modeling and in vivo analysis were conducted to investigate the artery stent-induced wall stresses, and subsequent biological inflammatory response. In particular, variations in stent design were investigated as a means of examining specific stent design criteria that minimize the mechanical impact of stenting. Collectively, these data indicate that stent designs that subject the artery wall to higher stress values result in significantly more neointimal tissue proliferation, therefore, confirming the aforementioned hypothesis. Moreover, this work provides valuable insight into the role that biomechanics can play in improving the success rate of this percutaneous therapy and overall patient care