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

The importance of vessel factors for stent deployment in diseased arteries

Finite element analyses have been carried out to investigate the effects of plaque thickness, plaque asymmetry and artery curvature on stent deployment in stenotic arteries. The Xience stent, one of the latest commercial metallic stents, was considered and its expansion was controlled by the inflation of a folded balloon. Results showed that it became a challenge to open arteries with thick plaque via stent expansion, as stresses and recoiling increased considerably with the increasing level of stenosis. Asymmetric plaque caused non-uniform stent expansion and uneven dogboning effect, with considerably high levels of vessel wall stresses developed in the regions covered by relatively thin layer of plaque. In a curved artery, a reduction in stent expansion was observed with the increase of artery curvature, accompanied by an elevation of stresses in the plaque and arterial layers. Consequently, particular care should be taken when implanting stents in diseased arteries with severe stenosis, unevenly distributed plaque layer and sharp curvature, as tissue damage might occur due to non-uniform expansion of the system

A computational study of stent performance by considering vessel anisotropy and residual stresses

Finite element simulations of stent deployment were carried out by considering the intrinsic anisotropic behaviour, described by a Holzapfel-Gasser-Ogden (HGO) hyperelastic anisotropic model, of individual artery layers. The model parameters were calibrated against the experimental stress-stretch responses in both circumferential and longitudinal directions. The results showed that stent expansion, system recoiling and stresses in the artery layers were greatly affected by vessel anisotropy. Following deployment, deformation of the stent was also modelled by applying relevant biomechanical forces, i.e. in-plane bending and radial compression, to the stent-artery system, for which the residual stresses generated during deployment were particularly accounted for. Residual stresses were found to have a significant influence on the deformation of the system, resulting in a re-distribution of stresses and a change of the system flexibility. The results were also utilised to interpret the mechanical performance of stent after deployment

A study of balloon type, system constraint and artery constitutive model used in finite element simulation of stent deployment

This paper carried out a comparative study of different practices used in finite element simulation of stent deployment, with a focus on the choice of balloon type, system constraint and artery constitutive model. Folded balloon produces sustained stent expansion under a lower pressure when compared to rubber balloon. The maximum stresses on the stent and stenotic artery are considerably higher for simulations using a folded balloon, due to the assumed elastic behaviour of the folded balloon which signified the contact stresses between the balloon and the stent. The achieved final diameter is larger for folded balloon than that for rubber balloon, with increased dogboning and decreased recoiling effects. Fully constrained artery reduces the final expansion when compared to a free artery and a partially constrained artery due to the increased recoiling effect. The stress on the plaque-artery system has similar distribution for all three types of artery constraints (full, partial and free of constraints), but the magnitude is higher for a free artery as a result of more severe stretch. Stenotic plaque model plays a dominant role in controlling stent expansion, and calcified plaque model leads to a considerably lower expansion than hypocellular plaque model. Simulations using Ogden and 6-parameter polynomial models generate different behaviour for stent expansion. For Ogden model, stent expansion approaches the saturation at a certain stage of balloon inflation, while saturation is not observed for 6- 2 parameter polynomial model due to the negligence of the second stretch invariant in the strain energy potential. The use of anisotropic model for the vessel layers reduced the expansion at peak pressure when compared to the simulation using an isotropic model, but the final diameter increased due to the significantly reduced recoiling effect. The stress distribution in the arteryplaque system is also different for different combination of artery and plaque constitutive models. In conclusion, folded balloon should be used in the simulation of stent deployment, with the artery partially constrained using spring elements with a proper stiffness constant. The blood vessel should be modelled as a three-layer structure using a hyperelastic potential that considers both the first and second stretch invariants as well as the anisotropy. The composition of the plaque also has to be considered due to its major effect on stent deployment

Effects of material, coating, design and plaque composition on stent deployment inside a stenotic artery-finite element simulation

Finite-element simulations have been carried out to study the effects of material choice, drug eluting coating and cell design on the mechanical behaviour of stents during deployment inside a stenotic artery. Metallic stents made of materials with lower yield stress and weaker strain hardening tend to experience higher deformation and stronger dogboning and recoiling, but less residual stresses. Drug eluting coatings have limited effect on stent expansion, recoiling, dogboning and residual stresses. Stent expansion is mainly controlled by the radial stiffness of the stent which is closely associated with the stent design. In particular, open-cell design tends to have easier expansion and higher recoiling than closed-cell design. Dogboning is stronger for slotted tube design and open-cell sinusoidal design, but reduced significantly for designs strengthened with longitudinal connective struts. After deployment, the maximum von Mises stress appears to locate at the U-bends of stent cell struts, with varying magnitude that depends on the materials and severity of plastic deformation. For the artery–plaque system, the stresses, especially in the plaque which is in direct contact with the stent, appear to be distinctly different for different stent designs and materials in terms of both distribution and magnitude. The plaque composition also strongly affects the expansion behaviour of the stent–artery system and modifies the stresses on the plaque

Crack initiation and propagation in ductile specimens with notches: experimental and numerical study

© 2015 Springer-Verlag Wien Failures of components and structures are often related to the presence of notches of different shapes. Damage modelling techniques have been proven capable of modelling the crack initiation and propagation in ductile materials (such as Al alloys). The Gurson–Tvergaard–Needleman (GTN) method and extended finite-element method (XFEM) are compared against original experiments to study the crack initiation and propagation processes in aluminium specimens with different notch shapes (V-shape, U-shape and square). Two regimes are considered in this study: quasi-static and impact uniaxial tensile loading. Results show that the load-bearing capability predicted with the two methods is somewhat lower compared to experiments; still, the crack shapes were predicted correctly, with the exception of the square-notch case, for which XFEM was unable to predict the correct shape due to limitations in the model formulation. This study provides information useful for the design of components with stress raisers that are exposed to different loading regimes and shows limitations in both the GTN- and XFEM-based approaches that in many cases underestimate the load-bearing capacity

Computational analysis of mechanical stress-strain interaction of a bioresorbable scaffold with blood vessel

Crimping and deployment of bioresorbable polymeric scaffold, Absorb, were modelled using finite element method, in direct comparison with Co-Cr alloy drug eluting stent, Xience V. Absorb scaffold has an expansion rate lower than Xience V stent, with a less outer diameter achieved after balloon deflation. Due to the difference in design and material properties, Absorb also shows a higher recoiling than Xience V, which suggests that additional post-dilatation is required to achieve effective treatment for patients with calcified plaques and stiff vessels. However, Absorb scaffold induces significantly lower stresses on the artery-plaque system, which can be clinically beneficial. Eccentric plaque causes complications to stent deployment, especially non-uniform vessel expansion. Also the stress levels in the media and adventitia layers are considerably higher for the plaque with high eccentricity, for which the choice of stents, in terms of materials and designs, will be of paramount importance. Our results imply that the benefits of Absorb scaffolds are amplified in these cases