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.