Analysis of the mechanical performance of a biodegradable magnesium stent in a remodelling artery
Boland, Enda L.
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Coronary stents made from degradable biomaterials such as magnesium alloy are an emerging technology in the treatment of coronary artery disease. Biodegradable stents provide mechanical support to the artery during the initial scaffolding period after which the artery will have remodelled. The subsequent resorption of the stent biomaterial by the body has potential to reduce the risk associated with long-term placement of these devices, such as in-stent restenosis, late stent thrombosis, and fatigue fracture. Computational modelling such as finite-element analysis has proven to be an extremely useful tool in the continued design and development of these medical devices. What is lacking in computational modelling literature is the representation of the active response of the arterial tissue in the weeks and months following stent implantation, i.e., neointimal remodelling. The phenomenon of neointimal remodelling is particularly interesting and significant in the case of biodegradable stents, when both stent degradation and neointimal remodelling can occur simultaneously, presenting the possibility of a mechanical interaction and transfer of load between the degrading stent and the remodelling artery. A computational modelling framework is developed that combines magnesium alloy degradation and neointimal remodelling, which is capable of simulating both uniform (best case) and localised pitting (realistic) stent corrosion in a remodelling artery. The framework is used to evaluate the effects of the neointima on the mechanics of the stent, when the stent is undergoing uniform or pitting corrosion, and to assess the effects of the neointimal formation rate relative to the overall stent degradation rate (for both uniform and pitting conditions). Experimental mechanical and corrosion testing is conducted to characterise the mechanical and corrosion behaviour of magnesium WE43 alloy, a candidate base material for biodegradable magnesium stents. Previously developed uniform and pitting corrosion models are calibrated based on in vitro mechanical and corrosion testing of magnesium WE43 alloy specimens. The calibrated pitting corrosion model can capture the mechanical and corrosion behaviour of magnesium WE43, including the vii experimentally observed non-linear reduction in failure strength with mass loss, whereas the uniform corrosion model is incapable of capturing this trend. An enhanced computational modelling framework is developed, building on the previous investigations, that combines magnesium alloy degradation and neointimal remodelling in order to simulate corrosion of a magnesium stent in a remodelling artery. The enhanced computational modelling framework, accounts for two major physiological stimuli responsible for neointimal remodelling and is combined with the magnesium stent pitting corrosion model, which has been calibrated for Mg WE43 alloy. The enhanced modelling framework is used to simulate different neointimal growth patterns and to explore the effects the neointimal remodelling has on the mechanical performance (scaffolding support) of a bioabsorbable magnesium stent. In conclusion, the work performed in this thesis utilising computational modelling and experimental corrosion testing has led to an enhanced understanding of the mechanical performance of biodegradable magnesium stents in a remodelling artery.
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