Conception et modélisation par la méthode des éléments finis d'une prothèse endovasculaire en alliage à mémoire de forme muni d'un dispositif de déploiement progressif

Thériault, Philippe (2004). Conception et modélisation par la méthode des éléments finis d'une prothèse endovasculaire en alliage à mémoire de forme muni d'un dispositif de déploiement progressif. Mémoire de maîtrise électronique, Montréal, École de technologie supérieure.

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Résumé

La maladie engendrant la majorité des infarctus s'appelle la sténose. Il s'agit d'un rétrécissement de la lumière d'une artère vasculaire. Un des traitements consiste à installer une prothèse endovasculaire (stent). Il permet la réouverture du diamètre d'artères partiellement obstruées. Cependant, la principale limitation de cette technologie est la resténose. La structure de la prothèse pénètre dans les parois de l'artère et la blesse. Il y a alors une prolifération cellulaire et une formation de tissu cicatriciel. Cette réaction au traumatisme subit par l'artère referme la lumière du conduit. Ce projet propose l'étude, par la méthode des éléments finis, d'un stent auto-déployant en Nitinol avec un système, en polyéthylène, d'expansion progressif dans le temps par fluage. Ce mécanisme permet un contact lent entre la prothèse et l'artère, diminuant le traumatisme. Le but de ce mémoire est de développer la technologie et la méthodologie pour l'analyse numérique de ce type de prothèses.

Titre traduit

Finite elements modeling of a progressive expanding shape memory stent

Résumé traduit

In North America, diseases of the circulatory system are the most important cause of mortality. More precisely, 35% of Canadians and 30% of Americans die each year due to a heart related illness [1] [2]. The affliction that affects the majority of infarction is called stenosis. It is characterised by the shortening of the lumen (the transversal section of the artery) of the vascular artery that nourishes the heart with blood. During physical activity, the myocardium (muscle composing the heart) lacks oxygen and dies rapidly without its vital support. The observed result is a heart attack.

Luckily, numerous treatments exist to cure stenosis. Since the 80s, one particularly promising treatment has been developed and has gained popularity amongst cardiac surgeons. This treatment involves the use of an endovascular prosthesis commonly referred to as a stent. A stent is a permanent metallic device that resembles a cylindrical grid. It permits the partially obstructed diameter of an artery to be reopened. As well as its low cost relative to other treatments, the procedure is non-invasive to the patient. In general, the patient can return to regular activities in the days following the intervention. However, like all new technologies, this treatment presents complications, namely restenosis. This involves the reclosing of the lumen after angioplasty or the insertion of a stent. The structure of the prosthesis penetrates into the walls of the patient's artery. The inflicted unsuccessfully attempts to heal. There is a proliferation of cells and the formation of scar tissue, similar to scarring of other organic tissue. This reaction to the trauma subjects the artery to closing of the lumen.

This project entitled "Finite elements modeling of a progressive expanding shape memory stent" proposes the study of a self-expanding stent made from Nitinol with a progressive expansion system in time by creep effect of polyethylene. This mechanism allows a gradual contact between the prosthesis and the artery. This allows the latter/artery the opportunity to adapt to the presence of a foreign object. The goal of this thesis is to develop the technology and the methodology for the numerical analysis of
these types of prosthesis.

After the analysis of the problem and the bibliographical review, the first step is to characterize the materials in order to obtain the necessary properties for modeling with finite element method. This numerical technique allows the quick and efficient study of complex geometric structures and the evaluation of materials exhibiting strong nonlinear properties. The second step consists of choosing and validating a law that represents as faithfully as possible the desired behaviour of the material. The superelasticity of Nitinol is represented through a multilinear elasticity law. This results in underestimating the stress involved upon loading (application of pressure/charging) and overestimating when unloading. The mechanical behaviour of polyethylene is represented by a multilinear isotropic hardening law using von Mises plasticity. This approximation is valid up to a certain level of strain, above which the material becomes highly anisotropic and the hypothesis results in errors. As for the creep of this material, a time hardening law is chosen for its simplicity. Finally, each of the modelled properties is validated through comparison of experimental analysis with its numerically reproduced counterpart. All simulations have been completed with ANSYS® software.

The simulations can be divided into two steps. The first simulation determines the geometry of the stent prior to the thermal treatment that renders its new geometry and superelastic properties. The second simulation examines the behaviour of the prosthesis during surgery and the four weeks following the intervention. The results demonstrate that a robust restraint device allows the positioning of the contracted stent to be maintained while being expelled from the positioning mechanism, but involve a bad deployment in terms of creep. ln contrast, a more delicate device does not retain the contracted stent as well, but offers an interesting deployment by creep. A better material than polyethylene should be found. This material should retain the stent at is contracted position and provide a better deployment by creep effect.

Type de document:	Mémoire ou thèse (Mémoire de maîtrise électronique)
Renseignements supplémentaires:	"Mémoire présenté à l'École de technologie supérieure comme exigence partielle à l'obtention de la maîtrise en génie mécanique". Bibliogr.: f. [202]-207.
Mots-clés libres:	Alliage, Cardiovasculaire, Comportement, Conception, Deploiement, Dispositif, Element, Endovasculaire, Fini, Fluage, Forme, Mecanique, Memoire, Methode, Modelisation, Polymere, Progressif, Propriete, Prothese, Semi-Cristallin, Stent, Superelastique
Directeur de mémoire/thèse:	Directeur de mémoire/thèse Terriault, Patrick
Programme:	Maîtrise en ingénierie > Génie mécanique
Date de dépôt:	04 mai 2011 18:56
Dernière modification:	22 oct. 2016 00:45
URI:	https://espace.etsmtl.ca/id/eprint/689

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