Investigation of aneurysm formation: a macro-structural approach
Student thesis: Doctoral Thesis
Among the cardiovascular diseases, the leading cause of death in Europe, is related to the aortic aneurysm. Despite several studies focus on its rupture risk, limited attention is reserved to aneurysm formation. The main purpose of this research study is to assess, from the macro-structural point of view, whether an aneurysm may be instigated in healthy model of aorta inflated by a supra-physiological pressure. In addition, a physiologic systolic pressure is applied to estimate the overall arterial response. The mechanical characterization of the healthy descending aorta is accomplished by means of uniaxial and equi-biaxial tensile tests performed on porcine tissue. Since no standard experimental protocol is available for soft tissues, a complete methodology based on up-to-date techniques is detailed. Several computational analyses have been performed to highlight the impact on numerical predictions of the pre-conditioning frequency,the sample shape and the sample thickness measurement. Furthermore, the fitting of experimental responses appears definitely crucial for numerical predictions. Several strain-energy functions are compared as part of the isotropic modelling based on uniaxial mechanical responses. In addition, a Fung orthotropic FE model is generated based one qui-biaxial mechanical behaviour. A cylindrical and a more physiologic FE geometry are compared, emphasizing how a different design leads to consistent differences in wall stress and radial stretch predictions. This study suggests that aneurysm formation is unlikely to be caused by supra-physiological pressure in a healthy model of descending aorta. Isotropic models predict the bulge appearance in the abdominal district only for selected mechanical responses. However, a localized geometric imperfection could lead to aneurysm formation in the thoracic district.
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