Advanced computer modeling of abdominal aortic aneurysms to predict risk of rupture

An abdominal aortic aneurysm (AAA) is an abnormal enlargement of the aorta which is related to weakness of the vessel wall (associated with degradation of connective tissue), and if left untreated will lead to rupture and cause death in 78% to 94% of cases. Approximately 7,000 deaths each year in the United Kingdom are caused by AAA rupture.AAA repair requires surgical intervention but the surgery itself has a mortality rate of about 5% in patients with stable AAA. The decision to undertake the surgery is made depending on the aortic maximum diameter of ≥5 cm. However, it is observed that rupture sometimes occurs in aneurysms with smaller diameter, thereby creating the need for better criteria for surgical intervention. Therefore, (biomechanical) indicators of AAA rupture were introduced as a superior criterion to the maximum diameter for predicting the risk AAA rupture. Several studies that have been conducted on abdominal aortic aneurysms have suggested that peak wall stress may be a more reliable predictor of the risk of AAA rupture.This thesis is a continuation of a previous study undertaken at the University of Hull which investigated a number of biomechanical factors that affect the AAA wall stress magnitude and distribution. Novel results were gained which may help in the understanding of AAA growth and rupture events. For the first time, it is proposed that aspect ratio has an effect on the stress magnitude, location and distribution of the outer wall of AAA. These findings were used to introduce an empirical relationship between the aneurysm aspect ratio and maximum wall stress. This empirical relationship could be used as an additional clinical indicator to predict the location and magnitude of maximum wall stress where a rupture may develop.Analysis of the porosity of the thrombus was introduced for the first time in this work using the simulation of mass transport of blood flow in an AAA, showing novel results for the possible role of blood flow on the site of growth and rupture for AAA. Furthermore, the results of this research may also explain the conflicting views on aneurysm shape and the role of the thrombus as previously reported in the literature.The work carried out in this research used simplified AAA geometries to allow the isolation of specific aneurysm parameters. Clearly, the next stage would include the application of the ideas and results developed here to more complex patient-specific geometries