Analysis of growth and rupture of fusiform abdominal aortic aneurysms

An aneurysm is, in general terms, a stretching, dilatation and bulging of an artery that can occur as a result of weakening of the arterial wall and loss of its inherent elastic properties. From an engineering perspective, the development of an arterial aneurysm is a classic case of material failure that may involve two factors: excessive applied load and inadequate material strength. The risk associated with an Abdominal Aortic Aneurysm (AAA) is high: if rupture of the aneurysm occurs, the chance of survival can be as low as 10%. Many research groups have attempted to understand the mechanisms involved in aneurysm growth and rupture so as to propose a model that describes and predicts the rupture potential of AAAs. To date their efforts have proved unsuccessful, and it is generally agreed that prediction of AAA growth and rupture is not possible given the current level of knowledge. The aim of this research programme was to develop both an analytical and numerical model which could determine the wall stress of fusiform AAAs for given physiological parameters, in order to assess the risk of rupture. A novel analytical model has been developed, which employs the Law of LaPlace and Bernoulli's equation for steady fluid flow, to predict the wall stress of fusiform AAAs. This model fundamentally assumes a constant volume wall and representative blood properties and flow conditions. Numerical models of fusiform AAAs have also been developed, using computational engineering tools such as Computational Fluid Dynamics (CFD) and Finite Element Analysis (FEA). A sequential coupled-field analysis approach was adopted to simulate the fluid- structural problem of blood flow and vascular wall stress in fusiform AAAs. The structural wall model incorporated clinically determined mechanical properties from uniaxial tensile tests of aneurysmal tissue. Comparison of the results obtained for wall stress in fusiform AAAs determined by the analytical and numerical models have shown good agreement, even though the model formulations are significantly different. Moreover, evaluation of wall stress results from both the analytical and numerical models with clinical data of aneurysmal ultimate tensile strength, has shown that it is possible to predict the diameter at which a fusiform AAA will rupture for given parameters. The numerical results predict that the aneurysm will rupture at approximately 85mm diameter, which is the size at which the clinically determined ultimate tensile strength value of approximately 1x106 N/m2 is exceeded. Further comparison with data of ruptured AAAs obtained from clinical studies has also shown very good general clinical agreement. In conclusion, an analytical and a numerical model have been proposed which are able to predict the diameter at which rupture of a fusiform abdominal aortic aneurysm may occur.