Influence of the Local Curvature on the Abdominal Aortic Aneurysm Wall Stress and New Methodologies for Manufacturing Realistic Phantoms

Abstract

An abdominal aortic aneurysm (AAA) is a focal dilation of the abdominal aorta that is at least 1.5 times its normal diameter. AAA rupture causes around 1.3% of deaths in developed countries among men aged 65-85. In clinical practice, uncertainty still remains about the correct time to operate, but the criterion of maximum diameter is commonly accepted as a rupture prediction factor. The general consensus is that patients with AAA diameters bigger than 5 cm warrant elective repair if they are reasonable operative candidates. However, the failure rate of this criterion is high, ranging from 10% to 25% of cases: 13% of aneurysms with a maximum diameter under 50 mm ruptured, while 60% of aneurysms with diameters over 50 mm remained intact. Several numerical studies have attempted to find new parameters that can help physicians estimate AAA rupture risk. In contrast, few experimental studies have been carried out due to the cost and time-consuming nature of manufacturing patient-specific AAA replicas. This thesis comprises both numerical and experimental studies. The numerical approach investigates new geometric parameters that influence AAA wall stress distribution, while the experimental approach studies new methodologies for manufacturing AAA replicas. In the numerical studies, 30 patient-specific AAA geometries were analyzed, and it was found that the local curvature significantly affects the wall stress distribution, which in turn affects the risk of AAA rupture. The results suggest that considering this parameter in future AAA rupture estimations can assist in clinical decision-making. In terms of the experimental studies, a new methodology for manufacturing more realistic AAA phantoms was developed via vacuum casting technique. This new methodology considers the regionally varying wall thickness and the anisotropic behavior of the AAA. Additionally, the multi-material additive manufacturing technology has been used to fabricate idealized AAA phantoms with anisotropic properties. The results of uniaxial and biaxial tests verify the suitability of both methodologies in manufacturing AAA replicas with properties similar to AAA tissue. Finally, an experimental study was run on the fabricated AAA phantoms and the AAA wall stress distribution was verified. This study was carried out at the University of Texas at San Antonio (UTSA) in collaboration with Dr. Ender Finol