Abstract:
"Towards an Improved Rupture Potential Index for Abdominal Aortic Aneurysms: Anisotropic Constitutive Modeling and Noninvasive Wall Strength Estimation"
This dissertation details the continued development of a rupture potential index (RPI) for abdominal aortic aneurysms (AAA) as a biomechanics-based alternative to the standard clinical criterion of maximum transverse diameter. The RPI is defined as the ratio of local wall stress to local wall strength. The study addresses two primary areas of improvement: more accurate stress estimation through anisotropic constitutive modeling and a more robust, noninvasive method for wall strength estimation. Biaxial tensile testing on excised AAA wall tissue and intraluminal thrombus (ILT) revealed that aneurysmal degeneration is associated with a significant increase in mechanical anisotropy, particularly with preferential stiffening in the circumferential direction. This anisotropic behavior was modeled using a new exponential strain energy function and implemented into a patient-specific finite element (FE) code (ABAQUS). The results showed a significant increase in peak wall stress when using the anisotropic model compared to a previously used isotropic model. The study also improved an existing statistical model for noninvasively estimating AAA wall strength, utilizing a larger data set and more reliable CT-based measurements. This new model successfully predicted a statistically weaker wall for ruptured AAAs compared to non-ruptured ones, outperforming both the previous strength model and the maximum diameter criterion. The final RPI, combining the improved stress and strength estimations, demonstrated enhanced capability in identifying AAAs at high risk of rupture. This work aims to provide surgeons with a more reliable, biomechanically-sound tool for clinical decision-making and patient management.
