Towards an improved rupture potential index for abdominal aortic aneurysms: Anisotropic constitutive modeling and noninvasive wall strength estimation

Abdominal aortic aneurysm (AAA), a localized dilation of the infrarenal aorta, represents a significant disease in the western population. The formation of aneurysm within the abdominal aorta presents a unique clinical dilemma, requiring surgeons to offer intervention when the risks of rupture outweigh those associated with the repairing the AAA. The gold standard for quantitatively assessing a AAAs risk of rupture is the maximum transverse diameter---with intervention typically recommended at a diameter of 5.5cm. This criterion, however, is not based on the sound physical properties governing the mechanical failure of the AAA wall---the stresses acting on the wall and the ability to withstand those stresses (its strength). The current work describes the continued improvement of a rupture potential index (RPI) which is defined as the ratio of local wall stress and strength.; The effect of mechanical anisotropy on the constitutive modeling and finite element analyses of AAA has been neglected in the literature. In order to address the assumption of isotropy, planar biaxial tensile testing was performed on AAA wall and intraluminal thrombus (ILT) tissue excised from patients undergoing elective open repair of their AAA. The peak stretch values and maximum tangential moduli for AAA versus nonaneurysmal tissue indicate a preferential circumferential stiffening of the abdominal aorta in the presence of aneurysm. It was concluded that aneurysmal degeneration of the abdominal aorta is associated with an increase in mechanical anisotropy, with preferential stiffening in the circumferential direction. This anisotropy was modeled using an exponential strain energy function which was able to minimize the covariance between model parameters. Implementation of this relation into a commercially available finite element code (ABAQUS) resulted in a more realistic estimation of in-vivo wall stress. There was a significant increase in peak wall stress in AAAs utilizing the anisotropic constitutive relation versus those using the previously derived isotropic relation (38.30 +/- 3.04, 36.06 +/- 2.73, p<0.001). This result was not universal, however, indicating the presence of anisotropy on peak wall stress may be patient-specific. (Abstract shortened by UMI.)...