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. There are approximately 200,000 patients in the US and 500,000 patients worldwide diagnosed with AAAs every year, and rupture of AAAs currently ranks as the 13th leading cause of death in the US. 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 golden 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 the 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. Previous work in our laboratory resulted in an initial statistical model for noninvasively estimating AAA wall strength. This model has currently been improved with several notable enhancements some of which include a larger construction data set and a CT-based method of local diameter measurement. This model contains four, non-invasively measurable predictors: the square root of local ILT thickness, normalized local diameter, patient's sex, and the patient's family history of AAA. The noninvasive statistical model for predicting AAA wall strength derived here predicted a statistically weaker wall for ruptured AAAs than for non-ruptured AAAs (119.41 ± 4.48 and 137.06 ± 1.49 N/cm2, p=0.02). In fact, the current model performed better than either the previously derived AAA wall strength model or the clinically utilized maximum cross sectional diameter in identifying ruptured AAAs. The currently developed rupture potential index resulted in an increased peak value of RPI for a set of electively repaired AAAs in comparison to the previously developed RPI technique (0.34 ± 0.03 vs. 0.22 ± 0.03, p < 0.001). In addition, comparisons of peak RPI values for ruptured and non-ruptured AAAs suggest an improvement in rupture prediction utilizing the current methodology (p=0.10) as opposed to the previously developed RPI (p=0.79) as well as the maximum diameter criterion (p=0.17). The locally acting AAA wall stress divided by the local AAA wall strength, termed the rupture potential index, has been introduced as an alternative to the maximum diameter criterion for AAA rupture assessment. The clinical relevance of this method for rupture assessment has yet to be validated, however its success will undoubtedly aid surgeons in clinical decision making and AAA patient management.