Simple geometric characteristics fail to reliably predict abdominal aorticaneurysm wall stresses

Purpose: The treatment of patients with abdominal aortic aneurysms (AAAs) i s typically based on the potential for rupture. Current rupture assessments are in turn based on statistics from aggregate populations and are incapab le of providing precise risk estimates for individual AAAs. Significant ben efit could be realized if rupture potential for individual AAAs could be re liably determined on the basis of simple geometric characteristics or the r esults of symmetric thin-shell analysis. This study seeks to determine whet her it is possible to estimate wall stresses by use of these simple measure s. Methods: Linear finite element analysis was used to estimate the distributi on of von Mises stresses in a series of homogeneous, isotropic, three-dimen sional AAA models subject to static loading and assumed to have zero residu al stresses. The magnitude of the peak stress was tabulated for each model along with the following characteristics: aneurysm volume; maximum diameter ; maximum radius; maximal wall distention; aspect ratio (ratio of greatest anteroposterior diameter to transverse diameter); local radii of curvature (in both longitudinal and circumferential directions); and maximum symmetri c thin-shell stress estimates (on the basis of the meridional contour). The relationship between peak stress and each of the characteristics was asses sed by use of Spearman rank correlation coefficients, with values less than 0.95 interpreted as signifying unreliable associations. Results: Peak stresses in the individual models ranged from 1.79 x 10(6) dy ne/cm(2) to 15.1 x 10(6) dyne/cm(2). The circumferential and longitudinal r adii of curvature were frequently able to predict the locations of high str ess, but were unreliable in predicting the magnitude of peak stress. The as pect ratio showed the strongest correlation with peak wall stress (r = 0.88 , 95% CI, 0.68-0.96), whereas the other characteristics showed even less co rrelation. Symmetric thin shell analysis accurately predicted stresses in a xially symmetric models, but it was incapable of predicting either the loca tion or magnitude of peak stress in asymmetric models. Conclusions: Simple geometric criteria and symmetric thin shell analyses ar e unreliable in predicting AAA stresses. Future attempts to estimate wall s tress and assess risk of rupture for individual AAAs may require detailed t hree-dimensional modeling.