Improving the Efficiency of Abdominal Aortic Aneurysm Wall Stress Computations

Abstract

An abdominal aortic aneurysm is a pathological dilation of the abdominal aorta, which carries a high mortality rate if ruptured. The most commonly used surrogate marker of rupture risk is the maximal transverse diameter of the aneurysm. More recent studies suggest that wall stress from models of patient-specific aneurysm geometries extracted, for instance, from computed tomography images may be a more accurate predictor of rupture risk and an important factor in AAA size progression. However, quantification of wall stress is typically computationally intensive and time-consuming, mainly due to the nonlinear mechanical behavior of the abdominal aortic aneurysm walls. These difficulties have limited the potential of computational models in clinical practice. To facilitate computation of wall stresses, we propose to use a linear approach that ensures equilibrium of wall stresses in the aneurysms. This proposed linear model approach is easy to implement and eliminates the burden of nonlinear computations. To assess the accuracy of our proposed approach to compute wall stresses, results from idealized and patient-specific model simulations were compared to those obtained using conventional approaches and to those of a hypothetical, reference abdominal aortic aneurysm model. For the reference model, wall mechanical properties and the initial unloaded and unstressed configuration were assumed to be known, and the resulting wall stresses were used as reference for comparison. Our proposed linear approach accurately approximates wall stresses for varying model geometries and wall material properties. Our findings suggest that the proposed linear approach could be used as an effective, efficient, easy-to-use clinical tool to estimate patient-specific wall stresses.

Aortic outflow occlusion predicts rupture of abdominal aortic aneurysm

“…Our group has previously published our protocol using a linearized formulation to determine PWS using systolic blood pressure. 5 Despite these limitations, we believe that AOO and PAD are reliable predictors of increased PWS and along with COPD may also be associated with decreased aortic wall strength. Together, these features can be used to identify patients with high-risk AAAs unsafe for surveillance.…”

“…A linear model previously published by our group using smallstrain theory was used to describe both the intraluminal thrombus and arterial wall properties. 5 Both ends of the AAA geometry (ie, renal and aortic bifurcations) were fixed to simulate tethering to the arterial system. PWS was calculated as the maximum von Mises wall stress (away from the model inlet and outlet to avoid boundary effects) from the FEA model.…”

Intraluminal thrombus is associated with early rupture of abdominal aortic aneurysm

“…A recently developed linear model using small-strain theory was used to approximate wall stresses in the deformed (CTA scan) configuration. 17 The proximal and distal ends of our FEA models were immobilized in space to simulate tethering and attachment to the arterial system. Blood pressure was applied normal to the lumen-ILT interface as a constant boundary condition.…”

The Relationship Between Surface Curvature and Abdominal Aortic Aneurysm Wall Stress

“…While this represents a limitation of the present work, future improvements in clinical imaging technology and segmentation algorithms are expected to mitigate such shortcomings. Some simplifications were made in the present study, which could limit the findings of the work to other AAA population groups: (1) although AAA tissue exhibits anisotropic behavior [32], the isotropic nonlinear model postulated by Raghavan and Vorp [27] was implemented in the FEA models; (2) thrombus and calcifications are usually present in 75% of AAAs [1,33], but were not included in the FEA models; (3) the initial stresses due to the diastolic pressure acting on the AAA wall during clinical imaging were not considered; and (4) the residual stresses were not modeled, although they should have reduced the stress gradient across the arterial wall [34], and prestretch conditions [35] were also not modeled. It should be mentioned that the purpose of this work is not to obtain the most accurate wall stress predictive model, but to analyze the impact of the local mean curvature on the wall stress.…”