Fluid-structure interaction in abdominal aortic aneurysms: effects of asymmetry and wall thickness

Abstract

BackgroundAbdominal aortic aneurysm (AAA) is a prevalent disease which is of significant concern because of the morbidity associated with the continuing expansion of the abdominal aorta and its ultimate rupture. The transient interaction between blood flow and the wall contributes to wall stress which, if it exceeds the failure strength of the dilated arterial wall, will lead to aneurysm rupture. Utilizing a computational approach, the biomechanical environment of virtual AAAs can be evaluated to study the affects of asymmetry and wall thickness on this stress, two parameters that contribute to increased risk of aneurysm rupture.MethodsTen virtual aneurysm models were created with five different asymmetry parameters ranging from β = 0.2 to 1.0 and either a uniform or variable wall thickness to study the flow and wall dynamics by means of fully coupled fluid-structure interaction (FSI) analyses. The AAA wall was designed to have a (i) uniform 1.5 mm thickness or (ii) variable thickness ranging from 0.5 – 1.5 mm extruded normally from the boundary surface of the lumen. These models were meshed with linear hexahedral elements, imported into a commercial finite element code and analyzed under transient flow conditions. The method proposed was then compared with traditional computational solid stress techniques on the basis of peak wall stress predictions and cost of computational effort.ResultsThe results provide quantitative predictions of flow patterns and wall mechanics as well as the effects of aneurysm asymmetry and wall thickness heterogeneity on the estimation of peak wall stress. These parameters affect the magnitude and distribution of Von Mises stresses; varying wall thickness increases the maximum Von Mises stress by 4 times its uniform thickness counterpart. A pre-peak systole retrograde flow was observed in the AAA sac for all models, which is due to the elastic energy stored in the compliant arterial wall and the expansion force of the artery during systole.ConclusionBoth wall thickness and geometry asymmetry affect the stress exhibited by a virtual AAA. Our results suggest that an asymmetric AAA with regional variations in wall thickness would be exposed to higher mechanical stresses and an increased risk of rupture than a more fusiform AAA with uniform wall thickness. Therefore, it is important to accurately reproduce vessel geometry and wall thickness in computational predictions of AAA biomechanics.

Three-band decomposition analysis in multiscale FSI models of abdominal aortic aneurysms

“…The results con¯rm that the healthy case reproduces a diameter change of about 2 mm in good agreement with clinical observations. 36 In the pathological scenarios, we recover radial displacements in agreement with the numerical results illustrated by Scotti et al 37 We carried out a model tuning also in terms of arterial wall stresses. Figure 5 shows the von Mises stress distributions for the di®erent models simulated.…”

Carotid Atheroma Rupture Observed In Vivo and FSI-Predicted Stress Distribution Based on Pre-rupture Imaging

“…(2) and material formulations discussed above, were solved over a cardiac cycle using the commercial finite element code ADINA (ADINDA R&D, Watertown, MA). ADINA has been used extensively to model systems exhibiting material and geometric nonlinearity, and has been successfully employed to solve arterial FSI problems 20,39,46,50,51. We use ADINA to solve the strongly coupled nonlinear fluid and solid systems of equations in an iterative manner using in-core direct sparse solvers.…”

The Role of Geometric and Biomechanical Factors in Abdominal Aortic Aneurysm Rupture Risk Assessment

“…However, in their work, the wall and ILT were considered linearly elastic materials. The nonlinear behavior of the wall and ILT as well as more complex pulsatile flow conditions were modeled in a series of studies conducted by Scotti et al, 74–76 which demonstrated the importance of considering the nonlinear elastic behavior of the structural domain. Moreover, the comparative study between FSI (coupled and decoupled) and structural analysis of patient-specific AAA performed by Scotti et al 75 show that the non-uniform pressure distribution on the inner AA surface due to the flow yielded a maximum peak wall stress up to 20% higher compared to that obtained with a static FEA when a uniform systolic pressure of 117 mmHg is applied.…”