Heterogeneous, Variable Wall-Thickness Modeling of a Ruptured Abdominal Aortic Aneurysm

Pressure-induced mechanical stress in ruptured abdominal aortic aneurysms (AAA) was investigated using a finite element model with measured variations in wall thickness and material properties. We harvested an 8-cm ruptured AAA from a cadaver and recorded its geometry on bi-plane photographs and three-dimensionally reconstructed. The wall thickness was measured at over 100 sites on the aneurysm surface using digital calipers. Regional mechanical property variation (failure strengths and hyperelastic material parameters) was determined using data from uniaxial extension tests of 19 test strips cut from various regions of the AAA surface. The measured data for wall thickness and mechanical properties were transferred to the 3D AAA surface model and nonlinearly interpolated on the surface to obtain the point-to-point regional distribution for thickness and material parameters. Modeling the AAA as a thick shell, finite element stress analysis was performed. The peak stress was found to be exactly at the rupture site and was substantially higher than population averages. The measured low wall thickness at the rupture site appeared to play a major role in elevating stresses indicating that localized thin wall may be an important risk factor for AAA rupture.