MECHANICAL STRESS IN ABDOMINAL ANEURYSM: INFLUENCE OF MATERIAL ANISOTROPY AND UNLOAD CONFIGURATION

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• Dr. Jose F. Rodriguez, Mechanical Engineering Department. Aragon Institute of Engineering Research (I3A). University of Zaragoza (Spain).
• Friday 13 May 2011, 11:00-12:00
• Venue to be confirmed.

If you have a question about this talk, please contact Dr. Teng.

Biomechanical studies suggest that risk for rupture of an abdominal aortic aneurysm (AAA) is more precisely related to mechanical wall stress. In this regard, a reliable and accurate stress analysis of an in vivo AAA requires the use of suitable constitutive models. To date, all stress analysis conducted on AAA have considered the tissue as isotropic. However, recent biaxial tensile tests conducted on AAA tissue samples demonstrate the anisotropic nature of this tissue. In addition, the standard procedure for calculating wall stress is by means of the finite element analysis (FEA) on patientspecific AAA models generated from computed tomography (CT) images. However, these models presents the vasculature in pressurized form, so assuming it as unloaded model and applying a real physiological pressure might overestimate the maximum stress in the AAA . To solve this problem, we propose a methodology to derive the unloaded geometries of structural and fluid domain, ready to be used for subsequent FEA , CFD or fluid-structure interaction (FSI) studies. The purpose of this work is to study the influence of geometry and material anisotropy and the zero pressure configuration on the magnitude and distribution of the peak wall stress in AAAs. CT scans were obtained retrospectively from 6 AAA subjects (3 ruptured and 3 unruptured aneurysms) treated at Allegheny General Hospital in Pittsburgh, Pennsylvania. The corresponding DICOM images were imported into an in-house Matlab based image segmentation code (VESSEG v.1.0.1, Carnegie Mellon University), for the lumen, inner wall, and outer wall segmentations. Three-node shell elements (quadratic, with 5 integration points through the thickness) were used for meshing the arterial wall, while ten-node tetrahedral elements (quadratic, with 4 integration points) were used to mesh the thrombus. A comparison between ruptured and unruptured wall mechanics on the basis of the anisotropic material model yields mean peak wall stresses (in kPa) of 977.9 ± 179.6 and 702.6 ± 166.2, for the ruptured and unruptured geometries, respectively. The anisotropic characteristics of the AAA wall, previously verified by means of biaxial tensile testing of AAA tissue specimens, also play a role in the distribution of wall stress yielding higher peak wall stresses that likely influence the assessment of rupture potential in individual AAAs. We finally compare the stress results obtained from CT image based geometry and unloaded geometry to understand the effect of the modeling assumption.

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