title: Importance of material model in wall stress prediction in abdominal aortic aneurysms.
abstract: BACKGROUND
  Results of biomechanical simulation of the abdominal aortic aneurysm (AAA) depend on the constitutive description of the wall. Based on in vitro and in vivo experimental data several constitutive models for the AAA wall have been proposed in the literature. Those models differ strongly from each other and their impact on the computed stress in biomechanical simulation is not clearly understood.
  
  
  METHODS
  Finite element (FE) models of AAAs from 7 patients who underwent elective surgical repair were used to compute wall stresses. AAA geometry was reconstructed from CT angiography (CT-A) data and patient-specific (PS) constitutive descriptions of the wall were derived from planar biaxial testing of anterior wall tissue samples. In total 28 FE models were used, where the wall was described by either patient-specific or previously reported study-average properties. This data was derived from either uniaxial or biaxial in vitro testing. Computed wall stress fields were compared on node-by-node basis.
  
  
  RESULTS
  Different constitutive models for the AAA wall cause significantly different predictions of wall stress. While study-average data from biaxial testing gives globally the same stress field as the patient-specific wall properties, the material model based on uniaxial test data overestimates the wall stress on average by 30 kPa or about 67% of the mean stress. A quasi-linear description based on the in vivo measured distensibility of the AAA wall leads to a completely altered stress field and overestimates the wall stress by about 75 kPa or about 167% of the mean stress.
  
  
  CONCLUSIONS
  The present study demonstrated that the constitutive description of the wall is crucial for AAA wall stress prediction. Consequently, results obtained using different models should not be mutually compared unless different stress gradients across the wall are not taken into account. Highly nonlinear material models should be preferred when the response of AAA to increased blood pressure is investigated, while the quasi-linear model with high initial stiffness produces negligible stress gradients across the wall and thus, it is more appropriate when response to mean blood pressure is calculated.
authors: Institute of Solid Mechanics, Mechatronics and Biomechanics, Brno University of Technology, Czech Republic. polzer@seznam.cz
authors: Stanislav
authors: S
authors: Polzer
authors: T Christian
authors: TC
authors: Gasser
authors: Jiri
authors: J
authors: Bursa
authors: Robert
authors: R
authors: Staffa
authors: Robert
authors: R
authors: Vlachovsky
authors: Vojtech
authors: V
authors: Man
authors: Pavel
authors: P
authors: Skacel
mesh_terms: D001012:Aorta, Abdominal; D017544:Aortic Aneurysm, Abdominal; D001696:Biomechanical Phenomena; D020342:Finite Element Analysis; D006801:Humans; D008297:Male; D013314:Stress, Mechanical
keywords: Abdominal aneurysm; FE analysis; Material model; Pre-stressing; Wall stress