Does elevated wall tension cause aortic aneurysm rupture? Investigation using a subject-specific heterogeneous model

ObjectiveTo investigate whether peak wall tension in abdominal aortic aneurysm occurs at the site of rupture to test for a causative relationship.MethodsFour ruptured and nine unruptured AAA were harvested whole from cadavers, followed by regional measurements of wall thickness, elastic parameters and failure tension. Finite element models were developed with subject-specific load-free AAA morphology and heterogeneous properties interpolated using a geodesic distance weighted approach from the measurements. The wall tension under uniform pressure and tension to failure tension ratio as an index of susceptibility to rupture were computed. As a secondary aim, the peak wall tension using this heterogeneous model approach was compared to the traditional homogeneous model approach in order to evaluate the reliability of the latter.ResultsThe average peak wall tension in the ruptured group was 43% higher than in the unruptured group without statistical significance even though it was 54% larger in diameter. The site of peak wall tension was in the vicinity of the site of rupture in two ruptured AAA. The peak tension did not breach failure tension at the rupture site in any of the AAA. The traditional population-wide homogeneous model approach overestimated peak wall tension by just 3% compared to the subject-specific heterogeneous model approach.ConclusionWe failed to find adequate evidence of a causative relationship between peak wall tension and AAA rupture. The findings are not conclusive owing to study limitations such as ignoring intraluminal thrombus, sparse distribution of specimens procured and small study population.     

Improving the Efficiency of Abdominal Aortic Aneurysm Wall Stress Computations

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.     

Regional distribution of wall thickness and failure properties of human abdominal aortic aneurysm

The regional distribution of wall thickness and failure properties in human abdominal aortic aneurysm (AAA) was explored. Three unruptured and one ruptured AAA were harvested as a whole during necropsy. Thickness was measured at about every 1.5 cm² wall surface area for an average of 100 measurement sites per AAA. Multiple longitudinally oriented rectangular specimen strips were cut at various locations from each AAA for a total of 48 strips. The strips were subjected to uniaxial extension until failure. Wall thickness varied regionally and between AAA from as low as 0.23 mm at a rupture site to 4.26 mm at a calcified site (median=1.48 mm). Wall thickness was slightly lower in the posterior and right regions. The failure tension (ultimate) of specimen strips varied regionally and between AAA from 5.5 N/cm close to a blister site in the ruptured AAA to 42.3 N/cm at the undilated neck of a 4 cm diameter unruptured AAA (median=14.8 N/cm). Failure stress (ultimate) varied from 33.6 to 235.1 N/cm² (median=126.6 N/cm²). There was no perceptible pattern in failure properties along the circumference. Failure tension of specimen strips at or close to blisters was mostly low. The rupture site in the ruptured aneurysm had the lowest recorded wall thickness of 0.23 mm with only slightly higher readings within a 1 cm radius. The failure tension of the specimen strip close to the rupture site was low (11.1 N/cm) compared to its neighborhood in the ruptured aneurysm.     

Mechanical stresses in abdominal aortic aneurysms: influence of diameter, asymmetry, and material anisotropy

Biomechanical studies suggest that one determinant of abdominal aortic aneurysm (AAA) rupture is related to the stress in the wall. In this regard, a reliable and accurate stress analysis of an in vivo AAA requires a suitable 3D constitutive model. To date, stress analysis conducted on AAA is mainly driven by isotropic tissue models. However, recent biaxial tensile tests performed on AAA tissue samples demonstrate the anisotropic nature of this tissue. The purpose of this work is to study the influence of geometry and material anisotropy on the magnitude and distribution of the peak wall stress in AAAs. Three-dimensional computer models of symmetric and asymmetric AAAs were generated in which the maximum diameter and length of the aneurysm were individually controlled. A five parameter exponential type structural strain-energy function was used to model the anisotropic behavior of the AAA tissue. The anisotropy is determined by the orientation of the collagen fibers (one parameter of the model). The results suggest that shorter aneurysms are more critical when asymmetries are present. They show a strong influence of the material anisotropy on the magnitude and distribution of the peak stress. Results confirm that the relative aneurysm length and the degree of aneurysmal asymmetry should be considered in a rupture risk decision criterion for AAAs.     

The association of wall mechanics and morphology: a case study of abdominal aortic aneurysm growth

The purpose of this study is to evaluate the potential correlation between peak wall stress (PWS) and abdominal aortic aneurysm (AAA) morphology and how it relates to aneurysm rupture potential. Using in-house segmentation and meshing software, six 3-dimensional (3D) AAA models from a single patient followed for 28 months were generated for finite element analysis. For the AAA wall, both isotropic and anisotropic materials were used, while an isotropic material was used for the intraluminal thrombus (ILT). These models were also used to calculate 36 geometric indices characteristic of the aneurysm morphology. Using least squares regression, seven significant geometric features (p?相似文献   

Numerical identification of the rupture locations in patient-specific abdominal aortic aneurysmsusing hemodynamic parameters

The rupture of an abdominal aortic aneurysm (AAA) is generally an unexpected event. Up to now, there is no agreement on an accurate criteria to predict the rupture risk of AAAs. This paper aims to numerically investigate the hemodynamics of three ruptured and one non-ruptured patient-specific AAA models to correlate local hemodynamic parameters with the rupture sites, and for the first time, this study introduced helicity as a potential index for the rupture potential of AAAs.3D reconstructions from CT scans were done. The simulation revealed that all the rupture sites were in regions of stagnation with near zero wall shear stress (WSS) but large WSS gradient (WSSG), which may explain the observation by the former researchers that the rupture site in the ruptured AAA has the lowest recorded wall thickness compared to other non-ruptured regions. Moreover, all the ruptures occurred at regions of zero helicity which represents a purely axial or circumferential flow. In addition, this study revealed that the double low region for the non-ruptured AAA was present with a thick layer of plaques, it suggests that the AAA rupture and the formation of atherosclerotic plaques may share a lot common physiological features. However, the fact that there are no plaques present in the walls of three RAAAs also indicates that AAA is not always a result of atherosclerosis. The current computational study may complement the maximum diameter, peak wall stress and other clinically relevant factors in AAA ruptures to identify the rupture sites of AAAs.     

Fluid structure interaction of patient specific abdominal aortic aneurysms: a comparison with solid stress models

Background  

Abdominal aortic aneurysm (AAA) is a dilatation of the aortic wall, which can rupture, if left untreated. Previous work has shown that, maximum diameter is not a reliable determinant of AAA rupture. However, it is currently the most widely accepted indicator. Wall stress may be a better indicator and promising patient specific results from structural models using static pressure, have been published. Since flow and pressure inside AAA are non-uniform, the dynamic interaction between the pulsatile flow and wall may influence the predicted wall stress. The purpose of the present study was to compare static and dynamic wall stress analysis of patient specific AAAs.     

Identification of rupture locations in patient-specific abdominal aortic aneurysms using experimental and computational techniques

In the event of abdominal aortic aneurysm (AAA) rupture, the outcome is often death. This paper aims to experimentally identify the rupture locations of in vitro AAA models and validate these rupture sites using finite element analysis (FEA). Silicone rubber AAA models were manufactured using two different materials (Sylgard 160 and Sylgard 170, Dow Corning) and imaged using computed tomography (CT). Experimental models were inflated until rupture with high speed photography used to capture the site of rupture. 3D reconstructions from CT scans and subsequent FEA of these models enabled the wall stress and wall thickness to be determined for each of the geometries. Experimental models ruptured at regions of inflection, not at regions of maximum diameter. Rupture pressures (mean±SD) for the Sylgard 160 and Sylgard 170 models were 650.6±195.1 mmHg and 410.7±159.9 mmHg, respectively. Computational models accurately predicted the locations of rupture. Peak wall stress for the Sylgard 160 and Sylgard 170 models was 2.15±0.26 MPa at an internal pressure of 650 mmHg and 1.69±0.38 MPa at an internal pressure of 410 mmHg, respectively. Mean wall thickness of all models was 2.19±0.40 mm, with a mean wall thickness at the location of rupture of 1.85±0.33 and 1.71±0.29 mm for the Sylgard 160 and Sylgard 170 materials, respectively. Rupture occurred at the location of peak stress in 80% (16/20) of cases and at high stress regions but not peak stress in 10% (2/20) of cases. 10% (2/20) of models had defects in the AAA wall which moved the rupture location away from regions of elevated stress. The results presented may further contribute to the understanding of AAA biomechanics and ultimately AAA rupture prediction.     

A decoupled fluid structure approach for estimating wall stress in abdominal aortic aneurysms

Abdominal aortic aneurysm (AAA) is a localized dilatation of the aortic wall. The lack of an accurate AAA rupture risk index remains an important problem in the clinical management of the disease. To accurately estimate AAA rupture risk, detailed information on patient-specific wall stress distribution and aortic wall tissue yield stress is required. A complete fluid structure interaction (FSI) study is currently impractical and thus of limited clinical value. On the other hand, isolated static structural stress analysis based on a uniform wall loading is a widely used approach for AAA rupture risk estimation that, however, neglects the flow-induced wall stress variation. The aim of this study was to assess the merit of a decoupled fluid structure analysis of AAA wall stress. Anatomically correct, patient specific AAA wall models were created by 3D reconstruction of computed tomography images. Flow simulations were carried out with inflow and outflow boundary conditions obtained from patient extracted data. Static structural stress analysis was performed applying both a uniform pressure wall loading and a flow induced non-uniform pressure distribution obtained during early systolic deceleration. For the structural analysis, a hyperelastic arterial wall model and an elastic intraluminal thrombus model were assumed. The results of this study demonstrate that although the isolated static structural stress analysis approach captures the gross features of the stress distribution it underestimates the magnitude of the peak wall stress by as much as 12.5% compared to the proposed decoupled fluid structure approach. Furthermore, the decoupled approach provides potentially useful information on the nature of the aneurysmal sac flow.     

Nonlinear anisotropic stress analysis of anatomically realistic cerebral aneurysms

BACKGROUND: Static deformation analysis and estimation of wall stress distribution of patient-specific cerebral aneurysms can provide useful insights into the disease process and rupture. METHOD OF APPROACH: The three-dimensional geometry of saccular cerebral aneurysms from 27 patients (18 unruptured and nine ruptured) was reconstructed based on computer tomography angiography images. The aneurysm wall tissue was modeled using a nonlinear, anisotropic, hyperelastic material model (Fung-type) which was incorporated in a user subroutine in ABAQUS. Effective material fiber orientations were assumed to align with principal surface curvatures. Static deformation of the aneurysm models were simulated assuming uniform wall thickness and internal pressure load of 100 mm Hg. RESULTS: The numerical analysis technique was validated by quantitative comparisons to results in the literature. For the patient-specific models, in-plane stresses in the aneurysm wall along both the stiff and weak fiber directions showed significant regional variations with the former being higher. The spatial maximum of stress ranged from as low as 0.30 MPa in a small aneurysm to as high as 1.06 MPa in a giant aneurysm. The patterns of distribution of stress, strain, and surface curvature were found to be similar. Sensitivity analyses showed that the computed stress is mesh independent and not very sensitive to reasonable perturbations in model parameters, and the curvature-based criteria for fiber orientations tend to minimize the total elastic strain energy in the aneurysms wall. Within this small study population, there were no statistically significant differences in the spatial means and maximums of stress and strain values between the ruptured and unruptured groups. However, the ratios between the stress components in the stiff and weak fiber directions were significantly higher in the ruptured group than those in the unruptured group. CONCLUSIONS: A methodology for nonlinear, anisotropic static deformation analysis of geometrically realistic aneurysms was developed, which can be used for a more accurate estimation of the stresses and strains than previous methods and to facilitate prospective studies on the role of stress in aneurysm rupture.     

Wall stress and flow dynamics in abdominal aortic aneurysms: finite element analysis vs. fluid–structure interaction

Abdominal aortic aneurysm (AAA) rupture is the clinical manifestation of an induced force exceeding the resistance provided by the strength of the arterial wall. This force is most frequently assumed to be the product of a uniform luminal pressure acting along the diseased wall. However fluid dynamics is a known contributor to the pathogenesis of AAAs, and the dynamic interaction of blood flow and the arterial wall represents the in vivo environment at the macro-scale. The primary objective of this investigation is to assess the significance of assuming an arbitrary estimated peak fluid pressure inside the aneurysm sac for the evaluation of AAA wall mechanics, as compared with the non-uniform pressure resulting from a coupled fluid–structure interaction (FSI) analysis. In addition, a finite element approach is utilised to estimate the effects of asymmetry and wall thickness on the wall stress and fluid dynamics of ten idealised AAA models and one non-aneurysmal control. Five degrees of asymmetry with uniform and variable wall thickness are used. Each was modelled under a static pressure-deformation analysis, as well as a transient FSI. The results show that the inclusion of fluid flow yields a maximum AAA wall stress up to 20% higher compared to that obtained with a static wall stress analysis with an assumed peak luminal pressure of 117 mmHg. The variable wall models have a maximum wall stress nearly four times that of a uniform wall thickness, and also increasing with asymmetry in both instances. The inclusion of an axial stretch and external pressure to the computational domain decreases the wall stress by 17%.     

Wall stress and flow dynamics in abdominal aortic aneurysms: finite element analysis vs. fluid-structure interaction

Abdominal aortic aneurysm (AAA) rupture is the clinical manifestation of an induced force exceeding the resistance provided by the strength of the arterial wall. This force is most frequently assumed to be the product of a uniform luminal pressure acting along the diseased wall. However fluid dynamics is a known contributor to the pathogenesis of AAAs, and the dynamic interaction of blood flow and the arterial wall represents the in vivo environment at the macro-scale. The primary objective of this investigation is to assess the significance of assuming an arbitrary estimated peak fluid pressure inside the aneurysm sac for the evaluation of AAA wall mechanics, as compared with the non-uniform pressure resulting from a coupled fluid-structure interaction (FSI) analysis. In addition, a finite element approach is utilised to estimate the effects of asymmetry and wall thickness on the wall stress and fluid dynamics of ten idealised AAA models and one non-aneurysmal control. Five degrees of asymmetry with uniform and variable wall thickness are used. Each was modelled under a static pressure-deformation analysis, as well as a transient FSI. The results show that the inclusion of fluid flow yields a maximum AAA wall stress up to 20% higher compared to that obtained with a static wall stress analysis with an assumed peak luminal pressure of 117 mmHg. The variable wall models have a maximum wall stress nearly four times that of a uniform wall thickness, and also increasing with asymmetry in both instances. The inclusion of an axial stretch and external pressure to the computational domain decreases the wall stress by 17%.     

Intraluminal thrombus and risk of rupture in patient specific abdominal aortic aneurysm – FSI modelling

Recent numerical studies of abdominal aortic aneurysm (AAA) suggest that intraluminal thrombus (ILT) may reduce the stress loading on the aneurysmal wall. Detailed fluid structure interaction (FSI) in the presence and absence of ILT may help predict AAA rupture risk better. Two patients, with varied AAA geometries and ILT structures, were studied and compared in detail. The patient specific 3D geometries were reconstructed from CT scans, and uncoupled FSI approach was applied. Complex flow trajectories within the AAA lumen indicated a viable mechanism for the formation and growth of the ILT. The resulting magnitude and location of the peak wall stresses was dependent on the shape of the AAA, and the ILT appeared to reduce wall stresses for both patients. Accordingly, the inclusion of ILT in stress analysis of AAA is of importance and would likely increase the accuracy of predicting AAA risk of rupture.     

Intraluminal thrombus and risk of rupture in patient specific abdominal aortic aneurysm - FSI modelling

Recent numerical studies of abdominal aortic aneurysm (AAA) suggest that intraluminal thrombus (ILT) may reduce the stress loading on the aneurysmal wall. Detailed fluid structure interaction (FSI) in the presence and absence of ILT may help predict AAA rupture risk better. Two patients, with varied AAA geometries and ILT structures, were studied and compared in detail. The patient specific 3D geometries were reconstructed from CT scans, and uncoupled FSI approach was applied. Complex flow trajectories within the AAA lumen indicated a viable mechanism for the formation and growth of the ILT. The resulting magnitude and location of the peak wall stresses was dependent on the shape of the AAA, and the ILT appeared to reduce wall stresses for both patients. Accordingly, the inclusion of ILT in stress analysis of AAA is of importance and would likely increase the accuracy of predicting AAA risk of rupture.     

Effects of wall calcifications in patient-specific wall stress analyses of abdominal aortic aneurysms

It is generally acknowledged that rupture of an abdominal aortic aneurysm (AAA) occurs when the stress acting on the wall over the cardiac cycle exceeds the strength of the wall. Peak wall stress computations appear to give a more accurate rupture risk assessment than AAA diameter, which is currently used for a diagnosis. Despite the numerous studies utilizing patient-specific wall stress modeling of AAAs, none investigated the effect of wall calcifications on wall stress. The objective of this study was to evaluate the influence of calcifications on patient-specific finite element stress computations. In addition, we assessed whether the effect of calcifications could be predicted directly from the CT-scans by relating the effect to the amount of calcification present in the AAA wall. For 6 AAAs, the location and extent of calcification was identified from CT-scans. A finite element model was created for each AAA and the areas of calcification were defined node-wise in the mesh of the model. Comparisons are made between maximum principal stress distributions, computed without calcifications and with calcifications with varying material properties. Peak stresses are determined from the stress results and related to a calcification index (CI), a quantification of the amount of calcification in the AAA wall. At calcification sites, local stresses increased, leading to a peak stress increase of 22% in the most severe case. Our results displayed a weak correlation between the CI and the increase in peak stress. Additionally, the results showed a marked influence of the calcification elastic modulus on computed stresses. Inclusion of calcifications in finite element analysis of AAAs resulted in a marked alteration of the stress distributions and should therefore be included in rupture risk assessment. The results also suggest that the location and shape of the calcified regions--not only the relative amount--are considerations that influence the effect on AAA wall stress. The dependency of the effect of the wall stress on the calcification elastic modulus points out the importance of determination of the material properties of calcified AAA wall.     

A fluid–structure interaction-based numerical investigation on the evolution of stress,strength and rupture potential of an abdominal aortic aneurysm

An abdominal aortic aneurysm (AAA) is an irreversible dilation of the abdominal artery. Once an aneurysm is detected by doctors, clinical intervention is usually recommended. The interventions involve traditional open surgery repair and endovascular aneurysm repair with a stent graft. Both types of prophylactic procedures are expensive and not without any risk to the patient. It is very difficult to balance the risk of aneurysm repair and the chance of rupture. The reason lies in that the changing trend of characteristic physical quantities with the evolution of AAA and the mechanisms that give rise to it are still not completely clear. In this study, computational 3D patient-specific model for investigating AAA development was established based on computed tomography (CT) images. Results showed that as the aneurysm evolved, peak wall stress and time-averaged wall shear stress distribution patterns changed. The expansion of AAA wall resulted in the increment of peak stress. The AAA wall compliance not only showed different magnitudes at different cross-sections of the aneurismal body, but also changed with the development of the aneurysm. Furthermore, minimum wall strength and rupture potential index during the three stages of AAA evolution were also investigated in detail. This study might provide valuable information on how to further explore the mechanical basis and the rupture potential during AAA evolution, and that it may assist clinical diagnostic procedures and avoid the potential risk of unnecessary surgical intervention.     

Haemodynamics and stresses in abdominal aortic aneurysms: A fluid-structure interaction study into the effect of proximal neck and iliac bifurcation angle

Our knowledge of how geometry influences abdominal aortic aneurysm (AAA) biomechanics is still developing. Both iliac bifurcation angle and proximal neck angle could impact the haemodynamics and stresses within AAA. Recent comparisons of the morphology of ruptured and intact AAA show that cases with large iliac bifurcation angles are less likely to rupture than those with smaller angles. We aimed to perform fluid-structure interaction (FSI) simulations on a range of idealised AAA geometries to conclusively determine the influence of proximal neck and iliac bifurcation angle on AAA wall stress and haemodynamics.Peak wall shear stress (WSS) and time-averaged WSS (TAWSS) in the AAA sac region only increased when the proximal neck angle exceeded 30°. Both peak WSS (p < 0.0001) and peak von Mises wall stress (p = 0.027) increased with iliac bifurcation angle, whereas endothelial cell activation potential (ECAP) decreased with iliac bifurcation angle (p < 0.001) and increased with increasing neck angle.These observations may be important as AAAs have been shown to expand, develop thrombus and rupture in areas of low WSS. Here we show that AAAs with larger iliac bifurcation angles have higher WSS, potentially reducing the likelihood of rupture. Furthermore, ECAP was lower in AAA geometries with larger iliac bifurcation angles, implying less likelihood of thrombus development and wall degeneration. Therefore our findings could help explain the clinical observation of lower rupture rates associated with AAAs with large iliac bifurcation angles.     

Metabolites secreted by human atherothrombotic aneurysms revealed through a metabolomic approach

Abdominal aortic aneurysm (AAA) is perma-nent and localized dilation of the abdominal aorta. Intraluminal thrombus (ILT) is involved in evolution and rupture of AAA. Complex biological processes associated with AAA include oxidative stress, proteolysis, neovascularization, aortic inflammation, cell death, and extracellular matrix breakdown. Biomarkers of growth and AAA rupture could give a more nuanced indication for surgery, unveil novel pathogenic pathways, and open possibilities for pharmacological inhibition of growth. Differential analysis of metabolites released by normal and pathological arteries in culture may help to find molecules that have a high probability of later being found in plasma and start signaling processes or be useful diagnostic/prognostic markers. We used a LC-QTOF-MS metabolomic approach to analyze metabolites released by human ILT (divided into luminal and abluminal layers), aneurysm wall (AW), and healthy wall (HW). Statistical analysis was used to compare luminal with abluminal ILT layer, ILT with AW, and AW with HW to select the metabolites exchanged between tissue and external medium. Identified compounds are related to inflammation and oxidative stress and indicate the possible role of fatty acid amides in AAA. Some metabolites (e.g., hippuric acid) had not been previously associated to aneurysm, others (fatty acid amides) have arisen, indicating a very promising line of research.     

Biomechanics of abdominal aortic aneurysm

Abdominal aortic aneurysm (AAA) is a condition whereby the terminal aorta permanently dilates to dangerous proportions, risking rupture. The biomechanics of AAA has been studied with great interest since aneurysm rupture is a mechanical failure of the degenerated aortic wall and is a significant cause of death in developed countries. In this review article, the importance of considering the biomechanics of AAA is discussed, and then the history and the state-of-the-art of this field is reviewed--including investigations into the biomechanical behavior of AAA tissues, modeling AAA wall stress and factors which influence it, and the potential clinical utility of these estimates in predicting AAA rupture.     

Biomechanical failure properties and microstructural content of ruptured and unruptured abdominal aortic aneurysms

PurposeTo test the hypothesis that ruptured abdominal aortic aneurysms (AAA) are globally weaker than unruptured ones.MethodsFour ruptured and seven unruptured AAA specimens were harvested whole from fresh cadavers during autopsies performed over an 18-month period. Multiple regionally distributed longitudinally oriented rectangular strips were cut from each AAA specimen for a total of 77 specimen strips. Strips were subjected to uniaxial extension until failure. Sections from approximately the strongest and weakest specimen strips were studied histologically and histochemically. From the load-extension data, failure tension, failure stress and failure strain were calculated. Rupture site characteristics such as location, arc length of rupture and orientation of rupture were also documented.ResultsThe failure tension, a measure of the tissue mechanical caliber was remarkably similar between ruptured and unruptured AAA (group mean±standard deviation of within-subject means: 11.2±2.3 versus 11.6±3.6 N/cm; p=0.866 by mixed model ANOVA). In post-hoc analysis, there was little difference between the groups in other measures of tissue mechanical caliber as well such as failure stress (95±28 versus 98±23 N/cm²; p=0.870), failure strain (0.39±0.09 versus 0.36±0.09; p=0.705), wall thickness (1.7±0.4 versus 1.5±0.4 mm; p=0.470) , and % coverage of collagen within tissue cross section (49.6±12.9% versus 60.8±9.6%; p=0.133). In the four ruptured AAA, primary rupture sites were on the lateral quadrants (two on left; one on left-posterior; one on right). Remarkably, all rupture lines had a longitudinal orientation and ranged from 1 to 6 cm in length.ConclusionThe findings are not consistent with the hypothesis that ruptured aortic aneurysms are globally weaker than unruptured ones.