Increased Peak Wall Stress, but Not Maximum Diameter, Is Associated with Symptomatic Abdominal Aortic Aneurysm

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 < 0.05) were found to characterize the AAA morphology during the surveillance period. By means of nonlinear regression, PWS estimated with the anisotropic material was found to be highly correlated with three of these features: maximum diameter (r = 0.992, p = 0.002), sac volume (r = 0.989, p = 0.003) and diameter to diameter ratio (r = 0.947, p = 0.033). The correlation of wall mechanics with geometry is nonlinear and reveals that PWS does not increase concomitantly with aneurysm diameter. This suggests that a quantitative characterization of AAA morphology may be advantageous in assessing rupture risk.

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A Methodology for the Derivation of Unloaded Abdominal Aortic Aneurysm Geometry With Experimental Validation

Journal of Biomechanical Engineering ◽

10.1115/1.4034425 ◽

2016 ◽

Vol 138 (10) ◽

Cited By ~ 5

Author(s):

Santanu Chandra ◽

Vimalatharmaiyah Gnanaruban ◽

Fabian Riveros ◽

Jose F. Rodriguez ◽

Ender A. Finol

Keyword(s):

Finite Element ◽

Abdominal Aortic Aneurysm ◽

Aortic Aneurysm ◽

Stress Component ◽

Wall Stress ◽

Patient Specific ◽

Element Analysis ◽

Nodal Surface ◽

Abdominal Aortic ◽

Peak Wall Stress

In this work, we present a novel method for the derivation of the unloaded geometry of an abdominal aortic aneurysm (AAA) from a pressurized geometry in turn obtained by 3D reconstruction of computed tomography (CT) images. The approach was experimentally validated with an aneurysm phantom loaded with gauge pressures of 80, 120, and 140 mm Hg. The unloaded phantom geometries estimated from these pressurized states were compared to the actual unloaded phantom geometry, resulting in mean nodal surface distances of up to 3.9% of the maximum aneurysm diameter. An in-silico verification was also performed using a patient-specific AAA mesh, resulting in maximum nodal surface distances of 8 μm after running the algorithm for eight iterations. The methodology was then applied to 12 patient-specific AAA for which their corresponding unloaded geometries were generated in 5–8 iterations. The wall mechanics resulting from finite element analysis of the pressurized (CT image-based) and unloaded geometries were compared to quantify the relative importance of using an unloaded geometry for AAA biomechanics. The pressurized AAA models underestimate peak wall stress (quantified by the first principal stress component) on average by 15% compared to the unloaded AAA models. The validation and application of the method, readily compatible with any finite element solver, underscores the importance of generating the unloaded AAA volume mesh prior to using wall stress as a biomechanical marker for rupture risk assessment.

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Computational Modeling of the Effects of Transient Blood Flow Characteristics and Wall Thickness on the Rupture of Abdominal Aortic Aneurysm

Volume 9: Mechanics of Solids, Structures and Fluids ◽

10.1115/imece2014-36150 ◽

2014 ◽

Author(s):

Pinaki Pal

Keyword(s):

Blood Flow ◽

Abdominal Aortic Aneurysm ◽

Aortic Aneurysm ◽

Wall Thickness ◽

Flow Model ◽

Wall Stress ◽

Flow Transition ◽

Abdominal Aortic ◽

Sst Model ◽

Peak Wall Stress

Precise estimation of wall stress distribution within an abdominal aortic aneurysm (AAA) is clinically useful for prediction of its rupture. In this paper a computational fluid dynamic model incorporating two-way coupled fluid-structure interaction is employed to investigate the role of laminar-turbulent flow transition and wall thickness in altering the distribution and magnitude of wall stress in an AAA. Blood flow in axially symmetric aneurysm models governed by a compliant wall mechanics was simulated. Menter’s hybrid k-epsilon/k-omega shear stress transport (SST) model with a correlation-based transition model was used to capture laminar-turbulent transition in the blood flow. Realistic physiological transient boundary conditions were prescribed. The numerical model was validated against experimental data available from the literature. Fluid flow analysis showed the formation of recirculating vortices at the proximal end of the aneurysm after the peak systole which then, moved towards the distal end of the aneurysm along with the bulk flow and were dissipated eventually due to viscous effects. These vortices interacted with the aortic wall and led to local pressure rise. Von Mises stress distribution on the aneurysm wall and location of its peak value were computed and compared with those of a separate numerical simulation performed using a laminar viscous flow model. The predicted peak wall stress was found to be significantly higher for the SST model as compared to the laminar flow model. The location of maximum stress shifted more towards the posterior end of the aneurysm when laminar-turbulent flow transition was considered. In addition, a small reduction of 0.4 mm in wall thickness resulted in the elevation of peak wall stress by a factor of 1.4. The present study showed that capturing flow transition in an AAA is essential to accurate prediction of its rupture. The proposed numerical model provides a robust computational framework to gain more insight into AAA biomechanics and to accurately estimate wall stresses in realistic aneurysm configurations.

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A Methodology for Verifying Abdominal Aortic Aneurysm Wall Stress

Journal of Biomechanical Engineering ◽

10.1115/1.4034710 ◽

2016 ◽

Vol 139 (1) ◽

Cited By ~ 5

Author(s):

Sergio Ruiz de Galarreta ◽

Aitor Cazón ◽

Raúl Antón ◽

Ender A. Finol

Keyword(s):

Abdominal Aortic Aneurysm ◽

Aortic Aneurysm ◽

Experimental Testing ◽

Wall Stress ◽

Intraluminal Pressure ◽

Maximum Diameter ◽

Numerical Studies ◽

Aneurysm Wall ◽

Abdominal Aortic ◽

Geometric Factors

An abdominal aortic aneurysm (AAA) is a permanent focal dilatation of the abdominal aorta of at least 1.5 times its normal diameter. Although the criterion of maximum diameter is still used in clinical practice to decide on a timely intervention, numerical studies have demonstrated the importance of other geometric factors. However, the major drawback of numerical studies is that they must be validated experimentally before clinical implementation. This work presents a new methodology to verify wall stress predicted from the numerical studies against the experimental testing. To this end, four AAA phantoms were manufactured using vacuum casting. The geometry of each phantom was subject to microcomputed tomography (μCT) scanning at zero and three other intraluminal pressures: 80, 100, and 120 mm Hg. A zero-pressure geometry algorithm was used to calculate the wall stress in the phantom, while the numerical wall stress was calculated with a finite-element analysis (FEA) solver based on the actual zero-pressure geometry subjected to 80, 100, and 120 mm Hg intraluminal pressure loading. Results demonstrate the moderate accuracy of this methodology with small relative differences in the average wall stress (1.14%). Additionally, the contribution of geometric factors to the wall stress distribution was statistically analyzed for the four phantoms. The results showed a significant correlation between wall thickness and mean curvature (MC) with wall stress.

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A Randomized Placebo Controlled Trial of the Effect of Preoperative Statin Use on Matrix Metalloproteinases and Tissue Inhibitors of Matrix Metalloproteinases in Areas of Low and Peak Wall Stress in Patients Undergoing Elective Open Repair of Abdominal Aortic Aneurysm

Annals of Vascular Surgery ◽

10.1016/j.avsg.2010.06.006 ◽

2011 ◽

Vol 25 (1) ◽

pp. 32-38 ◽

Cited By ~ 14

Author(s):

MNA. Abdul Rahman ◽

Junaid Alam Khan ◽

Fayyaz Ali Khan Mazari ◽

Katherine Mockford ◽

Peter Thomas McCollum ◽

...

Keyword(s):

Abdominal Aortic Aneurysm ◽

Aortic Aneurysm ◽

Matrix Metalloproteinases ◽

Open Repair ◽

Controlled Trial ◽

Wall Stress ◽

Tissue Inhibitors ◽

Abdominal Aortic ◽

Peak Wall Stress ◽

Statin Use

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Assessment of Abdominal Aortic Aneurysm Risk: Asymmetry as an Additional Diagnostic Tool?

ASME 2008 Summer Bioengineering Conference, Parts A and B ◽

10.1115/sbc2008-191585 ◽

2008 ◽

Author(s):

Barry J. Doyle ◽

Anthony Callanan ◽

Michael T. Walsh ◽

David A. Vorp ◽

Timothy M. McGloughlin

Keyword(s):

Risk Factors ◽

Abdominal Aortic Aneurysm ◽

Aortic Aneurysm ◽

Surgical Intervention ◽

Current Trend ◽

Wall Stress ◽

Posterior Wall ◽

Maximum Diameter ◽

Infrarenal Aorta ◽

Abdominal Aortic

An abdominal aortic aneurysm (AAA) can be defined as a permanent and irreversible localised dilation of the infrarenal aorta. This localised dilation is a result of a degradation of the elastic media of the arterial wall. This degradation of the aortic wall can be attributed to risk factors such as tobacco smoking, sex, age, hypertension, chronic obstructive pulmonary disease, hyperlipidaemia, and family history of the disorder [1]. With the recent advancements in medicine, more AAAs are being detected than ever. Approximately 500,000 new cases are diagnosed each year worldwide resulting in 15,000 deaths per year in the USA alone [2]. Currently, the rupture risk of AAAs is regarded as a continuous function of aneurysm size, with surgical intervention decided based on the maximum diameter of the AAA. Most AAA repairs are performed when the diameter exceeds 50–60mm. It has been shown that maximum diameter may not be a reliable predictor of rupture, as smaller AAAs can also rupture. It is believed by many researchers that there is a need to review the determination of the timing of surgical intervention based solely on aneurysm diameter, and include other relevant risk factors. These additional risk factors could, for example, include, AAA wall stress, AAA expansion rate, degree of asymmetry, presence of intraluminal thrombus (ILT), and hypertension. The addition of these parameters may aid the surgical decision-making process. Shifting the current trend towards more encompassing assessment of AAA rupture potential may help reduce the morbidity and mortality rates associated with AAA repair. It was previously reported [3] that 82% of AAA ruptures occur on the posterior wall. In this research, the asymmetry of the AAA is examined, with respect to both peak wall stress and posterior wall stress, in ten realistic cases, and a resulting threshold factor is presented.

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ABDOMINAL AORTIC ANEURYSM (AAA) MORPHOLOGY AS PREDICTOR OF PEAK WALL STRESS

Journal of Biomechanics ◽

10.1016/s0021-9290(12)70012-x ◽

2012 ◽

Vol 45 ◽

pp. S11

Author(s):

Guillermo Vilalta ◽

José A. Vilalta ◽

Félix Nieto ◽

María Á Pérez ◽

Germán Salgado ◽

...

Keyword(s):

Abdominal Aortic Aneurysm ◽

Aortic Aneurysm ◽

Wall Stress ◽

Abdominal Aortic ◽

Peak Wall Stress

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Automated Methodology for Determination of Stress Distribution in Human Abdominal Aortic Aneurysm

Journal of Biomechanical Engineering ◽

10.1115/1.1992530 ◽

2005 ◽

Vol 127 (5) ◽

pp. 868-871 ◽

Cited By ~ 30

Author(s):

Madhavan L. Raghavan ◽

Mark F. Fillinger ◽

Steven P. Marra ◽

Bernhard P. Naegelein ◽

Francis E. Kennedy

Keyword(s):

Abdominal Aortic Aneurysm ◽

Aortic Aneurysm ◽

Peak Stress ◽

Wall Stress ◽

Patient Specific ◽

Element Analysis ◽

Biomechanical Stress ◽

Abdominal Aortic ◽

Study Population ◽

Peak Wall Stress

Knowledge of impending abdominal aortic aneurysm (AAA) rupture can help in surgical planning. Typically, aneurysm diameter is used as the indicator of rupture, but recent studies have hypothesized that pressure-induced biomechanical stress may be a better predictor. Verification of this hypothesis on a large study population with ruptured and unruptured AAA is vital if stress is to be reliably used as a clinical prognosticator for AAA rupture risk. We have developed an automated algorithm to calculate the peak stress in patient-specific AAA models. The algorithm contains a mesh refinement module, finite element analysis module, and a postprocessing visualization module. Several aspects of the methodology used are an improvement over past reported approaches. The entire analysis may be run from a single command and is completed in less than 1h with the peak wall stress recorded for statistical analysis. We have used our algorithm for stress analysis of numerous ruptured and unruptured AAA models and report some of our results here. By current estimates, peak stress in the aortic wall appears to be a better predictor of rupture than AAA diameter. Further use of our algorithm is ongoing on larger study populations to convincingly verify these findings.

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