Trans-Thrombus Blood Pressure Effects in Abdominal Aortic Aneurysms

How much and how the thrombus supports the wall of an abdominal aortic aneurysm (AAA) is unclear. While some previous studies have indicated that thrombus lacks the mechanical integrity to support much load compared with the aneurysm wall, others have shown that removing thrombus in computational AAA models drastically changes aneurysm wall stress. Histopathological studies have shown that thrombus properties vary through the thickness and it can be porous. The goal of this study is to explore the variations in thrombus properties, including the ability to isolate pressure from the aneurysm wall, incomplete attachment, and their effects on aneurysm wall stress, an important parameter in determining risk for rupture. An analytical model comprised of cylinders and two patient specific models were constructed with pressurization boundary conditions applied at the lumen or the thrombus/aneurysm wall interface (to simulate complete transmission of pressure through porous thrombus). Aneurysm wall stress was also calculated in the absence of thrombus. The potential importance of partial thrombus attachment was also analyzed. Pressurizing at either surface (lumen versus interface) made little difference to mean von Mises aneurysm wall stress values with thrombus completely attached (3.1% analytic, 1.2% patient specific) while thrombus presence reduced mean von Mises stress considerably (79% analytic, 40–46% patient specific) in comparison to models without it. Peak von Mises stresses were similarly influenced with pressurization surface differing slightly (3.1% analytic, 1.4% patient specific) and reductions in stress by thrombus presence (80% analytic, 28–37% patient specific). The case of partial thrombus attachment was investigated using a cylindrical model in which there was no attachment between the thrombus and aneurysm wall in a small area (10 deg). Applying pressure at the lumen resulted in a similar stress field to fully attached thrombus, whereas applying pressure at the interface resulted in a 42% increase in peak aneurysm wall stress. Taken together, these results show that the thrombus can have a wall stress reducing role even if it does not shield the aneurysm wall from direct pressurization—as long as the thrombus is fully attached to the aneurysm wall. Furthermore, the potential for porous thrombus to transmit pressure to the interface can result in a considerable increase in aneurysm wall stress in cases of partial attachment. In the search for models capable of accurately assessing the risk for rupture, the nature of the thrombus and its attachment to the aneurysm wall must be carefully assessed.

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Evaluating patient-specific abdominal aortic aneurysm wall stress based on flow-induced loading

Biomechanics and Modeling in Mechanobiology ◽

10.1007/s10237-009-0163-4 ◽

2009 ◽

Vol 9 (2) ◽

pp. 127-139 ◽

Cited By ~ 22

Author(s):

A. Dorfmann ◽

C. Wilson ◽

E. S. Edgar ◽

R. A. Peattie

Keyword(s):

Abdominal Aortic Aneurysm ◽

Aortic Aneurysm ◽

Wall Stress ◽

Patient Specific ◽

Aneurysm Wall ◽

Abdominal Aortic ◽

Abdominal Aortic Aneurysm Wall

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Use of the Photoelastic Method to Determine the Wall Stress in Realistic Abdominal Aortic Aneurysm Models

ASME 2011 Summer Bioengineering Conference, Parts A and B ◽

10.1115/sbc2011-53268 ◽

2011 ◽

Author(s):

Barry J. Doyle ◽

Anthony Callanan ◽

John Killion ◽

Timothy M. McGloughlin

Keyword(s):

Wall Stress ◽

Aortic Aneurysms ◽

Maximum Diameter ◽

Patient Specific ◽

Western World ◽

Photoelastic Method ◽

Element Analysis ◽

Abdominal Aortic ◽

The Us ◽

Numerical Tools

Abdominal aortic aneurysms (AAAs) remain a significant cause of death in the Western world with over 15,000 deaths per year in the US linked to AAA rupture. Recent research [1] has questioned the use of maximum diameter as a definitive risk parameter as it is now believed that alternative factors may be important in rupture-prediction. Wall stress was shown to be a better predictor than diameter of rupture [1], with biomechanics-based rupture indices [2,3] and asymmetry also reported to have potential clinical applicability [4]. However, the majority of numerical methods used to form these alternative rupture parameters are without rigorous experimental validation, and therefore may not be as accurate as believed. Validated experiments are required in order to convince the clinical community of the worth of numerical tools such as finite element analysis (FEA) in AAA risk-prediction. Strain gauges have been used in the past to determine the strain on an AAA [5], however, the photoelastic method has also proved to be a useful tool in AAA biomechanics [6]. This paper examines the approach using three medium-sized patient-specific AAA cases at realistic pressure loadings.

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Impact of calcifications on patient-specific wall stress analysis of abdominal aortic aneurysms

Biomechanics and Modeling in Mechanobiology ◽

10.1007/s10237-010-0191-0 ◽

2010 ◽

Vol 9 (5) ◽

pp. 511-521 ◽

Cited By ~ 62

Author(s):

A. Maier ◽

M. W. Gee ◽

C. Reeps ◽

H.-H. Eckstein ◽

W. A. Wall

Keyword(s):

Stress Analysis ◽

Wall Stress ◽

Abdominal Aortic Aneurysms ◽

Aortic Aneurysms ◽

Patient Specific ◽

Abdominal Aortic

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On the Relative Impact of Intraluminal Thrombus Heterogeneity on Abdominal Aortic Aneurysm Mechanics

Journal of Biomechanical Engineering ◽

10.1115/1.4044143 ◽

2019 ◽

Vol 141 (11) ◽

Author(s):

Joseph R. Leach ◽

Evan Kao ◽

Chengcheng Zhu ◽

David Saloner ◽

Michael D. Hope

Keyword(s):

Finite Element Analysis ◽

Finite Element ◽

Aneurysm Rupture ◽

Aortic Aneurysms ◽

Patient Specific ◽

Rupture Risk ◽

Intraluminal Thrombus ◽

Element Analysis ◽

Aneurysm Wall ◽

Abdominal Aortic

Intraluminal thrombus (ILT) is present in the majority of abdominal aortic aneurysms (AAA) of a size warranting consideration for surgical or endovascular intervention. The rupture risk of AAAs is thought to be related to the balance of vessel wall strength and the mechanical stress caused by systemic blood pressure. Previous finite element analyses of AAAs have shown that ILT can reduce and homogenize aneurysm wall stress. These works have largely considered ILT to be homogeneous in mechanical character or have idealized a stiffness distribution through the thrombus thickness. In this work, we use magnetic resonance imaging (MRI) to delineate the heterogeneous composition of ILT in 7 AAAs and perform patient–specific finite element analysis under multiple conditions of ILT layer stiffness disparity. We find that explicit incorporation of ILT heterogeneity in the finite element analysis is unlikely to substantially alter major stress analysis predictions regarding aneurysm rupture risk in comparison to models assuming a homogenous thrombus, provided that the maximal ILT stiffness is the same between models. Our results also show that under a homogeneous ILT assumption, the choice of ILT stiffness from values common in the literature can result in significantly larger variations in stress predictions compared to the effects of thrombus heterogeneity.

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A Comparative Study of Biomechanical and Geometrical Attributes of Abdominal Aortic Aneurysms in the Asian and Caucasian Populations

Journal of Biomechanical Engineering ◽

10.1115/1.4045268 ◽

2020 ◽

Vol 142 (6) ◽

Cited By ~ 1

Author(s):

Tejas Canchi ◽

Sourav S. Patnaik ◽

Hong N. Nguyen ◽

E. Y. K. Ng ◽

Sriram Narayanan ◽

...

Keyword(s):

Wall Thickness ◽

Gaussian Curvature ◽

Wall Stress ◽

Aneurysm Rupture ◽

Aortic Aneurysms ◽

Patient Specific ◽

Backward Elimination ◽

Abdominal Aortic ◽

Highly Correlated ◽

Peak Wall Stress

Abstract In this work, we provide a quantitative assessment of the biomechanical and geometric features that characterize abdominal aortic aneurysm (AAA) models generated from 19 Asian and 19 Caucasian diameter-matched AAA patients. 3D patient-specific finite element models were generated and used to compute peak wall stress (PWS), 99th percentile wall stress (99th WS), and spatially averaged wall stress (AWS) for each AAA. In addition, 51 global geometric indices were calculated, which quantify the wall thickness, shape, and curvature of each AAA. The indices were correlated with 99th WS (the only biomechanical metric that exhibited significant association with geometric indices) using Spearman's correlation and subsequently with multivariate linear regression using backward elimination. For the Asian AAA group, 99th WS was highly correlated (R2 = 0.77) with three geometric indices, namely tortuosity, intraluminal thrombus volume, and area-averaged Gaussian curvature. Similarly, 99th WS in the Caucasian AAA group was highly correlated (R2 = 0.87) with six geometric indices, namely maximum AAA diameter, distal neck diameter, diameter–height ratio, minimum wall thickness variance, mode of the wall thickness variance, and area-averaged Gaussian curvature. Significant differences were found between the two groups for ten geometric indices; however, no differences were found for any of their respective biomechanical attributes. Assuming maximum AAA diameter as the most predictive metric for wall stress was found to be imprecise: 24% and 28% accuracy for the Asian and Caucasian groups, respectively. This investigation reveals that geometric indices other than maximum AAA diameter can serve as predictors of wall stress, and potentially for assessment of aneurysm rupture risk, in the Asian and Caucasian AAA populations.

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Porohyperelastic Simulation of Abdominal Aortic Aneurysms

ASME 2008 Summer Bioengineering Conference, Parts A and B ◽

10.1115/sbc2008-193147 ◽

2008 ◽

Author(s):

Avinash Ayyalasomayajula ◽

Bruce R. Simon ◽

Jonathan P. Vande Geest

Keyword(s):

Wall Stress ◽

Aortic Aneurysms ◽

Maximum Diameter ◽

Patient Specific ◽

Infrarenal Aorta ◽

Stress Studies ◽

Abdominal Aortic ◽

Peak Wall Stress ◽

Work Done

Abdominal aortic aneurysm (AAA) is a progressive dilation of the infrarenal aorta and results in a significant alteration in local hemodynamic environment [1]. While an aneurysmal diameter of 5.5cm is typically classified as being of high risk, recent studies have demonstrated that maximum wall stress could be a better indicator of an AAA rupture than maximum diameter [2]. The wall stress is greatly influenced by the blood pressure, aneurysm diameter, shape, wall thickness and the presence of thrombus. The work done by Finol et al. suggested that hemodynamic pressure variations have an insignificant effect on AAA wall stress and that primarily the shape of the aneurysm determines the stress distribution. They noted that for peak wall stress studies the static pressure conditions would suffice as the in vivo conditions. Wang et al have developed an isotropic hyperelastic constitutive model for the intraluminal thrombus (ILT). Such models have been used to study the stress distributions in patient specific AAAs [3, 4].

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Local Mechanical Properties of Abdominal Aortic Aneurysms

ASME 2008 Summer Bioengineering Conference, Parts A and B ◽

10.1115/sbc2008-192892 ◽

2008 ◽

Cited By ~ 1

Author(s):

Evelyne van Dam ◽

Marcel Rutten ◽

Frans van de Vosse

Keyword(s):

Mechanical Properties ◽

Stress Analysis ◽

Vessel Wall ◽

Computational Models ◽

Wall Stress ◽

Abdominal Aortic Aneurysms ◽

Aortic Aneurysms ◽

Patient Specific ◽

Rupture Risk ◽

Abdominal Aortic

Rupture risk of abdominal aortic aneurysms (AAA) based on wall stress analysis may be superior to the currently used diameter-based rupture risk prediction [4; 5; 6; 7]. In patient specific computational models for wall stress analysis, the geometry of the aneurysm is obtained from CT or MR images. The wall thickness and mechanical properties are mostly assumed to be homogeneous. The pathological AAA vessel wall may contain collageneous areas, but also calcifications, cholesterol crystals and large amounts of fat cells. No research has yet focused yet on the differences in mechanical properties of the components present within the degrading AAA vessel wall.

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A Predictive Analysis of Wall Stress in Abdominal Aortic Aneurysms Using a Neural Network Model

Journal of Biomechanical Engineering ◽

10.1115/1.4051905 ◽

2021 ◽

Author(s):

Balaji Rengarajan ◽

Sourav Patnaik ◽

Ender A. Finol

Keyword(s):

Neural Network ◽

Finite Element ◽

Finite Element Modeling ◽

Wall Stress ◽

Aortic Aneurysms ◽

Multivariate Adaptive Regression Splines ◽

Patient Specific ◽

Clinical Images ◽

Element Modeling ◽

Abdominal Aortic

Abstract In the present work, we investigated the use of geometric indices to predict patient-specific abdominal aortic aneurysm (AAA) wall stress by means of a novel neural network (NN) modeling approach. We conducted a retrospective review of existing clinical images of two patient groups: 98 asymptomatic and 50 symptomatic AAA. The images were subject to a protocol consisting of image segmentation, processing, volume meshing, finite element modeling, and geometry quantification, from which 53 geometric indices and the spatially averaged wall stress (SAWS) were calculated. We developed feed-forward NN models composed of an input layer, two dense layers, and an output layer using Keras, a deep learning library in Python. The NN models were trained, tested, and validated independently for both AAA groups using all geometric indices, as well as a reduced set of indices resulting from a variable reduction procedure. We compared the performance of the NN models with two standard machine learning algorithms (MARS: multivariate adaptive regression splines and GAM: generalized additive model) and a linear regression model (GLM: generalized linear model). The NN-based approach exhibited the highest overall mean goodness-of-fit and lowest overall relative error compared to MARS, GAM, and GLM, when using the reduced sets of indices to predict SAWS for both AAA groups. The use of NN modeling represents a promising alternative methodology for the estimation of AAA wall stress using geometric indices as surrogates, in lieu of finite element modeling.

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