Mechanical Stresses in Abdominal Aortic Aneurysms: Influence of Diameter, Asymmetry, and Material Anisotropy

2008 ◽  
Vol 130 (2) ◽  
Author(s):  
José F. Rodríguez ◽  
Cristina Ruiz ◽  
Manuel Doblaré ◽  
Gerhard A. Holzapfel
Keyword(s):  
Stress Analysis ◽  
Strain Energy Function ◽  
Peak Stress ◽  
Wall Stress ◽  
Aortic Aneurysms ◽  
Maximum Diameter ◽  
Material Anisotropy ◽  
Tissue Samples ◽  
Abdominal Aortic

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.

Porohyperelastic Simulation of Abdominal Aortic Aneurysms

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].

Fluid-Solid Interaction Simulation of Flow and Stress Pattern in Thoracoabdominal Aneurysms: A Patient Specific Study

2006 ◽  
Author(s):  
Alessandro Borghi ◽  
Nigel B. Wood ◽  
Raad H. Mohiaddin ◽  
X. Yun Xu
Keyword(s):  
Wall Stress ◽  
Aortic Aneurysms ◽  
Maximum Diameter ◽  
Patient Specific ◽  
Specific Flow ◽  
Hyperelastic Materials ◽  
Abdominal Aortic ◽  
Magnetic Resonance Imaging Mri ◽  
Fluid Solid Interaction

Thoracoabdominal aneurysm (TA) is a pathology that involves the enlargement of the aortic diameter in the inferior descending thoracic aorta and has risk factors including aortic dissection, aortitis or connective tissue disorders (Webb, T. H. and Williams, G. M. 1999). Abnormal flow patterns and stress on the diseased aortic wall are thought to play an important role in the development of this pathology and the internal wall stress has proved to be more reliable as a predictor of rupture than the maximum diameter for abdominal aortic aneurysms (Fillinger, M. F., et al. 2003). In the present study, two patients with TAs of different maximum diameters were scanned using magnetic resonance imaging (MRI) techniques. Realistic models of the aneurysms were reconstructed from the in vivo MRI data acquired from the patients, and subject-specific flow conditions were applied as boundary conditions. The wall and thrombus were modeled as hyperelastic materials and their properties were derived from the literature. Fully coupled fluid-solid interaction simulations were performed for both cases using ADINA 8.2. Results were obtained for both the flow and wall stress patterns within the aneurysms. The results show that the wall stress distribution and its magnitude are strongly dependent on the 3-D shape of the aneurysm and the distribution of thrombus.

Stress Characteristics of Non-Axisymmetric Synthetic Abdominal Aortic Aneurysm Models: How Real are They?

2004 ◽  
Vol 1-2 ◽  
pp. 245-250 ◽  
Author(s):  
Arindam Chaudhuri ◽  
Leslie E. Ansdell ◽  
Mohan Adiseshiah ◽  
Anthony J. Grass
Keyword(s):  
Blood Pressure ◽  
Inflection Point ◽  
Wall Stress ◽  
Aortic Aneurysms ◽  
In Vivo Studies ◽  
Maximum Diameter ◽  
Abdominal Aortic ◽  
Before And After ◽  
Peak Wall Stress

Abdominal aortic aneurysms (AAAs) are abnormal aortic dilatations that are prone to rupture, with fatal consequences. Synthetic aneurysm models are being used to assess in vivo stress characteristics of aneurysms before and after surgical reinforcement. This study seeks to assess peak wall stress characteristics in a latex life- like model. A life-like non-axisymmetric latex AAA model, constructed from a 3D computed tomographic reconstruction of a real AAA, was incorporated into a pulsatile flow unit (PFU) to simulate the cardiac output. Strain gauges were placed at the neck (n= 2 x 3), inflection point (the junction of neck and sac, n=4 x 3) and maximum anteroposterior diameter (n=4 x 3). The arterial pressure settings used were 130/90 and 140/100mmHg, termed the low and high setting respectively. Strain readings were obtained at 10Hz over 30 seconds using a data logger. Stress was derived using the relationship between stress and Young’s modulus (E= 5.151872 Nmm-2). Peak wall stresses were statistically analysed over the two pressure settings using ANOVA in Minitab 13. The highest stresses were noted at the inflection point and not at the maximum diameter, as might have been expected. Peak inflection point stress anteriorly measured 394.69 (SD 218.1) x10-4 N/cm2 in the low setting, increasing to 715.39(SD 230.32) in the high setting (p<0.001). Posteriorly, peak wall stress measured as high as 373.61(SD207.24) x10-4 Ncm-2 in the low setting, and increased to 1053.32 (SD 347.01) x10-4 Ncm-2 in the high setting (p<0.001). High posterior stress conforms to in vivo studies. Peak wall stresses were not as high in the sac (range 35.08-204.98 x 10-4 Ncm-2 in the low setting and 54.66- 322.73 x 10-4 Ncm-2 in the high setting). An increase in blood pressure caused an increase only in the anterior and lateral, but not the posterior aspect of the sac (p<0.05). Abdominal aortic wall stress is highest at the inflection point, and is affected by blood pressure, which suggests that it is the area most likely to rupture and is critical to reinforcement of the AAA. These readings are lower than stress noticed in vivo due to the lower E of latex as compared to aneurysmal aorta, which structurally is primarily a multilaminate of elastin and collagen; however, the trends themselves may parallel those that occur in AAAs before and after endovascular or open grafting, and therefore justify artificial stress modelling of AAAs.

Pulsatile Flow in Fusiform Models of Abdominal Aortic Aneurysms: Flow Fields, Velocity Patterns and Flow-Induced Wall Stresses

2004 ◽  
Vol 126 (4) ◽  
pp. 438-446 ◽  
Author(s):  
Robert A. Peattie ◽  
Tiffany J. Riehle ◽  
Edward I. Bluth
Keyword(s):  
Shear Stress ◽  
Pulsatile Flow ◽  
Maximum Shear Stress ◽  
Wall Stress ◽  
Abdominal Aortic Aneurysms ◽  
Flow Fields ◽  
Aortic Aneurysms ◽  
Abdominal Aortic ◽  
Risk Of Rupture

As one important step in the investigation of the mechanical factors that lead to rupture of abdominal aortic aneurysms, flow fields and flow-induced wall stress distributions have been investigated in model aneurysms under pulsatile flow conditions simulating the in vivo aorta at rest. Vortex pattern emergence and evolution were evaluated, and conditions for flow stability were delineated. Systolic flow was found to be forward-directed throughout the bulge in all the models, regardless of size. Vortices appeared in the bulge initially during deceleration from systole, then expanded during the retrograde flow phase. The complexity of the vortex field depended strongly on bulge diameter. In every model, the maximum shear stress occurred at peak systole at the distal bulge end, with the greatest shear stress developing in a model corresponding to a 4.3 cm AAA in vivo. Although the smallest models exhibited stable flow throughout the cycle, flow in the larger models became increasingly unstable as bulge size increased, with strong amplification of instability in the distal half of the bulge. These data suggest that larger aneurysms in vivo may be subject to more frequent and intense turbulence than smaller aneurysms. Concomitantly, increased turbulence may contribute significantly to wall stress magnitude and thereby to risk of rupture.

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

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.

scholarly journals High-frequency murine ultrasound provides enhanced metrics of BAPN-induced AAA growth

2019 ◽  
Vol 317 (5) ◽  
pp. H981-H990 ◽  
Author(s):  
Daniel J. Romary ◽  
Alycia G. Berman ◽  
Craig J. Goergen
Keyword(s):  
Surface Area ◽  
Murine Model ◽  
High Frequency ◽  
Wall Stress ◽  
Growth Patterns ◽  
Aortic Aneurysms ◽  
Maximum Diameter ◽  
Rupture Risk ◽  
Abdominal Aortic ◽  
Peak Wall Stress

An abdominal aortic aneurysm (AAA), defined as a pathological expansion of the largest artery in the abdomen, is a common vascular disease that frequently leads to death if rupture occurs. Once diagnosed, clinicians typically evaluate the rupture risk based on maximum diameter of the aneurysm, a limited metric that is not accurate for all patients. In this study, we worked to evaluate additional distinguishing factors between growing and stable murine aneurysms toward the aim of eventually improving clinical rupture risk assessment. With the use of a relatively new mouse model that combines surgical application of topical elastase to cause initial aortic expansion and a lysyl oxidase inhibitor, β-aminopropionitrile (BAPN), in the drinking water, we were able to create large AAAs that expanded over 28 days. We further sought to develop and demonstrate applications of advanced imaging approaches, including four-dimensional ultrasound (4DUS), to evaluate alternative geometric and biomechanical parameters between 1) growing AAAs, 2) stable AAAs, and 3) nonaneurysmal control mice. Our study confirmed the reproducibility of this murine model and found reduced circumferential strain values, greater tortuosity, and increased elastin degradation in mice with aneurysms. We also found that expanding murine AAAs had increased peak wall stress and surface area per length compared with stable aneurysms. The results from this work provide clear growth patterns associated with BAPN-elastase murine aneurysms and demonstrate the capabilities of high-frequency ultrasound. These data could help lay the groundwork for improving insight into clinical prediction of AAA expansion. NEW & NOTEWORTHY This work characterizes a relatively new murine model of abdominal aortic aneurysms (AAAs) by quantifying vascular strain, stress, and geometry. Furthermore, Green-Lagrange strain was calculated with a novel mapping approach using four-dimensional ultrasound. We also compared growing and stable AAAs, finding peak wall stress and surface area per length to be most indicative of growth. In all AAAs, strain and elastin health declined, whereas tortuosity increased.

Effect of Intraluminal Thrombus on Wall Stress and Growth Rate of Abdominal Aneurysms

2010 ◽  
Author(s):  
Lambert Speelman ◽  
E. Marielle H. Bosboom ◽  
Geert Willem H. Schurink ◽  
Jaap Buth ◽  
Marcel Breeuwer ◽  
...  
Keyword(s):  
Risk Estimation ◽  
Wall Stress ◽  
Aortic Aneurysms ◽  
Maximum Diameter ◽  
Intraluminal Thrombus ◽  
Abdominal Aortic ◽  
Risk Of Rupture ◽  
Peak Wall Stress ◽  
Higher Sensitivity ◽  
Main Factor

In the decision for surgical repair of abdominal aortic aneurysms (AAAs), the risk of rupture is weighed carefully against the risk of the surgical procedure. Currently, AAA diameter is the main factor that determines the decision for surgery. However, in rupture risk estimation AAA wall stress has higher sensitivity and specificity than maximum diameter [1]. Moreover, peak wall stress was higher for ruptured than for non-ruptured or asymptomatic AAAs [2, 3].

Local Mechanical Properties of Abdominal Aortic Aneurysms

2008 ◽  
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.

Strain Mapping From Four-Dimensional Ultrasound Reveals Complex Remodeling in Dissecting Murine Abdominal Aortic Aneurysms

2019 ◽  
Vol 141 (6) ◽  
Author(s):  
Hannah L. Cebull ◽  
Arvin H. Soepriatna ◽  
John J. Boyle ◽  
Sean M. Rothenberger ◽  
Craig J. Goergen
Keyword(s):  
Three Dimensional ◽  
Small Animal ◽  
Aortic Aneurysms ◽  
Maximum Diameter ◽  
High Frequency Ultrasound ◽  
4D Ultrasound ◽  
Abdominal Aortic ◽  
Lagrange Strain ◽  
Aortic Strain

Current in vivo abdominal aortic aneurysm (AAA) imaging approaches tend to focus on maximum diameter but do not measure three-dimensional (3D) vascular deformation or strain. Complex vessel geometries, heterogeneous wall compositions, and surrounding structures can all influence aortic strain. Improved understanding of complex aortic kinematics has the potential to increase our ability to predict aneurysm expansion and eventual rupture. Here, we describe a method that combines four-dimensional (4D) ultrasound and direct deformation estimation to compute in vivo 3D Green-Lagrange strain in murine angiotensin II-induced suprarenal dissecting aortic aneurysms, a commonly used small animal model. We compared heterogeneous patterns of the maximum, first-component 3D Green-Lagrange strain with vessel composition from mice with varying AAA morphologies. Intramural thrombus and focal breakage in the medial elastin significantly reduced aortic strain. Interestingly, a dissection that was not detected with high-frequency ultrasound also experienced reduced strain, suggesting medial elastin breakage that was later confirmed via histology. These results suggest that in vivo measurements of 3D strain can provide improved insight into aneurysm disease progression. While further work is needed with both preclinical animal models and human imaging studies, this initial murine study indicates that vessel strain should be considered when developing an improved metric for predicting aneurysm growth and rupture.

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