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Effect of thrombus on abdominal aortic aneurysm wall dilation and stress

Authors:
Mano J. Thubrikar
Mano J. Thubrikar
F Robicsek
F Robicsek
F Robicsek
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Michel R Labrosse at University of Ottawa
Michel R Labrosse
V Chervenkoff
V Chervenkoff
V Chervenkoff
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Abstract

Our goal is to understand how a mural thrombus may influence the pressure transmitted to and the dilation experienced by the abdominal aortic aneurysm (AAA) wall. Two intact AAAs with mural thrombus were removed from patients and pressurized to 100 mmHg. The pressure was measured using a micro-tip needle transducer inserted in the aneurysm wall and advanced through the thrombus. In 1 patient with AAA, similar measurements were made in vivo. Also, in vitro, in the two aneurysms the dilation as a function of pressure was measured using the markers on the surface before and after the thrombus was removed. Both, in vitro and in vivo, in the presence of the thrombus the pressure transmitted to the aneurysm wall was 91+/-10% of luminal pressure and at 6 mm from the wall it was 96+/-5%. The aneurysm dilated more in the pressure range of 0-40 mmHg (2-8%) than in the range of 40-100 mmHg (0.4-1.8%). Upon removal of the thrombus these dilations increased significantly to 4-15% and 0.9-3.3%, respectively. Overall, the strains (dilation) in the circumferential and longitudinal directions were similar before the thrombus was removed. Even though the thrombus allows the transmission of luminal pressure to the aneurysm wall, it may prevent the aneurysm from rupture by diminishing the strain on the wall. Consistent with this, a mechanical model of the thrombus proposed is "a thrombus as a fibrous network adherent to the aneurysm wall".
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Effect of thrombus on abdominal aortic aneurysm wall dilation and stress
M J Thubrikar; F Robicsek; M Labrosse; V Chervenkoff; B L Fowler
Journal of Cardiovascular Surgery; Feb 2003; 44, 1; ProQuest Nursing & Allied Health Source
pg. 67
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
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... Intraluminal thrombus (ILT) is one of the common substances in AAA. ILT has been suggested in previous studies to reduce mural stress in AAA and thus may prevent rupture of AAA (10)(11)(12). In contrast, some other studies has found that ILT can act as an inflammatory lesion of protein hydrolysis and aortic wall degeneration, thus increasing the risk of AAA rupture (13)(14)(15). ...
... But the effect of ILT on the risk of AAA rupture is still controversial. Some studies suggest that ILT reduces aortic wall shear stress, thus acting as a mechanical cushion (10,23,24), being negatively correlated with the rate of AAA growth (25). Nevertheless, ILT may also create an inflammatory environment in which neutrophils, cytokines, proteases and reactive oxygen species are sequestered (26), which may cause a decrease in wall strength. ...
High-density thrombus and maximum transverse diameter on multi-spiral computed tomography angiography combine to predict abdominal aortic aneurysm rupture
Article
Full-text available
  • Sep 2022
Objective We attempted to measure maximum transverse diameter (MTD) of and CT values of ILT by using multi-spiral computed tomography angiography (MSCTA) to investigate the predictive value of MTD with different CT values of thrombus on the risk of AAA rupture. Methods Forty-five intact abdominal aortic aneurysms (IAAA) and 17 ruptured abdominal aortic aneurysms (RAAA) were included in this study. MTD and CT values in their planes were measured from MSCTA images and aneurysm lumen and thrombus volumes were calculated for the range of different CT values. Results The median of maximum CT value of thrombus at the plane of MTD was higher in RAAA (107.0 HU) than the median in IAAA (84.5 HU) ( P < 0.001). Univariate logistic regression analysis showed that the maximum CT value was a risk factor for RAAA ( P < 0.001). It was further found that the area under the ROC curve for thrombus maximum CT value in the MTD plane to predict RAAA was 0.848 ( P < 0.001), with a cut-off value of 97.5 HU, a sensitivity of 82.35%, and a specificity of 84.44%. And the MTD of the abnormal lumen combined with the maximum CT value at its plane predicted RAAA with an area under the ROC curve of 0.901, a sensitivity of 76.47%, and a specificity of 97.78%. The further analysis of thrombus volume in the range of different CT value showed that median thrombus volume in RAAA in the range of 30 HU~150 HU was 124.2 cm ³ which was higher than the median of 81.4 cm ³ in IAAA ( P = 0.005). To exclude confounding factors (aneurysm volume), we calculated the standardized thrombus (ILT volume/total aneurysm volume), and the thrombus volume in the range of 30 HU~150 HU in RAAA was positively correlated with the standardized thrombus volume (ρ = 0.885, P < 0.001), while the thrombus volume in the range of −100 HU~30 HU was not correlated with it (ρ = 0.309, P = 0.228). Conclusions High-density ILT shown on MSCTA in AAAs is associated with aneurysm rupture, and its maximum transverse diameter combined with the maximum CT value in its plane is a better predictor of RAAA.
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... [12] There are contradicting views on the role of ILT in the risk of rupture, with studies showing either an increase or a decrease in the risk. [13,14] While ILT may prevent aneurysm rupture as a mechanical barrier, there are also opinions that it reduces wall resistance due to the inflammatory cells in its existing structure. [15] It has also been reported that ILT causes aortic degeneration in some animal models, and platelet inhibitors prevent the growth of ILT and suppress aneurysm formation. ...
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... ILT has been shown to reduce peak wall stress in AAA and thus have a protective effect on the arterial wall, which prevents rupture [9,[128][129][130]. However, other studies suggest that ILT may act as an inflammatory foci of proteolytic and enzymatic degeneration of the aortic wall, thereby increasing the risk of rupture [10,50,131,132]. ...
The Detrimental Role of Intraluminal Thrombus Outweighs Protective Advantage in Abdominal Aortic Aneurysm Pathogenesis: The Implications for the Anti-Platelet Therapy
Article
Full-text available
  • Jul 2022
Abdominal aortic aneurysm (AAA) is a common cardiovascular disease resulting in morbidity and mortality in older adults due to rupture. Currently, AAA treatment relies entirely on invasive surgical treatments, including open repair and endovascular, which carry risks for small aneurysms (diameter < 55 mm). There is an increasing need for the development of pharmacological intervention for early AAA. Over the last decade, it has been increasingly recognized that intraluminal thrombus (ILT) is involved in the growth, remodeling, and rupture of AAA. ILT has been described as having both biomechanically protective and biochemically destructive properties. Platelets are the second most abundant cells in blood circulation and play an integral role in the formation, expansion, and proteolytic activity of ILT. However, the role of platelets in the ILT-potentiated AAA progression/rupture remains unclear. Researchers are seeking pharmaceutical treatment strategies (e.g., anti-thrombotic/anti-platelet therapies) to prevent ILT formation or expansion in early AAA. In this review, we mainly focus on the following: (a) the formation/deposition of ILT in the progression of AAA; (b) the dual role of ILT in the progression of AAA (protective or detrimental); (c) the function of platelet activity in ILT formation; (d) the application of anti-platelet drugs in AAA. Herein, we present challenges and future work, which may motivate researchers to better explain the potential role of ILT in the pathogenesis of AAA and develop anti-platelet drugs for early AAA.
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... This would account for the clinical observation that AAAs treated by iliac ligation and extra-anatomical bypass-inducing thrombosis in the aneurysm-can still rupture post-operatively [74][75][76]. One explanation for a reduction in wall stress despite transmitting systemic pressure to the aortic wall is provided by Thubrikar et al. who found experimentally that the longitudinal and circumferential strains induced by pressurising explanted AAA were significantly higher after removal of the thrombus [77], indicating that adherent ILT reinforces the AAA wall. Polzer et al. demonstrated this computationally by modelling the ILT as a poroelastic material [78]. ...
Micromechanical and Ultrastructural Properties of Abdominal Aortic Aneurysms
Article
Full-text available
  • Mar 2022
Abdominal aortic aneurysms are a common condition of uncertain pathogenesis that can rupture if left untreated. Current recommended thresholds for planned repair are empirical and based entirely on diameter. It has been observed that some aneurysms rupture before reaching the threshold for repair whilst other larger aneurysms do not rupture. It is likely that geometry is not the only factor influencing rupture risk. Biomechanical indices aiming to improve and personalise rupture risk prediction require, amongst other things, knowledge of the material properties of the tissue and realistic constitutive models. These depend on the composition and organisation of the vessel wall which has been shown to undergo drastic changes with aneurysmal degeneration, with loss of elastin, smooth muscle cells, and an accumulation of isotropically arranged collagen. Most aneurysms are lined with intraluminal thrombus, which has an uncertain effect on the underlying vessel wall, with some authors demonstrating a reduction in wall stress and others a reduction in wall strength. The majority of studies investigating biomechanical properties of ex vivo abdominal aortic aneurysm tissues have used low-resolution techniques, such as tensile testing, able to measure the global material properties at the macroscale. High-resolution engineering techniques such as nanoindentation and atomic force microscopy have been modified for use in soft biological tissues and applied to vascular tissues with promising results. These techniques have the potential to advance the understanding and improve the management of abdominal aortic aneurysmal disease.
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... AAA rupture occurs when the wall stress exceeds the strength of the aortic wall. The intraluminal thrombus (ILT), practically found in every aneurysm of clinically meaningful 2 of 10 size, is suggested to have a protective role against aneurysm rupture, by diminishing the tension on the wall [7] and the peak wall stress (PWS) [8]. Other research associated the ILT thickness with optimal nidus for creating an inflammatory environment, apoptosis of vascular smooth muscle cells, and elastin degradation [9]. ...
Abdominal Aortic Aneurysm Morphology as an Essential Criterion for Stratifying the Risk of Aneurysm Rupture
Article
Full-text available
  • Feb 2022
The current stratification model of aneurysm rupture seems to be insufficient in some clinical cases. In our study, we determined the differences in wall structure between ruptured and unruptured aneurysms. We obtained computed tomography angiograms and categorized them into the following three groups, consisting of 49 patients each: the group with ruptured abdominal aortic aneurysms (rAAA), symptomatic (sAAA), and asymptomatic (aAAA). The three-dimensional AAA anatomy was digitally reconstructed for each patient through semi-automatically obtained segmentation, and each aneurysm was distinguished by the following three parameters: AFL (aneurysm flow lumen), ILT (intraluminal thrombus), and calcifications. The AFL volume was greater in rAAA compared with aAAA (p = 0.004), the ILT volume was greater in aAAA than in rAAA (p = 0.013), and the AFL/ILT surface ratio was bigger in rAAA than in aAAA (p < 0.001), sAAA than in aAAA (p = 0.033), and rAAA than in sAAA (p = 0.016). AFL/ILT surface*100 was defined as an independent predictive factor of rAAA to aAAA (OR 1.187; 95% CI 1.099–1.281), to sAAA (OR 1.045; 95% CI 1.004–1.087), and in sAAA vs. aAAA (OR 1.067; 95% CI 1.017–1.119). Consequently, the wall of rAAA differs significantly from unruptured aneurysms. The AFL/ILT surface ratio might indicate an increased risk of aneurysm rupture and the occurrence of symptoms in AAA.
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... However, although the presence of intraluminal thrombus in TAA is a rare phenomenon compared with abdominal AA, 50 regional strain assessment instead of averaging strain values over the whole segmented wall might be favorable considering the known impact of thrombosis on vessel mechanical properties. 51,52 The observed smaller principal strains in patients with a TAA in this study suggest that aorta strain could be monitored during follow-up and have an additional clinical value in assessing the TAA rupture risk. If such a hypothesis becomes clinically proven in the future, we can assume that a decrease in TAA deformability over some thresholds (which will need to be defined) during the follow-up of a patient could lead to a preventive intervention to avoid a potentially fatal rupture. ...
Deformability of ascending thoracic aorta aneurysms assessed using ultrafast ultrasound and a principal strain estimator: In vitro evaluation and in vivo feasibility
Article
Full-text available
  • Jan 2022
  • MED PHYS
Background Noninvasive vascular strain imaging under conventional line‐by‐line scanning has a low frame rate and lateral resolution and depends on the coordinate system. It is thus affected by high deformations due to image decorrelation between frames. Purpose To develop an ultrafast time‐ensemble regularized tissue‐Doppler optical‐flow principal strain estimator for aorta deformability assessment in a long‐axis view. Methods This approach alleviated the impact of lateral resolution using image compounding and that of the coordinate system dependency using principal strain. Accuracy and feasibility were evaluated in two aorta‐mimicking phantoms first, and then in four age‐matched individuals with either a normal aorta or a pathological ascending thoracic aorta aneurysm (TAA). Results Instantaneous aortic maximum and minimum principal strain maps and regional accumulated strains during each cardiac cycle were estimated at systolic and diastolic phases to characterize the normal aorta and TAA. In vitro, principal strain results matched sonomicrometry measurements. In vivo, a significant decrease in maximum and minimum principal strains was observed in TAA cases, whose range was respectively 7.9 ± 6.4% and 8.2 ± 2.6% smaller than in normal aortas. Conclusions The proposed principal strain estimator showed an ability to potentially assess TAA deformability, which may provide an individualized and reliable evaluation method for TAA rupture risk assessment.
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... These differences in definition of the pressure load between the models implemented using Non-LISA and BioPARR reflect state of the art where the exact role of ILT and its effects on stress in AAA aortic wall remain an active research topic. 23,[41][42][43] In many computational biomechanics studies of AAA, the pressure loading has been applied to ILT luminal surface. 8,26,44,45 However, an alternative approach in which the pressure is directly applied to AAA aortic wall has been also used. ...
On Stress in Abdominal Aortic Aneurysm: Linear vs Non‐linear Analysis and Aneurysms Rupture Risk
Article
  • Nov 2021
We present comprehensive biomechanical analyses of abdominal aortic aneurysms (AAA) for 43 patients. We compare stress magnitudes and stress distributions within arterial walls of abdominal aortic aneurysms (AAA) obtained using two simulation and modelling methods: 1) Fully automated and computationally very efficient linear method embedded in the software platform BioPARR Biomechanics based Prediction of Aneurysm Rupture Risk, freely available from https://bioparr.mech.uwa.edu.au/; 2) More complex and much more computationally demanding Non-LISA Non-Linear Iterative Stress Analysis that uses a non-linear inverse iterative approach and strongly non-linear material model. Both methods predicted localised high stress zones with over 90% of AAA model volume fraction subjected to stress below 20% of the 99th percentile maximum principal stress. However, for the non-linear iterative method, the peak maximum principal stress (and 99th percentile maximum principal stress) was higher and the stress magnitude in the low stress area lower than for the automated linear method embedded in BioPARR. Differences between the stress distributions obtained using the two methods tended to be particularly pronounced in the areas where the AAA curvature was large. Performance of the selected characteristic features of the stress fields (we used 99th percentile maximum principal stress) obtained using BioPARR and Non-LISA in distinguishing between the AAAs that would rupture and remain intact was for practical purposes the same for both methods. This article is protected by copyright. All rights reserved.
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A quarter of a century biomechanical rupture risk assessment of abdominal aortic aneurysms. Achievements, clinical relevance, and ongoing developments
Article
Full-text available
  • Mar 2022
Abdominal aortic aneurysm (AAA) disease, the local enlargement of the infrarenal aorta, is a serious condition that causes many deaths, especially in men exceeding 65 years of age. Over the past quarter of a century, computational biomechanical models have been developed towards the assessment of AAA risk of rupture, technology that is now on the verge of being integrated within the clinical decision‐making process. The modeling of AAA requires a holistic understanding of the clinical problem, in order to set appropriate modeling assumptions and to draw sound conclusions from the simulation results. In this article we summarize and critically discuss the proposed modeling approaches and report the outcome of clinical validation studies for a number of biomechanics‐based rupture risk indices. Whilst most of the aspects concerning computational mechanics have already been settled, it is the exploration of the failure properties of the AAA wall and the acquisition of robust input data for simulations that has the greatest potential for the further improvement of this technology. Computational biomechanical models to predict abdominal aortic aneurysms risk of rupture are now on the verge of being integrated within the clinical decision‐making process. We review the proposed modeling approaches and report the outcome of clinical validation studies. Computational approaches are well developed, and it is the exploration of the vessel wall failure properties and the acquisition of robust input data for simulations that has the greatest potential for the further improvement of this technology.
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Quantifying the effects of intraluminal thrombi and their poroelastic properties on abdominal aortic aneurysms
Article
Full-text available
  • Sep 2021
  • ARCH APPL MECH
We introduce a poroelastic model for intraluminal thrombus (ILT) that captures both the flow within ILT and its deformation. The model for ILT is coupled with blood flow and arterial wall deformation and used to study the biomechanics in image-based abdominal aortic aneurysm (AAA) models. Three different patient-specific geometries were considered in this study. Using finite element analysis, numerical simulations were performed to investigate the role of ILT on the risk of AAA rupture as assessed by Peak Wall Stress (PWS). Our results indicate that the presence of ILT may reduce wall stress in AAA. However, our results indicate that ILT permeability has little effect on AAA PWS, and that similar values of PWS are obtained with both porous and nonporous ILT models.
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CT of abdominal aortic aneurysms–Aneurysmal size and thickness of intraaneurysmal thrombus as risk factors or rupture
Article
  • Apr 1991
In order to seek for the factors to suggest a risk of rupture of abdominal aortic aneurysms (AAA), we measured the largest diameter of AAA and the thickness of intra-luminal thrombus on CT in 72 patients. The mean aneurysmal size was 64 mm in diameter in 9 patients with ruptured aneurysm and 61 mm in diameter in 63 patients with non-ruptured aneurysm respectively (p less than 0.90). The rupture of AAA was seen in 3 of 30 patients with AAA of the small size (less than 50 mm in diameter), in 3 of 16 patients with AAA of the intermediate size (51-66 mm in diameter) and in 3 of 26 patients with AAA of the large size (more than 70 mm in diameter), respectively. The mean intra-luminal thrombus was 9 mm in 9 patients with ruptured aneurysm and 19 mm in thickness in 63 patients with non-ruptured aneurysm respectively (p = 0.05). We concluded from the above results that the aneurysmal size was not important, but the thickness of intra-luminal thrombus was useful for suggestion of a risk of rupture of AAA.
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Biomechanical factors in abdominal aortic aneurysm rupture
Article
  • Dec 1993
  • Eur J Vasc Surg
Hitherto the size of abdominal aortic aneurysms (AAA) has been considered the most important factor in determining the risk of rupture. For this reason most interest has been devoted to physical, echographic and tomographic analyses of the shape of AAA. However, it is known that rupture can also occur in small AAA. Other factors must be considered to have an important role in the natural history of aneurysms. The aim of this study was to characterise the mechanical stress in the wall of an AAA due to pressure in the presence of atherosclerosis, intraluminal thrombus and anatomical restraints. The Finite Elements Method (FEM) was used to determine wall stress distribution. Due to the simplicity of the AAA structure an axisymmetric model has been built. The results of the structural analysis confirms that maximum stress increases with diameter. These effects may be reduced by the presence of intraluminal thrombus, which in the models reduces maximum stress by up to 30%; however this is not the case for dissecting thrombus. On the other hand atherosclerotic plaques cause stress concentration and a significant increase in maximum wall stress. The risk of rupture can increase by about 200%. Finally the investigation shows the FEM is a versatile tool for studying the mechanics of vascular structures. It enables the influence of various parameters on wall stress to be quantified in diagnostic settings, and so could be useful for predicting the rupture of AAA, although at present such predictions are limited by data leakage and by the approximations used in the model.
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Stress Distributions in Vascular Aneurysms: Factors Affecting Risk of Aneurysm Rupture
Article
  • Sep 1993
Aneurysm rupture occurs when local wall stresses exceed the tensile strength of vascular tissues. Knowledge of vascular wall stresses, and insight into the factors that change wall stresses, will lead to a better understanding of how aneurysms grow and rupture. In this study, stress distributions in the walls of small aneurysms were calculated using finite element analysis (FEA), a numerical technique able to predict stress distributions with greater accuracy than the Law of Laplace. Stresses were calculated for an initial small aneurysm and compared to stresses produced by increasing the aneurysm diameter, decreasing the wall thickness, and changing the material properties of the aneurysm wall. FEA calculations indicate that wall stresses are generally greatest on the inner surface of an aneurysm, and decrease nonlinearly as the outer wall is approached. Maximum wall stresses occur along the region of greatest diameter, and circumferential stresses tend to be significantly greater than longitudinal stresses. Doubling the diameter of an aneurysm produced a twofold increase in the maximum wall stress. Decreasing the wall thickness by half also produced a doubling of the maximum wall stress. Changing material properties produced no appreciable change in wall stresses. However, weaker materials fail at lower stresses, thus halving material strength would be equivalent to doubling wall stresses. We conclude that the Law of Laplace is inaccurate in predicting the complicated stress distributions that exist in aneurysm walls, and that more sophisticated tools, such as FEA, will be needed to understand this complex phenomenon. We also conclude that proportional changes in the diameter, wall thickness, or aneurysm tissue strength have roughly equivalent effects on aneurysm growth and rupture.
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Ex vivo biomechanical behavior of abdominal aortic aneurysm: Assessment using a new mathematical model
Article
  • Sep 1996
Knowledge of the biomechanical behavior of abdominal aortic aneurysm (AAA) as compared to nonaneurysmal aorta may provide information on the natural history of this disease. We have performed uniaxial tensile testing of excised human aneurysmal and nonaneurysmal abdominal aortic specimens. A new mathematical model that conforms to the fibrous structure of the vascular tissue was used to quantify the measured elastic response. We determined for each specimen the yield σy and ultimate σu strengths, the separate contribution to total tissue stiffness by elastin (E E) and collagen (E C) fibers, and a collagen recruitment parameter (A), which is a measure of the tortuosity of the collagen fibers. There was no significant difference in any of these mechanical properties between longitudinal and circumferential AAA specimens, nor inE E andE C between longitudinally oriented aneurysmal and normal specimens.A, σy, and σu were all significantly higher for the normal than for the aneurysmal group:A=0.223±0.046versus A=0.091±0.009 (mean ± SEM;p<0.0005), σyversus σy (p<0.05), and σuversus σu (p<0.0005), respectively. Our findings suggest that the AAA tissue is isotropic with respect to these mechanical properties. The observed difference inA between aneurysmal and normal aorta may be due to the complete recruitment and loading of collagen fibers at lower extensions in the former. Our data indicate that AAA rupture may be related to a reduction in tensile strength and that the biomechanical properties of AAA should be considered in assessing the severity of an individual aneurysm.
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Potential influence of intraluminal thrombus on abdominal aortic aneurysm as assessed by a new non-invasive method
Article
  • Dec 1996
  • Cardiovasc Surg
Intraluminal thrombus may play a role in abdominal aortic aneurysm pathogenesis and rupture. The purpose of this work was to demonstrate the feasibility of a new non-invasive method for the determination of the biomechanical features of the aortic wall and luminal boundary in abdominal aortic aneurysm containing intraluminal thrombus. Automated ultrasonographic measures of infrarenal aortic cross-sectional area (A) were obtained on-line along with non-invasive arterial pressure (p) from eight patients of mean (s.e.m.) age 74(3) years, with abdominal aortic aneurysm (mean dimensions 5.9(0.4) x 5.3(0.5) cm) containing intraluminal thrombus. Luminal boundary and abdominal aortic aneurysm wall were scanned separately. Compliance (C) was computed as C = (Amax - Amin)/[Amax(Pmax - Pmin)], where 'max and 'min' represent maximum and minimum values, respectively. Mean compliance was lower for the abdominal aortic aneurysm wall alone than for the luminal surface enclosed by intraluminal thrombus: 4.0(0.9) x 10(-4)/mmHg versus 9.8(1.7) x 10(-4)/mmHg (P < 0.01). Intraluminal thrombus area was nearly constant over the cardiac cycle, indicating that the thrombus is virtually incompressible. This noninvasive method to assess biomechanical features of abdominal aortic aneurysm has potential to further the understanding of the influences of intraluminal thrombus on aneurysm disease.
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Possible objectification of a critical maximum diameter for elective surgery in abdominal aortic aneurysms based on one- and three-dimensional ratios
Article
  • Jan 1998
The purpose of this study was to evaluate the ratio of aneurysm size to aortic diameter (dm/ds) and newly defined volumetric parameters and to objectify by means of these parameters a critical maximum diameter (dm) as indicator for elective surgery of abdominal aortic aneurysms (AAA). Based on contrast-enhanced CT scans one- and three-dimensional parameters were measured in 3 groups of n=58 patients selected for a) emergency surgery (n=13) in case of contained rupture and b) elective surgery (n=29) or c) non operative follow-up (n=16) if the AAA was bigger or smaller than 4.5 cm in dm. Ignoring these groups the statistically independent ratios dm/ds and thrombus to aneurysm volume (TV/AV) of all patients were retrospectively subjected to cluster and regression analyses in order to find out possible critical values confirming a dm-based decision. Thrombus (TV) and aneurysm volume (AV) correlated with increasing diameter. The ratio TV/AV predominantly increased from 0.42 to 0.65 in aneurysms 5 to 8 cm in diameter, reaching a maximum of 0.8 in AAA with contained rupture. A TV/AV of 0.45 and a dm/ds of 2.0 were found to be separators of two distinguished groups confirming a critical dm of about 5 cm. The newly defined volumetric ratio TV/AV and dm/ds may objectify the dm-based decision for elective surgery or follow-up treatment. Prospective studies have to evaluate this approach.
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Biomechanics of abdominal aortic aneurysm in the presence of endoluminal thrombus: Experimental characterisation and structural static computational analysis
Article
  • May 1998
  • EUR J VASC ENDOVASC
To evaluate the role played by biomechanical and geometrical parameters of endoluminal thrombus and of aortic wall on abdominal aortic aneurysm (AAA) behaviour. Tensile tests on 21 AAA thrombus specimens from six patients undergoing AAA repair and numerical evaluation of aneurysmal aortic wall stress and strain distribution. Parameters of the analysis were lumen eccentricity, thrombus Young's Modulus and the aortic wall constitutive equation. There was a linear stress/strain for all the thrombus specimens. The numerical analyses show the mechanical behaviour of AAA as a function of lumen eccentricity and biomechanical parameters. Well organised thrombus reduces the effect of the pressure load on the aneurysmal aortic wall.
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Abdominal aortic aneurysm rupture rates: A 7-year follow-up of the entire abdominal aortic aneurysm population detected by screening
Article
  • Aug 1998
  • J VASC SURG
The goal of the current study was to identify the risk of rupture in the entire abdominal aortic aneurysm (AAA) population detected through screening and to review strategies for surgical intervention in light of this information. Two hundred eighteen AAAs were detected through ultrasound screening of a family practice population of 5394 men and women aged 65 to 80 years. Subjects with an AAA of less than 6.0 cm in diameter were followed prospectively with the use of ultrasound, according to our protocol, for 7 years. Patients were offered surgery if symptomatic, if the aneurysm expanded more than 1.0 cm per year, or if aortic diameter reached 6.0 cm. The maximum potential rupture rate (actual rupture rate plus elective surgery rate) for small AAAs (3.0 to 4.4 cm) was 2.1% per year, which is less than most reported operative mortality rates. The equivalent rate for aneurysms of 4.5 to 5.9 cm was 10.2% per year. The actual rupture rate for aneurysms up to 5.9 cm using our criteria for surgery was 0.8% per year In centers with an operative mortality rate of greater than 2%, (1) surgical intervention is not indicated for asymptomatic AAAs of less than 4.5 cm in diameter, and (2) elective surgery should be considered only for patients with aneurysms between 4.5 and 6 cm in diameter that are expanding by more than 1 cm per year or for patients in whom symptoms develop. In centers with elective mortality rates of greater than 10% for abdominal aortic aneurysm (AAA) repair, the benefit to the patient of any surgical intervention for an asymptomatic AAA of less than 6.0 cm in diameter is questionable.
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Effect of Intraluminal Thrombus Thickness and Bulge Diameter on the Oxygen Diffusion in Abdominal Aortic Aneurysm
Article
  • Nov 1998
The intraluminal thrombus (ILT) commonly found within abdominal aortic aneurysm (AAA) may serve as a barrier to oxygen diffusion from the lumen to the inner layers of the aortic wall. The purpose of this work was to address this hypothesis and to assess the effects of AAA bulge diameter (dAAA) and ILT thickness (delta) on the oxygen flow. A hypothetical, three-dimensional, axisymmetric model of AAA containing ILT was created for computational analysis. Commercial software was utilized to estimate the volume flow of O2 per cell, which resulted in zero oxygen tension at the AAA wall. Solutions were generated by holding one of the two parameters fixed while varying the other. The supply of O2 to the AAA wall increases slightly and linearly with dAAA for a fixed delta. This slight increase is due to the enlarged area through which diffusion of O2 may take place. The supply of O2 was found to decrease quickly with increasing delta for a fixed dAAA due to the increased resistance to O2 transport by the ILT layer. The presence of even a thin, 3 mm ILT layer causes a diminished O2 supply (less than 4 x 10(-10) mumol/min/cell). Normally functioning smooth muscle cells require a supply of 21 x 10(-10) mumol/min/cell. Thus, our analysis serves to support our hypothesis that the presence of ILT alters the normal pattern of O2 supply to the AAA wall. This may lead to hypoxic cell dysfunction in the AAA wall, which may further lead to wall weakening and increased potential for rupture.
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Wall stress distribution on three-dimensionally reconstructed models of human abdominal aortic aneurysm
Article
  • May 2000
  • J VASC SURG
Abdominal aortic aneurysm (AAA) rupture is believed to occur when the mechanical stress acting on the wall exceeds the strength of the wall tissue. Therefore, knowledge of the stress distribution in an intact AAA wall could be useful in assessing its risk of rupture. We developed a methodology to noninvasively estimate the in vivo wall stress distribution for actual AAAs on a patient-to-patient basis. Six patients with AAAs and one control patient with a nonaneurysmal aorta were the study subjects. Data from spiral computed tomography scans were used as a means of three-dimensionally reconstructing the in situ geometry of the intact AAAs and the control aorta. We used a nonlinear biomechanical model developed specifically for AAA wall tissue. By means of the finite element method, the stress distribution on the aortic wall of all subjects under systolic blood pressure was determined and studied. In all the AAA cases, the wall stress was complexly distributed, with distinct regions of high and low stress. Peak wall stress among AAA patients varied from 29 N/cm(2) to 45 N/cm(2) and was found on the posterior surface in all cases studied. The wall stress on the nonaneurysmal aorta in the control subject was relatively low and uniformly distributed, with a peak wall stress of 12 N/cm(2). AAA volume, rather than AAA diameter, was shown by means of statistical analysis to be a better indicator of high wall stresses and possibly rupture. The approach taken to estimate AAA wall stress distribution is completely noninvasive and does not require any additional involvement or expense by the AAA patient. We believe that this methodology may allow for the evaluation of an individual AAA's rupture risk on a more biophysically sound basis than the widely used 5-cm AAA diameter criterion.
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