Evaluating ascending aortic aneurysm tissue toughness: Dependence on collagen and elastin contents

Region-specific delamination strength of ascending thoracic aortic aneurysm of elderly hypertensive patients with bicuspid and tricuspid aortic valves

Both ageing and hypertension are clinical factors that may lead to a higher propensity for dissection or rupture of ascending thoracic aortic aneurysms (ATAAs). This study sought to investigate effect of valve morphology on regional delamination strength of ATAAs in the elderly hypertensive patients. Whole fresh ATAA samples were harvested from 23 hypertensive patients (age, 71 ± 8 years) who underwent elective aortic surgery. Peeling tests were performed to measure region-specific delamination strengths of the ATAAs, which were compared between patients with bicuspid aortic valve (BAV) and tricuspid aortic valve (TAV). The regional delamination strengths of the ATAAs were further correlated with patient ages and aortic diameters for BAV and TAV groups. In the anterior and right lateral regions, the longitudinal delamination strengths of the ATAAs were statistically significantly higher for BAV patients than TAV patients (33 ± 7 vs. 23 ± 8 mN/mm, p = 0.01; 30 ± 7 vs. 19 ± 9 mN/mm, p = 0.02). For both BAV and TAV patients, the left lateral region exhibited significantly higher delamination strengths in both directions than the right lateral region. Histology revealed that disruption of elastic fibers in the right lateral region of the ATAAs was more severe for the TAV patients than the BAV patients. A strong inverse correlation between longitudinal delamination strength and age was identified in the right lateral region of the ATAAs of the TAV patients. Results suggest that TAV-ATAAs are more vulnerable to aortic dissection than BAV-ATAAs for the elderly hypertensive patients. Regardless of valve morphotypes, the right lateral region may be a special quadrant which is more likely to initiate dissection when compared with other regions.

Evidence in Clinical Studies for the Role of Wall Thickness in Ascending Thoracic Aortic Aneurysms: A Scoping Review

BACKGROUND

Ascending thoracic aortic aneurysm is a chronic degenerative pathology characterized by dilatation of this segment of the aorta. Clinical guidelines use aortic diameter and growth rate as predictors of rupture and dissection. However, these guidelines neglect the effects of tissue remodeling, which may affect wall thickness. The present study aims to systematically review observational studies to examine to what extent wall thickness is considered and measured in clinical practice.

METHODS

Using PubMed and Web of Science, studies were identified with data on ascending aortic wall thickness, morphology, aortic diameter, and measurement techniques.

RESULTS

15 included studies report several methods by which wall thickness is measured. No association was observed between wall thickness and aortic diameter across included studies. Wall thickness values appear not materially different between aneurysmatic aortas and non-aneurysmal aortas.

CONCLUSIONS

The effects on and consequences of wall thickness changes during ATAA formation are ill-defined. Wall thickness values for aneurysmatic aortas can be similar to aortas with normal diameters. Given the existing notion that wall thickness is a determinant of mechanical stress homeostasis, our review exposes a clear need for consistent as well as clinically applicable methods and studies to quantify wall thickness in ascending aortic aneurysm research.

Effect of hypertension on the delamination and tensile strength of ascending thoracic aortic aneurysm with a focus on right lateral region

Hypertension is a major predisposing factor to initiate thoracic aortopathy. The objective of this study is to investigate effect of hypertension on delamination and tensile strength of ascending thoracic aortic aneurysms (ATAAs). A total of 35 fresh ATAA samples were harvested from 19 hypertensive and 16 non-hypertensive patients during elective aortic surgery. Peeling tests with two extension rates were performed to determine delamination strength, while uniaxial tensile (UT) tests were employed to measure failure stresses. The delamination strength and failure stresses of the ATAAs were further correlated with patient ages for hypertensive and non-hypertensive groups. The delamination strength to peel apart the ATAA tissue along the longitudinal direction was statistically significantly lower for the hypertensive patients than that of the non-hypertensive patients (35 ± 11 vs. 49 ± 9 mN/mm, p = 0.02). A higher delamination strength was measured if peeling was performed with a higher extension rate. The circumferential failure stresses were significantly lower for the hypertensive ATAAs than those of the non-hypertensive ATAAs (1.03 ± 0.27 vs. 1.43 ± 0.38 MPa, p = 0.02). Histology showed that laminar structures of elastic fibers were mainly disrupted in the hypertensive ATAAs. The longitudinal delamination strength of the ATAAs was significantly decreased and strongly correlated with ages for the hypertensive patients. Strong inverse correlations were also identified between the circumferential and longitudinal failure stresses of the ATAAs and ages for the hypertensive patients. Results suggest that the ATAAs of the elderly hypertensive patients may have a higher propensity for dissection or rupture. The dissection properties of the ATAA tissue are rate dependent.

A classification approach to improve out of sample predictability of structure-based constitutive models for ascending thoracic aortic tissue

In this research, a pipeline was developed to assess the out-of-sample predictive capability of structure-based constitutive models of ascending aortic aneurysmal tissue. The hypothesis being tested is that a biomarker can help establish similarities among tissues sharing the same level of a quantifiable property, thus enabling the development of biomarker-specific constitutive models. Biomarker-specific averaged material models were constructed from biaxial mechanical tests of specimens that shared similar biomarker properties such as level of blood-wall shear stress or microfiber (elastin or collagen) degradation in the extracellular matrix. Using a cross-validation strategy commonly used in classification algorithms, biomarker-specific averaged material models were assessed in contrast to individual tissue mechanics of out of sample specimens that fell under the same category but did not contribute to the averaged model's generation. The normalized root means square errors (NRMSE) calculated on out-of-sample data were compared with average models when no categorization was performed versus biomarker-specific models and among different level of a biomarker. Different biomarker levels exhibited statistically different NRMSE when compared among each other, indicating more common features shared by the specimens belonging to the lower error groups. However, no specific biomarkers reached a significant difference when compared to the average model created when No Categorization was performed, possibly on account of unbalanced number of specimens. The method developed could allow for the screening of different biomarkers or combinations/interactions in a systematic manner leading the way to larger datasets and to more individualized constitutive approaches.

Interaction of the Blood Components with Ascending Thoracic Aortic Aneurysm Wall: Biomechanical and Fluid Analyses

Background: Ascending thoracic aortic aneurysm (ATAA) is an asymptomatic localized dilation of the aorta that is prone to rupture with a high rate of mortality. While diameter is the main risk factor for rupture assessment, it has been shown that the peak wall stress from finite element (FE) simulations may contribute to refinement of clinical decisions. In FE simulations, the intraluminal boundary condition is a single-phase blood flow that interacts with the thoracic aorta (TA). However, the blood is consisted of red blood cells (RBCs), white blood cells (WBCs), and plasma that interacts with the TA wall, so it may affect the resultant stresses and strains in the TA, as well as hemodynamics of the blood. Methods: In this study, discrete elements were distributed in the TA lumen to represent the blood components and mechanically coupled using fluid–structure interaction (FSI). Healthy and aneurysmal human TA tissues were subjected to axial and circumferential tensile loadings, and the hyperelastic mechanical properties were assigned to the TA and ATAA FE models. Results: The ATAA showed larger tensile and shear stresses but smaller fluid velocity compared to the ATA. The blood components experienced smaller shear stress in interaction with the ATAA wall compared to TA. The computational fluid dynamics showed smaller blood velocity and wall shear stress compared to the FSI. Conclusions: This study is a first proof of concept, and future investigations will aim at validating the novel methodology to derive a more reliable ATAA rupture risk assessment considering the interaction of the blood components with the TA wall.

Translation of mechanical strain to a scalable biomanufacturing process for acellular matrix production from full thickness porcine bladders

Bladder acellular matrix has promising applications in urological and other reconstructive surgery as it represents a naturally compliant, non-immunogenic and highly tissue-integrative material. As the bladder fills and distends, the loosely-coiled bundles of collagen fibres in the wall become extended and orientate parallel to the lumen, resulting in a physical thinning of the muscular wall. This accommodating property can be exploited to achieve complete decellularisation of the full-thickness bladder wall by immersing the distended bladder through a series of hypotonic buffers, detergents and nucleases, but the process is empirical, idiosyncratic and does not lend itself to manufacturing scale up. In this study we have taken a mechanical engineering approach to determine the relationship between porcine bladder size and capacity, to define the biaxial deformation state of the tissue during decellularisation and to apply these principles to the design and testing of a scalable novel laser-printed flat-bed apparatus in order to achieve reproducible and full-thickness bladder tissue decellularisation. We demonstrate how the procedure can be applied reproducibly to fresh, frozen or twice-frozen bladders to render8×8 cm²patches of DNA-free acellular matrix suitable for surgical applications.

Relative strain is a novel predictor of aneurysmal degeneration of the thoracic aorta: An ex vivo mechanical study

OBJECTIVE

Current guidelines for prophylactic replacement of the thoracic aorta, primarily based on size alone, may not be adequate in identifying patients at risk for either progression of disease or aortic catastrophe. We undertook the current study to determine whether the mechanical properties of the aorta might be able to predict aneurysmal dilatation of the aorta using a clinical database and benchtop mechanical testing of human aortic tissue.

METHODS

Using over 400 samples from 31 patients, mechanical properties were studied in (a) normal aorta and then (b) between normal and diseased aorta using linear mixed-effects models. A machine learning technique was used to predict aortic growth rate over time using mechanical properties and baseline clinical characteristics.

RESULTS

Healthy aortic tissue under in vivo loading conditions, after accounting for aortic segment location, had lower longitudinal elastic modulus compared with circumferential elastic modulus: -166.8 kPa (95% confidence interval [CI]: -210.8 to -122.7, P < .001). Fracture toughness was also lower in the longitudinal vs circumferential direction: -201.2 J/m3 (95% CI: -272.9 to -129.5, P < .001). Finally, relative strain was lower in the longitudinal direction compared with the circumferential direction: -0.01 (95% CI: -0.02 to -0.004, P = .002). Patients with diseased aorta, after accounting for segment location and sample direction, had decreased toughness compared with normal aorta, -431.7 J/m3 (95% CI: -628.6 to -234.8, P < .001), and increased relative strain, 0.09 (95% CI: 0.04 to 0.14, P = .003).

CONCLUSIONS

Increasing relative strain was identified as a novel independent predictor of aneurysmal degeneration. Noninvasive measurement of relative strain may aid in the identification and monitoring of patients at risk for aneurysmal degeneration. (JVS-Vascular Science 2021;2:1-12.).

CLINICAL RELEVANCE

Aortic aneurysm surveillance and prophylactic surgical recommendations are based on computed tomographic angiogram aortic dimensions and growth rate measurements. However, aortic catastrophes may occur at small sizes, confounding current risk stratification models. Herein, we report that increasing aortic relative strain, that is, greater distensibility, is associated with growth over time, thus potentially identifying patients at risk for dissection/rupture.

Biomechanical properties of ascending aortic aneurysms: Quantification of inter- and intra-patient variability

This study investigates the biomechanical properties of ascending aortic aneurysms focusing on the inter-patient differences vs. the heterogeneity within a patient's aneurysm. Each specimen was tested on a biaxial testing device and the resulting stress-strain response was fitted to a four-parameter Fung constitutive model. We postulate that the inter-patient variability (differences between patients) blurs possible intra-patient variability (regional heterogeneity) and, thus, that both effects must be considered to shed light on the role of heterogeneity in aneurysm progression. We propose, demonstrate, and discuss two techniques to assess differences by, first, comparing conventional biomechanical properties and, second, the overall constitutive response. Results show that both inter- and intra-patient variability contribute to errors when using population averaged models to fit individual tissue behaviour. When inter-patient variability was accounted for and its effects excluded, intra-patient heterogeneity could be assessed, showing a wide degree of heterogeneity at the individual patient level. Furthermore, the right lateral region (from the patient's perspective) appeared different (stiffer) than the other regions. We posit that this heterogeneity could be a consequence of maladaptive remodelling due to altered loading conditions that hastens microstructural changes naturally occurring with age. Further validation of these results should be sought from a larger cohort study.

Targeted Gold Nanoparticles as an Indicator of Mechanical Damage in an Elastase Model of Aortic Aneurysm

Elastin is a key structural protein and its pathological degradation deterministic in aortic aneurysm (AA) outcomes. Unfortunately, using current diagnostic and clinical surveillance techniques the integrity of the elastic fiber network can only be assessed invasively. To address this, we employed fragmented elastin-targeting gold nanoparticles (EL-AuNPs) as a diagnostic tool for the evaluation of unruptured AAs. Electron dense EL-AuNPs were visualized within AAs using micro-computed tomography (micro-CT) and the corresponding Gold-to-Tissue volume ratios quantified. The Gold-to-Tissue volume ratios correlated strongly with the concentration (0, 0.5, or 10 U/mL) of infused porcine pancreatic elastase and therefore the degree of elastin damage. Hyperspectral mapping confirmed the spatial targeting of the EL-AuNPs to the sites of damaged elastin. Nonparametric Spearman's rank correlation indicated that the micro-CT-based Gold-to-Tissue volume ratios had a strong correlation with loaded (ρ = 0.867, p-val = 0.015) and unloaded (ρ = 0.830, p-val = 0.005) vessel diameter, percent dilation (ρ = 0.976, p-val = 0.015), circumferential stress (ρ = 0.673, p-val = 0.007), loaded (ρ = - 0.673, p-val = 0.017) and unloaded (ρ = - 0.697, p-val = 0.031) wall thicknesses, circumferential stretch (ρ = - 0.7234, p-val = 0.018), and lumen area compliance (ρ = - 0.831, p-val = 0.003). Likewise, in terms of axial force and axial stress vs. stretch, the post-elastase vessels were stiffer. Collectively, these findings suggest that, when combined with CT imaging, EL-AuNPs can be used as a powerful tool in the non-destructive estimation of mechanical and geometric features of AAs.

Biomechanics of aortic wall failure with a focus on dissection and aneurysm: A review

Aortic dissections and aortic aneurysms are fatal events characterized by structural changes to the aortic wall. The maximum diameter criterion, typically used for aneurysm rupture risk estimations, has been challenged by more sophisticated biomechanically motivated models in the past. Although these models are very helpful for the clinicians in decision-making, they do not attempt to capture material failure. Following a short overview of the microstructure of the aorta, we analyze the failure mechanisms involved in the dissection and rupture by considering also traumatic rupture. We continue with a literature review of experimental studies relevant to quantify tissue strength. More specifically, we summarize more extensively uniaxial tensile, bulge inflation and peeling tests, and we also specify trouser, direct tension and in-plane shear tests. Finally we analyze biomechanically motivated models to predict rupture risk. Based on the findings of the reviewed studies and the rather large variations in tissue strength, we propose that an appropriate material failure criterion for aortic tissues should also reflect the microstructure in order to be effective. STATEMENT OF SIGNIFICANCE: Aortic dissections and aortic aneurysms are fatal events characterized by structural changes to the aortic wall. Despite the advances in medical, biomedical and biomechanical research, the mortality rates of aneurysms and dissections remain high. The present review article summarizes experimental studies that quantify the aortic wall strength and it discusses biomechanically motivated models to predict rupture risk. We identified contradictory observations and a large variation within and between data sets, which may be due to biological variations, different sample sizes, differences in experimental protocols, etc. Based on the findings of the reviewed literature and the rather large variations in tissue strength, it is proposed that an appropriate criterion for aortic failure should also reflect the microstructure.

Bicuspid valve aortopathy is associated with distinct patterns of matrix degradation

OBJECTIVE

To explore the micromechanical, biochemical, and microstructural differences between bicuspid aortic valve aneurysm (BAV-A) and tricuspid aortic valve idiopathic degenerative aneurysm (DA), compared with normal aorta.

METHODS

Aortic tissue was obtained from patients undergoing aneurysmal repair surgery (BAV-A; n = 15 and DA; n = 15). Control tissue was obtained from aortic punch biopsies during coronary artery bypass graft surgery (n = 9). Nanoindentation was used to determine the elastic modulus on the medial layer. Glycosaminoglycan, collagen, and elastin levels were measured using biochemical assays. Verhoeff Van Gieson-stained cross-sections were imaged for elastin microstructural quantification.

RESULTS

The elastic modulus was more than 20% greater for BAV-A relative to control and DA (signifying a loss of compliance). No significance difference between control and DA were observed. Collagen levels for BAV-A (36.9 ± 7.4 μg/mg) and DA (49.9 ± 10.9 μg/mg) were greater compared with the control (30.2 ± 13.1 μg/mg). Glycosaminoglycan and elastin levels were not significant between the groups. Elastin segments were uniform throughout the control. Aneurysmal tissues had less elastin segments close to the intima and adventitia layers. Both BAV-A and DA had elastin segments compacted in the media; however, elastin segments were highly fragmented in DA.

CONCLUSIONS

BAV-A has a greater loss of aortic wall compliance relative to DA and the control. Although elastin levels were equal for all groups, spatial distribution of elastin provided a unique profile of matrix degradation for BAV-A. Elastin compaction within the media of BAV-A may have resulted from the altered hemodynamic pressure against the wall, which could explain for the stiffness of the tissue.

Does the Knowledge of the Local Thickness of Human Ascending Thoracic Aneurysm Walls Improve Their Mechanical Analysis?

Ascending thoracic aortic aneurysm (ATAA) ruptures are life threatening phenomena which occur in local weaker regions of the diseased aortic wall. As ATAAs are evolving pathologies, their growth represents a significant local remodeling and degradation of the microstructural architecture and thus their mechanical properties. To address the need for deeper study of ATAAs and their failure, it is required to analyze the mechanical behavior at the sub-millimeter scale by making use of accurate geometrical and kinematical measurements during their deformation. For this purpose, we propose a novel methodology that combined an accurate tool for thickness distribution measurement of the arterial wall, digital image correlation to assess local strain fields and bulge inflation to characterize the physiological and failure response of flat unruptured human ATAA specimens. The analysis of the heterogeneity of the local thickness and local physiological stress and strain was carried out for each investigated subject. At the subject level, our results state the presence of a non-consistent relationship between the local wall thickness and the local physiological strain field and high heterogeneity of the variables. At the inter-subject level, thicknesses were studied in relation to physiological strain and stress and load at rupture. The rupture pressure was correlated with neither the average thickness nor the lowest thickness of the specimens. Our results confirm that intrinsic material strength (hence structure) differs a lot from a subject to another and even within the same subject.

Investigation of the observed rupture lines in abdominal aortic aneurysms using crack propagation simulations

OBJECTIVE

To use crack propagation simulation to study the rupture site characteristics in ruptured abdominal aortic aneurysms (AAA).

METHODS

Rupture lines were precisely documented in four ruptured AAA harvested whole from cadavers. Wall thickness and material parameters were experimentally determined. Using subject-specific 3D geometry and subject-specific finite elastic model parameters, crack propagation simulations were conducted based on basic fracture mechanics principles to investigate if and how localized weak spots may have led to the observed rupture lines.

RESULTS AND CONCLUSION

When an initial crack was imposed at the site of peak wall stress, the propagated path did not match the observed rupture line. This indicates that in this study population, the peak wall stress was unlikely to have caused the observed rupture. When cracks were initiated at random locations in the AAA along random orientations for random initial lengths, the orientation of the resulting propagated rupture line was consistently longitudinal. This suggests that the AAA morphology predisposes the AAA to rupture longitudinally, which is consistent with observations. It was found that, in this study population, rupture may have initiated at short segments of less than about 1 cm that then propagated to form the observed rupture lines. This suggests that ex vivo experimental and in vivo elastography studies should seek a spatial resolution (approx. 1 cm) to reliably identify weak spots in AAA. The small study population and lack of a reliable failure model for AAA tissue make these findings preliminary.

Local variations in material and structural properties characterize murine thoracic aortic aneurysm mechanics

We recently developed an approach to characterize local nonlinear, anisotropic mechanical properties of murine arteries by combining biaxial extension-distension testing, panoramic digital image correlation, and an inverse method based on the principle of virtual power. This experimental-computational approach was illustrated for the normal murine abdominal aorta assuming uniform wall thickness. Here, however, we extend our prior approach by adding an optical coherence tomography (OCT) imaging system that permits local reconstructions of wall thickness. This multimodality approach is then used to characterize spatial variations of material and structural properties in ascending thoracic aortic aneurysms (aTAA) from two genetically modified mouse models (fibrillin-1 and fibulin-4 deficient) and to compare them with those from angiotensin II-infused apolipoprotein E-deficient and wild-type control ascending aortas. Local values of stored elastic energy and biaxial material stiffness, computed from spatial distributions of the best fit material parameters, varied significantly with circumferential position (inner vs. outer curvature, ventral vs. dorsal sides) across genotypes and treatments. Importantly, these data reveal an inverse relationship between material stiffness and wall thickness that underlies a general linear relationship between stiffness and wall stress across aTAAs. OCT images also revealed sites of advanced medial degeneration, which were captured by the inverse material characterization. Quantification of histological data further provided high-resolution local correlations among multiple mechanical metrics and wall microstructure. This is the first time that such structural defects and local properties have been characterized mechanically, which can better inform computational models of aortopathy that seek to predict where dissection or rupture may initiate.

Fatigue exhaustion of the mitral valve tissue

Sudden failure and rupture of the tissue is a rare but serious short-term complication after the mitral valve surgical repair. Excessive cyclic loading on the suture line of the repair can progressively damage the surrounding tissue and finally cause tissue rupture. Moreover, mechanical over-tension, which occurs in a diseased mitral valve, gradually leads to tissue floppiness, mitral annular dilation, and leaflet rupture. In this work, the rupture mechanics of mitral valve is studied by characterizing the fracture toughness exhaustion of healthy tissue. Results of this study show that fracture toughness of the posterior mitral valve is lower than its anterior counterpart, indicating that posterior tissue is more prone to failure. Moreover, the decrease in fracture toughness by increasing the number of fatigue cycles shows that excessive mechanical loading leads to progressive failure and rupture of mitral valve tissue within a damage accumulative process.

Structural modeling reveals microstructure-strength relationship for human ascending thoracic aorta

High lethality of aortic dissection necessitates accurate predictive metrics for dissection risk assessment. The not infrequent incidence of dissection at aortic diameters <5.5 cm, the current threshold guideline for surgical intervention (Nishimura et al., 2014), indicates an unmet need for improved evidence-based risk stratification metrics. Meeting this need requires a fundamental understanding of the structural mechanisms responsible for dissection evolution within the vessel wall. We present a structural model of the repeating lamellar structure of the aortic media comprised of elastic lamellae and collagen fiber networks, the primary load-bearing components of the vessel wall. This model was used to assess the role of these structural features in determining in-plane tissue strength, which governs dissection initiation from an intimal tear. Ascending aortic tissue specimens from three clinically-relevant patient populations were considered: non-aneurysmal aorta from patients with morphologically normal tricuspid aortic valve (CTRL), aneurysmal aorta from patients with tricuspid aortic valve (TAV), and aneurysmal aorta from patients with bicuspid aortic valve (BAV). Multiphoton imaging derived collagen fiber organization for each patient cohort was explicitly incorporated in our model. Model parameters were calibrated using experimentally-measured uniaxial tensile strength data in the circumferential direction for each cohort, while the model was validated by contrasting simulated tissue strength against experimentally-measured strength in the longitudinal direction. Orientation distribution, controlling the fraction of loaded collagen fibers at a given stretch, was identified as a key feature governing anisotropic tissue strength for all patient cohorts.

Bio-chemo-mechanics of thoracic aortic aneurysms

Most thoracic aortic aneurysms (TAAs) occur in the ascending aorta. This review focuses on the unique bio-chemo-mechanical environment that makes the ascending aorta susceptible to TAA. The environment includes solid mechanics, fluid mechanics, cell phenotype, and extracellular matrix composition. Advances in solid mechanics include quantification of biaxial deformation and complex failure behavior of the TAA wall. Advances in fluid mechanics include imaging and modeling of hemodynamics that may lead to TAA formation. For cell phenotype, studies demonstrate changes in cell contractility that may serve to sense mechanical changes and transduce chemical signals. Studies on matrix defects highlight the multi-factorial nature of the disease. We conclude that future work should integrate the effects of bio-chemo-mechanical factors for improved TAA treatment.

Investigation on the Regional Loss Factor and Its Anisotropy for Aortic Aneurysms

An aortic aneurysm is a lethal arterial disease that mainly occurs in the thoracic and abdominal regions of the aorta. Thoracic aortic aneurysms are prevalent in the root/ascending parts of the aorta and can lead to aortic rupture resulting in the sudden death of patients. Understanding the biomechanical and histopathological changes associated with ascending thoracic aortic aneurysms (ATAAs), this study investigates the mechanical properties of the aorta during strip-biaxial tensile cycles. The loss factor-defined as the ratio of dissipated energy to the energy absorbed during a tensile cycle-the incremental modulus, and their anisotropy indexes were compared with the media fiber compositions for aneurysmal (n = 26) and control (n = 4) human ascending aortas. The aneurysmal aortas were categorized into the aortas with bicuspid aortic valves (BAV) and tricuspid aortic valves (TAV). The strip-biaxial loss factor correlates well with the diameter of the aortas with BAV and TAV (for the axial direction, respectively, R² = 0.71, p = 0.0022 and R² = 0.54, p = 0.0096). The loss factor increases significantly with patients' age in the BAV group (for the axial direction: R² = 0.45, p = 0.0164). The loss factor is isotropic for all TAV quadrants, whereas it is on average only isotropic in the anterior and outer curvature regions of the BAV group. The results suggest that loss factor may be a useful surrogate measure to describe the histopathology of aneurysmal tissue and to demonstrate the differences between ATAAs with the BAV and TAV.