Regular ArticleLocalization of Aortic Disease Is Associated with Intrinsic Differences in Aortic Structure
Abstract
Purpose. While localization of atherosclerosis and aneurysms to the infrarenal aorta has been attributed, in part, to hemodynamic factors, anatomic differences between the proximal and the distal aorta may also be important. Our purpose was to determine the changes in content and organization of major structural proteins (elastin and collagen) throughout the normal human aorta. Methods . Biochemical analysis for desmosine-isodesmosine (elastin) and hydroxyproline (collagen) content was done by HPLC on complete 1-cm transverse rings removed from the ascending and descending thoracic aorta and abdominal supraceliac, suprarenal, and midinfrarenal aorta. Elastin and collagen content was normalized to lumenal surface area and compared by ANOVA. Light microscopy and optical micrometry were used to determine changes in intimal, medial, and adventitial thickness and number of elastin lamellae at each level. Results . Both collagen/cm2 and elastin/cm2 decrease from the proximal to distal aorta. Collagen content did not differ among the three abdominal segments, but there was a 58% decrease in elastin between the suprarenal and the infrarenal aorta. The proportion of elastin and collagen does not differ throughout the aorta except in the infrarenal aorta where there is decreased elastin relative to collagen. Conclusion. Collagen and elastin in the distal aorta hear an increased load as compared to the proximal aorta. The infrarenal aorta differs biochemically and histologically from the remainder of the aorta. A decrease in infrarenal elastin without a corresponding decrease in collagen may effect the compliance and integrity of the distal aorta. These anatomic differences may be important in predisposing the infrarenal aorta to atherosclerosis and aneurysm formation.
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Ex vivo biaxial load testing analysis of aortic biomechanics demonstrates variation in elastic energy distribution across the aortic zone zero
2023, Journal of Thoracic and Cardiovascular SurgeryWe hypothesized that tissue characteristics vary significantly along zone zero, which may be reflected by regional differences in stored elastic energy. Our objectives were to (1) characterize the regional variation in stored elastic energy within tissues of the aortic zone zero and (2) identify the association between this variation and patient characteristics.
From February 2018 to January 2021, 123 aortic tissue samples were obtained from the aortic root and proximal and distal ascending aortas of 65 adults undergoing elective ascending aorta replacement. Biaxial biomechanics testing was performed to obtain tissue elastic energy at the inflection point and compared with patient demographics and preoperative computed tomography imaging. Coefficient models were fit using B-spline to interrogate the relationship among elastic energy, region, and patient characteristics.
Mean elastic energy at inflection point was 24.3 ± 15.6 kJ/m3. Elastic energy increased significantly between the root and proximal, and root and distal ascending aorta and decreased with increasing age. Differences due to history of connective tissue disorder and bicuspid aortic valve were significant but diminished when controlled for other patient characteristics. Among covariates, age and region were found to be the most important predictors for elastic energy.
Aortic tissue biomechanical metrics varied across regions and with patient characteristics within the aortic zone zero. Assessment of endovascular outcomes in the ascending aorta must closely consider the region of deployment and variable tissue qualities along the length of the landing zone. Regional variation in tissue characteristics should be incorporated into existing patient-specific models of aortic mechanics.
Multimodal Structural Analysis of the Human Aorta: From Valve to Bifurcation
2022, European Journal of Vascular and Endovascular SurgeryThe aims of the present study were to assess the relative proportion of collagen and elastin in the arterial wall and to evaluate the collagen microstructure from the aortic root to the external iliac artery.
Arterial wall tissue samples sampled during post-mortem examination from 16 sites in 14 individuals without aneurysm disease were fixed and stained for collagen and elastin. Stained sections were imaged and analysed to calculate collagen and elastin content as a percentage of overall tissue area. Scanning electron microscopy was used to quantify the collagen microstructure at six specific arterial regions.
From the aortic root to the level of the suprarenal aorta, the percentages (area fractions) of collagen (ascending, descending, and suprarenal aorta respectively with 95% confidence interval [CI] 37.5%, 31.7 – 43.2; 38.9%, 33.1 – 44.7; 44.8%, 37.4 – 52.1) and elastin (43.0%, 37.3 – 48.8; 40.3%, 34.8 – 46.1; 32.4%, 25.2 – 39.6) in the aortic wall were similar. From the suprarenal aorta to the internal iliac arteries, the percentage of collagen increased (abdominal aorta, common and internal iliac arteries and external iliac artery respectively with 95% CI 50.6%, 42.7 – 58.7; 51.2%, 45.5 – 56.9; 49.2%, 42.0 – 56.4) reaching a double percentage for elastin (23.6%, 15.7 – 31.6; 20.8%, 15.1 – 26.5; 22.2%, 14.9 – 29.5). Mean collagen fibre diameter (MFD) and average segment length (ASL) were significantly larger in the external iliac artery (MFD 6.03, 95% CI 5.95 – 6.11; ASL 22.21, 95% CI 20.80 – 23.61) than in the ascending aorta (MFD 5.81, 5.72 – 5.89; ASL 19.47, 18.07 – 20.88) and the abdominal aorta (MFD 5.92, 5.84 – 6.00; ASL 21.10, 19.69 – 22.50).
In subjects lacking aneurysmal disease, the aorta and iliac arteries are not structurally uniform along their length. There is an increase in collagen percentage and decrease in elastin percentage progressing distally along the aorta. Mean collagen fibre diameter and average segment length are larger in the external iliac artery, compared with the ascending and the abdominal aorta.
Cellular mechanisms of aging and their impact on the aortic/arterial wall
2022, Textbook of Arterial Stiffness and Pulsatile Hemodynamics in Health and DiseaseVascular aging is associated with altered biochemical, mechanical, and structural feature changes of the vascular wall and by a gradual loss of vascular integrity leading to reduced arterial compliance. Vascular aging is associated with increased risk for cardiovascular disease, the leading cause of death worldwide. From a clinical point of view, it can be defined as a gradually developing process characterized by age-related alterations in structural properties of the vascular wall accompanied by decline in endothelial function resulting in loss of arterial elasticity and increased arterial stiffness. In the presence of comorbidities and/or other diseased conditions related to inflammation, the rate of vascular aging significantly increases. Thus, understanding the exact mechanisms mediating these processes is important for modulation of disease states and potential treatment interventions. Experimental models and human studies have identified interconnected hallmark processes driving aging, which include oxidative stress, vascular inflammation, endothelial dysfunction, and calcification, while from the molecular signaling pathways the role of nuclear factor erythroid 2-related factor 2 (Nrf2) and sirtuins is introduced. In this chapter, we discuss detailed roles of the most important cellular and molecular pathways involved in vascular aging, their association with arterial wall stiffening, and their combined impact on the aortic wall structure in comparison with other vascular beds and in the presence of comorbidities.
Comparison of existing aneurysm models and their path forward
2021, Computer Methods and Programs in Biomedicine UpdateThe two most important aneurysm types are cerebral aneurysms and abdominal aortic aneurysms, accounting together for over 80% of all fatal aneurysm incidences. To minimise aneurysm-related deaths, clinicians need several tools to accurately assess the risk of rupture. For both aneurysm types the current state-of-the-art tools to evaluate rupture risks are identified and evaluated in terms of clinical applicability. We perform a comprehensive literature review, using Web-of-Science. The identified records (3127) are clustered by their modeling approach and aneurysm location in a meta-analysis to quantify scientific relevance of the respective approach and to extract modeling patterns. We further assess the different modeling approaches according to PRISMA guidelines (179 full text screens). We identify and systematically evaluate four major modeling approaches on aneurysm rupture risk: (i) finite element analysis and (ii) computational fluid dynamics as deterministic approaches and (iii) machine learning and (iv) assessment-tools as stochastic approaches. In doing so, we evaluated ten approaches for their applicability in clinical practice, cost, accuracy, and other characteristics. Assessment-tools receive the highest score in the evaluation of their potential as a clinical application for rupture prediction due to their readiness level and user friendliness. Deterministic approaches are less likely to be applied in a clinical environment because of their high model complexity. This is mainly due to the fact that for a very accurate simulation the aneurysm behaviour, detailed assumptions are necessary, which physicians often don’t have time to reflect on. However, since deterministic approaches consider underlying mechanisms for aneurysm rupture, they have improved capability to account for unusual patient-specific characteristics, compared to stochastic approaches. In addition to the specific evaluation of current models to determine the rupture probability of aneurysms, we show that an increased interdisciplinary exchange between specialists can boost comprehension of this disease to design tools for a clinical environment. By combining deterministic and stochastic models, advantages of both approaches can improve the accessibility for clinicians and prediction quality for rupture risk.
Time-course of axial residual strain remodeling and layer-specific thickening during aging along the human aorta
2020, Journal of BiomechanicsDetailed estimation of axial residual strains in the human aorta is necessary when performing biomechanical analyses of physiologic functions and pathologic conditions. We recently published such data for autopsied aortas and the present aim was to measure axial residual stretches at different wall depths, along with layer thicknesses on images borrowed from that work. Residual stretches at the external surface and medial-adventitial interface increased along the aorta’s ascending course, decreased along its descending course, and increased from the diaphragm toward the iliac arteries. Residual stretches at the intimal-medial interface and internal surface decreased down the distal one-third of the aorta. A continuous decrease in medial thickness was witnessed along the vessel, whereas intimal and adventitial thickness remained fairly stable. At some axial locations, smaller were the axial residual stretches of the outer than those of the other quadrants, with minor differences in layer-specific thicknesses among quadrants. Adventitial thickness did not vary with age, while the intima and media thickened considerably with different time-courses. The observed intimal thickening solely between young (≤40 yr) and middle-aged subjects (40–60 yr) is consistent with the increased circumferential residual stretches previously established by our group between those subject groups and the minimal further increase in old subjects (≥60 yr). The observed medial thickening between middle-aged and old subjects was accompanied by decreased axial residual stretches that were not seen between young and middle-aged subjects. These observations suggest distinct roles for the intima and media in determining circumferential and axial residual stretches that merit further attention.
Mechanical and structural changes in human thoracic aortas with age
2020, Acta BiomaterialiaAortic mechanical and structural characteristics have profound effects on pathophysiology, but many aspects of physiologic stress-stretch state and intramural changes due to aging remain poorly understood in human tissues. While difficult to assess in vivo due to residual stresses and pre-stretch, physiologic stress-stretch characteristics can be calculated using experimentally-measured mechanical properties and constitutive modeling. Mechanical properties of 76 human descending thoracic aortas (TA) from 13 to 78-year-old donors (mean age 51±18 years) were measured using multi-ratio planar biaxial extension. Constitutive parameters were derived for aortas in 7 age groups, and the physiologic stress-stretch state was calculated. Intramural characteristics were quantified from histological images and related to aortic morphometry and mechanics. TA stiffness increased with age, and aortas became more nonlinear and anisotropic. Systolic and diastolic elastic energy available for pulsation decreased with age from 30 to 8 kPa and from 18 to 5 kPa, respectively. Cardiac cycle circumferential stretch dropped from 1.14 to 1.04, and circumferential and longitudinal physiologic stresses decreased with age from 90 to 72 kPa and from 90 to 17 kPa, respectively. Aortic wall thickness and radii increased with age, while the density of elastin in the tunica media decreased. The number of elastic lamellae and circumferential physiologic stress per lamellae unit remained constant with age at 102±10 and 0.85±0.04 kPa, respectively. Characterization of mechanical, physiological, and structural features in human aortas of different ages can help understand aortic pathology, inform the development of animal models that simulate human aging, and assist with designing devices for open and endovascular aortic repairs.
This manuscript describes mechanical and structural changes occurring in human thoracic aortas with age, and presents material parameters for 4 commonly used constitutive models. Presented data can help better understand aortic pathology, inform the development of animal models that simulate human aging, and assist with designing devices for open and endovascular aortic repairs.