Finite element modeling of the thoracic aorta: including aortic root motion to evaluate the risk of aortic dissection

The correlation study between the length and angle of ascending aortic and the incidence risk of acute type A aortic dissection

Objective

This study utilized computed tomography angiography (CTA) to assess the risk of acute type A aortic dissection (ATAAD) by analyzing the imaging morphology indicators of the ascending aorta, along with the relevant risk factors associated with aortic dissection.

Methods

The study utilized a retrospective observational research design. The population consisted of 172 patients who received treatment in the Department of Cardiothoracic Surgery at Qilu Hospital, Shandong University, from January 2018 to December 2022. The patients were divided into two groups: the ATAAD group (n = 97) and the thoracic aortic aneurysm group (TAA, n = 75). Demographic data and ascending aorta CTA measurements were collected from all patients. Single factor and multivariate logistic regression were employed to analyze the statistical differences in clinical data and ascending aorta CTA imaging morphology indicators between the two groups.

Results

The variables were included in logistic multivariate analysis for further screening, indicating that the length of the ascending aorta (LAA) before ATAAD (OR = 3.365; 95% CI :1.742-6.500, P<0.001), ascending arch angle (asc-arch angle, OR = 0.902; 95% CI: 0.816-0.996, P = 0.042) and the maximum aortic diameter (MAD) before ATAAD, (OR = 0.614; 95% CI: 0.507-0.743, P<0.001) showed statistically significant differences.

Conclusions

This study suggests that increased LAA and MAD, as well as a smaller asc-arch angle may be high-risk factors for the onset of ATAAD.

An anatomically-realistic computational framework for evaluating the efficacy of protective plates in mitigating non-penetrating ballistic impacts

BACKGROUND

A major threat in combat scenarios is the 'behind armor blunt trauma' (BABT) of a non-penetrating ballistic impact with a ballistic protective plate (BPP). This impact results in pressure waves that propagate through tissues, potentially causing life-threatening damage. To date, there is no standardized procedure for rapid virtual testing of the effectiveness of BPP designs. The objective of this study was to develop a novel, anatomically-accurate, finite element modeling framework, as a decision-making tool to evaluate and rate the biomechanical efficacy of BPPs in protecting the torso from battlefield-acquired non-penetrating impacts.

METHODS

To simulate a blunt impact with a BPP, two types of BPPs representing generic designs of threat-level III and IV plates, and a generic 5.56 mm bullet were modeled, based on their real dimensions, physical and mechanical characteristics (plate level-III is smaller, thinner, and lighter than plate level-IV). The model was validated by phantom testing.

RESULTS

Plate level-IV induced greater strains and stresses in the superficial tissues post the ballistic impact, due to the fact that it is larger, thicker and heavier than plate level-III; the shock wave which is transferred to the superficial tissues behind the BPP is greater in the case of a non-penetrating impact. For example - the area under volumetric tissue exposure histograms of strains and stresses for the skin and adipose tissues were 16.6-19.2% and 17.3-20.3% greater in the case of plate level-IV, for strains and stresses, respectively. The validation demonstrates a strong agreement between the physical phantom experiment and the simulation, with only a 6.37% difference between them.

CONCLUSIONS

Our modelling provides a versatile, powerful testing framework for both industry and clients of BPPs at the prototype design phase, or for quantitative standardized evaluations of candidate products in purchasing decisions and bids.

Is location a significant parameter in the layer dependent dissection properties of the aorta?

Proper characterisation of biological tissue is key to understanding the effect of the biomechanical environment in the physiology and pathology of the cardiovascular system. Aortic dissection in particular is a prevalent and sometimes fatal disease that still lacks a complete comprehension of its progression. Its development and outcome, however, depend on the location in the vessel. Dissection properties of arteries are frequently studied via delamination tests, such as the T-peel test and the mixed-mode peel test. So far, a study that performs both tests throughout different locations of the aorta, as well as dissecting several interfaces, is missing. This makes it difficult to extract conclusions in terms of vessel heterogeneity, as a standardised experimental procedure cannot be assured for different studies in literature. Therefore, both dissection tests have been here performed on healthy porcine aortas, dissecting three interfaces of the vessels, i.e., the intima-media, the media-adventitia and the media within itself, considering different locations of the aorta, the ascending thoracic aorta (ATA), the descending thoracic aorta and the infrarenal abdominal aorta (IAA). Significant differences were found for both, layers and location. In particular, dissection forces in the ATA were the highest and the separation of the intima-media interface required significantly the lowest force. Moreover, dissection in the longitudinal direction of the vessel generally required more force than in the circumferential one. These results emphasise the need to characterise aortic tissue considering the specific location and dissected layer of the vessel.

Hemodynamic Study of Stanford Type B Aortic Dissection Based on Ex Vivo Porcine Aorta Models

Background:

In this study, we aimed to evaluate hemodynamic influence of the dissected aortic system via various ex vivo type B aortic dissection (AD) models.

Methods:

Twenty-four raw porcine aortas were harvested and randomly divided into 4 groups to create various aortic models. Model A was the control group, while models B to D indicated the AD group, where models B and C presented a proximal primary entry with the false lumen (FL) lengths of 15 and 20 cm, respectively, and model D presented a 20 cm FL with a proximal primary entry and a distal reentry. All the aortic models were connected to a mock circulation loop to attain the realistic flow and pressure status. The flow distribution rate (FDR) of the aortic branches was calculated. Doppler ultrasound was applied to visualize the AD structure and to attain the velocity of flow in both the true and false lumens. Several sections of the AD were stained with hematoxylin and eosin for histologic evaluation after the experiment.

Results:

This study demonstrated that higher pressures were found for the AD group compared with the control group. The mean systolic pressures at the inlet of models A to D were 113.34±0.81, 120.58±0.52, 117.76±0.82, and 115.87±0.42 mm Hg, respectively. The FDRs of the celiac artery in models A to D were 8.65%, 8.32%±0.15%, 7.87%±0.13%, and 8.03%±0.21%, respectively. By ultrasound visualization, the velocity of the flow at the entry to the FL in the AD group ranged in 10 to 92 cm/s. The dissection flap presented pulsatile movement, especially in the models B and C which contained 1 primary entry without distal reentries. Histological examinations indicated that AD was located between the intimal and medial layers.

Conclusions:

Our ex vivo models demonstrated that the configuration of the dissected aorta influenced the pressure distribution. Moreover, the dissection flap affected the FDR of the aortic branches that possibly inducing malperfusion syndrome.

Review of Current Advances in the Mechanical Description and Quantification of Aortic Dissection Mechanisms

Aortic dissection is a life-threatening event associated with a very poor outcome. A number of complex phenomena are involved in the initiation and propagation of the disease. Advances in the comprehension of the mechanisms leading to dissection have been made these last decades, thanks to improvements in imaging and experimental techniques. However, the micro-mechanics involved in triggering such rupture events remains poorly described and understood. It constitutes the primary focus of the present review. Towards the goal of detailing the dissection phenomenon, different experimental and modeling methods were used to investigate aortic dissection, and to understand the underlying phenomena involved. In the last ten years, research has tended to focus on the influence of microstructure on initiation and propagation of the dissection, leading to a number of multiscale models being developed. This review brings together all these materials in an attempt to identify main advances and remaining questions.

Multimodality Imaging-Based Characterization of Regional Material Properties in a Murine Model of Aortic Dissection

Chronic infusion of angiotensin-II in atheroprone (ApoE-/-) mice provides a reproducible model of dissection in the suprarenal abdominal aorta, often with a false lumen and intramural thrombus that thickens the wall. Such lesions exhibit complex morphologies, with different regions characterized by localized changes in wall composition, microstructure, and properties. We sought to quantify the multiaxial mechanical properties of murine dissecting aneurysm samples by combining in vitro extension-distension data with full-field multimodality measurements of wall strain and thickness to inform an inverse material characterization using the virtual fields method. A key advance is the use of a digital volume correlation approach that allows for characterization of properties not only along and around the lesion, but also across its wall. Specifically, deformations are measured at the adventitial surface by tracking motions of a speckle pattern using a custom panoramic digital image correlation technique while deformations throughout the wall and thrombus are inferred from optical coherence tomography. These measurements are registered and combined in 3D to reconstruct the reference geometry and compute the 3D finite strain fields in response to pressurization. Results reveal dramatic regional variations in material stiffness and strain energy, which reflect local changes in constituent area fractions obtained from histology but emphasize the complexity of lesion morphology and damage within the dissected wall. This is the first point-wise biomechanical characterization of such complex, heterogeneous arterial segments. Because matrix remodeling is critical to the formation and growth of these lesions, we submit that quantification of regional material properties will increase the understanding of pathological mechanical mechanisms underlying aortic dissection.

The role of the angle of the ascending aortic curvature on the development of type A aortic dissection: ascending aortic angulation and dissection

OBJECTIVES

Type A aortic dissection (TAD), which consists of an intimal tear in the aorta, necessitates emergency surgery. Various risk factors related to aortic dissection have been defined in the literature. According to our hypothesis, a narrower angle of ascending aortic curvature (AAAC) may be an additional risk factor in relation to aortic dissection due to the increased force applied to the aortic wall.

METHODS

Patients undergoing ascending aortic surgery due to an ascending aortic aneurysm (AsAA) (n = 105) and patients undergoing such surgery because of the occurrence of TAD (n = 101) were enrolled in this study. The AAAC was measured using Cobb's method; the measurements were made on all patients by just 1 cardiovascular radiologist using 3-dimensional computerized tomographic imaging. This measurement was made indirectly by using the aortic valve and brachiocephalic artery to avoid obtaining misleading data as a result of distortions due to dissection. A statistical comparison was also performed relating the traditional risk factors for TAD to other clinical and echocardiographic parameters: the diameter of the ascending aorta and the AAAC.

RESULTS

The AAAC was found to be narrower statistically in the TAD group (α = 76.2° ± 17.5°) than it was in the AsAA group (α = 92.9° ± 13°) (P < 0.001). Furthermore, mean ascending aortic diameter (P = 0.019), the presence of a bicuspid aorta (P = 0.007) and aortic valve stenosis (P = 0.005) were higher in the AsAA group. According to multivariable analyses, a narrower AAAC is a significant predictor for the development of TAD (odds ratio 0.93, 95% confidence interval 0.91-0.95; P < 0.001). Overall hospital mortality from various causes including stroke, myocardial infarction, bleeding or renal failure was 13% in the TAD group and 7% in the AsAA group.

CONCLUSIONS

According to this study, the AAAC was significantly smaller in aortic dissection patients than in aortic aneurysm patients. This may be related to higher shear stress and elevated pressure on the ascending aorta in patients with a narrower AAAC. Thus, a narrower AAAC may be an additional risk factor in the development of TAD. Therefore, we may need to be more careful in terms of looking for the development of aortic dissection in patients with narrower AAAC.

Penetrating Atherosclerotic Ulcers of the Abdominal Aorta: A Case Report and Review of the Literature

Penetrating atherosclerotic ulcers (PAUs) of the aorta are defined as atherosclerotic lesions with aortic intima and media ulceration, which may lead to a complete rupture of the adventitial wall. The present article aimed to report an unusual case of a surgically treated patient with abdominal aorta PAU with an illustration of the key features and to review and analyze the existing literature data. PAUs typically develop in elderly and hypertensive patients and in patients with advanced atherosclerosis. Although originally described for the descending thoracic aorta, a similar clinicopathological entity also occurs in the abdominal aorta. Patients with symptoms of a PAU should be treated immediately if they are fit for surgery. Exceptive observation by imaging modalities is necessary in patients with asymptomatic small (<2 cm) PAU, with or without focal dissection.

Impact of the bicuspid aortic valve on aortic root haemodynamics: three-dimensional computed fluid dynamics simulation

OBJECTIVES

The aim was to evaluate the impact of a bicuspid aortic valve (BAV) on local shear stress and on the pressure profile on the elements of the aortic root (AoR).

METHODS

The experiment setup included a BAV with aortic valve stenosis (n = 5 pigs, 67 ± 3.5 kg) and insufficiency (n = 5 pigs, 66.7 ± 4.4 kg). By implanting 6 high-fidelity microsonometric crystals in each AoR, we determined the 3-dimensional (3D) geometry of the AoR. Experimental and geometric data were used to create a 3D time- and pressure-dependent computed fluid dynamic model of the AoR with the BAV.

RESULTS

3D AoR geometry was determined by AoR tilt (α) and rotation angle (β). Both values were maximal at the end of diastole: 24.41 ± 1.70° (α) and 20.90 ± 2.11° (β) for BAV with stenosis and 31.92 ± 11.51° (α) and 20.84 ± 9.80° (β) for BAV with insufficiency and minimal at peak ejection 23.42 ± 1.65° (α), 20.38 ± 1.61° (β) for stenosis and 26.62 ± 7.86° (α), 19.79 ± 8.45° (β) for insufficiency. In insufficiency, low shear stress of 0-0.08 Pa and moderate pressure (60-80 mmHg) were present. In BAV with stenosis, low shear stress of 0-0.5 Pa and moderate pressure (0-20 mmHg) were present at diastole; at peak ejection high shear stress >2 Pa and elevated pressure of >80 mmHg were present.

CONCLUSIONS

In a BAV with aortic valve stenosis, the haemodynamics are less favourable. The elevated pressure with elevated shear stress may over the long term promote degenerative processes in the leaflets and consequently valve function failure.

Computational modeling of bicuspid aortopathy: Towards personalized risk strategies

This paper describes current advances on the application of in-silico for the understanding of bicuspid aortopathy and future perspectives of this technology on routine clinical care. This includes the impact that artificial intelligence can provide to develop computer-based clinical decision support system and that wearable sensors can offer to remotely monitor high-risk bicuspid aortic valve (BAV) patients. First, we discussed the benefit of computational modeling by providing tangible examples of in-silico software products based on computational fluid-dynamic (CFD) and finite-element method (FEM) that are currently transforming the way we diagnose and treat cardiovascular diseases. Then, we presented recent findings on computational hemodynamic and structural mechanics of BAV to highlight the potentiality of patient-specific metrics (not-based on aortic size) to support the clinical-decision making process of BAV-associated aneurysms. Examples of BAV-related personalized healthcare solutions are illustrated.

Investigating heartbeat-related in-plane motion and stress levels induced at the aortic root

Background

The axial motion of aortic root (AR) due to ventricular traction was previously suggested to contribute to ascending aorta (AA) dissection by increasing its longitudinal stress, but AR in-plane motion effects on stresses have never been studied. The objective is to investigate the contribution of AR in-plane motion to AA stress levels.

Methods

The AR in-plane motion was assessed on magnetic resonance imagining data from 25 healthy volunteers as the movement of the AA section centroid. The measured movement was prescribed to the proximal AA end of an aortic finite element model to investigate its influences on aortic stresses. The finite element model was developed from a patient-specific geometry using LS-DYNA solver and validated against the aortic distensibility. Fluid–structure interaction (FSI) approach was also used to simulate blood hydrodynamic effects on aortic dilation and stresses.

Results

The AR in-plane motion was 5.5 ± 1.7 mm with the components of 3.1 ± 1.5 mm along the direction of proximal descending aorta (PDA) to AA centroid and 3.0 ± 1.3 mm perpendicularly under the PDA reference system. The AR axial motion elevated the longitudinal stress of proximal AA by 40% while the corresponding increase due to in-plane motion was always below 5%. The stresses at proximal AA resulted approximately 7% less in FSI simulation with blood flow.

Conclusions

The AR in-plane motion was comparable with the magnitude of axial motion. Neither axial nor in-plane motion could directly lead to AA dissection. It is necessary to consider the heterogeneous pressures related to blood hydrodynamics when studying aortic wall stress levels.

Electronic supplementary material

The online version of this article (10.1186/s12938-019-0632-7) contains supplementary material, which is available to authorized users.

Etiology of aortic dissection

We discuss the etiology of aortic dissection (AD) from various points of view. The development of AD requires two pathological conditions: medial degeneration and mechanical wall stress. First, histopathological findings of medial degeneration are hypothesized to be due to a loss of elastic fibers and interconnecting elastic fibers. Damage to the vasa vasorum plays a key role in creating an entry site. The clinical causes of medial degeneration include hypertension, aortic aneurysms, obstructive sleep apnea, and connective tissue disorders. Second, mechanical wall stress is supposedly induced by shear stress caused by blood flow, together with hypertension and aortic root movement. Further investigation is necessary in the search for mechanisms responsible for medial degeneration prior to AD development.

Cardiopulmonary-induced deformations of the thoracic aorta following thoracic endovascular aortic repair

Objectives

Thoracic endovascular aortic repair has become a preferred treatment strategy for thoracic aortic aneurysms and dissections. Yet, it is not well understood if the performance of endografts is affected by physiologic strain due to cyclic aortic motion during cardiac pulsation and respiration. We aim to quantify cardiac- and respiratory-induced changes of the postthoracic endovascular aortic repair thoracic aorta and endograft geometries.

Methods

Fifteen thoracic endovascular aortic repair patients (66 ± 10 years) underwent cardiac-resolved computed tomography angiographies during inspiratory/expiratory breath holds. The computed tomography angiography images were utilized to build models of the aorta, and lumen centerlines and cross-sections were extracted. Arclength and curvature were computed from the lumen centerline. Effective diameter was computed from cross-sections of the thoracic aorta. Deformation was computed from the mid-diastole to end-systole (cardiac deformation) and expiration to inspiration (respiratory deformation).

Results

Cardiac pulsation induced significant changes in arclength, mean curvature, maximum curvature change, and effective diameter of the ascending aorta, as well as effective diameter of the stented aortic segment. Respiration, however, induced significant change in mean curvature and effective diameter of the ascending aorta only. Cardiac-induced arclength change of the ascending aorta was significantly greater than respiratory-induced arclength change.

Conclusions

Deformations are present across the thoracic aorta due to cardiopulmonary influences after thoracic endovascular aortic repair. The geometric deformations are greatest in the ascending aorta and decline at the stented thoracic aorta. Additional investigation is warranted to correlate aortic deformation to endograft performance.

Gene expression profiling of acute type A aortic dissection combined with in vitro assessment

OBJECTIVES

The mechanisms underlying aortic dissection remain to be fully elucidated. We aimed to identify key molecules driving dissection through gene expression profiling achieved by microarray analysis and subsequent in vitro experiments using human aortic endothelial cells (HAECs) and aortic vascular smooth muscle cells (AoSMCs).

METHODS

Total RNA, including microRNA (miRNA), was isolated from the intima-media layer of dissected ascending aorta obtained intraoperatively from acute type A aortic dissection (ATAAD) patients without familial thoracic aortic disease (n = 8) and that of non-dissected ascending aorta obtained from transplant donors (n = 9). Gene expression profiling was performed with mRNA and miRNA microarrays, and results were confirmed by quantitative polymerase chain reaction (qPCR). Target genes and miRNA were identified by gene ontology analysis and a literature search. To reproduce the in silico results, HAECs and AoSMCs were stimulated in vitro by upstream cytokines, and expression of target genes was assessed by qPCR.

RESULTS

Microarray analysis revealed 1536 genes (3.6%, 1536/42 545 probes) and 41 miRNAs (3.0%, 41/1368 probes) that were differentially expressed in the ATAAD group (versus donor group). The top 15 related pathways included regulation of inflammatory response, growth factor activity and extracellular matrix. Gene ontology analysis identified JAK2 (regulation of inflammatory response), PDGFA, TGFB1, VEGFA (growth factor activity) and TIMP3, TIMP4, SERPINE1 (extracellular matrix) as the target genes and miR-21-5p, a TIMP3 repressor, as target miRNA that interacts with the target genes. Validation qPCR confirmed the altered expression of all 7 target genes and miR-21-5p in dissected aorta specimens (all genes, P < 0.05). Ingenuity pathway analysis showed TNF-α and TGF-β to be upstream cytokines for the target genes. In vitro experiments showed these cytokines inhibit TIMP3 expression (P < 0.05) and enhance VEGFA expression (P < 0.01) in AoSMCs but not HAECs. miR-21-5p expression increases in AoSMCs under TNF-α and TGF-β stimulation (fold change: 1.36; P = 0.011).

CONCLUSIONS

Results of our novel approach, integrating in vitro assessment into gene expression profiling, implicated chronic inflammation characterized by MMP-TIMP dysregulation, increased VEGFA expression, and TGF-β signalling in the development of dissection. Further investigation may reveal novel diagnostic biomarkers and uncover the mechanism(s) underlying ATAAD.

Hemodynamic assessments of the ascending thoracic aortic aneurysm using fluid-structure interaction approach

Current assessment and management of ascending thoracic aortic aneurysm (ATAA) rely heavily on the diameter of the ATAA and blood pressure rather than biomechanical and hemodynamic parameters such as arterial wall deformation or wall shear stress. The objective of the current study was to develop an accurate computational method for modeling the mechanical responses of the ATAA to provide additional information in patient evaluations. Fully coupled fluid structure interaction simulations were conducted using data from cases with ATAA with measured geometrical parameters in order to evaluate and analyze the change in biomechanical responses under normotensive and hypertensive conditions. Anisotropic hyperelastic material property estimates were applied to the ATAA data which represented three different geometrical configurations of ATAAs. The resulting analysis showed significant variations in maximum wall shear stress despite minimal differences in flow velocity between two blood pressure conditions. Additionally, the three different ATAA conditions identified different aortic expansions that were not uniform under pulsatile pressure. The elevated wall stress with hypertension was also geometry-dependent. The developed models suggest that ATTA cases have unique characteristic in biomechanical and hemodynamic evaluations that can be useful in risk management.

When and How to Enlarge the Small Aortic Root

Successful enlargement of the small aortic root in children has remained a management challenge, particularly in the neonates and small infants. Achieving this aim requires thorough understanding of the anatomic features of the left ventricular outflow tract, careful patient selection, and skilful execution of complex surgery. This article reviews the anatomical principles upon which the surgical techniques rely, the decision-making, the timing of surgery, the surgical options, and the outcomes.

Layer- and region-specific material characterization of ascending thoracic aortic aneurysms by microstructure-based models

Material characterization of ascending thoracic aortic aneurysms is indispensable for the determination of stress distributions across wall thickness and the different aneurysm regions that may be responsible for their catastrophic rupture or dissection, but only few studies have addressed this issue hitherto. In this article, we are presenting our findings of implementing microstructure-based formulations for characterizing layer- and region-specific variations in wall properties, which is a reasonable consensus today. Together, we performed image-based analysis to derive collagen-fiber orientation angles that may serve as validation of the preferred candidate for a fiber-reinforced constitutive descriptor. We considered a four-fiber model with dispersions of fiber angles about the main directions, based on our histological observations, demonstrating a wide distribution of fiber orientations spanning circumferential to longitudinal directions, and its successful implementation to our biomechanical data from tensile testing. However, an in-depth parametric analysis showed that a condensed model without longitudinal-fiber family described the data just as well and did not omit essential histological organization of collagen fibers, while reserving a smaller number of parameters, which makes it advantageous for computational applications. A major aberration from almost all existing models in the literature is the hypothesis made that fibers can support compressive stresses. Such a hypothesis needs further examination but it has the benefits of allowing improved fits to the vanishing transverse stresses under uniaxial test conditions and of properly reflecting the exponential nature of the compressive stress-strain response of aortic tissue, being consistent with observations of collagen being under compression in the unloaded wall.

Quantification of Respiratory Movement of the Aorta and Side Branches

Purpose: To assess and quantify the magnitude and direction of respiratory movement of the aorta and origins of its side branches. Methods: A quantitative 3-dimensional (3D) subtraction analysis of computed tomography (CT) scans during inspiration and expiration was performed to determine the respiratory geometric movements of the aorta and side branches in 60 patients. During breath-hold expiration and inspiration, 1-mm-thick CT slices of the aorta were acquired in unenhanced and contrast-enhanced scans. The datasets were compared using dedicated multiplanar reformation image subtraction software to determine the change in position of relevant anatomic sections, including the ascending thoracic aorta (AA), the origins of the brachiocephalic artery (BA) and left subclavian artery (LSA), the descending thoracic aorta (DTA) at the level of the tenth thoracic vertebra, as well as the origins of the celiac trunk, superior mesenteric artery, and the renal arteries. Results: Complex movement was visible during inspiration; the regions of interest in the thoracic aorta and side branches moved in the anterior, medial, and caudal directions compared with the expiration state. Mean 3D movement vectors (± standard deviation) were 8.9±3.6 mm (AA), 12.0±4.1 mm (BA), 11.1±3.9 mm (LSA), and 4.9±2.5 mm (DTA). Abdominal side branches moved in the caudal direction 1.3±1.1 mm. There was significantly less movement in the DTA compared to AA (p<0.001). The correlation coefficient between the extent of LSA movement and thoracic excursion was 0.78. Conclusion: The aorta and side branches undergo considerable respiratory movement. The results from this study provide an important contribution to understanding aortic dynamics.

A computational study of the role of the aortic arch in idiopathic unilateral vocal-fold paralysis

Unilateral vocal-fold paralysis (UVP) occurs when one of the vocal folds becomes paralyzed due to damage to the recurrent laryngeal nerve (RLN). Individuals with UVP experience problems with speaking, swallowing, and breathing. Nearly two-thirds of all cases of UVP is associated with impaired function of the left RLN, which branches from the vagus nerve within the thoracic cavity and loops around the aorta before ascending to the larynx within the neck. We hypothesize that this path predisposes the left RLN to a supraphysiological, biomechanical environment, contributing to onset of UVP. Specifically, this research focuses on the identification of the contribution of the aorta to onset of left-sided UVP. Important to this goal is determining the relative influence of the material properties of the RLN and the aorta in controlling the biomechanical environment of the RLN. Finite element analysis was used to estimate the stress and strain imposed on the left RLN as a function of the material properties and loading conditions. The peak stress and strain in the RLN were quantified as a function of RLN and aortic material properties and aortic blood pressure using Spearman rank correlation coefficients. The material properties of the aortic arch showed the strongest correlation with peak stress [ρ = -0.63, 95% confidence interval (CI), -1.00 to -0.25] and strain (ρ = -0.62, 95% CI, -0.99 to -0.24) in the RLN. Our results suggest an important role for the aorta in controlling the biomechanical environment of the RLN and potentially in the onset of left-sided UVP that is idiopathic.

Biomechanical roles of medial pooling of glycosaminoglycans in thoracic aortic dissection

Spontaneous dissection of the human thoracic aorta is responsible for significant morbidity and mortality, yet this devastating biomechanical failure process remains poorly understood. In this paper, we present finite element simulations that support a new hypothesis for the initiation of aortic dissections that is motivated by extensive histopathological observations. Specifically, our parametric simulations show that the pooling of glycosaminoglycans/proteoglycans that is singularly characteristic of the compromised thoracic aorta in aneurysms and dissections can lead to significant stress concentrations and intra-lamellar Donnan swelling pressures. We submit that these localized increases in intramural stress may be sufficient both to disrupt the normal cell-matrix interactions that are fundamental to aortic homeostasis and to delaminate the layered microstructure of the aortic wall and thereby initiate dissection. Hence, pathologic pooling of glycosaminoglycans/proteoglycans within the medial layer of the thoracic aortic should be considered as a possible target for clinical intervention.

Impact of shear stress and atherosclerosis on entrance-tear formation in patients with acute aortic syndromes

Weak aortic media layers can lead to intimal tear (IT) in patients with overt aortic dissection (AD), and aortic plaque rupture is thought to progress to penetrating atherosclerotic ulcer (PAU) with intramural hematoma (IMH). However, the influences of shear stress and atherosclerosis on IT and PAU have not been fully examined. Ninety-eight patients with overt AD and 30 patients with IMH and PAU admitted to our hospital from 2002 to 2007 were enrolled. The greater curvatures of the aorta, including the anterior and right portions of the ascending aorta and anterior portion of the aortic arch, were defined as sites of high shear stress. The other portions of the aorta were defined as sites of low shear stress based on anatomic and hydrodynamic theories. Aortic calcified points (ACPs) were manually counted on computed tomography slices of the whole aorta every 10 mm from the top of the arch to the abdominal bifurcation point. IT was more often observed at sites of high shear stress in overt AD than in PAU (73.5 vs 20.0 %, P < 0.0001). Significantly more ACPs were present in PAU than in overt AD (18.6 ± 8 vs 13.3 ± 10, P = 0.007). The present study suggests that high shear stress and less severe atherosclerosis could induce the occurrence of an IT, thereafter progressing to overt AD, and that low shear stress and more severe atherosclerosis could proceed to PAU with IMH. These findings may help to identify the entrance-tear site.

Possible mechanical roles of glycosaminoglycans in thoracic aortic dissection and associations with dysregulated transforming growth factor-β

BACKGROUND

Four distinguishing histopathological characteristics of thoracic aortic aneurysms and dissections (TAADs) are the fragmentation or degradation of elastic fibers, loss of smooth muscle, pooling of glycosaminoglycans, and remodeling of fibrillar collagens. Of these, pooling of glycosaminoglycans appears to be unique to these lesions.

METHODS

This review acknowledges the importance of dysregulated transforming growth factor-β (TGF-β) in TAADs and offers a complementary hypothesis that increased TGF-β could contribute to the accumulation of glycosaminoglycans in the media of the proximal thoracic aorta. Regardless, observed pools of glycosaminoglycans could decrease tensile strength, cause stress concentrations, and increase intralamellar swelling pressure, all of which could initiate local delaminations that could subsequently propagate as dissections and result in a false lumen or rupture.

CONCLUSIONS

There is a pressing need to investigate potential mechanical as well as biological consequences of accumulated glycosaminoglycans in TAADs and to elucidate responsible signaling pathways, with particular attention to synthetic cells of nonmesodermal lineage. Such research could provide insight into the mechanisms of dissection and the seemingly paradoxical role of the over-expression of a cytokine that is typically associated with fibrosis but is implicated in a degenerative disease of the aorta that can result in a catastrophic mechanical failure.

Pathogenesis of acute aortic dissection: a finite element stress analysis

BACKGROUND

Type A and type B aortic dissections typically result from intimal tears above the sinotubular junction and distal to the left subclavian artery (LSA) ostium, respectively. We hypothesized that this pathology results from elevated pressure-induced regional wall stress.

METHODS

We identified 47 individuals with normal thoracic aortas by electrocardiogram-gated computed tomography angiography. The thoracic aorta was segmented, reconstructed, and triangulated to create a geometric mesh. Finite element analysis using a systolic pressure load of 120 mm Hg was performed to predict regional thoracic aortic wall stress.

RESULTS

There were local maxima of wall stress above the sinotubular junction in the ascending aorta and distal to the ostia of the supraaortic vessels, including the LSA, in the aortic arch. No local maximum of wall stress was found in the descending thoracic aorta. Comparison of the mean peak wall stress above the sinotubular junction (0.43 ± 0.07 MPa), distal to the LSA (0.21 ± 0.07 MPa), and in the descending thoracic aorta (0.06 ± 0.01 MPa) showed a significant effect for wall stress by aortic region (p < 0.001).

CONCLUSIONS

In the normal thoracic aorta, there are peaks in wall stress above the sinotubular junction and distal to the LSA ostium. This stress distribution may contribute to the pathogenesis of aortic dissections, given their colocalization. Future investigations to determine the utility of image-derived biomechanical calculations in predicting aortic dissection are warranted, and therapies designed to reduce the pressure load-induced wall stress in the thoracic aorta are rational.

Regional dependency of the vascular smooth muscle cell contribution to the mechanical properties of the pig ascending aortic tissue

BACKGROUND

Dilation and dissection of aneurysmal ascending aortic tissues occur preferentially at the outer curvature of the vessel. In this study we hypothesize that the density and contractile properties of the vascular smooth muscle cells (VSMCs) of the pig ascending aorta (AA) are heterogeneous and could explain the non-uniform remodeling and weakening of the AA during aneurysm formation.

METHODS

Eleven pig AA rings were collected. Two square samples of 15 x 15 mm were taken from each ring from the inner and outer curvature of the AA. Each sample was subjected to equi-biaxial tensile testing in Krebs-Ringer solution maintained at 37 degrees C. Each test consisted of 8 cycles of preconditioning followed by one experimental run from 0% to 30% strain. Phenylephrine (10(-5) M) was added to contract VSMCs. After biaxial testing, samples were paraffin-embedded and stained with hematoxylin-phloxine-saffron (HPS) to quantify VSMC density.

RESULTS

Significant differences in cell density, maximum contractile stress resultant magnitude (MCSRM) and orientation (theta(MCSR)) were found between the inner and outer curvature. The inner curvature had the greatest contraction. The outer curvature had the highest VSMC density with the maximum contraction stress resultant oriented towards the axial direction.

CONCLUSION

VSMC activation with phenylephrine had a significant effect on the stiffness of the pig AA. This effect was independent of location and direction. However, cell orientation, density and contractile properties were dependent on location and suggest variations in the remodeling capabilities, tissue strain and cell phenotype between locations.

Heartbeat-related displacement of the thoracic aorta in patients with chronic aortic dissection type B: quantification by dynamic CTA

PURPOSE

The purpose of this study was to characterize the heartbeat-related displacement of the thoracic aorta in patients with chronic aortic dissection type B (CADB).

MATERIALS AND METHODS

Electrocardiogram-gated computed tomography angiography was performed during inspiratory breath-hold in 11 patients with CADB: Collimation 16 mm x 1 mm, pitch 0.2, slice thickness 1mm, reconstruction increment 0.8 mm. Multiplanar reformations were taken for 20 equidistant time instances through both ascending (AAo) and descending aorta (true lumen, DAoT; false lumen, DAoF) and the vertex of the aortic arch (VA). In-plane vessel displacement was determined by region of interest analysis.

RESULTS

Mean displacement was 5.2+/-1.7 mm (AAo), 1.6+/-1.0 mm (VA), 0.9+/-0.4 mm (DAoT), and 1.1+/-0.4mm (DAoF). This indicated a significant reduction of displacement from AAo to VA and DAoT (p<0.05). The direction of displacement was anterior for AAo and cranial for VA.

CONCLUSION

In CADB, the thoracic aorta undergoes a heartbeat-related displacement that exhibits an unbalanced distribution of magnitude and direction along the thoracic vessel course. Since consecutive traction forces on the aortic wall have to be assumed, these observations may have implications on pathogenesis of and treatment strategies for CADB.