Salvatore Pasta

Difference in hemodynamic and wall stress of ascending thoracic aortic aneurysms with bicuspid and tricuspid aortic valve

The aortic dissection (AoD) of an ascending thoracic aortic aneurysm (ATAA) initiates when the hemodynamic loads exerted on the aneurysmal wall overcome the adhesive forces holding the elastic layers together. Parallel coupled, two-way fluid-structure interaction (FSI) analyses were performed on patient-specific ATAAs obtained from patients with either bicuspid aortic valve (BAV) or tricuspid aortic valve (TAV) to evaluate hemodynamic predictors and wall stresses imparting aneurysm enlargement and AoD. Results showed a left-handed circumferential flow with slower-moving helical pattern in the aneurysms center for BAV ATAAs whereas a slight deviation of the blood flow toward the anterolateral region of the ascending aorta was observed for TAV ATAAs. Blood pressure and wall shear stress were found key hemodynamic predictors of aneurysm dilatation, and their dissimilarities are likely associated to the morphological anatomy of the aortic valve. We also observed discontinues, wall stresses on aneurysmal aorta, which was modeled as a composite with two elastic layers (i.e., inhomogeneity of vessel structural organization). This stress distribution was caused by differences on elastic material properties of aortic layers. Wall stress distribution suggests AoD just above sinotubular junction. Moreover, abnormal flow and lower elastic material properties that are likely intrinsic in BAV individuals render the aneurysm susceptible to the initiation of AoD.

Effect of aneurysm on the mechanical dissection properties of the human ascending thoracic aorta

OBJECTIVES The acute dissection of an ascending thoracic aortic aneurysm (ATAA) represents a devastating separation of elastic layers occurring when the hemodynamic loads on the diseased wall exceed the adhesive strength between layers. At present, the mechanics underlying aortic dissection are largely unclear, and the biomechanical delamination properties of the aneurysmal aorta are not defined. Individuals with bicuspid aortic valve (BAV) are particularly predisposed to ascending aortic aneurysm formation, with a marked risk of aortic dissection. The purpose of this study was to evaluate and compare the dissection properties of nonaneurysmal and aneurysmal human ascending thoracic aorta from patients with BAV morphology or normal tricuspid aortic valve (TAV) morphology using biomechanical delamination testing. METHODS The influence on the delamination strength (S(d)) of the aorta associated with BAV was compared with that in patients with TAV. After complete delamination of ATAA tissue samples, tensile tests were performed on each delaminated half for comparison of their tensile strengths. RESULTS The results showed that the aneurysmal aortas with BAV and TAV have lower S(d) than nonaneurysmal aortas and that ATAA with BAV has a lower S(d) than that with TAV. We have found a significant difference in S(d) between longitudinal and circumferential directions of the nondiseased aorta, suggesting anisotropic dissection properties. CONCLUSIONS The tensile testing results suggest that the weaker intimal half of the aortic wall might fail before the outer adventitial half. Scanning electron microscope analyses suggest different failure modalities of dissection between the two morphologies, and the lower S(d) in ATAAs appears to be associated with a disorganized microstructure. BAV ATAAs have a lower S(d) than TAV ATAAs, suggesting a greater propensity for aortic dissection.

Computer Modeling for the Prediction of Thoracic Aortic Stent Graft Collapse

OBJECTIVE To assess the biomechanical implications of excessive stent protrusion into the aortic arch in relation to thoracic aortic stent graft (TASG) collapse by simulating the structural load and quantifying the fluid dynamics on the TASG wall protrusion extended into a model arch. METHODS One-way coupled fluid-solid interaction analyses were performed to investigate the flow-induced hemodynamic and structural loads exerted on the proximal protrusion of the TASG and aortic wall reconstructed from a patient who underwent traumatic thoracic aortic injury repair. Mechanical properties of a Gore TAG thoracic endoprosthesis (W. L. Gore and Assoc, Flagstaff, Ariz) were assessed via experimental radial compression testing and incorporated into the computational modeling. The TASG wall protrusion geometry was characterized by the protrusion extension (PE) and by the angle (θ) between the TASG and the lesser curvature of the aorta. The effect of θ was explored with the following four models with PE fixed at 1.1 cm: θ = 10 degrees, 20 degrees, 30 degrees, and 40 degrees. The effect of PE was evaluated with the following four models with θ fixed at 10 degrees: PE = 1.1 cm, 1.4 cm, 1.7 cm and 2.0 cm. RESULTS The presence of TASG wall protrusion into the aortic arch resulted in the formation of swirling, complex flow regions in the proximal luminal surface of the endograft. High PE values (PE = 2.0 cm) led to a markedly reduced left subclavian flow rate (0.27 L/min), low systolic perfusion pressure (98 mm Hg), and peak systolic TASG diameter reduction (2 mm). The transmural pressure load across the TASG was maximum for the model with the highest PE and θ, 15.2 mm Hg for the model with PE = 2.0 cm and θ = 10 degrees, and 11.6 mm Hg for PE = 1.1 cm and θ = 40 degrees. CONCLUSIONS The findings of this study suggest that increased PE imparts an apparent risk of distal end-organ malperfusion and proximal hypertension and that both increased PE and θ lead to a markedly increased transmural pressure across the TASG wall, a load that would portend TASG collapse. Patient-specific computational modeling may allow for identification of patients with high risk of TASG collapse and guide preventive intervention. CLINICAL RELEVANCE A potentially devastating complication that may occur after endovascular repair of traumatic thoracicaortic injuries is stent graft collapse. Although usually asymptomatic, stent graft collapse may be accompanied by adverse hemodynamic consequences. Numerous anatomic and device-related factors contribute to the development of collapse, but predictive factors have not yet been clearly defined. In the present study, we assessed the relevant hemodynamics and solid mechanics underlying stent graft collapse using a computational fluid-structure interaction framework of stent graft malapposition. Our findings suggest that both increased stent graft angle and extension into the aortic arch lead to a markedly increased transmural pressure across the stent graft wall, portending collapse. Patient-specific computational modeling may allow for identification of patients at high risk for collapse and aid in planning for an additional, prophylactic intervention to avert its occurrence.

Fiber micro-architecture in the longitudinal-radial and circumferential-radial planes of ascending thoracic aortic aneurysm media

It was recently demonstrated by our group that the delamination strength of ascending thoracic aortic aneurysms (ATAA) was lower than that of control (CTRL, non-aneurysmal) ascending thoracic aorta (ATA), and the reduced strength was more pronounced among bicuspid (BAV) vs. tricuspid aortic valve (TAV) patients, suggesting a different risk of aortic dissection for BAV patients. We hypothesized that aortic valve morphologic phenotype predicts fiber micro-architectural anomalies in ATA. To test the hypothesis, we characterized the micro-architecture in the longitudinal-radial (Z-RAD) and circumferential-radial (Θ-RAD) planes of human ATA tissue that was artificially dissected medially. The outer and inner-media of CTRL-ATA, BAV-ATAA and TAV-ATAA were imaged using multi-photon microscopy in the Z-RAD and Θ-RAD planes to observe collagen and elastin. Micrographs were processed using an image-based tool to quantify several micro-architectural characteristics. In the outer-media of BAV-ATAA, elastin was more undulated and less aligned about the Θ-axis when compared with CTRL-ATA, which is consistent with increased tensile stretch at inflection point of Θ-strips of adventitial-medial half of BAV-ATAA (1.28) when compared with CTRL-ATA (1.13). With increasing age, collagen became more undulated about the Z-axis within the outer-media of TAV-ATAA, and elastin became more oriented in the Z-axis and collagen less radially-oriented within the inner-media of TAV-ATAA. This discrepancy in the micro-architecture with fibers in the inner layers being more stretched and with disrupted radially-oriented components than fibers in the outer layers may be associated with the development, progression and vascular remodeling in aneurysms arising in TAV patients.

AA6082-T6 Friction Stir Welded Joints Fatigue Resistance: Influence of Process Parameters:

Abstract In the paper the results of a wide range of experiments on friction stir welding (FSW) of aluminium alloys are reported. In particular, the AA6082-T6 butt joints fatigue resistance was investigated by varying the most relevant process parameters. In addition, a revolutionary pitch was utilized in order to investigate the effects of the tool rotating speed and the tool feed rate. Observations of the fracture insurgence were developed for different levels of applied load.

A mechanistic model on the role of “radially-running” collagen fibers on dissection properties of human ascending thoracic aorta.

Aortic dissection (AoD) is a common condition that often leads to life-threatening cardiovascular emergency. From a biomechanics viewpoint, AoD involves failure of load-bearing microstructural components of the aortic wall, mainly elastin and collagen fibers. Delamination strength of the aortic wall depends on the load-bearing capacity and local micro-architecture of these fibers, which may vary with age, disease and aortic location. Therefore, quantifying the role of fiber micro-architecture on the delamination strength of the aortic wall may lead to improved understanding of AoD. We present an experimentally-driven modeling paradigm towards this goal. Specifically, we utilize collagen fiber micro-architecture, obtained in a parallel study from multi-photon microscopy, in a predictive mechanistic framework to characterize the delamination strength. We then validate our model against peel test experiments on human aortic strips and utilize the model to predict the delamination strength of separate aortic strips and compare with experimental findings. We observe that the number density and failure energy of the radially-running collagen fibers control the peel strength. Furthermore, our model suggests that the lower delamination strength previously found for the circumferential direction in human aorta is related to a lower number density of radially-running collagen fibers in that direction. Our model sets the stage for an expanded future study that could predict AoD propagation in patient-specific aortic geometries and better understand factors that may influence propensity for occurrence.

Regional variation of wall shear stress in ascending thoracic aortic aneurysms

The development of an ascending thoracic aortic aneurysm is likely caused by excessive hemodynamic loads exerted on the aneurysmal wall. Computational fluid-dynamic analyses were performed on patient-specific ascending thoracic aortic aneurysms obtained from patients with either bicuspid aortic valve or tricuspid aortic valve to evaluate hemodynamic and wall shear parameters, imparting aneurysm enlargement. Results showed an accelerated flow along the outer aortic wall with helical flow in the aneurysm center for bicuspid aortic valve ascending thoracic aortic aneurysms. In a different way, tricuspid aortic valve ascending thoracic aortic aneurysms exhibited normal systolic flow without substantial secondary pattern. Analysis of wall shear parameters evinced a high and locally varying wall shear stress on the outer aortic wall and high temporal oscillations in wall shear stress (oscillatory shear index) on either left or right side of aneurysmal aorta. These findings may explain the asymmetric dilatation typically observed in ascending thoracic aortic aneurysms. Simulations of a hypertensive scenario revealed an increase in wall shear stress upon 44% compared to normal systemic pressure models. Computational fluid-dynamics–based analysis may allow identification of wall shear parameters portending aneurysm dilatation and hence guide preventative intervention.

Role of Computational Modeling in Thoracic Aortic Pathology: A Review

Thoracic aortic diseases are life‐threatening conditions causing significant mortality and morbidity despite advances in diagnostic and surgical treatments. Computational methods combined with imaging techniques provide quantitative information of disease progression, which may improve clinical treatments and therapeutic strategies for clinical practice. Since hemodynamic and wall mechanics play important roles in the natural history and progression of aortic diseases, we reviewed the potential application of computational modeling of the thoracic aorta. We placed emphasis on the clinical relevance of these techniques for the assessment of aortic dissection, thoracic aortic aneurysm, and aortic coarctation. Current clinical guidelines and treatment are also described. doi: 10.1111/jocs.12413 (J Card Surg 2014;29:653–662)

Biomechanics and Pathobiology of Aortic Aneurysms

Biomechanical weakening of the aorta leads to aneurysm formation and/or dissection and total biomechanical failure results in rupture, which is often fatal. The most common aneurysm is the abdominal aortic aneurysm (AAA) whereas thoracic aortic aneurysms (TAA) involve the ascending or descending segments of the aorta. Biomechanical strength of the aorta is maintained in part via balance between the integrity of the aortic medial and adventitial extracellular matrix and the health of the mural cells. From a biomechanical perspective, aneurysms rupture or dissect when wall stresses locally exceed the wall strength. Pathobiologic mechanisms, pre-disposing disorders and variability of patient demographic characteristics can weaken the aortic wall while increased blood pressure and dilatation increase the stress acting on it, leading to further aneurysm expansion. Thoracic and abdominal aortic aneurysms arise from very different pathophysiologies that ultimately result in a final common outcome of matrix degeneration and biomechanical failure. Therefore, the patient-specific knowledge of both wall stress and wall strength distributions for a given aneurysm will greatly improve the ability to identify those aortic aneurysms that are at highest risk of rupture. Towards this end, the biomechanics of AAA has been studied extensively by many groups whereas TAA biomechanics has not been fully considered. This chapter articulates the state-of-the-art of aortic biomechanics, including the modeling of tensile strength and wall stress distributions and the biological mechanisms which influence them. The potential clinical utility of these biomechanical estimates in predicting AAA rupture is also discussed.

Structural analysis of woody species in Mediterranean old fields

Abstract The study analyses the changes in vegetation structure and composition within a sere of secondary succession at Pantelleria Island (Sicilian Channel, Italy). It aims to show that not only phytosociological data but also structural parameters, like woody species’ height and spatial distribution indices are useful to describe and interpret renaturation processes. Woody species structure was recorded on abandoned terraces, both on north-facing and on south-facing slopes. Relevés were made in fallows representing five different stages of succession. The pace of succession, measured through the analysis of woody species cover, basal area, height distribution and spatial indices, resulted quite rapid: already after 50 years of abandonment terraces are covered by dense maquis communities. Our study also revealed that different plant species or groups prevail during colonisation dynamics, mostly depending on exposition, a factor which strongly influences also the speed of colonisation by woody species. In this case study, human activity seems to be unnecessary to accelerate the process of renaturation, except in some unfavourable contexts. Species turnover rate, biodiversity value, and structural evolution along progressive succession must be taken into account in nature management and conservation policies of terraced landscapes, which are nowadays one of the most endangered landscape types throughout the Mediterranean area.