Effect of External Load on the Stresses in the Artery

V první části práce byla provedená rozsáhlá studie odborných článku s ohledem na uložení arterie během FEM (Finite element method) analýzy. Byly zjištěny dva základní typy uložení, a to fixní ukotvení proximálního a distálního konce anebo axiální fixace obou konců vyjmuté tepny. Žádná z prostudovaných studii neuvažuje s možností objemového uložení arterie V druhé části práce byla navržena nová okrajová podmínka zahrnující síť kolagenových vláken vycházejících z vnější arteriální stěny. Pro určení základních parametrů nové okrajové podmínky byla použita citlivostní analýza.In the first part of the thesis, a large study of specialized articles was performed, which were focused on the anchoring of the artery during the FEM (Finite element method) analysis. Two basic types of anchoring were identified, namely proximal and distal ends anchorage, or axial anchoring of both ends of the extracted artery. None of the studies considered an option for anchoring every point of the abdominal aortic bifurcation using collagen fibers. In the second part of the thesis a new boundary condition was proposed including a network of collagen fibers coming from the external arterial wall. A sensitivity analysis was used to determine the basic parameters of the new boundary condition330 - Katedra aplikované mechanikyvýborn

FINITE ELEMENT ANALYSIS OF AORTAL BIFURCATION

Arterial bifurcations loaded by internal pressure represent significant stress concentrators. Increased mechanical stress inside arterial wall probably accelerates pathogenic processes at these places. Stress concentration factor (SCF) depends mainly on geometry, loading and material. This work presents a map of SCFs calculated by FEM at aortic bifurcation (AB) loaded by static internal pressure. Influence of geometry (aortic diameter, wall thickness, bifurcation angle, "non-planarity" angle and radius of apex), material properties and internal pressure were evaluated statistically by regression of FEM results. Two variants of materials were used (linear Hook and hyper elastic Ogden). Viscoelastic behaviour, anisotropy and prestrain were neglected. Results indicate that the highest Mises stress appears in the inner side of AB apex and that the SCF is negatively correlated with bifurcation angle and with internal pressure. The SCF varies from 4,5 to 7,5 (Hook) and from 7 to 21 (Ogden)

A structural approach including the behavior of collagen cross-links to model patient-specific human carotid arteries

The final publication is available at Springer via http://dx.doi.org/10.1007/s10439-014-0995-7The objective of this work is to develop a remodeling model for biological matter coupling two different processes in a 3D framework: reorientation of the preferential direction of a given fibered structure and reorientation of the fibrils or filaments that make up such a structure. This work uses the microsphere-based approach to take into account the micro mechanics involved in biological fibered structures regarding both their passive behavior and the reorientation of their micro constituents. Moreover, the macro behavior of the material as a whole is obtained by means of homogenizing the underlying micro response. We associate the orientation space of the integration directions to the physical space of micro-fibrils. To approximate the directional distribution of the fibrils within each fiber bundle, a Bingham probability orientation density function is introduced into the Helmholtz energy function. With all these assumptions, the problem is studied from an energetic point of view, describing the dissipation inherent to remodeling processes, and the evolution equations for both reorientations (change in preferential direction of the network and change in shape of the fibril distribution) re obtained. The model is included in a finite element code which allows computing different geometries and boundary value problems. This results in a complete methodology for characterizing the reorientation evolution of different fibered biological structures, such as cells. Our results show remodeling of fibered structures in two different scales, presenting a qualitatively good agreement with experimental findings in cell mechanics. Hierarchical structures align in the direction of the maximum principal direction of the considered stimulus and narrow in the perpendicular direction. The dissipation rates follows predictable trends although there are no experimental findings to date for comparison. The incorporation of metabolic processes and an insight into cell-oriented mechano-sensing processes can help to overcome the limitations involved.Peer ReviewedPostprint (author's final draft

Development of a novel uncovered stent system for the management of complex aortic aneurysms

Endovascular aortic repair (EVAR) is a minimally invasive alternative to open surgery for the treatment of aortic aneurysms (AA). However, standard EVAR is not applicable to complex AA with involvement of vital branches, which could be occluded by the endograft. As an emerging technique, the concept of multiple overlapping uncovered stents (MOUS) have been proposed to manage complex lesions. MOUS was used to modulate the flow pattern inside the aneurysm sac, and promote the thrombus formation followed by the aneurysm shrinkage. In this dissertation, we sought to investigate the mechanism of MOUS-induced flow modulation and key factors associated with the success of this novel technique: - The mechanical behaviour of AA was characterised by uniaxial material tests (Chapter 4). A Bayesian framework was proposed for material constants identification. They were found correlated to the microstructure of tissue fibre network and were capable in differentiating tissue types. - Solid-to-solid interaction and one-way fluid-solid interaction (FSI) analysis was performed based on patient-specific computer tomography angiography (Chapters 5&6). Structural stress concentrations were observed within the landing zones, which increased with the number of stents deployed. In the parameter studies (Chapter 6), the overall porosity was identified as the dominant factor of the flow-diverting outcome, while cross-stent structures of MOUS had limited influence. - The pathological effect of structural stress concentration induced by an implanted device was further studied in rabbit models (Chapter 7). The wall structural stress and fluid shear stress were obtained from FSI analysis based on magnetic resonance imaging (MRI), and correlated to plaque characteristics. Both high structural stress and low fluid shear stress were found correlated to plaque initialisation and increased inflammation. Overall, MOUS modulates the blood flow with robust performance under different overlapping patterns. Image-based biomechanical analysis can optimise MOUS design and can contribute to personalised pre-surgery planning.Shuo Wang was sponsored by China Scholarship Council (2014-2018)

Numerical simulations of patient-specific models with multiple plaques in human peripheral artery: a fluid-structure interaction analysis

© 2020, The Author(s). Atherosclerotic plaque in the femoral is the leading cause of peripheral artery disease (PAD), the worse consequence of which may lead to ulceration and gangrene of the feet. Numerical studies on fluid-structure interactions (FSI) of atherosclerotic femoral arteries enable quantitative analysis of biomechanical features in arteries. This study aims to investigate the hemodynamic performance and its interaction with femoral arterial wall based on the patient-specific model with multiple plaques (calcified and lipid plaques). Three types of models, calcification-only, lipid-only and calcification-lipid models, are established. Hyperelastic material coefficients of the human femoral arteries obtained from experimental studies are employed for all simulations. Oscillation of WSS is observed in the healthy downstream region in the lipid-only model. The pressure around the plaques in the two-plaque model is lower than that in the corresponding one-plaque models due to the reduction of blood flow domain, which consequently diminishes the loading forces on both plaques. Therefore, we found that stress acting on the plaques in the two-plaque model is lower than that in the corresponding one-plaque models. This finding implies that the lipid plaque, accompanied by the calcified plaque around, might reduce its risk of rupture due to the reduced the stress acting on it

THE CLINICAL IMPACT OF INTRAVASCULAR ULTRASOUND DERIVED VIRTUAL HISTOLOGY ON PERCUTANEOUS CORONARY INTERVENTION.

It has been shown that early interventional treatment of patients with high risk acute coronary syndromes (ACS) has a favourable effect on mortality. It is also known that coronary plaque rupture and atherothrombosis creates a mileu of necrotic and thrombotic material, which is difficult to treat. Moreover, angiographic assessment of coronary artery disease is highly flawed; 2-dimensional luminal silhouettes are not ideal templates to guide very important interventions. Intravascular ultrasound (IVUS) is the gold standard imaging tool able to delineate vessel dimensions, plaque burden, length and now with virtual histology (VH) - plaque composition. Despite optimal medical care and urgent revascularisation, 12-20% of ACS patients will suffer a further major adverse cardiac event (MACE) at 30 months. Our aim was to evaluate the angiographic treatment of high risk ACS patients by performing IVUS-VH pre and post-intervention, with the operators blinded to the images. Our hypotheses for this work were as follows: 1. Significant compositional and structural differences exist between culprit, non-culprit and stable plaques when analysed by Virtual Histology. 2. The histologically most unstable plaque does not occur at the site of maximum angiographic stenosis (in culprit lesions). 3. Angiographically guided stent length selection and positioning is flawed, leaving unstable plaque behind in the reference segments. Following recruitment, 135 lesions split into: 70 ACS culprit; 20 ACS non-culprit and 35 Stable lesions underwent analysis for inter-observer and intra-observer variability. We were able to show good standard markers of correlation but a large repeatability co-efficent, for some outputs from the analysis. This has raised questions with regard to the ability of the technique to detect differences between plaque types. In relation to our main hypothesis, we have been able to show structural and compositional volume differences between “active” ACS plaques (n=70) and “stable” angina plaques (n=35). We have used the most important of these to generate a plaque risk score based upon ROC statistics and logistic regression. The most important discriminators were: Remodelling index at the minimum lumen area; Plaque Burden; Presence of VH-TCFA; minimum lumen area (MLA) <4mm2 and necrotic core to dense calcium ratio (NC/DC). The subsequent risk model was tested on an independent, blinded cohort of plaques from the Thoraxcentre, Rotterdam (n=50). This confirmed good discriminatory power for the equation (AUC – 0.71). Within this hypothesis we also explored the differences in individual areas of plaque. At two separate sites (MLA and MAX NC) in each lesion type (ACS and stable) n=210, we showed that the MAX NC site lies proximal to the MLA in most cases and contains more positively remodelled plaque disease with less calcification. This is important as positively remodeled plaque disease is often not visible on a plain coronary angiogram when treating ACS lesions. Finally, as a follow on from thie previous chapter, we examined the treatment of ACS lesions by blinded IVUS examination. The operator completed their stent procedure with only angiographic guidance. We were able to show in 56 ACS lesions that systematic errors of judgement occur related to sizing of the vessel, the choice of stent sizes and the subsequent stent deployment. 36%, 40% and 65.5% of stents met three separate standard criteria for good stent deployment. Moreover, between 5-40% of stents had some form of significant abnormality. With regard to the sizing of stents to the vessel, the mean reference vessel size was 10.58mm2 (±2.51) yet the minimum stent area achieved was only 6.79mm2 (±2.43). If the stent that was chosen had been symmetrically deployed to its nominal size (e.g 2.5; 3.0; 3.5) then the stent area achievable should have been 8.87mm2 (±2.68). This has allowed us to calculate for the first time an estimated “under deployment area”. This was 2.08mm2 (±1.87)

Biomechanical Modeling of Atherosclerotic Plaques for Risk Assessment

A healthy arterial wall comprises three layers: the adventitia, the media and the intima (Figure 1.1, left side). The adventitia is the outermost layer, mainly composed of collagen. The media underlies the adventitia and is the middle layer in the arterial wall. It is made up of concentrically arranged smooth muscle cells and collagen fibers. The intima is the innermost layer. It is a thin sheet of endothelial cells attached to a basal membrane. Atherosclerosis is a systemic, inflammatory disease of the arterial system characterized by local thickening of vessel walls. Thickened arterial segments are called atherosclerotic plaques (Figure 1.1, right side). During atherogenesis - progression of an atherosclerotic plaque- the major changes take place in the intima due to infiltration of lipids and inflammatory cells from the luminal side, smooth muscle cell migration and proliferation, extracellular matrix deposition, and intraplaque hemorrhage. From a thin cell layer, the intima transforms into a thick layer (Figure 1.1) with the possible structural components being smooth muscle cells, collagen and elastin fibers, and lipids. Besides changes in the intima, atherosclerosis causes differentiation in the media and adventitia layers. Fibrosis, atrophy and inflammation may take place in the media and adventitia during atherogenesis