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  • 1. Auer, M.
    et al.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Reconstruction and Finite Element Mesh Generation of Abdominal Aortic Aneurysms From Computerized Tomography Angiography Data With Minimal User Interactions2010In: IEEE Transactions on Medical Imaging, ISSN 0278-0062, E-ISSN 1558-254X, Vol. 29, no 4, p. 1022-1028Article in journal (Refereed)
    Abstract [en]

    Evaluating rupture risk of abdominal aortic aneurysms is critically important in reducing related mortality without unnecessarily increasing the rate of elective repair. According to the current clinical practice aneurysm rupture risk is (mainly) estimated from its maximum diameter and/or expansion rate; an approach motivated from statistics but known to fail often in individuals. In contrast, recent research demonstrated that patient specific biomechanical simulations can provide more reliable diagnostic parameters, however current structural model development is cumbersome and time consuming. This paper used 2D and 3D deformable models to reconstruct aneurysms from computerized tomography angiography data with minimal user interactions. In particular, formulations of frames and shells, as known from structural mechanics, were used to define deformable modes, which in turn allowed a direct mechanical interpretation of the applied set of reconstruction parameters. Likewise, a parallel finite element implementation of the models allows the segmentation of clinical cases on standard personal computers within a few minutes. The particular topology of the applied 3D deformable models supports a fast and simple hexahedral-dominated meshing of the arising generally polyhedral domain. The variability of the derived segmentations (luminal: 0.50(SD 0.19) mm; exterior 0.89(SD 0.45) mm) with respect to large variations in elastic properties of the deformable models was in the range of the differences between manual segmentations as performed by experts (luminal: 0.57(SD 0.24) mm; exterior: 0.77(SD 0.58) mm), and was particularly independent from the algorithm's initialization. The proposed interaction of deformable models and mesh generation defines finite element meshes suitable to perform accurate and efficient structural analysis of the aneurysm using mixed finite element formulations.

  • 2.
    Auer, Martin
    et al.
    Develoment, Vascops GmbH, Graz, Austria.
    Gasser, Thomas Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Portugaller, R.
    Automatic Displacement and Strain measuring in the Aorta from dynamic electrocardiographically-gated Computed Tomographic Angiography2010Conference paper (Refereed)
    Abstract [en]

    Introduction

    Image modalities like Duplex Ultrasound, Transesophageal Echocardiography, Intravascular Ultrasound, Computed Tomography and Magnetic Resonance provide vascular interventionists and surgeons with useful diagnostic information for treatment planning. Recent developments in cross-sectional imaging, including multi-modality image fusion and new contrast agents have resulted in improved spatial resolution. Specifically, dynamic Electrocardiographically-Gated Computed Tomographic Angiography (ECG-gated CTA) provides valuable information regarding motion and deformation of the normal and diseased aorta during the cardiac cycle. Extracting and presenting (visualization) of accurate quantitative information from the recorded image data, however remains a challenging task of image post processing.

    Method

    The algorithm proposed within this paper processes ECG-gated CTA data (here goes the scanner model and manufacturer) in DICOM (digital imaging and communication in medicine) format, within which the user manually defines an Eulerian Region of Interest (ROI). 2D deformable (active) contour models are used to pre-segment the luminal surfaces of the selected vessels at an arbitrary time point during the cardiac cycle. A tessellation algorithm is used to define the initial configuration of a 3D deformable (active) contour model, which in turn is used for the final segmentation of the luminal surfaces continuously during the cardiac cycle. Specifically, Finite Element (FE) formulations [1] for frames and shells, as known from structural mechanics, are used to define the deformable contour modes. This allows a direct mechanical interpretation of the applied set of reconstruction parameters and leads to an efficient FE implementation of the models [2]; parallel processor architecture is used to solve the global set of non-linear FE equations. Finally displacement and strain measures are derived from the dynamic segmentations and color coded plots are used to visualize them.

    Results and Conclusions

    The clinical relevance of dynamic imaging has not been fully exploited and accurate and fast image processing tools are critical to extract valuable information from ECG-gated CTA data. Such information is not only of direct clinical relevance but also critical to process our current understanding regarding normal and pathological aortic motions and deformations. The image processing concept proposed in this paper leads to efficient and clinically applicable software that facilitates an analysis of the entire aorta on a standard Personal Computer within a few minutes. Deformable (active) contour models are known to be more accurate compared to threshold based segmentation concepts [3] and the accuracy of the present approach is in the range of the in-plane image resolution. Apart from direct diagnostic information the extracted geometrical data could also be used (once enriched by accurate pressure measurements) for none invasive (minimal invasive) estimation of biomechanical aortic tissue properties.

    References

    [1] O. C. Zienkiewicz and R. L. Taylor, vol.1,2, 5th ed. Oxford: Butterworth Heinemann, 2000.

    [2] M. Auer and T. C. Gasser,

    IEEE T. Med. Imaging, 2010 (in press).

    [3] M. Sonka and J. M. Fitzpatrick, editors.,

    Bellingham: Spie press, 2000

  • 3.
    Biasetti, Jacopo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Auer, Martin
    VASCOPS GmbH, Graz, Austria.
    Hedin, Ulf
    Department of Molecular Medicine and Surgery, Karolinska Universty Hospital, Stockholm, Sweden.
    Labruto, Fausto
    Department of Radoilogy, Karolinska University Hospital, Stockholm, Sweden.
    Hemodynamics of the Normal Aorta Compared to Fusiform and Saccular Abdominal Aortic Aneurysms with Emphasis on a Potential Thrombus Formation Mechanism2010In: Annals of Biomedical Engineering, ISSN 0090-6964, E-ISSN 1573-9686, Vol. 38, no 2, p. 380-390Article in journal (Refereed)
    Abstract [en]

    Abdominal Aortic Aneurysms (AAAs), i.e., focal enlargements of the aorta in the abdomen are frequently observed in the elderly population and their rupture is highly mortal. An intra-luminal thrombus is found in nearly all aneurysms of clinically relevant size and multiply affects the underlying wall. However, from a biomechanical perspective thrombus development and its relation to aneurysm rupture is still not clearly understood. In order to explore the impact of blood flow on thrombus development, normal aortas (n = 4), fusiform AAAs (n = 3), and saccular AAAs (n = 2) were compared on the basis of unsteady Computational Fluid Dynamics simulations. To this end patient-specific luminal geometries were segmented from Computerized Tomography Angiography data and five full heart cycles using physiologically realistic boundary conditions were analyzed. Simulations were carried out with computational grids of about half a million finite volume elements and the Carreau-Yasuda model captured the non-Newtonian behavior of blood. In contrast to the normal aorta the flow in aneurysm was highly disturbed and, particularly right after the neck, flow separation involving regions of high streaming velocities and high shear stresses were observed. Naturally, at the expanded sites of the aneurysm average flow velocity and wall shear stress were much lower compared to normal aortas. These findings suggest platelets activation right after the neck, i.e., within zones of pronounced recirculation, and platelet adhesion, i.e., thrombus formation, downstream. This mechanism is supported by recirculation zones promoting the advection of activated platelets to the wall.

  • 4.
    Biasetti, Jacopo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Gasser, Thomas Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    A Blood Flow based model for Platelet Activation in Abdominal Aortic Aneurisms2010Conference paper (Refereed)
    Abstract [en]

    Introduction

    Thrombus formation is the physiological response to vascular injury, it prevents loss of blood and permits wound healing, however, it is also associated with pathological conditions like hypoxia, anoxia and infarction [1]. Consequently, thrombus development must be carefully modulated to avoid uncontrolled growth, which in turn could lead to organ malfunctions. Specifically, an Intra-Luminal Thrombus (ILT) is found in almost all larger (clinically relevant) Abdominal Aortic Aneurysms (AAAs) and multiple biochemical [2] and biomechanical [3] implications on the underlying wall tissue have been reported. Despite the dominant role played by the ILT in AAA disease little is known regarding its development, and hence, the present study investigates ILT formation with particular emphasis on platelet activation triggered by biomechanical and biochemical field variables.

    Method

    The proposed model assumes that platelet activation is defined by a single field variable

    representing the accumulation of mechanical [4] and chemical [5] factors as the platelet moves along its path line. Platelet activation is given as soon asovercomes a certain threshold thought to be a constitutive property of blood. Specifically, the rate of the activation variable is determined by the maximum shear stress and the local concentrations of agonists and antagonists. To implement the model the fluid mechanical problem was solved in (COMSOL, COMSOL AB) and a particle tracking analysis (MATLAB, The MathWorks) was applied as a post processing step. The flow in a circular tube and the Backward Facing Step (BFS) problem under varying initial conditions were used for a basic investigation of the model and to relate its predictions to available data in the literature. Finally, platelet activation in patient specific AAAs was predicted and related to ILT development, which was estimated from Computer Tomography-Angiography (CT-A) data recorded from patient follow-up studies.

    Results and Conclusions

    The platelet activation variable

     is complex distributed (highly heterogeneous) in the flow field, where, specifically, at the boundary of vortexes [6] and in the boundary layer of the non- endothelialized wall highest values were predicted. Continuous release of antagonists from the endothelialized wall lowers  in its vicinity, and hence, despite the high shear stress platelet activation

    is prevented. The proposed model links biomechanical and biochemical mechanisms of platelet activation and is able to predict the onset of thrombus formation of the BFS problem. The model is also able to predict some features of ILT development in the AAA, however, the change in luminal geometry is a cumulative effect of ILT growth, wall growth and their mechanical interactions, and hence, data recorded form patient follow-up studies needs to be analyzed carefully when validating the present model.

    References

    [1] J. D. Humphrey,

    Springer-Verlag, New York, 2002.

    [2] M. Kazi, et. al.

    J. Vasc. Surg., 38:1283-1292, 2003.

    [3] W. R. Mower et. al.,

    J. Vasc. Surg., 33:602-608, 1997.

    [4] J. D. Hellums,

    Ann. Biomed. Eng., 22: 445-455, 1994.

    [5] B. Alberts et. al.

    Molecular Biology of the cell, 2002.

    [6] J. Biasetti et. al.

    Ann. Biomed. Eng., 38: 380–390 2010.

  • 5.
    Biasetti, Jacopo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Gasser, Thomas Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    A Fluid-chemical model of thrombus formation2011In: CMBE2011: 2nd International Conference on Mathematical and Computational Biomedical Engineering / [ed] P. Nithiarasu, R. Löhner, 2011Conference paper (Refereed)
    Abstract [en]

    Our understanding of the genesis and evolution of Abdominal Aortic Aneurysms (AAAs), withparticular emphasis on Intra-Luminal Thrombus’ evolution, may be improved by studying thecomplex interplay between fluid-dynamics and biochemistry. To investigate the evolution of prothromboticchemicals inside the blood flow, in particular thrombin (factor IIa), a fluido-chemicalmodel has been developed. To this end a series of convection-diffusion-reaction (CDR) equationsdescribing the tissue factor pathway to thrombin have been solved on top of the biofluiddynamics problem. The proposed model integrates biochemistry and fluids dynamics, and hence,supports a comprehensive understanding of how ILT in AAAs may develop.

  • 6.
    Biasetti, Jacopo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Hussain, Fazle
    Department of Mechanical Engineering, University of Houston, Houstohn, TX, USA.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Blood flow and coherent vortices in the normal and aneurysmatic aortas: a fluid dynamical approach to intraluminal thrombus formation2011In: Journal of the Royal Society Interface, ISSN 1742-5689, E-ISSN 1742-5662, Vol. 8, no 63, p. 1449-1461Article in journal (Refereed)
    Abstract [en]

    Abdominal aortic aneurysms (AAAs) are frequently characterized by the development of an intra-luminal thrombus (ILT), which is known to have multiple biochemical and biomechanical implications. Development of the ILT is not well understood, and shear-stress-triggered activation of platelets could be the first step in its evolution. Vortical structures (VSs) in the flow affect platelet dynamics, which motivated the present study of a possible correlation between VS and ILT formation in AAAs. VSs educed by the lambda(2)-method using computational fluid dynamics simulations of the backward-facing step problem, normal aorta, fusiform AAA and saccular AAA were investigated. Patient-specific luminal geometries were reconstructed from computed tomography scans, and Newtonian and Carreau-Yasuda models were used to capture salient rheological features of blood flow. Particularly in complex flow domains, results depended on the constitutive model. VSs developed all along the normal aorta, showing that a clear correlation between VSs and high wall shear stress (WSS) existed, and that VSs started to break up during late systole. In contrast, in the fusiform AAA, large VSs developed at sites of tortuous geometry and high WSS, occupying the entire lumen, and lasting over the entire cardiac cycle. Downward motion of VSs in the AAA was in the range of a few centimetres per cardiac cycle, and with a VS burst at that location, the release (from VSs) of shear-stress-activated platelets and their deposition to the wall was within the lower part of the diseased artery, i.e. where the thickest ILT layer is typically observed. In the saccular AAA, only one VS was found near the healthy portion of the aorta, while in the aneurysmatic bulge, no VSs occurred. We present a fluid-dynamics-motivated mechanism for platelet activation, convection and deposition in AAAs that has the potential of improving our current understanding of the pathophysiology of fluid-driven ILT growth.

  • 7.
    Biasetti, Jacopo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Spazzini, Pier Giorgio
    Mechanics Division, National Institute of Metrological Research (INRiM), Turin, Italy.
    Gasser, Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    An integrated fluido-chemical model towards modeling the formation of intra-luminal thrombus in abdominal aortic aneurysms2011Article in journal (Other academic)
    Abstract [en]

    Abdominal Aortic Aneurysms (AAAs) are frequently characterized by the presence of an Intra-Luminal Thrombus (ILT) known to influence biochemically and biomechanically their evolution. ILT progression mechanism is still unclear and little is known regarding the impact of chemicals transported by blood flow. It is expected that chemical agonists and antagonists of platelets activation, aggregation, and adhesion play an important role in ILT development. Starting fromthis assumption, the evolution of chemical species related to the coagulation cascade (CC), their relation to coherent vortical structures (VSs) and their effect on ILT evolution have been studied. To this end a fluido-chemical model that simulates the CC through a series of convection-diffusion-reaction (CDR) equations and considers blood as a non-Newtonian incompressible fluid has been developed. In addition to the relation between VSs and thrombin distribution, high thrombin concentrations at the distal portion of the AAA were observed, i.e. the region where the thickest ILT is usually seen. The proposed model, due to its ability to couple the fluid and chemical domains, provides an integrated mechanochemical picture that potentially could unveil mechanisms of ILT formation and development.

  • 8.
    Biasetti, Jacopo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Spazzini, Pier Giorgio
    Mechanics Division, National Institute of Metrological Research (INRiM), Turin, Italy.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Shear-induced migration of red blood cells in the abdominal aorta and thecarotid bifurcation: considerations on oxygen transport2013Report (Other academic)
    Abstract [en]

    Shear-induced migration of red blood cells (RBCs) is a well known phenomenon characterizing blood flow in the small vessels (micron to mm size) of the cardiovascular system. In large vessels, like the abdominal aorta and the carotid artery (mm to cm size), the extent of this migration has not been fully elucidated. RBCs migration exerts its influence primarily on platelet concentration, oxygen transport and oxygen availability at the luminal surface; this being of primary importance in, for example, intra-luminal thrombus (ILT) growth, atherosclerosis and intima hyperplasia. Phillips’ shear-induced particle migration model coupled to the Quemada viscosity model was employed to simulate the macroscopic behavior of RBCs in four patient-specific geometries: a normal abdominal aorta, an abdominal aortic aneurysm (AAA), a normal carotid bifurcation and a stenotic carotid bifurcation. Simulations show a migration of RBCs from the near wall region with a lowering of wall hematocrit (volume fraction of RBCs) on the posterior side of the normal aorta and in the iliac arteries. A marked migration is observed on the outer wall of the carotid sinus, the inner curvature wall of the common carotid artery and in the carotid stenosis. No significant migration is observed in the AAA. The spatial and temporal patterns of wall hematocrit are correlated with the near-wall shear layer and with the secondary flow induced by the vessel curvature. The results reinforce data in literature showing a decrease in oxygen partial pressure on the inner curvature wall of the carotid sinus and, more in general, on the inner curvature wall. The lowering of wall hematocrit is postulated to induce a decrease in oxygen availability at the luminal surface through a diminished concentration of oxyhemoglobin, hence contributing, with the lowered oxygen partial pressure, to local hypoxia.

  • 9.
    Biasetti, Jacopo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Spazzini, Pier Giorgio
    Mechanics Division, National Institute of Metrological Research, Turin, Italy.
    Swedenborg, Jesper
    Department of Molecular Medicine and Surgery, Center for Molecular Medicine, Karolinska Institutet, Stockholm, Sweden.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    An Integrated Fluid-Chemical Model Toward Modeling the Formation of Intra-Luminal Thrombus in Abdominal Aortic Aneurysms2012In: Frontiers in Physiology, ISSN 1664-042X, E-ISSN 1664-042X, Vol. 3, no 266Article in journal (Refereed)
    Abstract [en]

    Abdominal Aortic Aneurysms (AAAs) are frequently characterized by the presence of an Intra-Luminal Thrombus (ILT) known to influence their evolution biochemically and biomechanically. The ILT progression mechanism is still unclear and little is known regarding the impact of the chemical species transported by blood flow on this mechanism. Chemical agonists and antagonists of platelets activation, aggregation, and adhesion and the proteins involved in the coagulation cascade (CC) may play an important role in ILT development. Starting from this assumption, the evolution of chemical species involved in the CC, their relation to coherent vortical structures (VSs) and their possible effect on ILT evolution have been studied. To this end a fluid-chemical model that simulates the CC through a series of convection-diffusion-reaction (CDR) equations has been developed. The model involves plasma-phase and surface-bound enzymes and zymogens, and includes both plasma-phase and membrane-phase reactions. Blood is modeled as a non-Newtonian incompressible fluid. VSs convect thrombin in the domain and lead to the high concentration observed in the distal portion of the AAA. This finding is in line with the clinical observations showing that the thickest ILT is usually seen in the distal AAA region. The proposed model, due to its ability to couple the fluid and chemical domains, provides an integrated mechanochemical picture that potentially could help unveil mechanisms of ILT formation and development.

  • 10. Bäck, Magnus
    et al.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Michel, Jean-Baptiste
    Caligiuri, Giuseppina
    Biomechanical factors in the biology of aortic wall and aortic valve diseases2013In: Cardiovascular Research, ISSN 0008-6363, E-ISSN 1755-3245, Vol. 99, no 2, p. 232-241Article, review/survey (Refereed)
    Abstract [en]

    The biomechanical factors that result from the haemodynamic load on the cardiovascular system are a common denominator of several vascular pathologies. Thickening and calcification of the aortic valve will lead to reduced opening and the development of left ventricular outflow obstruction, referred to as aortic valve stenosis. The most common pathology of the aorta is the formation of an aneurysm, morphologically defined as a progressive dilatation of a vessel segment by more than 50% of its normal diameter. The aortic valve is exposed to both haemodynamic forces and structural leaflet deformation as it opens and closes with each heartbeat to assure unidirectional flow from the left ventricle to the aorta. The arterial pressure is translated into tension-dominated mechanical wall stress in the aorta. In addition, stress and strain are related through the aortic stiffness. Furthermore, blood flow over the valvular and vascular endothelial layer induces wall shear stress. Several pathophysiological processes of aortic valve stenosis and aortic aneurysms, such as macromolecule transport, gene expression alterations, cell death pathways, calcification, inflammation, and neoangiogenesis directly depend on biomechanical factors.

  • 11. Comellas, E.
    et al.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Bellomo, F. J.
    Oller, S.
    A homeostatic-driven turnover remodelling constitutive model for healing in soft tissues2016In: Journal of the Royal Society Interface, ISSN 1742-5689, E-ISSN 1742-5662, Vol. 13, no 116, article id 20151081Article in journal (Refereed)
    Abstract [en]

    Remodelling of soft biological tissue is characterized by interacting biochemical and biomechanical events, which change the tissue's microstructure, and, consequently, its macroscopic mechanical properties. Remodelling is a well-defined stage of the healing process, and aims at recovering or repairing the injured extracellular matrix. Like other physiological processes, remodelling is thought to be driven by homeostasis, i.e. it tends to re-establish the properties of the uninjured tissue. However, homeostasis may never be reached, such that remodelling may also appear as a continuous pathological transformation of diseased tissues during aneurysm expansion, for example. A simple constitutive model for soft biological tissues that regards remodelling as homeostatic-driven turnover is developed. Specifically, the recoverable effective tissue damage, whose rate is the sum of a mechanical damage rate and a healing rate, serves as a scalar internal thermodynamic variable. In order to integrate the biochemical and biomechanical aspects of remodelling, the healing rate is, on the one hand, driven by mechanical stimuli, but, on the other hand, subjected to simple metabolic constraints. The proposed model is formulated in accordance with continuum damage mechanics within an open-system thermodynamics framework. The numerical implementation in an in-house finite-element code is described, particularized for Ogden hyperelasticity. Numerical examples illustrate the basic constitutive characteristics of the model and demonstrate its potential in representing aspects of remodelling of soft tissues. Simulation results are verified for their plausibility, but also validated against reported experimental data.

  • 12. Erhart, P.
    et al.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Auer, M.
    Böckler, D.
    Hyhlik-Dürr, A.
    Finite-Elemente-Analyse abdomineller Aortenaneurysmen: Aktuelle Wertigkeit als Ergänzung zur herkömmlichen Diagnostik2015In: Gefässchirurgie, ISSN 0948-7034, E-ISSN 1434-3932, Vol. 20, no 7, p. 503-507Article in journal (Refereed)
    Abstract [en]

    Finite element analysis (FEA) of abdominal aortic aneurysms (AAA) could enable a more precise patient-specific risk assessment of AAA rupture. Further clinical studies are needed to validate this model as a clinical decision-making tool. The A4clinics™ software provides a simple and detailed FEA simulation. After implementation of a FEA workstation in a high volume university vascular center, relevant studies for further model validation are expected to be carried out.

  • 13. Erhart, P.
    et al.
    Hyhlik-Dürr, A.
    Geisbüsch, P.
    Kotelis, D.
    Müller-Eschner, M.
    Gasser, Thomas Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    von Tengg-Kobligk, H.
    Böckler, D.
    Finite Element Analysis in Asymptomatic, Symptomatic, and Ruptured Abdominal Aortic Aneurysms: In Search of New Rupture Risk Predictors2015In: European Journal of Vascular and Endovascular Surgery, ISSN 1078-5884, E-ISSN 1532-2165, Vol. 49, no 3, p. 239-245Article in journal (Refereed)
    Abstract [en]

    Objectives: To compare biomechanical rupture risk parameters of asymptomatic, symptomatic and ruptured abdominal aortic aneurysms (AAA) using finite element analysis (FEA). Study design: Retrospective biomechanical single center analysis of asymptomatic, symptomatic, and ruptured AAAs. Comparison of biomechanical parameters from FEA. Materials and methods: From 2011 to 2013 computed tomography angiography (CTA) data from 30 asymptomatic, 15 symptomatic, and 15 ruptured AAAs were collected consecutively. FEA was performed according to the successive steps of AAA vessel reconstruction, segmentation and finite element computation. Biomechanical parameters Peak Wall Rupture Risk Index (PWRI), Peak Wall Stress (PWS), and Rupture Risk Equivalent Diameter (RRED) were compared among the three subgroups. Results: PWRI differentiated between asymptomatic and symptomatic AAAs (p < .0004) better than PWS (p < .1453). PWRI-dependent RRED was higher in the symptomatic subgroup compared with the asymptomatic subgroup (p < .0004). Maximum AAA external diameters were comparable between the two groups (p < .1355). Ruptured AAAs showed the highest values for external diameter, total intraluminal thrombus volume, PWS, RRED, and PWRI compared with asymptomatic and symptomatic AAAs. In contrast with symptomatic and ruptured AAAs, none of the asymptomatic patients had a PWRI value >1.0. This threshold value might identify patients at imminent risk of rupture: Conclusions: From different FEA derived parameter, PWRI distinguishes most precisely between asymptomatic and symptomatic AAAs. If elevated, this value may represent a negative prognostic factor for asymptomatic AAAs.

  • 14. Federico, Salvatore
    et al.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Nonlinear elasticity of biological tissues with statistical fibre orientation2010In: Journal of the Royal Society Interface, ISSN 1742-5689, E-ISSN 1742-5662, Vol. 7, no 47, p. 955-966Article in journal (Refereed)
    Abstract [en]

    The elastic strain energy potential for nonlinear fibre-reinforced materials is customarily obtained by superposition of the potentials of the matrix and of each family of fibres. Composites with statistically oriented fibres, such as biological tissues, can be seen as being reinforced by a continuous infinity of fibre families, the orientation of which can be represented by means of a probability density function defined on the unit sphere (i.e. the solid angle). In this case, the superposition procedure gives rise to an integral form of the elastic potential such that the deformation features in the integral, which therefore cannot be calculated a priori. As a consequence, an analytical use of this potential is impossible. In this paper, we implemented this integral form of the elastic potential into a numerical procedure that evaluates the potential, the stress and the elasticity tensor at each deformation step. The numerical integration over the unit sphere is performed by means of the method of spherical designs, in which the result of the integral is approximated by a suitable sum over a discrete subset of the unit sphere. As an example of application, we modelled the collagen fibre distribution in articular cartilage, and used it in simulating displacement-controlled tests: the unconfined compression of a cylindrical sample and the contact problem in the hip joint.

  • 15.
    Forsell, Caroline
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Björck, Hanna M.
    Eriksson, Per
    Franco-Cereceda, Anders
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Biomechanical Properties of the Thoracic Aneurysmal Wall: Differences Between Bicuspid Aortic Valve and Tricuspid Aortic Valve Patients2014In: Annals of Thoracic Surgery, ISSN 0003-4975, E-ISSN 1552-6259, Vol. 98, no 1, p. 65-71Article in journal (Refereed)
    Abstract [en]

    Background. The prevalence for thoracic aortic aneurysms (TAAs) is significantly increased in patients with a bicuspid aortic valve (BAV) compared with patients who have a normal tricuspid aortic valve (TAV). TAA rupture is a life-threatening event, and biomechanics-based simulations of the aorta may help to disentangle the molecular mechanism behind its development and progression. The present study used polarized microscopy and macroscopic in vitro tensile testing to explore collagen organization and mechanical properties of TAA wall specimens from BAV and TAV patients. Methods. Circumferential sections of aneurysmal aortic tissue from BAV and TAV patients were obtained during elective operations. The distribution of collagen orientation was captured by a Bingham distribution, and finite element models were used to estimate constitutive model parameters from experimental load-displacement curves. Results. Collagen orientation was almost identical in BAV and TAV patients, with a highest probability of alignment along the circumferential direction. The strength was almost two times higher in BAV samples (0.834 MPa) than in TAV samples (0.443 MPa; p < 0.001). The collagen-related stiffness (C-f) was significantly increased in BAV compared with TAV patients (C-f = 7.45 MPa vs 3.40 MPa; p = 0.003), whereas the elastin-related stiffness was similar in both groups. A trend toward a decreased wall thickness was seen in BAV patients (p = 0.058). Conclusions. The aneurysmal aortas of BAV patients show a higher macroscopic strength, mainly due to an increased collagen-related stiffness, compared with TAV patients. The increased wall stiffness in BAV patients may contribute to the higher prevalence for TAAs in this group.

  • 16.
    Forsell, Caroline
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Eriksson, Per
    France-Cereceda, Anders
    Gasser, Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Failure properties for the thoracic aneurysm wall: Differences between BicuspidAortic Valve (BAV) and Tricuspid Aortic Valve (TAV) patientsManuscript (preprint) (Other academic)
  • 17.
    Forsell, Caroline
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Gasser, Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Numerical simulation of the failure of ventricular tissue due to deep penetration: The impact of constitutive properties2011In: Journal of Biomechanics, ISSN 0021-9290, E-ISSN 1873-2380, Vol. 44, no 1, p. 45-51Article in journal (Refereed)
    Abstract [en]

    Lead perforation is a rare but serious clinical complication of pacemaker implantation, and towards understanding this malfunction, the present study investigated myocardial failure due to deep penetration by an advancing rigid punch. To this end, a non-linear Finite Element model was developed that integrates constitutive data published in the literature with information from in vitro tensile testing in cross-fibre direction of porcine myocardial tissue. The Finite Element model considered non-linear, isotropic and visco-elastic properties of the myocardium, and tissue failure was phenomenologically described by a Traction Separation Law. In vitro penetration testing of porcine myocardium was used to validate the Finite Element model, and a particular objective of the study was to investigate the impact of different constitutive parameters on the simulated results. Specifically, results demonstrated that visco-elastic properties of the tissue strongly determine the failure process, whereas dissipative effects directly related to failure had a minor impact on the simulation results. In addition, non-linearity of the bulk material did not change the predicted peak penetration force and the simulations did not reveal elastic crack-tip blunting. The performed study provided novel insights into ventricular failure due to deep penetration, and provided useful information with which to develop numerical failure models.

  • 18.
    Forsell, Caroline
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Impact of material anisotropy on deformation of myocardial tissue due to pacemaker electrodes2011In: ASME 2011 Summer Bioengineering Conference, SBC 2011, 2011, no PARTS A AND B, p. 789-790Conference paper (Refereed)
    Abstract [en]

    A Pacemaker electrode can penetrate the heart wall, and to design a penetration-resistent lead tip sound knowledge regarding failure of ventricular tissue is required. Numerical simulations can be particular helpful in that respect, but depend on a reliable constitutive description for ventricular tissue. In this study an anisotropic hyperelastic model for the myocardium has been implemented and compared to predictions from an isotropic description. Specifically, the response due to pushing a rigid punch into the myocardium was studied. Results between anisotropic and isotropic descriptions of the myocardium differed significantly, which justified the implementation of an anisotropic model for the myocardium.

  • 19.
    Forsell, Caroline
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Modeling of myocardial splitting due to deep penetration2010In: CONSTITUTIVE MODELS FOR RUBBER VI, BOCA RATON: CRC PRESS-TAYLOR & FRANCIS GROUP , 2010, p. 449-452Conference paper (Refereed)
    Abstract [en]

    The risk for pacemaker lead perforation, a rare but serious clinical complication, is thought to be minimized by perforation resistant device design. Fracture properties of ventricular tissue play a central role in such optimization studies, however, this information is currently not provided by the open literature; even failure models for soft biological tissue in general are rare. Incompressible finite deformations, material nonlinearity and time-dependent anisotropic properties require sophisticated approaches to identify and model failure of such a material. In this study we investigated myocardial failure due to deep penetration, where previously collected data from in-vitro experiments are integrated in a non-linear Finite Element model. In details, the proposed model describes tissue splitting by a cohesive process zone, and hence, tissue failure is modeled as a gradual process, where all inelastic phenomena are accumulated and mathematically captured by a traction separation law. The cohesive zone is embedded in a fibrous bulk material thought to capture the properties of passive myocardial tissue, where a transversely isotropic hyper-elastic constitutive description proposed in the literature was utilized. The developed numerical model integrates latest experimental data and is able to replicate quantitative and qualitative data from ventricular penetration experiments.

  • 20.
    Forsell, Caroline
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    The impact of constitutive properties on myocardial tissue perforation2010Conference paper (Refereed)
    Abstract [en]

    Introduction

    Acute or delayed lead perforation is a rare but serious complication of pacemaker implantation with numerous case reports and case series known [1]. A perforation-resistant lead tip design requires detailed knowledge regarding the mechanical failure of the ventricular wall. Like many other soft biological tissues, ventricular tissue exhibits complex mechanical properties like incompressibility, finite deformability, inhomogeneity, material non-linearity, anisotropy, strain rate-dependency, and a constitutive model should reflect that to the required degree of completeness. Within this work we investigate the failure mechanisms of myocardial penetration by advancing a rigid punch, conditions thought to be related to pacemaker lead perforation. Specifically, the impact of constitutive parameters related to the bulk material and the failure zone is analyzed.

    Method

    A single penetration site of our previous penetration experiment of biaxially-stretched myocardial tissue [2] was models by the non-linear Finite Element Method (ABAQUS,

    Dassault Systèmes). To this end a visco-elastic description was applied and a previously reported anisotropic constitutive model for myocardial tissue [3] was implemented using the user material model interface. All failure related inelastic deformations were lumped into a fracture process zone and captured by a triangular cohesive traction separation law. To this end the cohesive strength of ventricular tissue was experimentally determined by tensile testing in cross-fiber direction of porcine myocardial tissue. Simulated results with different visco-elastic and failure properties, i.e. by varying the associated sets of constitutive parameters of the myocardial tissue were investigated.

    Results and Conclusions

    Results demonstrated that visco-elastic properties of the myocardial tissue strongly determine the failure of myocardial tissue due to deep penetration. This finding is in line with failure of rubber-like materials, where visco-elastic energy dissipation in front of the crack tip was found to be an important factor of energy dissipation [4]. In contrast dissipative effects which are directly related to failure (i.e. captured by the cohesive zone model) had a minor impact on the simulated penetration force displacement characteristics. Likewise, non-linearity and anisotropy of the bulk material did not change the predicted peak penetration force and the simulations did not reveal elastic crack-tip blunting.

    The proposed numerical model integrates experimental data from different studies and allows a detailed investigation of failure related to pacemaker lead perforation. Results from the study provided novel insights into ventricular failure due to deep penetration, which might also be related to other soft biological tissues and helpful to design penetration resistant pacemaker leads.

    References

    [1] M.N. Khan, et. al.

    Pacing and Clinical Electrophysiology, 28, 251-253, 2005.

    [2] T.C. Gasser et. al.

    J. Biomech., 42, 626-633, 2009.

    [3] J. D. Humphrey et al.,

    J. Biomech. Engrg., 112, 340-346, 1990.

    [4] B.N.J. Persson,et. Al

    ., J.Phys. Condens. Matter., 17, R1071-R1142, 2005.

  • 21.
    Forsell, Caroline
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Swedenborg, Jesper
    Roy, Joy
    Gasser, Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    The Quasi-Static Failure Properties of the Abdominal Aortic Aneurysm Wall Estimated by a Mixed Experimental-Numerical Approach2012In: Annals of Biomedical Engineering, ISSN 0090-6964, E-ISSN 1573-9686, Vol. 41, no 7, p. 1554-1566Article in journal (Refereed)
    Abstract [en]

    Assessing the risk for abdominal aortic aneurysm (AAA) rupture is critical in the management of aneurysm patients and an individual assessment is possible with the biomechanical rupture risk assessment. Such an assessment could potentially be improved by a constitutive AAA wall model that accounts for irreversible damage-related deformations. Because of that the present study estimated the elastic and inelastic properties of the AAA wall through a mixed experimental-numerical approach. Specifically, finite element (FE) models of bone-shaped tensile specimens were used to merge data from failure testing of the AAA wall with their measured collagen orientation distribution. A histo-mechanical constitutive model for collagen fibers was employed, where plastic fibril sliding determined not only remaining deformations but also weakening of the fiber. The developed FE models were able to replicate the experimentally recorded load-displacement property of all 16 AAA wall specimens that were investigated in the study. Tensile testing in longitudinal direction of the AAA defined a Cauchy strength of 569(SD 411) kPa that was reached at a stretch of 1.436(SD 0.118). The stiffness and strength of specimens decreased with the wall thickness and were elevated (p = 0.018; p = 0.030) in patients with chronic obstructive pulmonary disease (COPD). Smoking affected the tissue parameters that were related to the irreversible deformation response, and no correlation with gender and age was found. The observed effects on the biomechanical properties of the AAA wall could have long-term consequences for the management of aneurysm patients, i.e., specifically they might influence future AAA rupture risk assessments. However, in order to design appropriate clinical validation studies our findings should firstly be verified in a larger patient cohort.

  • 22.
    Gasser, Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Histomechanical modeling of thewall of abdominal aortic aneurysm2016In: Structure-Based Mechanics of Tissues and Organs, Springer, 2016, p. 57-78Chapter in book (Other academic)
    Abstract [en]

    Vascular diseases are already the leading cause of death in the industrialized countries and many of the associated risk factors are increasing. A multidisciplinary approach including biomechanics is needed to better understand and more effectively treat these diseases. Specifically, constitutive modeling is critical in understanding the biomechanics of the vascular wall and to uncover pathologies like Abdominal Aortic Aneurysms (AAAs), i.e. local dilatations of the infrarenal aorta. Aneurysms are formed through irreversible pathological remodeling of the vascular wall and integrating this biological process in the constitutive description could improve our current understanding of aneurysm disease. It might also increase the predictability of biomechanical simulations towards augmenting clinical decisions. The present chapter develops histomechanical constitutive models for the AAA wall according to Lanir’s pioneering approach. Consequently, macroscopic properties were derived through an integration of distributed fibers, where collagen was regarded as the most important protein of the aneurysmatic Extra Cellular Matrix (ECM). Collagen organization was quantified through Polarized Light Microscopy (PLM) of picrosirius red stained histological slices from tissue samples harvested during elective open AAA repair. This histological information was either directly integrated in the constitutive description or used to qualitatively validate the predicted remodeling of the AAA wall. Specifically, two descriptions for the AAA wall were used, where collagen was regarded either as a purely passive entity of the ECM or as an active entity. The suggested constitutive models were able to successfully capture salient features of the AAA wall, but a rigorous validation against detailed experimental data was beyond the scope of this chapter.

  • 23.
    Gasser, Christian
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Forsell, Caroline
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    The numerical implementation of invariant-based viscoelastic formulations at finite strains. An anisotropic model for the passive myocardium2011In: Computer Methods in Applied Mechanics and Engineering, ISSN 0045-7825, E-ISSN 1879-2138, Vol. 200, no 49-52, p. 3637-3645Article in journal (Refereed)
    Abstract [en]

    The present study developed a conceptual framework for finite strain viscoelasticity thought to be suitable to capture the salient features of a class of passive soft biological tissues like the myocardium. A superposition of a Maxwell Body and an Elastic Body defines the viscoelastic continuum, and its deformation is related to two independent reference configurations. The reference configuration of the Maxwell Body moves in space as it is described (apart from rigid body rotation) by a rate equation in strain space, and stores the history of the deformation. At thermodynamic equilibrium the reference configuration of the Maxwell Body coincides with the current configuration of the continuum. The Helmholtz free energy is expressed as a function of two independent strain variables and entirely renders the constitution of the viscoelastic body. Although this view is to some extent different from reported viscoelastic concepts for finite strains, its linearization around the thermodynamic equilibrium coincides with earlier suggested viscoelastic models. The linearized viscoelastic model has been implemented for a particular anisotropic constitutive model for the passive myocardium. Non-negative dissipation of the model is guaranteed. Material parameters were estimated from in vitro testing of porcine myocardium and the response due to pushing a rigid punch into the myocardium was studied. Results between anisotropic and isotropic descriptions of the myocardium differed significantly, which justified the implementation of an anisotropic model for the myocardium.

  • 24.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    An irreversible constitutive model for fibrous soft biological tissue: A 3-D microfiber approach with demonstrative application to abdominal aortic aneurysms2011In: ACTA BIOMATERIALIA, ISSN 1742-7061, Vol. 7, no 6, p. 2457-2466Article in journal (Refereed)
    Abstract [en]

    Understanding the failure and damage mechanisms of soft biological tissue is critical to a sensitive and specific characterization of tissue injury tolerance and its relation to biological responses. Despite increasing experimental and analytical efforts, failure-related irreversible effects of soft biological tissue are still poorly understood. There is still no clear definition of what "damage" of a soft biological material is, and conventional macroscopic indicators, as known from damage of engineering materials for example, may not identify the tissue's tolerance to injury appropriately. To account for the complex three-dimensional arrangement of collagen, a microfiber model approach is applied, where constitutive relations for collagen fibers are integrated over the unit sphere, which in turn defines the tissue's macroscopic properties. A collagen fiber is represented by a bundle of proteoglycan cross-linked collagen fibrils that undergoes irreversible deformations when exceeding its elastic tensile limit. The proposed constitutive model is able to predict strain stiffening at physiological strain levels and does not exhibit a clear macroscopic elastic limit, two typical features known from soft biological tissue testing. An elastic-predictor/plastic-corrector implementation of the model is followed and constitutive parameters are estimated from in vitro test data from a particular abdominal aortic aneurysm (AAA). Damage-based structural instabilities of the AAA under different inflation conditions are investigated, where the collagen orientation density has been estimated from its in vivo stress state.

  • 25.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Bringing vascular biomechanics into clinical practice. Simulation-based decisions for elective abdominal aortic aneurysms repair2012In: Lecture Notes in Computational Vision and Biomechanics, ISSN 2212-9391, E-ISSN 2212-9413, Vol. 5, p. 1-37Article in journal (Refereed)
    Abstract [en]

    Vascular diseases are the leading cause of death in the industrialized countries and some of the associated risk factors are increasing. A multi-disciplinary approach including biomechanics is needed to better understand and more effectively treat these diseases. Despite the tremendous progress made in modeling the biomechanics of the vasculature, so far this research has accomplished only very limited clinical relevance or acceptance. Establishing vascular biomechanical simulations in the clinical work-flow requires integrating (i) a robust reconstruction of vascular bodies from medical images, (ii) a non-linear biomechanical analysis and (iii) a clinically relevant interpretation of the derived results. Such an approach is outlined for the biomechanical rupture risk assessment of Abdominal Aortic Aneurysms (AAAs), i.e. a local dilatation of the infrarenal aorta that may form through irreversible pathological remodeling of the aortic wall. Rupture of an AAA is a frequent cause of death in the elderly male population and assessing this risk plays a central role in the clinical management of aneurysms. Specifically, the present chapter details an operator-insensitive method to reconstruct vascular bodies from Computer Tomography-Angiography data. The approach is based on beam and shell-like deformable (active) contour models and allows a hexahedral-dominated mesh generation for an efficient Finite Element computation. Laboratory experiments and histo-mechanical constitutive modeling of AAA tissue are reviewed. Finally, the clinical application of the biomechanical rupture risk assessment is demonstrated through the especially developed software A4clinics. Most critically, individual biomechanical parameters are related to the ‘average AAA patient’, which in turn provides a biomechanics-based index for elective AAA repair indication. © 2012, Springer Science+Business Media Dordrecht.

  • 26.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    The biomechanical rupture risk assessment of abdominal aortic aneurysms—method and clinical relevance2018In: Lecture Notes in Applied and Computational Mechanics, Springer Verlag , 2018, p. 233-253Conference paper (Refereed)
    Abstract [en]

    An Abdominal Aortic Aneurysm (AAA) is an enlargement of the infrarenal aorta, a serious condition whose clinical treatment requires assessing its risk of rupture. This chapter reviews the current state of the Biomechanical Rupture Risk Assessment (BRRA), a non-invasive diagnostic method to calculate such AAA rupture risk, and emphasizes on constitutive modeling of AAA tissues. Histology and mechanical properties of the normal and aneurysmatic walls are summarized and related to proposed constitutive descriptions. Models for the passive vessel wall as well as their adaptation in time are discussed. Reported clinical BRRA validation studies are summarized and their clinical relevance is discussed. Despite open problems in AAA biomechanics, like robust modeling vascular tissue adaptation to mechanical and biochemical environments, a significant body of current validation evidence suggests integrating the BRRA method into the clinical decision-making process. 

  • 27.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Validation of 3D crack propagation in plain concrete. Part II: Computational modeling and predictions of the PCT3D test2007In: Computers and Concrete, ISSN 1598-8198, Vol. 4, no 1, p. 67-82Article in journal (Refereed)
    Abstract [en]

    The discrete crack-concept is applied to study the 3D propagation of tensile-dominated failure in plain concrete. To this end the Partition of Unity Finite Element Method (PUFEM) is utilized and the strong discontinuity approach is followed. A consistent linearized implementation of the PUFEM is combined with a predictor-corrector algorithm to track the crack path, which leads to a robust numerical description of concrete cracking. The proposed concept is applied to study concrete failure during the PCT3D test and the predicted numerical results are compared to experimental data. The proposed numerical concept provides a clear interface for constitutive models and allows an investigation of their impact on concrete cracking under 3D conditions, which is of significant scientific interests to interpret results from 3D experiments.

  • 28.
    Gasser, T. Christian
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Auer, Martin
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Labruto, F.
    Swedenborg, J.
    Roy, J.
    Biomechanical Rupture Risk Assessment of Abdominal Aortic Aneurysms: Model Complexity versus Predictability of Finite Element Simulations2010In: European Journal of Vascular and Endovascular Surgery, ISSN 1078-5884, E-ISSN 1532-2165, Vol. 40, no 2, p. 176-185Article in journal (Refereed)
    Abstract [en]

    Objective: Investigation of the predictability of finite element (FE) models regarding rupture risk assessment of abdominal aortic aneurysms (AAAs). Materials and materials: Peak wall stress (PWS) and peak wall rupture risk (PWRR) of ruptured (n = 20) and non-ruptured (n = 30) AAAs were predicted by four FE models of different complexities derived from computed tomography (CT) data. Two matching sub-groups of ruptured and non-ruptured aneurysms were used to investigate the usability of different FE models to discriminate amongst them. Results: All FE models exhibited a strong positive correlation between PWS and PWRR with the maximum diameter. FE models, which excluded the intra-luminal thrombus (ILT) failed to discriminate between ruptured and non-ruptured aneurysms. The predictability of all applied FE models was strengthened by including wall strength data, that is, computing the PWRR. The most sophisticated FE model applied in this study predicted PWS and PWRR 1.17 (p = 0.021) and 1.43 (p = 0.016) times higher in ruptured than diameter-matched non-ruptured aneurysms, respectively. Conclusions: PWRR reinforces PWS as a biomechanical rupture risk index. The ILT has a major impact on AAA biomechanics and rupture risk, and hence, needs to be considered in meaningful FE simulations. The applied FE models, however, could not explain rupture in all analysed aneurysms.

  • 29.
    Gasser, T. Christian
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Gallinetti, Sara
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Swedenborg, Jesper
    Roy, Joy
    Collagen fiber orientation in Abdominal Aortic Aneurysms wall2010Conference paper (Refereed)
    Abstract [en]

    Introduction

    Collagen is the most abundant protein in mammals and gives mechanical strength, stiffness and toughness to biological tissues like skin, tendon, bone, and vasculature [1]. Collagen fibrils of about 0.1 micrometers in diameters are the basic building blocks of fibrous collagenous tissues and their organization into suprafibrilar structures determines the tissue’s macroscopic mechanical properties. For example, detailed data regarding the organization of strong bundles of collagen might be critical to predict the onset of tissue failure, as it is clinically motivated by a rupture risk assessment of Abdominal Aortic Aneurysm (AAA). Previously proposed structural constitutive models for soft biological tissues [2, 3] integrated information regarding the collagen orientation, and regardless of their popularity, the requested microstructural information is not yet available in the open literature.

    Method and Materials

    The present study investigated the collagen formation in 12 AAA wall specimens stemming from 9 patients and harvested during elective aneurysm repair at Karolinska University Hospital, Stockholm, Sweden. Specimens of about 1.0 x 1.0 centimeter were squeezed between Plexiglas plates and fixated in formaldehyde for 24 hours. Fixated specimens were dehydrated and embedded in paraffin (Tissue Tek VIP 3000,

    Sakura)and sliced at a thickness of 7.0 micrometers (HM 360, Microm). To reinforce the birefringend properties of collagen the slices were stained with Picrus Sirius red before three-dimensional collagen fiber orientations were identified in a polarized light microscope (BX 50, Olympus) equipped with an Universal Rotary Stage (Zeiss). Specifically, the collagen orientations were measured at 36 points at each slice, where three slices across the thickness of the AAA wall were considered. The derived structural information was included in two different structural constitutive models and reported macroscopic mechanical data [4] was used to estimate mechanical parameters of the constitutive formulations.

    Results and Conclusions

    Collagen fiber orientation in the AAA wall is considerably spread out and no difference amongst medial and adventitial layers could be identified; a result in line with the layered structure of, e.g., cerebral aneurysms [5] but in clear contrast to the structural differences amongst the layers of normal arteries [6]. Collagen fibers in the AAA wall are predominantly aligned in circumferential direction, which might explain its higher stiffness along that direction [4]. Naturally, the complex collagen formation cannot be captured by a single (or two) families of collagen fibers and associated constitutive models are not applicable. Collagen turnover is thought to be mediated by the local stress or strain state [7] and the supra-physiological stresses in the AAA wall might cause the identified pathological collagen orientation.

    References

    [1] P. Fratzl, editor.

    Springer-Verlag, New York, 2008.

    [2] T. C. Gasser, et. al.

    J. R. Soc. Interface, 3:15–35, 2006.

    [3] S. Federico and T. C. Gasser.

    J. R.Soc. Interface, 2009.

    [4] J. P. Vande Geest et al..

    J Biomech. 39, 1324--1334, 2006.

    [5] P. B. Canham, et al..

    Neurological Res., 21, 618--626, 1999.

    [6] P. B. Canham, et al.

    Cardiovasc. Res. 23, 973-982, 1989.

    [7] J. D. Humphrey,

    Springer-Verlag, New York, 2002.

  • 30.
    Gasser, T. Christian
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Gallinetti, Sara
    Xing, Xiao
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Forsell, Caroline
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Swedenborg, Jesper
    Roy, Joy
    Spatial orientation of collagen fibers in the abdominal aortic aneurysm's wall and its relation to wall mechanics2012In: Acta Biomaterialia, ISSN 1742-7061, E-ISSN 1878-7568, Vol. 8, no 8, p. 3091-3103Article in journal (Refereed)
    Abstract [en]

    Collagen is the most abundant protein in mammals and provides the abdominal aortic aneurysm (AAA) wall with mechanical strength, stiffness and toughness. Specifically, the spatial orientation of collagen fibers in the wall has a major impact on its mechanical properties. Apart from valuable microhistological information, this data can be integrated by histomechanical constitutive models thought to improve biomechanical simulations, i.e. to improve the biomechanical rupture risk assessment of AAAs. Tissue samples (n = 24) from the AAA wall were harvested during elective AAA repair, fixated, embedded, sectioned and investigated by polarized light microscopy. The birefringent properties of collagen were reinforced by picrosirius red staining and the three-dimensional collagen fiber orientations were identified with a universal rotary stage. Two constitutive models for collagen fibers were used to integrate the identified structural information in a macroscopic AAA wall model. The collagen fiber orientation in the AM wall was widely dispersed and could be captured by a Bingham distribution function (kappa(1) = 11.6, kappa(2) = 9.7). The dispersion was much larger in the tangential plane than in the cross-sectional plane, and no significant difference between the medial and adventitial layers could be identified. The layered directional organization of collagen in normal aortas was not evident in the AAA. The collagen organization identified, combined with constitutive descriptions of collagen fibers that depend on its orientation, explain the anisotropic (orthotropic) mechanical properties of the AAA wall. The mechanical properties of collagen fibers depend largely on their undulation, which is an important structural parameter that requires further experimental investigation.

  • 31.
    Gasser, T. Christian
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Gorgulu, G.
    Folkesson, M.
    Swedenborg, J.
    Failure properties of intraluminal thrombus in abdominal aortic aneurysm under static and pulsating mechanical loads2008In: Journal of Vascular Surgery, ISSN 0741-5214, E-ISSN 1097-6809, Vol. 48, no 1, p. 179-188Article in journal (Refereed)
    Abstract [en]

    Objectives: It has been suggested that mechanical failure of intraluminal thrombus (ILT) could play a key role in the rupture of abdominal aortic aneurysms (AAAs), and in the present study, this hypothesis has been investigated. An in vitro experimental approach has been proposed, which provides layer-specific failure data of ILT tissue under static and pulsatile mechanical loads. Methods. In total, 112 bone-shaped test specimens are prepared from luminal, medial, and abluminal layers of eight ILTs harvested during open elective AAA repair. Three different types of mechanical experiments, denoted as control test, ultimate strength test, and fatigue test were performed in Dulbecco's modified eagle's medium (DMEM) supplemented with fetal calf serum, L-ascorbic acid, and antibiotics at 37 degrees C and pH 7.0. In detail, fatigue tests, which are experiments, where the ILT tissue is loaded. in pulsatile manner, were carried out at three different load levels with a natural frequency of 1.0 Hz. Results. ILT's ultimate strength (156.5 kPa, 92.0 kPa, and 47.7 kPa for luminal, medial, and abluminal layers, respectively) and referential stiffness (62.88 kPa, 47.52 kPa, and 41.52 kPa, for luminal, medial, and abluminal layers, respectively) continuously decrease from the inside to the outside. ILT tissue failed within less than 1 hour under pulsatile loading at a load level of 60% ultimate strength, while a load level of about 40% ultimate strength did not cause failure within 13.9 hours. Conclusions. ILT tissue is vulnerable against fatigue failure and shows significant decreasing strength with respect to the number of load cycles. Hence, after a reasonable time of pulsating loading ILT's strength is far below its ultimate strength, and when compared with stress predictions from finite element (FE) studies, this indicates the likelihood of fatigue failure in vivo. Failure within the ILT could propagate towards the weakened vessel wall behind it and could initialize AAA failure thereafter.

  • 32.
    Gasser, T. Christian
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Gudmundson, Peter
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Dohr, Gottfried
    Med Univ Graz, Inst Cell Biol, Histol & Embryol & Ctr Mol Med.
    Failure mechanisms of ventricular tissue due to deep penetration2009In: Journal of Biomechanics, ISSN 0021-9290, E-ISSN 1873-2380, Vol. 42, no 5, p. 626-633Article in journal (Refereed)
    Abstract [en]

    Lead perforation is a rare but serious complication of pacemaker implantations, and in the present study the associated tissue failure was investigated by means of in-vitro penetration of porcine and bovine ventricular tissue. Rectangular patches from the right ventricular free wall and the interventricular were separated, bi-axially stretched and immersed in physiological salt solution at 37 C before load displacement curves of m total 891 penetrations were recorded. To this end flat-bottomed cylindrical punches of different diameters were used, and following mechanical testing the penetration were histological analyzed using light and electron microscopes. Penetration pressure, i.e. penetration force divided by punch cross-sectional area decreased slightly from 2.27(SD 0.66) to 1.76 (SD 0.46) N mm(2) for punches of 1.32 to 2.30 mm in diameter, respectively. Deep penetration formed cleavages aligned with the local fiber orientation of the tissue, and hence, a mode-I crack developed, where the crack faces were wedged open by the advancing punch. The performed study derived novel failure data from ventricular tissue due to deep penetration and uncovered associated failure mechanisms. This provides information to derive mechanical failure models, which are essential to enrich our current understanding of failure of soft biological tissues and to guide medical device development.

  • 33.
    Gasser, T. Christian
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Holzapfel, Gerhard A.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    3D Crack propagation in unreinforced concrete. A two-step algorithm for tracking 3D crack paths2006In: Computer Methods in Applied Mechanics and Engineering, ISSN 0045-7825, E-ISSN 1879-2138, Vol. 195, no 37-40, p. 5198-5219Article in journal (Refereed)
    Abstract [en]

    Tensile failure of unreinforced concrete involves progressive micro-cracking, and the related strain-softening can coalesce into geometrical discontinuities, which separate the material. Advanced mechanical theories and numerical schemes are required to efficiently and adequately represent crack propagation in 3D. In this paper we use the concept of strong discontinuities to model concrete failure. We introduce a cohesive fracture process zone, which is characterized by a transversely isotropic traction-separation law. We combine the cohesive crack concept with the partition of unity finite element method, where the finite element space is enhanced by the Heaviside function. The concept is implemented for tetrahedral elements and the failure initialization is based on the simple (non-local) Rankine criterion. For each element we assume the embedded discontinuity to be flat in the reference configuration, which leads to a non-smooth crack surfaces approximation in 3D, in general; different concepts for tracking non-planar cracks in 3D are reviewed. In addition, we propose a two-step algorithm for tracking the crack path, where a predictor step defines discontinuities according to the (non-local) failure criterion and a corrector step draws in non-local information of the existing discontinuities in order to predict a 'closed' 3D crack surface; implementation details are provided. The proposed framework is used to analyze the predictability of concrete failure by two benchmark examples, i.e. the Nooru-Moharned test, and the Brokenshire test. We compare our numerical results, which are mesh independent, with experimental data and numerical results adopted from the literature.

  • 34.
    Gasser, T. Christian
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Holzapfel, Gerhard A.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    A numerical framework to model 3-D fracture in bone tissue with application to failure of the proximal femur2007In: IUTAM SYMPOSIUM ON DISCRETIZATION METHODS FOR EVOLVING DISCONTINUITIES / [ed] Combescure, A; DeBorst, R; Belytschko, T, DORDRECHT: SPRINGER , 2007, Vol. 5, p. 199-211Conference paper (Refereed)
    Abstract [en]

    Bone can be regarded as a quasi-brittle material. Under excessive loading nonlinear fracture zones may occur ahead the crack tips, where, typically, cohesive mechanisms are activated. The finite element method provides a powerful tool to analyze fracture formations on a numerical basis, and to better understand failure mechanisms within complex structures. The present work aims to introduce a particular numerical framework to investigate bone failure. We combine the partition of unity finite element method with the cohesive crack concept, and a two-step predictor-corrector algorithm for tracking 3-D non-interacting crack paths. This approach renders a numerically efficient tool that is able to capture the strong discontinuity kinematics in an accurate way. The prediction of failure propagation in the proximal part of the femur under compressive load demonstrates the suitability of the proposed concept. A 3-D finite element model, which accounts for inhomogeneous fracture properties, was used for the prediction of the 3-D crack surface. The achieved computational results were compared with experimental data available in the literature.

  • 35. Gasser, T. Christian
    et al.
    Holzapfel, Gerhard A.
    A rate-independent elastoplastic constitutive model for biological fiber-reinforced composites at finite strains: continuum basis, algorithmic formulation and finite element implementation2002In: Computational Mechanics, ISSN 0178-7675, E-ISSN 1432-0924, Vol. 29, no 05-apr, p. 340-360Article in journal (Refereed)
    Abstract [en]

    This paper presents a rate-independent elastoplastic constitutive model for (nearly) incompressible biological fiber-reinforced composite materials. The constitutive framework, based on multisurface plasticity, is suitable for describing the mechanical behavior of biological fiber-reinforced composites in finite elastic and plastic strain domains. A key point of the constitutive model is the use of slip systems, which determine the strongly anisotropic elastic and plastic behavior of biological fiber-reinforced composites. The multiplicative decomposition of the deformation gradient into elastic and plastic parts allows the introduction of an anisotropic Helmholtz free-energy function for determining the anisotropic response. We use the unconditionally stable backward-Euler method to integrate the flow rule and employ the commonly used elastic predictor/plastic corrector concept to update the plastic variables. This choice is expressed as an Eulerian vector update the Newton's type, which leads to a numerically stable and efficient material model. By means of a representative numerical simulations the performance of the proposed constitutive framework is investigated in detail.

  • 36.
    Gasser, T. Christian
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Holzapfel, Gerhard A.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Finite element modeling of balloon angioplasty by considering overstretch of remnant non-diseased tissues in lesions2007In: Computational Mechanics, ISSN 0178-7675, E-ISSN 1432-0924, Vol. 40, no 1, p. 47-60Article in journal (Refereed)
    Abstract [en]

    The paper deals with the modeling of balloon angioplasty by considering the balloon-induced overstretch of remnant non-diseased tissues in atherosclerotic arteries. A stenotic artery is modeled as a heterogenous structure composed of adventitia, media and a model plaque, and residual stresses are considered. The constitutive models are able to capture the anisotropic elastic tissue response in addition to the inelastic phenomena associated with tissue stretches beyond the physiological domain. The inelastic model describes the experimentally-observed changes of the wall during balloon inflation, i.e. non-recoverable deformation, and tissue weakening. The contact of the artery with a balloon catheter is simulated by a point-to-surface strategy. The states of deformations and stresses within the artery before, during and after balloon inflation are computed, compared and discussed. The 3D stress states at physiological loading conditions before and after balloon inflation differ significantly, and even compressive normal stresses may occur in the media after dilation.

  • 37. Gasser, T. Christian
    et al.
    Holzapfel, Gerhard A.
    Geometrically non-linear and consistently linearized embedded strong discontinuity models for 3D problems with an application to the dissection analysis of soft biological tissues2003In: Computer Methods in Applied Mechanics and Engineering, ISSN 0045-7825, E-ISSN 1879-2138, Vol. 192, no 47-48, p. 5059-5098Article in journal (Refereed)
    Abstract [en]

    Three different finite element formulations with embedded strong discontinuities are derived on the basis of the enhanced assumed strain method. According to the work by Jirasek and Zimmermann [Int. J. Numer. Methods Engrg. 50 (2001) 1269] they are referred to as statically optimal symmetric (SOS), kinematically optimal symmetric (KOS) and statically and kinematically optimal non-symmetric (SKON) formulations. The effect of the discontinuities are characterized by additional degrees of freedom on the element level. Modifications to the standard KOS and SKON formulations are proposed in order to achieve consistency with the employed type of a three-field Hu-Washizu principle under mode-I condition. Under this condition the formulation satisfies the internal compatibility at the discontinuity, i.e. the relation between the stress in the bulk material and the traction across the discontinuity surface, which is not the case for the classical KOS formulation. We propose a suitable explicit expression for a transversely isotropic traction law in form of a displacement-energy function and assume that softening phenomena in the cohesive zone are modeled by a damage law, which depends on the maximum gap displacement of the deformation path. A linearization of all quantities, which are related to the non-linear problem, leads to new closed form expressions. In particular, we focus attention on the linearization of the cohesive traction vector. The associated element residua and stiffness matrices are provided. Standard static condensation of the internal degree of freedom leads to a generalized displacement model. A comparative study of the modified formulations, carried out by means of two numerical examples, show the performance of the individual approach. We employ constant-strain tetrahedral elements with a single discontinuity embedded. Among the known stress locking phenomena associated with the SOS formulation, we recognized that the (non-symmetric) SKON formulation was not able to provide meaningful results for the dissection process of an arterial layer in three-dimensions on distorted meshes. For both numerical examples the (symmetric) KOS formulation seems to be most suitable for representing the embedded discontinuities.

  • 38. Gasser, T. Christian
    et al.
    Holzapfel, Gerhard A.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Modeling 3D crack propagation in unreinforced concrete using PUFEM2005In: Computer Methods in Applied Mechanics and Engineering, ISSN 0045-7825, E-ISSN 1879-2138, Vol. 194, no 25-26, p. 2859-2896Article in journal (Refereed)
    Abstract [en]

    Concrete is a quasi-brittle material, where tensile failure involves progressive micro-cracking, debounding and other complex irreversible processes of internal damage. Strain-softening is a dominate feature and advanced numerical schemes have to be applied in order to circumvent the ill-posdness of the Boundary-Value Problem to deal with. Throughout the paper we pursue the cohesive zone approach, where initialization and coalescence of micro-cracks is lumped into the cohesive fracture process zone in terms of accumulation of damage. We develop and employ a (discrete) constitutive description of the cohesive zone, which is based on a transversely isotropic traction separation law. The model reflects an exponential decreasing traction with respect to evolving opening displacement and is based on the theory of invariants. Non-negativeness of the damage dissipation is proven and the associated numerical embedded representation is based on the Partition of Unity Finite Element Method. A consistent linearization of the method is presented, where particular attention is paid to the (cohesive) traction terms. Based on the proposed concept three numerical examples are studied in detail, i.e. a double-notched specimen under tensile loading, a four point shear test and a pull-out test of unreinforced concrete. The computational results show mesh-independency and good correlation with experimental results. © 2004 Elsevier B.V. All rights reserved.

  • 39.
    Gasser, T. Christian
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Holzapfel, Gerhard A.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Modeling plaque fissuring and dissection during balloon angioplasty intervention2007In: Annals of Biomedical Engineering, ISSN 0090-6964, E-ISSN 1573-9686, Vol. 35, no 5, p. 711-723Article in journal (Refereed)
    Abstract [en]

    Balloon angioplasty intervention is traumatic to arterial tissue. Fracture mechanisms such as plaque fissuring and/or dissection occur and constitute major contributions to the lumen enlargement. However, these types of mechanically-based traumatization of arterial tissue are also contributing factors to both acute procedural complications and chronic restenosis of the treatment site. We propose physical and finite element models, which are generally useable to trace fissuring and/or dissection in atherosclerotic plaques during balloon angioplasty interventions. The arterial wall is described as an anisotropic, heterogeneous, highly deformable, nearly incompressible body, whereas tissue failure is captured by a strong discontinuity kinematics and a novel cohesive zone model. The numerical implementation is based on the partition of unity finite element method and the interface element method. The later is used to link together meshes of the different tissue components. The balloon angioplasty-based failure mechanisms are numerically studied in 3D by means of an atherosclerotic-prone human external iliac artery, with a type V lesion. Image-based 3D geometry is generated and tissue-specific material properties are considered. Numerical results show that in a primary phase the plaque fissures at both shoulders of the fibrous cap and stops at the lamina elastica interna. In a secondary phase, local dissections between the intima and the media develop at the fibrous cap location with the smallest thickness. The predicted results indicate that plaque fissuring and dissection cause localized mechanical trauma, but prevent the main portion of the stenosis from high stress, and hence from continuous tissue damage.

  • 40.
    Gasser, T. Christian
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Holzapfel, Gerhard A.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Modeling the propagation of arterial dissection2006In: European journal of mechanics. A, Solids, ISSN 0997-7538, E-ISSN 1873-7285, Vol. 25, no 4, p. 617-633Article in journal (Refereed)
    Abstract [en]

    Arterial dissections are frequently observed in clinical practice and during road traffic accidents. In particular, the lamellarly arrangement of elastin, collagen, in addition to smooth muscle cells in the middle arterial layer, the media, favors dissection failure. Experimental studies and related biomechanical models are rare in the literature. Finite strain kinematics is employed, and the discontinuity in the displacement field accounts for tissue separation. Dissection is regarded as a gradual process in which separation between incipient material surfaces is resisted by cohesive traction. Two variational statements together with their consistent linearizations form the basis for a finite element implementation. We combine the cohesive crack concept with the partition of unity finite element method, where nodal degrees of freedom adjacent to the discontinuity are enhanced. The developed continuum mechanical and numerical frameworks allow the analysis of the propagation of dissections within general nonlinear boundary-value problems, where the constitutive description for the continuous and the cohesive material is considered independent from each other. The continuous material is modeled as a fiber-reinforced composite with the fibers corresponding to the collagenous component which are assumed to be embedded in a non-collagenous isotropic groundmatrix. Dispersion of the collagen fiber orientation is considered in a continuum sense by one structure parameter. A novel cohesive potential per unit undeformed area is used to derive a traction separation law appropriate for the description of the mechanical properties of medial dissection. The cohesive stiffness contribution to the element stiffness matrix is explicitly derived. In particular, the dissection propagation of a rectangular strip of a human aortic media is investigated. Cohesive material properties are quantified by comparing the experimentally measured load with the computed dissection load.

  • 41.
    Gasser, T. Christian
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Holzapfel, Gerhard A.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Physical and numerical modeling of dissection propagation in arteries caused by balloon angioplasty2005In: Proceedings of the Third IASTED International Conference on BIOMECHANICS / [ed] Hamza, MH, 2005, p. 229-233Conference paper (Refereed)
    Abstract [en]

    Arterial dissections Caused by balloon angioplasty has been implicated as a contributing factor to both acute procedural complications and chronic restenosis of the treatment site. However, no related biomechanical studies are known in the literature. The mechanical properties of the arterial wall are controlled by the rubber-like protein elastin, fibrous protein collagen and smooth muscle cells. In the media of elastic arteries these constituents are found in thin layers that are arranged in repeating lamellar units and favor dissection type of failure. The presented approach models the dissection of the media by means of strong discontinuities and the application of the theory of cohesive zones. Thereby, the dissection is regarded as a gradual process in which separation between incipient material surfaces is resisted by cohesive traction. The applied numerical frame is based on the Partition of Unity Finite Element Method (PUFEM) and has been utilized for tetrahedral elements. A tracking algorithm for 3D non-planar cracks captures the evolution of multiple non-interacting dissections. The proposed concept is applied to investigate the dissection of the media due to balloon angioplasty, where the associated material parameters are determined from failure experiments on human tissue.

  • 42.
    Gasser, T. Christian
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Martufi, Giampaolo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Auer, M.
    Folkesson, M.
    Swedenborg, J.
    Micromechanical Characterization of Intra-luminal Thrombus Tissue from Abdominal Aortic Aneurysms2010In: Annals of Biomedical Engineering, ISSN 0090-6964, E-ISSN 1573-9686, Vol. 38, no 2, p. 371-379Article in journal (Refereed)
    Abstract [en]

    The reliable assessment of Abdominal Aortic Aneurysm rupture risk is critically important in reducing related mortality without unnecessarily increasing the rate of elective repair. Intra-luminal thrombus (ILT) has multiple biomechanical and biochemical impacts on the underlying aneurysm wall and thrombus failure might be linked to aneurysm rupture. Histological slices from 7 ILTs were analyzed using a sequence of automatic image processing and feature analyzing steps. Derived microstructural data was used to define Representative Volume Elements (RVE), which in turn allowed the estimation of microscopic material properties using the non-linear Finite Element Method. ILT tissue exhibited complex microstructural arrangement with larger pores in the abluminal layer than in the luminal layer. The microstructure was isotropic in the abluminal layer, whereas pores started to orient along the circumferential direction towards the luminal site. ILT's macroscopic (reversible) deformability was supported by large pores in the microstructure and the inhomogeneous structure explains in part the radially changing macroscopic constitutive properties of ILT. Its microscopic properties decreased just slightly from the luminal to the abluminal layer. The present study provided novel microstructural and micromechanical data of ILT tissue, which is critically important to further explore the role of the ILT in aneurysm rupture. Data provided in this study allow an integration of structural information from medical imaging for example, to estimate ILT's macroscopic mechanical properties.

  • 43.
    Gasser, T. Christian
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Martufi, Giampaolo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Auer, Martin
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    A constitutive model for vascular tissue that integrates fibril, fiber and continuum levels2011In: CMBE 2011: Proceedings of 2nd International Conference on Computational & Mathematical Biomedical Engineering, 2011Conference paper (Refereed)
  • 44.
    Gasser, T. Christian
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Nchimi, A.
    Swedenborg, J.
    Roy, J.
    Sakalihasan, N.
    Böckler, D.
    Hyhlik-Dürr, A.
    A novel strategy to translate the biomechanical rupture risk of abdominal aortic aneurysms to their equivalent diameter risk: Method and retrospective validation2014In: European Journal of Vascular and Endovascular Surgery, ISSN 1078-5884, E-ISSN 1532-2165, Vol. 47, no 3, p. 288-295Article in journal (Refereed)
    Abstract [en]

    Objective: To translate the individual abdominal aortic aneurysm (AAA) patient's biomechanical rupture risk profile to risk-equivalent diameters, and to retrospectively test their predictability in ruptured and non-ruptured aneurysms. Methods: Biomechanical parameters of ruptured and non-ruptured AAAs were retrospectively evaluated in a multicenter study. General patient data and high resolution computer tomography angiography (CTA) images from 203 non-ruptured and 40 ruptured aneurysmal infrarenal aortas. Three-dimensional AAA geometries were semi-automatically derived from CTA images. Finite element (FE) models were used to predict peak wall stress (PWS) and peak wall rupture index (PWRI) according to the individual anatomy, gender, blood pressure, intraluminal thrombus (ILT) morphology, and relative aneurysm expansion. Average PWS diameter and PWRI diameter responses were evaluated, which allowed for the PWS equivalent and PWRI equivalent diameters for any individual aneurysm to be defined. Results: PWS increased linearly and PWRI exponentially with respect to maximum AAA diameter. A size-adjusted analysis showed that PWS equivalent and PWRI equivalent diameters were increased by 7.5 mm (p = .013) and 14.0 mm (p < .001) in ruptured cases when compared to non-ruptured controls, respectively. In non-ruptured cases the PWRI equivalent diameters were increased by 13.2 mm (p < .001) in females when compared with males. Conclusions: Biomechanical parameters like PWS and PWRI allow for a highly individualized analysis by integrating factors that influence the risk of AAA rupture like geometry (degree of asymmetry, ILT morphology, etc.) and patient characteristics (gender, family history, blood pressure, etc.). PWRI and the reported annual risk of rupture increase similarly with the diameter. PWRI equivalent diameter expresses the PWRI through the diameter of the average AAA that has the same PWRI, i.e. is at the same biomechanical risk of rupture. Consequently, PWRI equivalent diameter facilitates a straightforward interpretation of biomechanical analysis and connects to diameter-based guidelines for AAA repair indication. PWRI equivalent diameter reflects an additional diagnostic parameter that may provide more accurate clinical data for AAA repair indication.

  • 45.
    Gasser, T. Christian
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Ogden, R. W.
    Holzapfel, G. A.
    Hyperelastic modelling of arterial layers with distributed collagen fibre orientations2006In: Journal of the Royal Society Interface, ISSN 1742-5689, E-ISSN 1742-5662, Vol. 3, no 6, p. 15-35Article, review/survey (Refereed)
    Abstract [en]

    Constitutive relations are fundamental to the solution of problems in continuum mechanics, and are required in the study of, for example, mechanically dominated clinical interventions involving soft biological tissues. Structural continuum constitutive models of arterial layers integrate information about the tissue morphology and therefore allow investigation of the interrelation between structure and function in response to mechanical loading. Collagen fibres are key ingredients in the structure of arteries. In the media (the middle layer of the artery wall) they are arranged in two helically distributed families with a small pitch and very little dispersion in their orientation (i.e. they are aligned quite close to the circumferential direction). By contrast, in the adventitial and intimal layers, the orientation of the collagen fibres is dispersed, as shown by polarized light microscopy of stained arterial tissue. As a result, continuum models that do not account for the dispersion are not able to capture accurately the stress-strain response of these layers. The purpose of this paper, therefore, is to develop a structural continuum framework that is able to represent the dispersion of the collagen fibre orientation. This then allows the development of a new hyperelastic free-energy function that is particularly suited for representing the anisotropic elastic properties of adventitial and intimal layers of arterial walls, and is a generalization of the fibre-reinforced structural model introduced by Holzapfel & Gasser (Holzapfel & Gasser 2001 Comput. Meth. Appl. Mech. Eng. 190, 4379-4403) and Holzapfel et al. (Holzapfel et al. 2000 J. Elast. 61, 1-48). The model incorporates an additional scalar structure parameter that characterizes the dispersed collagen orientation. An efficient finite element implementation of the model is then presented and numerical examples show that the dispersion of the orientation of collagen fibres in the adventitia of human iliac arteries has a significant effect on their mechanical response.

  • 46. Gasser, T. Christian
    et al.
    Schulze-Bauer, C. A. J.
    Holzapfel, Gerhard A.
    A three-dimensional finite element model for arterial clamping2002In: Journal of Biomechanical Engineering, ISSN 0148-0731, E-ISSN 1528-8951, Vol. 124, no 4, p. 355-363Article in journal (Refereed)
    Abstract [en]

    Clamp induced injuries of the arterial wall may determine the outcome of surgical procedures. Thus, it is important to investigate the underlying mechanical effects. We present a three-dimensional finite element model, which allows the study of the mechanical response of an artery-treated as a two-layer tube-during arterial clamping. The important residual stresses, which are associated with the load free configuration of the artery, are also considered. In particular, the finite element analysis of the deformation process of a clamped artery and the associated stress distribution is presented. Within the clamping area a zone of axial tensile peak-stresses was identified, which (may) cause intimal and medial injury. This is an additional injury mechanism, which clearly differs from the commonly assumed wall damage occurring due to compression between the jaws of the clamp. The proposed numerical model provides essential insights into the mechanics of the clamping procedure and the associated injury mechanisms. It allows detailed parameter studies on a virtual clamped artery, which can not be performed with other methodologies. This approach has the potential to identify the most appropriate clamps for certain types of arteries and to guide optimal clamp design.

  • 47.
    Gasser, Thomas Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Aorta2017In: Biomechanics of Living Organs: Hyperelastic Constitutive Laws for Finite Element Modeling, Elsevier, 2017, p. 169-191Chapter in book (Refereed)
    Abstract [en]

    The aorta is a dynamic structure that is able to maintain conditions for optimal mechanical operation through the continuous turnover of its internal structure. The aorta's properties are critical to the entire cardiovascular system, and the study of its biomechanics may help us to better understand the role of tissue stress and strain in aortic aging and pathology, help to optimize medical devices, and improve therapeutic and diagnostic methods that are currently used in clinics. The present chapter reviews aortic wall histology and morphology in relation to its key mechanical properties. Specifically, the biomechanical role of cells (endothelial cells, smooth muscle cells, fibroblasts, etc.), as well as the extracellular matrix components (elastin, collagen, proteoglycans, water, etc.), will be discussed. Then this information is related to reported constitutive descriptions for aortic tissues. The focus is on histo-mechanical approaches and modeling frames, related to hyperelasticity as well as a superposition of fiber contributions according to a general theory of fibrous connective tissue. Concluding remarks relate to open problems in aorta biomechanics, such as uncertainty and variability of input information. Remarks are also made on the admissible degree of complexity in aortic simulations, in the context of such uncertainties.

  • 48.
    Gasser, Thomas Christian
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Auer, M.
    Biasetti, Jacopo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Structural and Hemodynamical analysis of Aortic Aneurysms from Computerized Tomography Angiography data2010In: WORLD CONGRESS ON MEDICAL PHYSICS AND BIOMEDICAL ENGINEERING, VOL 25, PT 4: IMAGE PROCESSING, BIOSIGNAL PROCESSING, MODELLING AND SIMULATION, BIOMECHANICS, 2010, p. 1584-1587Conference paper (Refereed)
    Abstract [en]

    Evaluating rupture risk of Abdominal Aortic Aneurysms is critically important in reducing related mortality without unnecessarily increasing the rate of elective repair. According to the current clinical practice aneurysm rupture risk is (mainly) estimated from its maximum diameter and/or expansion rate; an approach motivated from statistics but known to fail often in individuals. In particular, the role of the Intraluminal Thrombus is unclear and further research is required to investigate and understand its multiple impacts on aneurysm disease. Biomechanical simulations might be helpful to explore this question, however, model development is time consuming and operator-variability limits their reliability. In this study we propose an automatic procedure to develop hemodynamic and structural models of healthy and diseased abdominal aortas, where Deformable Models segment Computerized Tomography Angiography data. In total 29 numerical models of the health and diseased abdominal aorta have been developed to investigate aneurysm's rupture risk and hemodynamic consequences of aneurismal dilations. The derived results underline the suitability of biomechanical simulations to enrich diagnostic information and to uncover mechanisms of aneurysm pathology.

  • 49. Georgakarakos, Efstratios
    et al.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Xenos, Michalis
    Kontopodis, Nikolaos
    Georgiadis, George S.
    Ioannou, Chris V.
    Applying Findings of Computational Studies in Vascular Clinical Practice: Fact, Fiction, or Misunderstanding?2014In: Journal of Endovascular Therapy, ISSN 1526-6028, E-ISSN 1545-1550, Vol. 21, no 3, p. 434-438Article in journal (Other academic)
  • 50.
    Giampaolo, Martufi
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Gasser, Thomas Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Auer, Martin
    Labruto, Fausto
    Swedenborg, Jesper
    Abdominal Aortic Aneurysm development over time: Experimental evidence and constitutive modeling2010In: Proceedings of the 6th World Congress of Biomechanics, Springer, 2010Conference paper (Refereed)
    Abstract [en]

    Abdominal Aortic Aneurysms (AAAs) are defined as a localized permanent dilatation of the infrarenal aorta at least 50 % of its normal diameter. AAAs are frequently diagnosed in the elderly male population and evaluating rupture risk is critically important as aneurysm rupture carries high mortality rates. Growth predictors might be helpful to assess AAA rupture risk and could therefore give a better graded indication for elective repair in order to reduce related mortality without unnecessarily increasing the rate of interventions. Factors associated with AAA growth are still limited but there are some evidence that higher initial AAA diameter is related to faster AAA expansion [1]. The initial dilatation is dependent on elastin degradation, but strength of the AAA is maintained by increased production of collagen. It has been suggested that rupture occurs when collagen production is insufficient to counteract load-bearing at high pressure [2].

    AAA growth quantification

    30 patients with infrarenal AAAs were included in this study. Criteria for inclusion were 1-year follow-up and availability of at least two high-resolution Computer Tomography-Angiography (CTA) scans. Consequently, 60 CT-A scans were systematically segmented, reconstructed and analyzed (A4research, VASCOPS GmbH), in order to investigate geometrical and mechanical factors likely to be correlated with AAA growth. Derived results were analyzed with an especially developed (automatic) analyzing schema (MatLab, The MathWorks), and the derived information aims at guiding the development of an analytical growth model for AAAs.

    Constitutive Modeling

    Collagen is a structural protein responsible for the mechanical strength, stiffness and toughness of biological tissues like skin, tendon, bone, cornea, lung and vasculature. In the present study we considered the enlargement of the aneurysm as a consequence of a pathological degradation and synthesis of collagen, i.e. malfunction of collagen turn-over. Consequently, the vascular wall is modeled by an (inert) matrix material representing the elastin, which is reinforced by a dynamic structure of bundles of collagen. Specifically, collagen is formed by a continuous stress-mediated process and deposited in the current configuration [3] and removed by a constant degradation rate. Finally the micro-plane concept [4] is used for the Finite Element implementation [5] of the constitutive model.

    Results and conclusions

    The quantitative description of AAA growth by examining patient follow-up data revealed novel insights into the natural history of this disease. Most interestingly not all portions of the AAA seem to enlarge, some might be stable or even shrink over time; a feature that has not yet been considered by models reported in the literature. The model proposed within this study has a

    strong biological motivation and captures saline feature of AAA growth. Besides that, the micro-plane approach allows a straight forward FE implementation and preliminary results indicate its numerical robustness.

    References

    [1]

    F.J.V. Schlösser, et al., J Vasc Surg, 47:1127–1133 2008.

    [2]

    E. Choke, et al., Eur.j.Vasc.endovasc.surg, 30(3):227-44 2005.

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    J.D.Humphrey, J Biomech Eng, 121:591–597 1999.

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