| 61. |
- Gasser, T. Christian, et al.
(författare)
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A novel strategy to translate the biomechanical rupture risk of abdominal aortic aneurysms to their equivalent diameter risk : Method and retrospective validation
- 2014
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Ingår i: European Journal of Vascular and Endovascular Surgery. - : Elsevier BV. - 1078-5884 .- 1532-2165. ; 47:3, s. 288-295
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 62. |
- Gasser, T. Christian, et al.
(författare)
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A numerical framework to model 3-D fracture in bone tissue with application to failure of the proximal femur
- 2007
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Ingår i: IUTAM SYMPOSIUM ON DISCRETIZATION METHODS FOR EVOLVING DISCONTINUITIES. - DORDRECHT : SPRINGER. - 9781402065293 ; , s. 199-211
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 63. |
- Gasser, T. Christian, et al.
(författare)
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A quarter of a century biomechanical rupture risk assessment of abdominal aortic aneurysms. Achievements, clinical relevance, and ongoing developments
- 2022
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Ingår i: International Journal for Numerical Methods in Biomedical Engineering. - : Wiley. - 2040-7939 .- 2040-7947.
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Tidskriftsartikel (refereegranskat)abstract
- Abdominal aortic aneurysm (AAA) disease, the local enlargement of the infrarenal aorta, is a serious condition that causes many deaths, especially in men exceeding 65 years of age. Over the past quarter of a century, computational biomechanical models have been developed towards the assessment of AAA risk of rupture, technology that is now on the verge of being integrated within the clinical decision-making process. The modeling of AAA requires a holistic understanding of the clinical problem, in order to set appropriate modeling assumptions and to draw sound conclusions from the simulation results. In this article we summarize and critically discuss the proposed modeling approaches and report the outcome of clinical validation studies for a number of biomechanics-based rupture risk indices. Whilst most of the aspects concerning computational mechanics have already been settled, it is the exploration of the failure properties of the AAA wall and the acquisition of robust input data for simulations that has the greatest potential for the further improvement of this technology.
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| 64. |
- Gasser, T. Christian, et al.
(författare)
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A rate-independent elastoplastic constitutive model for biological fiber-reinforced composites at finite strains : continuum basis, algorithmic formulation and finite element implementation
- 2002
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Ingår i: Computational Mechanics. - : Springer Science and Business Media LLC. - 0178-7675 .- 1432-0924. ; 29:05-apr, s. 340-360
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 65. |
- Gasser, T. Christian, et al.
(författare)
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A three-dimensional finite element model for arterial clamping
- 2002
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Ingår i: Journal of Biomechanical Engineering. - : ASME International. - 0148-0731 .- 1528-8951. ; 124:4, s. 355-363
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 66. |
- Gasser, T. Christian
(författare)
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An irreversible constitutive model for fibrous soft biological tissue : A 3-D microfiber approach with demonstrative application to abdominal aortic aneurysms
- 2011
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Ingår i: ACTA BIOMATERIALIA. - : Elsevier BV. - 1742-7061. ; 7:6, s. 2457-2466
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 67. |
- Gasser, T. Christian, et al.
(författare)
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Biomechanical modeling the adaptation of soft biological tissue
- 2017
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Ingår i: Current Opinion in Biomedical Engineering. - : Elsevier B.V.. - 2468-4511. ; 1, s. 71-77
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Tidskriftsartikel (refereegranskat)abstract
- External (mechanical) stimuli influence cell function at the level of gene expression and thereby contribute to the overall control of Soft Biological Tissues' (SBT) structure and function. SBT seem to adapt towards stable homeostatic mechanical conditions, and failure of reaching homeostasis may result in pathologies. SBT adaptation has to obey basic physical principles, and even within these constraints, a large number of SBT adaptation models have been proposed. Recent SBT models integrated the tissue's microstructure and directly addressed length scales of individual tissue constituents, which in turn allowed linking biomechanical and biochemical adaptation aspects. Despite adaptation models being based on very different hypotheses, many of them lead to physically reasonable results. Most interestingly, the recently developed homogenized Constrained Mixture Model reported very similar predictions than the original Constrained Mixture Model. This key observation indicates that the simpler kinematics-based approach is indeed able to capture the overall consequences of the continuous production and degradation of SBT constituents. However, mainly due to the scarcity of relevant experiment data, not a single model has been thoroughly validated against clearly specified modeling objectives. Consequently, much more interdisciplinary experimental work is required to guide SBT modeling activities. Nevertheless, predictive biomechanical SBT adaption models would not only be of considerable scientific interest, but would also have a large number of practical applications.
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| 68. |
- Gasser, T. Christian
(författare)
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Biomechanical Rupture Risk Assessment
- 2016
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Ingår i: AORTA. - : Thieme Medical Publishers, Inc.. - 2325-4637. ; 4:2, s. 42-60
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Tidskriftsartikel (refereegranskat)abstract
- Abdominal aortic aneurysm (AAA) rupture is a local event in the aneurysm wall that naturally demands tools to assess the risk for local wall rupture. Consequently, global parameters like the maximum diameter and its expansion over time can only give very rough risk indications; therefore, they frequently fail to predict individual risk for AAA rupture. In contrast, the Biomechanical Rupture Risk Assessment (BRRA) method investigates the wall's risk for local rupture by quantitatively integrating many known AAA rupture risk factors like female sex, large relative expansion, intraluminal thrombus-related wall weakening, and high blood pressure. The BRRA method is almost 20 years old and has progressed considerably in recent years, it can now potentially enrich the diameter indication for AAA repair. The present paper reviews the current state of the BRRA method by summarizing its key underlying concepts (i.e., geometry modeling, biomechanical simulation, and result interpretation). Specifically, the validity of the underlying model assumptions is critically disused in relation to the intended simulation objective (i.e., a clinical AAA rupture risk assessment). Next, reported clinical BRRA validation studies are summarized, and their clinical relevance is reviewed. The BRRA method is a generic, biomechanics-based approach that provides several interfaces to incorporate information from different research disciplines. As an example, the final section of this review suggests integrating growth aspects to (potentially) further improve BRRA sensitivity and specificity. Despite the fact that no prospective validation studies are reported, a significant and still growing body of validation evidence suggests integrating the BRRA method into the clinical decision-making process (i.e., enriching diameter-based decision-making in AAA patient treatment).
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| 69. |
- Gasser, T. Christian, et al.
(författare)
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Biomechanical Rupture Risk Assessment of Abdominal Aortic Aneurysms : Model Complexity versus Predictability of Finite Element Simulations
- 2010
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Ingår i: European Journal of Vascular and Endovascular Surgery. - : Elsevier BV. - 1078-5884 .- 1532-2165. ; 40:2, s. 176-185
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 70. |
- Gasser, T. Christian
(författare)
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Bringing vascular biomechanics into clinical practice. Simulation-based decisions for elective abdominal aortic aneurysms repair
- 2012
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Ingår i: Lecture Notes in Computational Vision and Biomechanics. - Dordrecht : Springer Netherlands. - 2212-9391 .- 2212-9413. ; 5, s. 1-37
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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