| 1. |
- Ambrosi, D., et al.
(författare)
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Perspectives on biological growth and remodeling
- 2011
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Ingår i: Journal of the mechanics and physics of solids. - : Elsevier BV. - 0022-5096 .- 1873-4782. ; 59:4, s. 863-883
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Forskningsöversikt (refereegranskat)abstract
- The continuum mechanical treatment of biological growth and remodeling has attracted considerable attention over the past fifteen years. Many aspects of these problems are now well-understood, yet there remain areas in need of significant development from the standpoint of experiments, theory, and computation. In this perspective paper we review the state of the field and highlight open questions, challenges, and avenues for further development.
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| 2. |
- Bauer, Margit, et al.
(författare)
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Assessment of the in vivo biomechanical properties of the human uterine cervix in pregnancy using the aspiration test A feasibility study
- 2009
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Ingår i: European Journal of Obstetrics, Gynecology, and Reproductive Biology. - : Elsevier BV. - 0301-2115 .- 1872-7654. ; 144, s. S77-S81
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Tidskriftsartikel (refereegranskat)abstract
- Objective: To date no diagnostic tool is yet available to objectively assess the in vivo biomechanical properties of the uterine cervix during gestation. Methods: We show the first clinical application of an aspiration device to assess the in vivo biomechanical properties of the cervix in pregnancy with the aim to describe the physiological biomechanical changes throughout gestation in order to eventually detect pregnant women at risk for cervical insufficiency (CI). Results: Out of 15 aspiration measurements, 12 produced valid results. The stiffness values were in the range between 0.013 and 0.068 bar/mm. The results showed a good reproducibility of the aspiration test. In our previous test series on non-pregnant cervices our repetitive measurements showed a standard deviation of > 20% compared to <+/- 10% to our data on pregnant cervices. Stiffness values are decreasing with gestational age which indicates a progressive softening of cervical tissue towards the end of pregnancy. Three pregnant women had two subsequent measurements within a time interval of four weeks. Decreasing stiffness values in the range of 20% were recorded. Discussion: This preliminary study on the clinical practicability of aspiration tests showed promising results in terms of reproducibility (reliability) and clinical use (feasibility). Ongoing studies will provide further insights on its usefulness in clinical practice and in the detection of substantial changes of the cervix in pregnancy indicative for threatened preterm birth or cervical insufficiency.
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| 3. |
- Holzapfel, Gerhard A., et al.
(författare)
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Constitutive modelling of arteries
- 2010
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Ingår i: Proceedings of the Royal Society. Mathematical, Physical and Engineering Sciences. - : The Royal Society. - 1364-5021 .- 1471-2946. ; 466:2118, s. 1551-1596
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Forskningsöversikt (refereegranskat)abstract
- This review article is concerned with the mathematical modelling of the mechanical properties of the soft biological tissues that constitute the walls of arteries. Many important aspects of the mechanical behaviour of arterial tissue can be treated on the basis of elasticity theory, and the focus of the article is therefore on the constitutive modelling of the anisotropic and highly nonlinear elastic properties of the artery wall. The discussion focuses primarily on developments over the last decade based on the theory of deformation invariants, in particular invariants that in part capture structural aspects of the tissue, specifically the orientation of collagen fibres, the dispersion in the orientation, and the associated anisotropy of the material properties. The main features of the relevant theory are summarized briefly and particular forms of the elastic strain-energy function are discussed and then applied to an artery considered as a thickwalled circular cylindrical tube in order to illustrate its extension-inflation behaviour. The wide range of applications of the constitutive modelling framework to artery walls in both health and disease and to the other fibrous soft tissues is discussed in detail. Since the main modelling effort in the literature has been on the passive response of arteries, this is also the concern of the major part of this article. A section is nevertheless devoted to reviewing the limited literature within the continuum mechanics framework on the active response of artery walls, i.e. the mechanical behaviour associated with the activation of smooth muscle, a very important but also very challenging topic that requires substantial further development. A final section provides a brief summary of the current state of arterial wall mechanical modelling and points to key areas that need further modelling effort in order to improve understanding of the biomechanics and mechanobiology of arteries and other soft tissues, from the molecular, to the cellular, tissue and organ levels.
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| 4. |
- Kroon, Martin, et al.
(författare)
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Elastic properties of anisotropic vascular membranes examined by inverse analysis
- 2009
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Ingår i: Computer Methods in Applied Mechanics and Engineering. - : Elsevier BV. - 0045-7825 .- 1879-2138. ; 198:45-46, s. 3622-3632
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Tidskriftsartikel (refereegranskat)abstract
- An inverse method for estimating the distributions of the nonlinear elastic properties of inhomogeneous and anisotropic vascular membranes such as cerebral aneurysms is proposed. The material description of the membrane is based on a versatile structural model able to represent multiple collagen layers and the passive response of the vascular wall. Each individual layer is assumed to behave transversely isotropic following exponential stiffening with increasing loading. The model includes four parameters to be explainable physically: two initial stiffnesses of the collagen fabric, a parameter related to the nonlinearity of the collagen fabric, angle between the principal directions of the collagen fabric and a reference coordinate system. For this finite deformation problem a finite element framework for membranous structures considering pressure boundary loading is outlined. i.e. the principle of virtual work, its linearisation and the related spatial discretisation. The estimation procedure consists of the following three steps: (i) in vivo or in vitro approaches record the mechanical responses of membranous structures whose properties are to be determined; (ii) define a corresponding finite element model; (iii) minimise an error function (regarding the unknown parameters) that quantifies the deviation of the numerical prediction from the recorded data. To achieve a robust parameter estimation, an element partition method is employed. The outcome of the procedure is affected by the number of nodes defined on the membrane surface and the number of load steps. In a numerical example, the proposed procedure is assessed by reestablishing given reference distributions in a reference membrane. The deviations of the estimated material parameter distributions from the related reference fields are within just a few percent. In most of the investigated cases the standard deviation for the resulting maximum principal stress was even below 1%, which is accurate enough for rupture risk assessment of vascular membranes.
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| 5. |
- Mortier, P., et al.
(författare)
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A Novel Simulation Strategy for Stent Insertion and Deployment in Curved Coronary Bifurcations : Comparison of Three Drug-Eluting Stents
- 2010
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Ingår i: Annals of Biomedical Engineering. - : Springer Science and Business Media LLC. - 0090-6964 .- 1573-9686. ; 38:1, s. 88-99
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Tidskriftsartikel (refereegranskat)abstract
- The introduction of drug-eluting stents (DES) has reduced the occurrence of restenosis in coronary arteries. However, restenosis remains a problem in stented coronary bifurcations. This study investigates and compares three different second generation DESs when being implanted in the curved main branch of a coronary bifurcation with the aim of providing better insights into the related changes of the mechanical environment. The 3D bifurcation model is based on patient-specific angiographic data that accurately reproduce the in vivo curvatures of the vessel segments. The layered structure of the arterial wall and its anisotropic mechanical behavior are taken into account by applying a novel algorithm to define the fiber orientations. An innovative simulation strategy considering the insertion of a folded balloon catheter over a guide wire is proposed in order to position the stents within the curved vessel. Straightening occurs after implantation of all stents investigated. The resulting distributions of the wall stresses are strongly dependent on the stent design. Using a parametric modeling approach, two design modifications, which reduce the predicted maximum values of the wall stress, are proposed and analyzed.
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| 6. |
- Pierce, David M., et al.
(författare)
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DT-MRI Based Computation of Collagen Fiber Deformation in Human Articular Cartilage : A Feasibility Study
- 2010
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Ingår i: Annals of Biomedical Engineering. - : Springer Science and Business Media LLC. - 0090-6964 .- 1573-9686. ; 38:7, s. 2447-2463
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Tidskriftsartikel (refereegranskat)abstract
- Accurate techniques for simulating the deformation of soft biological tissues are an increasingly valuable tool in many areas of biomechanical analysis and medical image computing. To model the complex morphology and response of articular cartilage, a hyperviscoelastic (dispersed) fiber-reinforced constitutive model is employed to complete two specimen-specific finite element (FE) simulations of an indentation experiment, with and without considering fiber dispersion. Ultra-high field Diffusion Tensor Magnetic Resonance Imaging (17.6 T DT-MRI) is performed on a specimen of human articular cartilage before and after indentation to similar to 20% compression. Based on this DT-MRI data, we detail a novel FE approach to determine the geometry (edge detection from first eigenvalue), the meshing (semi-automated smoothing of DTI measurement voxels), and the fiber structural input (estimated principal fiber direction and dispersion). The global and fiber fabric deformations of both the un-dispersed and dispersed fiber models provide a satisfactory match to that estimated experimentally. In both simulations, the fiber fabric in the superficial and middle zones becomes more aligned with the articular surface, although the dispersed model appears more consistent with the literature. In the future, a multi-disciplinary combination of DT-MRI and numerical simulation will allow the functional state of articular cartilage to be determined in vivo.
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| 7. |
- Prot, V., et al.
(författare)
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On modelling and analysis of healthy and pathological human mitral valves : Two case studies
- 2010
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Ingår i: JOURNAL OF THE MECHANICAL BEHAVIOR OF BIOMEDICAL MATERIALS. - : Elsevier BV. - 1751-6161. ; 3:2, s. 167-177
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Tidskriftsartikel (refereegranskat)abstract
- Biomechanical data and related constitutive modelling of the mitral apparatus served as a basis for finite element analyses to better understand the physiology of mitral valves in health and disease. Human anterior and posterior leaflets and chordae tendinae from an elderly heart showing no disease and a hypertrophic obstructive cardiomyopathic heart (HOCM) were mechanically tested by means of uniaxial cyclic extension tests under quasi-static conditions. Experimental data for the leaflets and the chordae tendinae showed highly nonlinear mechanical behaviours and the leaflets were anisotropic. The mitral valve from the HOCM heart exhibited a significantly softer behaviour than the valve from the healthy one. A comparison with porcine data was included because many previous mitral modelling studies have been based on porcine data. Some differences in mechanical response were observed. Material parameters for hyperelastic, transversely isotropic constitutive laws were determined. The experimental data and the related model parameters were used in two finite element studies to investigate the effects of the material properties on the mitral valve response during systole. The analyses showed that during systole the mitral valve from the HOCM heart bulged into the left atrium by taking on the shape of a balloon, whereas the anterior leaflet of the healthy valve remained in the left ventricle.
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| 8. |
- Sommer, G., et al.
(författare)
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Biaxial mechanical properties of intact and layer-dissected human carotid arteries at physiological and supraphysiological loadings
- 2010
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Ingår i: American Journal of Physiology. Heart and Circulatory Physiology. - : American Physiological Society. - 0363-6135 .- 1522-1539. ; 298:3, s. H898-H912
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Tidskriftsartikel (refereegranskat)abstract
- Sommer G, Regitnig P, Koltringer L, Holzapfel GA. Biaxial mechanical properties of intact and layer-dissected human carotid arteries at physiological and supraphysiological loadings. Am J Physiol Heart Circ Physiol 298: H898-H912, 2010. First published December 24, 2009; doi:10.1152/ajpheart.00378.2009.-Specimens of intact wall tubes of human common carotid arteries (CCA), internal carotid arteries (ICA) (n = 11, age 77.6 yr, SD 6.3), and related adventitia and media-intima tubes are mechanically examined. Cyclic, quasi-static extension-inflation tests at different axial stretches are performed on preconditioned tube specimens. Stress-free configurations show significant stress releases in the circumferential direction of the intact CCA and ICA walls and in the axial directions of the intact CCA walls and the CCA and ICA adventitias. All investigated tissues exhibit strong nonlinear, pseudoelastic mechanical behavior with small hysteresis. The "inversion" feature, where the pressure/axial stretch relationship becomes a vertical line, is found only for intact walls. Axial "inversion stretches" are 1.15 (SD 0.06) for CCA and 1.14 (SD 0.06) for ICA, and related external axial forces are 0.43 N (SD 0.15) and 0.30 N (SD 0.22), respectively. Significant negative correlations between age and axial inversion stretches for CCA (r = -0.67, P = 0.03) and ICA (r = -0.29, P = 0.04) are identified. Adventitias are very compliant at low pressures, but change into stiff tubes at high pressures. The burst pressure of the adventitia is beyond 250 kPa. A relatively low burst pressure of similar to 60 kPa is found in the media-intima tubes, in which the pressure/circumferential stretch relationships are almost independent of the axial stretches. Stress analyses indicate a high degree of material anisotropy for all investigated tissues. High circumferential and axial stresses occur in the media-intima tubes at physiological conditions. The obtained data are intended to serve for an improvement of constitutive laws, determination of constitutive parameters, and enhancing our knowledge of the mechanical functions of arteries and their associated layers in specific pathophysiological and clinical problems, such as hypertension and angioplasty with stenting.
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| 9. |
- Valentín, A., et al.
(författare)
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Constrained mixture models as tools for testing competing hypotheses in arterial biomechanics : A brief survey
- 2012
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Ingår i: Mechanics research communications. - : Elsevier BV. - 0093-6413 .- 1873-3972. ; 42, s. 126-133
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Forskningsöversikt (refereegranskat)abstract
- Hypothesis testing via numerical models has emerged as a powerful tool which permits the verification of theoretical frameworks against canonical experimental and clinical observations. Cleverly designed computational experiments also inspire new methodologies by elucidating important biological processes and restricting parametric spaces. Constrained mixture models of arterial growth and remodeling (G&R) can facilitate the design of computational experiments which can bypass technical limitations in the laboratory, by considering illustrative special cases. The resulting data may then inform the design of focused experimental techniques and the development of improved theories. This work is a survey of computational hypothesis-testing studies, which exploit the unique abilities offered by the constrained mixture theory of arterial G&R. Specifically, we explore the core hypotheses integrated in these models, review their basic mathematical conceptualizations, and recapitulate their most salient and illuminating findings. We then assess how a decade's worth of constrained mixture models have contributed to a lucid, emerging picture of G&R mechanisms.
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