Biomedical subjects

Gerhard A Holzapfel

Publications and source records attributed to Gerhard A Holzapfel.

17 recordsLinked to original sources

PubMed2006

Computational stress-deformation analysis of arterial walls including high-pressure response.

BACKGROUND: Changes in the mechanical behavior of arteries after balloon angioplasty cause cell reactions that may be responsible for restenosis. Hence, the study of the stress-deformation changes in arterial walls following supraphysiological tissue loading is an essential task. METHODS: A normal LAD coronary artery was modeled and computationally analyzed as a two-layer, thick-walled, anisotropic and inelastic circular tube including residual strains. Each layer was treated as a fibre-matrix composite. The tube was subjected to an axial stretch of 1.1 and a transmural pressure of 750 mm Hg. Since overstretch of remnant non-diseased tissue in lesions is a primary mechanism of lumen enlargement this model approach represents a reasonable first step. RESULTS: At physiological loading, the residual stresses led to a significant reduction of the high circumferential stress values at the inner wall, and the stress gradients. At low pressure level the media was the mechanically relevant layer, while at supraphysiological loading, the adventitia was the predominant load-carrying constituent providing a stiff support for 'redistribution' of soft plaque components by means of radial compression. After unloading to physiological loading conditions the stress state in the arterial wall differed significantly from that before inflation; the stress gradient in the media even changed its sign. Complete unloading indicated lumen enlargement, material softening and energy dissipation, which is in agreement with experimental studies. CONCLUSIONS: This method may be useful to improve interventional protocols for reducing the dilatational trauma, and thereby the adverse biological reaction in arterial walls following balloon angioplasty.

Angioplasty, Balloon, Coronary↗

PubMed2006

Hyperelastic modelling of arterial layers with distributed collagen fibre orientations.

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.

Animals↗

PubMed2006

3-D reconstruction of tissue components for atherosclerotic human arteries using ex vivo high-resolution MRI.

Automatic computer-based methods are well suited for the image analysis of the different components in atherosclerotic plaques. Although several groups work on such analysis some of the methods used are oversimplified and require improvements when used within a computational framework for predicting meaningful stress and strain distributions in the heterogeneous arterial wall under various loading conditions. Based on high-resolution magnetic resonance imaging of excised atherosclerotic human arteries and a series of two-dimensional (2-D) contours we present a segmentation tool that permits a three-dimensional (3-D) reconstruction of the most important tissue components of atherosclerotic arteries. The underlying principle of the proposed approach is a model-based snake algorithm for identifying 2-D contours, which uses information about the plaque composition and geometric data of the tissue layers. Validation of the computer-generated tissue boundaries is performed with 100 MR images, which are compared with the results of a manual segmentation performed by four experts. Based on the Hausdorff distance and the average distance for computer-to-expert differences and the interexpert differences for the outer boundary of the adventitia, the adventitia-media, media-intima, intima-lumen and calcification boundaries are less than 1 pixel (0.234 mm). The percentage statistic shows similar results to the modified Williams index in terms of accuracy. Except for the identification of lipid-rich regions the proposed algorithm is automatic. The nonuniform rational B-spline-based computer-generated 3-D models of the individual tissue components provide a basis for clinical and computational analysis.

Algorithms↗

PubMed2005

Mechanical properties of the human uterine cervix: an in vivo study.

Experimental results of in vivo measurements to characterize the mechanical behaviour of human uterine cervices are documented. Aspiration experiments were performed on eight uteri in vivo, before vaginal/abdominal hysterectomy, and four uteri were also tested ex vivo, approximately 1.5h after extraction. The reproducibility of the mechanical data from the in vivo aspiration experiments has been analysed. For an introduced "stiffness parameter" the organ specific SD is 22%, so that the proposed experimental procedure allows detections of 30% changes with respect to a reference value of the stiffness parameter. A comparison of in vivo and ex vivo data from the same organ has shown that: (i) the ex vivo mechanical response of the uterine cervix tissue does not differ considerably from that observed in vivo; (ii) some differences can be identified in tissue pre-conditioning with ex vivo showing a stronger history dependence with respect to in vivo; (iii) the differences in the time dependence of the mechanical response are not significant and might be masked by the variability of the measured data. This study represents a first step of a clinical application aiming at analysing the mechanical response of normal cervical tissue at different gestational ages, and identifying the mechanical properties that characterize pathologic conditions such as cervical insufficiency leading to preterm delivery.

Biomechanical Phenomena↗

PubMed2005

Determination of material models for arterial walls from uniaxial extension tests and histological structure.

An approach is proposed that allows the determination of material models from uniaxial tests and histostructural data including fiber orientation of the tissue. A combination of neo-Hookean and Fung-type strain-energy functions is utilized, and inequality constraints imposed on the constitutive parameters are derived providing strict local convexity and preferred fiber orientations. It is shown how the Fung-type model gets a pseudo-structural aspect inherent in the phenomenological model; a correlation between the fiber structure and the parameters of the Fung-type model is explicitly provided. In order to apply the proposed approach, quasi-static uniaxial extension tests of preconditioned prepared strips from the intima, media and adventitia of a human aorta with non-atherosclerotic intimal thickening are acquired in axial and circumferential directions; structural information from histological analyses for each aortic tissue are documented. Data reveal a remarkable thickness, load-bearing capacity and stiffness of the intimal samples in comparison with the media and adventitia. Constitutive parameters for each aortic tissue layer are determined by solving the constrained problem using a penalty function method; a new approach for the estimation of appropriate start values is proposed. Finally, the predictivity and efficacy of the material models is shown by comparing model data with data from the uniaxial extension tests and histological image analyses.

Aorta↗

PubMed2005

Determination of layer-specific mechanical properties of human coronary arteries with nonatherosclerotic intimal thickening and related constitutive modeling.

At autopsy, 13 nonstenotic human left anterior descending coronary arteries [71.5 +/- 7.3 (mean +/- SD) yr old] were harvested, and related anamnesis was documented. Preconditioned prepared strips (n = 78) of segments from the midregion of the left anterior descending coronary artery from the individual layers in axial and circumferential directions were subjected to cyclic quasi-static uniaxial tension tests, and ultimate tensile stresses and stretches were documented. The ratio of outer diameter to total wall thickness was 0.189 +/- 0.014; ratios of adventitia, media, and intima thickness to total wall thickness were 0.4 +/- 0.03, 0.36 +/- 0.03, and 0.27 +/- 0.02, respectively; axial in situ stretch of 1.044 +/- 0.06 decreased with age. Stress-stretch responses for the individual tissues showed pronounced mechanical heterogeneity. The intima is the stiffest layer over the whole deformation domain, whereas the media in the longitudinal direction is the softest. All specimens exhibited small hysteresis and anisotropic and strong nonlinear behavior in both loading directions. The media and intima showed similar ultimate tensile stresses, which are on average three times smaller than ultimate tensile stresses in the adventitia (1,430 +/- 604 kPa circumferential and 1,300 +/- 692 kPa longitudinal). The ultimate tensile stretches are similar for all tissue layers. A recently proposed constitutive model was extended and used to represent the deformation behavior for each tissue type over the entire loading range. The study showed the need to model nonstenotic human coronary arteries with nonatherosclerotic intimal thickening as a composite structure composed of three solid mechanically relevant layers with different mechanical properties. The intima showed significant thickness, load-bearing capacity, and mechanical strength compared with the media and adventitia.

Aged↗

PubMed2005

Role of facet curvature for accurate vertebral facet load analysis.

The curvature of vertebral facet joints may play an important role in the study of load-bearing characteristics and clinical interventions such as graded facetectomy. In previously-published finite element simulations of this procedure, the curvature was either neglected or approximated with a varying degree of accuracy. Here we study the effect of the curvature in three different load situations by using a numerical model which is able to represent the actual curvature without any loss of accuracy. The results show that previously-used approximations of the curvature lead to good results in the analysis of sagittal moment/rotation. However, for sagittal shear-force/displacement and for the contact stress distribution, previous results deviate significantly from our results. These findings are supported through related convergence studies. Hence we can conclude that in order to obtain reliable results for the analysis of sagittal shear-force/displacement and the contact stress distribution in the facet joint, the curvature must not be neglected. This is of particular importance for the numerical simulation of the spine, which may lead to improved diagnostics, effective surgical planning and intervention. The proposed method may represent a more reliable basis for optimizing the biomedical engineering design for tissue engineering or, for example, for spinal implants.

Biomechanical Phenomena↗

PubMed2005

An automatic nonrigid registration for stained histological sections.

Automatic computer-based analyses of histological sections which are differently stained require that they are related to each other. Most registration methods are only able to perform rigid-body motion and are sensitive to noise and artifacts. Histological images, however, are accompanied by several artifacts and different contrasts, which require a nonrigid registration. In this paper, we present a hierarchical nonrigid registration algorithm able to align images, which contain minor image artifacts. The algorithm requires no a priori knowledge of the true image. The hierarchical design of the algorithm enhances robustness and accuracy, and saves computational costs. The proposed algorithm is decomposed into a fast, coarse, rigid registration step and a slower, but finer, nonrigid step. For the coarse registration, we use image pyramids, while for the second step, we combine a point-based registration with an elastic thin-plate spline interpolation. Accuracy tests, performed for 20 histological images obtained from human arteries, have shown that the error measure is acceptable, and that the image noise does not cause a problem. The associated convergence rate of the mean pixel displacement error during the rigid and nonrigid registrations is satisfying. The algorithm can be applied to various multicontrast elastic registration problems in medical imaging and may be extended to three dimensions.

Algorithms↗

PubMed2005

Changes in the mechanical environment of stenotic arteries during interaction with stents: computational assessment of parametric stent designs.

Clinical studies have identified factors such as the stent design and the deployment technique that are one cause for the success or failure of angioplasty treatments. In addition, the success rate may also depend on the stenosis type. Hence, for a particular stenotic artery, the optimal intervention can only be identified by studying the influence of factors such as stent type, strut thickness, geometry of the stent cell, and stent-artery radial mismatch with the wall. We propose a methodology that allows a set of stent parameters to be varied, with the aim of evaluating the difference in the mechanical environment within the wall before and after angioplasty with stenting. Novel scalar quantities attempt to characterize the wall changes inform of the contact pressure caused by the stent struts, and the stresses within the individual components of the wall caused by the stent. These quantities are derived numerically and serve as indicators, which allow the determination of the correct size and type of the stent for each individual stenosis. In addition, the luminal change due to angioplasty may be computed as well. The methodology is demonstrated by using a full three-dimensional geometrical model of a postmortem specimen of a human iliac artery with a stenosis using imaging data. To describe the material behavior of the artery, we considered mechanical data of eight different vascular tissues, which formed the stenosis. The constitutive models for the tissue components capture the typical anisotropic, nonlinear and dissipative characteristics under supra-physiological loading conditions. Three-dimensional stent models were parametrized in such a way as to enable new designs to be generated simply with regard to variations in their geometric structure. For the three-dimensional stent-artery interaction we use a contact algorithm based on smooth contact surfaces of at least C-continuity, which prevents numerical problems known from standard facet-based contact algorithm. The proposed methodology has the potential to provide a scientific basis for optimizing treatment procedures and stent geometries and materials, to help stent designers examine new stent designs "virtually," and to assist clinicians in choosing the most suitable stent for a particular stenosis.

Adult↗

PubMed2005

Computer vision analysis of collagen fiber bundles in the adventitia of human blood vessels.

Numerical simulations, which are based on reliable biomechanical models of blood vessels, can help to get a better understanding of cardiovascular diseases such as atherosclerosis, and can be used to develop optimal medical treatment strategies.The adventitia is the outer most layer of blood vessels and its mechanical properties are essentially determined by the three-dimensional, structural arrangement of collagen fibre bundles embedded in the tissue. Global information such as the orientation statistics of the fibre bundles as well as detailed information as the crimp of the single fibres within the bundles is of particular interest in biomechanical modeling.In order to obtain a sufficiently large amount of data for biomechanical modeling, a fully automatic method for the structural analysis of the soft tissue is required. In this contribution we present methods based on computer vision to fulfill this task. We start by discussing proper tissue preparation and imaging techniques that have to be used to obtain data, which reliably represents the real three-dimensional tissue structure. The next step is concerned with algorithms that robustly segment the collagen fibre bundles and cope with various kinds of artifacts. Novel segmentation techniques for robust segmentation of individual fibril bundles and methods for estimation of their parameters, such as location, shape, mean fibril orientation, crimp of fibrils, etc, is discussed. The proposed algorithms are based on novel perceptual grouping methods operating on the extracted orientation data of fibrils.Finally, we demonstrate the results obtained by our fully automatic method on real data. In addition, for a more quantitative assessment, we introduce a generative structural model that enables the synthesis of three-dimensional fibre bundles with well-defined characteristics.

Adventitia↗

PubMed2004

Comparison of a multi-layer structural model for arterial walls with a fung-type model, and issues of material stability.

The goals of this paper are (i) to re-examine the constitutive law for the description of the (passive) highly nonlinear and anisotropic response of healthy elastic arteries introduced recently by the authors, (ii) to show how the mechanical response of a carotid artery under inflation and extension predicted by the structural model compares with that for a three-dimensional form of Fung-type strain-energy function, (iii) to provide a new set of material parameters that can be used in a finite element program, and (iv) to show that the model has certain mathematical features that are important from the point of view of material and numerical stability.

Animals↗

PubMed2004

Anisotropic mechanical properties of tissue components in human atherosclerotic plaques.

Knowledge of the biomechanical properties of human atherosclerotic plaques is of essential importance for developing more insights in the pathophysiology of the cardiovascular system and for better predicting the outcome of interventional treatments such as balloon angioplasty. Available data are mainly based on uniaxial tests, and most of the studies investigate the mechanical response of fibrous plaque caps only. However, stress distributions during, for example, balloon angioplasty are strongly influenced by all components of atherosclerotic lesions. A total number of 107 samples from nine human high-grade stenotic iliac arteries were tested; associated anamnesis of donors reported. Magnetic resonance imaging was employed to test the usability of the harvested arteries. Histological analyses has served to characterize the different tissue types. Prepared strips of 7 different tissue types underwent cyclic quasistatic uniaxial tension tests in axial and circumferential directions; ultimate tensile stresses and stretches were documented. Experimental data of individual samples indicated anisotropic and highly nonlinear tissue properties as well as considerable interspecimen differences. The calcification showed, however a linear property, with about the same stiffness as observed for the adventitia in high stress regions. The stress and stretch values at calcification fracture are smaller (179 +/- 56 kPa and 1.02 +/- 0.005) than for each of the other tissue components. Of all intimal tissues investigated, the lowest fracture stress occurred in the circumferential direction of the fibrous cap (254.8 +/- 79.8 kPa at stretch 1.182 +/- 0.1). The adventitia demonstrated the highest and the nondiseased media the lowest mechanical strength on average.

Aged↗

PubMed2003

Passive biaxial mechanical response of aged human iliac arteries.

Inflation and extension tests of arteries are essential for the understanding of arterial wall mechanics. Data for such tests of human arteries are rare. At autopsy we harvested 10 non-diseased external iliac arteries of aged subjects (52-87 yrs). Structural homogeneity was ensured by means of ultrasound imaging, and anamneses of patients were recorded. We measured the axial in situ stretches, load-free geometries and opening angles. Passive biaxial mechanical responses of preconditioned cylindrical specimens were studied in 37 degrees C calcium-free Tyrode solution under quasistatic loading conditions. Specimens were subjected to pressure cycles varying from 0 to 33.3 kPa (250 mmHg) at nine fixed axial loads, varying from 0 to 9.90N. For the description of the load-deformation behavior we employed five "two-dimensional" orthotropic strain-energy functions frequently used in arterial wall mechanics. The associated constitutive models were compared in regard to their ability of representing the experimental data. Histology showed that the arteries were of the muscular type. In contrast to animal arteries they exhibited intimal layers of considerable thickness. The average ratio of wall thickness to outer diameter was 7.7, which is much less than observed for common animal arteries. We found a clear correlation between age and the axial in situ stretch lambda is (r = -0.72, P = 0.03), and between age and distensibility of specimens, i.e. aged specimens are less distensible. Axial in situ stretches were clearly smaller (1.07 +/- 0.09, mean +/- SD) than in animal arteries. For one specimen lambda is was even smaller than 1.0, i.e. the vessel elongated axially upon excision. The nonlinear and anisotropic load-deformation behavior showed small hystereses. For the majority of specimens we observed axial stretches smaller than 1.3 and circumferential stretches smaller than 1.1 for the investigated loading range. Data from in situ inflation tests showed a significant increase of the axial stretch with intraluminal pressure. Thus, for this type of artery the axial in situ stretch of a non-pressurized vessel is not representative of the axial in vivo stretch. None of the constitutive models were able to represent the deformation behavior of the entire loading range. For the physiological loading range, however, some of the models achieved good agreement with the experimental data.

Age Factors↗

PubMed2002

Biomechanics.

Explore the source record for details and available documents.

Biomechanical Phenomena↗

PubMed2002

A layer-specific three-dimensional model for the simulation of balloon angioplasty using magnetic resonance imaging and mechanical testing.

A detailed understanding of the mechanical procedure of balloon angioplasty requires three-dimensional (3D) modeling and efficient numerical simulations. We have developed a 3D model for eight distinct arterial components associated with specific mechanical responses. The 3D geometrical model is based on in vitro magnetic resonance imaging of a human stenotic postmortem artery and is represented by nonuniform rational B-spline surfaces. Mechanical tests of the corresponding vascular tissues provide a fundamental basis for the formulation of large strain constitutive laws, which model the typical anisotropic, highly nonlinear, and inelastic mechanical characteristics under supraphysiological loadings. The 3D finite-element realization considers the balloon-artery interaction and accounts for vessel-specific axial in situ prestretches. 3D stress states of the investigated artery during balloon expansion and stent deployment were analyzed. Furthermore, we studied the changes of the 3D stress state due to model simplifications, which are characterized by neglecting axial in situ prestretch, assuming plane strain states, and isotropic material responses, as commonly utilized in previous works. Since these simplifications lead to maximum stress deviations of up to 600%-where even the stress character may interchange-the associated models are, in general, inappropriate. The proposed approach provides a tool that has the potential (i) to improve procedural protocols and the design of interventional instruments on a lesion-specific basis, and (ii) to determine postangioplasty mechanical environments, which may be correlated with restenosis responses.

Aged↗

PubMed2002

A three-dimensional finite element model for arterial clamping.

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.

Anisotropy↗

PubMed2002

Mechanics of the human femoral adventitia including the high-pressure response.

Adventitial mechanics were studied on the basis of adventitial tube tests and associated stress analyses utilizing a thin-walled model. Inflation tests of 11 nonstenotic human femoral arteries (79.3 +/- 8.2 yr, means +/- SD) were performed during autopsy. Adventitial tubes were separated anatomically and underwent cyclic, quasistatic extension-inflation tests using physiological pressures and high pressures up to 100 kPa. Associated circumferential and axial stretches were typically <20%, indicating "adventitiosclerosis." Adventitias behaved nearly elastically for both loading domains, demonstrating high tensile strengths (>1 MPa). The anisotropic and strongly nonlinear mechanical responses were represented appropriately by two-dimensional Fung-type stored-energy functions. At physiological pressure (13.3 kPa), adventitias carry ~25% of the pressure load in situ, whereas their circumferential and axial stresses were similar to the total wall stresses (~50 kPa in both directions), supporting a "uniform stress hypothesis." At higher pressures, they became the mechanically predominant layer, carrying >50% of the pressure load. These significant load-carrying capabilities depended strongly on circumferential and axial in-vessel prestretches (mean values: 0.95 and 1.08). On the basis of these results, the mechanical role of the adventitia at physiological and hypertensive states and during balloon angioplasty was characterized.

Aged↗