| 1. |
- Hyhlik-Dürr, A., et al.
(författare)
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Finite Element Analysis of Abdominal Aortic Aneurysms : Preliminary Results of Intra and Inter observer Validation
- 2010
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Konferensbidrag (refereegranskat)abstract
- Objective: Treatment of abdominal aortic aneurysm (AAA) is indicated if risk for rupture exceeds the risk for aortic repair. Estimation of the individual risk for rupture in AAA is therefore essential. The diameter of AAA is known as an independent risk factor for rupture and therefore the base of indication for surgical or endovascular therapy. For more sensitive patient selection, other morphological or hemodynamic predictors such as volume or peak wall stress have to be evaluated. The purpose of this study was to analyze the reproducibility of diameter measurement, volume estimation and peak wall stress calculation in AAA by finite element analysis. Methods: Computed tomography angiography (CTA) scans of 10 patients with AAA and 4 volunteers with healthy infrarenal aortas were analyzed by three independent investigators. A semiautomatic reconstruction using two- and three-dimensional deformable (active) contour models was used to segment vascular bodies from CTA data. Centreline calculated maximal diameter and volume measurements, as extracted from the reconstructed abdominal aorta, as well as peak wall stress, as predicted by three-dimensional non-linear finite element models, were analyzed. Specifically, aortic wall and thrombus tissue were captured by isotropic, non-linear and finite strain constitutive models. Likewise, mean arterial pressure was applied at the luminal surface, the vessels were fixed at the renal arteries and the aortic bifurcation and no contact with surrounding organs was considered. Inter- and intra-observer variabilities for diameter, volume and peak wall stress measurements were assessed by calculating the coefficient of variation (CV=SD*100/mean in %) of the five fold determinations. The methodological variation was expressed as deviation of diameter (mm), volume (ml) and peak wall stress (kPa) amongst the three observers. Results: Reproducibility measurements in healthy vessels of aortic diameters between 16.1mm to 16.6mm varied from CV=2.5% to CV=4.9%. Abdominal aortic volumes of 14ml to 15ml were measured in the healthy cohort with a reproducibility of CV=5.8% to CV=11.5%. Peak wall stress varied between 53 kPa and 55 kPa, where CV ranged from 3-13%. Inter-observer variation was <10% for diameter, volume and peak stress in healthy volunteers. Aortic diameter in three AAAs was measured to 58.9 mm; 54.6 mm; and 71.2 mm respectively. The coefficient of variation showed high agreement with values less than 5%. AAA volume varied between 130 ml and 300 ml (CV < 10%) and Peak wall stress was predicted between 172 kPa and 296 kPa (CV <10%). Variability between the 3 observers in AAA measurements was 0.7 mm – 6.0 mm for diameter, 11 – 28 ml for volume and 4-27 kPa for peak wall stress, respectively. Conclusions: Volume and diameter measurements based on geometrical models reconstructed from CTA scans showed quit good reproducibility for serial measurements in normal and degenerative arteries. Peak wall stress predictions exhibited high accordance between different observers, and in serial measurements within one observer. Volume and peak wall stress analysis could be an additionally module for assessment of individual rupture risk in AAA in the future, which however needs to be validated by additional studies.
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| 2. |
- Hyhlik-Dürr, A., et al.
(författare)
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Finite-Elemente-Analyse abdomineller Aortenaneurysmen : Erste Ergebnisse der Intra- und Interobserver Validierung
- 2010
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Konferensbidrag (refereegranskat)abstract
- Hintergrund: Die Therapie des abdominellen Aortenaneurysmas (AAA) ist indiziert, wenn das Rupturrisiko das Risiko der elektiven Operation übersteigt. Die Abschätzung des individuellen Rupturrisikos gilt als Basis der Indikationsstellung zur offenen oder endovaskulären Chirurgie. Bisher wird der Durchmesser des AAA als maßgeblicher Risikofaktor für die Ruptur herangezogen. Für eine sensitivere Indikationsstellung sollten jedoch andere morphologische oder biomechanische Faktoren wie die Volumenveränderung im Verlauf und/oder die Wandspannung im Aneurysma untersucht werden. Ziel dieser Studie ist die Analyse der Reproduzierbarkeit der Durchmesserbestimmung sowie der Volumen- und Wandspannungsberechnung anhand eines geometrischen Modells, basierend auf der Finite Elemente Methode. Methode: Computertomographische Daten von vier gesunden und zehn Patienten mit infrarenalen abdominellen Aneurysmen werden von drei unabhängigen Untersuchern analysiert. Die abdominelle Aorta wird semiautomatisch von Computertomographie-Angiographie (CTA) Bilddaten segmentiert, wobei zwei und drei-dimensionale aktive Konturmodelle, wie sie aus der Bildverarbeitung bekannt sind, zum Einsatz kommen. Der maximale Durchmesser (cernterline-basiert) sowie das aortale Volumen werden aus den rekonstruierten dreidimensionalen Modellen berechnet. Zusätzlich werden nicht-lineare Finite Elemente Modelle verwendet, um die mechanische Spannung in der Aortenwand zwischen der Aortenbifurkation und den Nierenarterien zu bestimmen. Zu diesen Zweck wird der mittlere arterielle Druck als Belastung angenommen und nicht-lineare isotrope Materialmodelle erfassen die mechanischen Eigenschaften der Aortenwand und des Thrombusgewebes. Die Intra- und Interobserver Variabilität der fünf Messungen des maximalen Durchmessers, des Volumens und der maximalen Wandspannung wurden durch die Berechnung des Variationskoeffizienten (CV=SD*100/Arithmethisches Mittel in %) ausgedrückt. Die methodische Variation berechnet sich aus der Abweichung des Duchmessers (mm), des Volumens (ml) und der maximalen Wandspannung (kPA) zwischen den drei Untersuchern. Ergebnisse: Die Reproduzierbarkeit gesunder Gefäßen lag bei einem Durchmesser zwischen 16.1mm und 16.6mm zwischen CV=2,5% und CV=4,9%. Das aortale Volumen lag zwischen 14ml und 15ml, die Reproduzierbarkeit bei den gesunden Gefäßen streute zwischen CV=5.8% und CV=11.5%. Die maximale Wandspannung variierte zwischen 53 kPA and 55 kPa, der CV% lag hierbei zwischen 3 und 13. Die Interobserver Variabilität lag < 10% für den Durchmesser, die Volumenbestimmung und die Bestimmung der maximale Wandspannung. Der maximale Durchmesser der Aorta bei 3 Patienten mit infrarenalem Aneurysma wurde mit durchschnittlich 58.9mm, 54.6mm und 71.2mm berechnet (Stand bei Abstracteinreichung). Der Variationskoeffizient zeigte dabei eine hohe Übereinstimmung mit Werten unter 5%. Das Volumen der Aneurysmen schwankte zwischen 130 ml und 300 ml (CV<10%), die berechnete Wandspannung lag zwischen 172 kPA und 296 kPA (CV<10%). Die Variabilität zwischen den drei Untersuchern betrug 0,7-6,0 mm für den Durchmesser, 11-28 ml für das Volumen und 4-27 kPA für die maximale Wandspannung. Zusammenfassung: Sowohl an gesunden als auch an degenerativ veränderten Gefäßen ergibt die Reproduzierbarkeit des Aortendurchmessers und des aortalen Volumens basierend auf dem dreidimensionalen rekonstruierten Modellen eine hohe Übereinstimmung. Die berechnete Wandspannung basierend auf den Finiten Elemente Modellen zeigt einen geringen Grad an Variabilität sowohl zwischen verschiedenen Untersuchern als auch bei wiederholter Messung. Daher könnten die Volumenbestimmung und die Analyse der Wandspannung zusätzliche Größen bei der Bestimmung des individuellen Rupturrisikos bei Patienten mit Aortenaneurysmen darstellen, um eine präzisere Indikationsstellung zu ermöglichen.
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| 3. |
- Forsell, Caroline, et al.
(författare)
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Impact of material anisotropy on deformation of myocardial tissue due to pacemaker electrodes
- 2011
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Ingår i: ASME 2011 Summer Bioengineering Conference, SBC 2011. - 9780791854587 ; , s. 789-790
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 4. |
- Forsell, Caroline, et al.
(författare)
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Modeling of myocardial splitting due to deep penetration
- 2010
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Ingår i: CONSTITUTIVE MODELS FOR RUBBER VI. - BOCA RATON : CRC PRESS-TAYLOR & FRANCIS GROUP. - 9780415563277 ; , s. 449-452
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 5. |
- Forsell, Caroline, et al.
(författare)
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The impact of constitutive properties on myocardial tissue perforation
- 2010
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 6. |
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| 7. |
- 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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| 8. |
- Gasser, T. Christian, et al.
(författare)
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Collagen fiber orientation in Abdominal Aortic Aneurysms wall
- 2010
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 9. |
- Gasser, T. Christian
(författare)
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Modeling the Structural and Mechanical Properties of the Normal and Aneurysmatic Aortic Wall
- 2020
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Ingår i: Studies in Mechanobiology, Tissue Engineering and Biomaterials. - Cham : Springer. ; , s. 55-82
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Konferensbidrag (refereegranskat)abstract
- The structural properties of the Extracellular Matrix change in response to many factors, such as mechanical stress, age, disease and lifestyle. The ECM ensures not only the vessel wall’s structural integrity, but it also defines the micro-mechanical environment within which vascular cells are embedded and to which they respond. Its mechanical properties are governed by the delicate interaction of elastin, collagen, ProteoGlycans, fibronectin, fibrilin and other, constituents which are synthesized by vascular cells and degraded, mainly by Matrix MetalloProteinases. The present chapter discusses the structural organization of the vessel wall towards the multi-scale mechanical characterization of the aneurysmatic aorta. It is assumed that aneurysmatic vessel wall properties are mainly governed by collagen fibrils, with their undulation and orientation being the most influential micro-histological parameters. Purely passive constitutive descriptions are further complemented by collagen turnover kinetics, and all models are set-up such that they may be used for organ-level vascular biomechanics simulations.
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| 10. |
- Gasser, T. Christian, et al.
(författare)
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Physical and numerical modeling of dissection propagation in arteries caused by balloon angioplasty
- 2005
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Ingår i: Proceedings of the Third IASTED International Conference on BIOMECHANICS. - 0889865329 ; , s. 229-233
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Konferensbidrag (refereegranskat)abstract
- 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.
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