Imre Bojtár

Impact of aneurysmal geometry on intraaneurysmal flow: a computerized flow simulation study.

IntroductionThis study was performed to assess the effect of aneurysm geometry on parameters that may have an impact on the natural history of intracranial aneurysms, such as intraaneurysmal flow pressure and shear stress.MethodsFlow was simulated in 21 randomly selected aneurysms using finite volume modeling. Ten aneurysms were classified as side-wall aneurysms, with either single-sided or circumferential involvement of the parent artery wall, and 11 as bifurcation aneurysms (symmetric or asymmetric), with an axis either perpendicular or parallel to the parent artery. The flow patterns were classified as either jet or vortex types (with regular or irregular vortex flow). Pressures and shear stresses were characterized as evenly or unevenly distributed over the aneurysm wall and neck.ResultsAll side-wall and four of the bifurcation aneurysms with a perpendicular axis had a vortex type flow pattern and seven bifurcation aneurysms with a parallel axis (four symmetric and two asymmetric) had a jet flow pattern. Jet type flow was associated with an uneven pressure distribution in seven out of seven aneurysms. Vortex type flow resulted in an even pressure distribution in five out of six aneurysms with an irregular flow pattern and six out of eight with a regular flow pattern. No firm relationship could be established between any geometrical type and shear stress distribution. Only 1 of 14 aneurysms with a perpendicular axis, but 4 of 7 aneurysms with a parallel axis, had ruptured.ConclusionAneurysm geometry does have an impact on flow conditions. Aneurysms with a main axis parallel to the parent artery have a tendency to have a jet flow pattern and uneven distribution of unsteady pressure. These aneurysms may have a higher rate of rupture as than those with a main axis perpendicular to the parent artery.

Comparison of the mechanical properties of the anterior lens capsule following manual capsulorhexis and femtosecond laser capsulotomy.

PURPOSE To evaluate and compare the mechanical properties of anterior capsule openings performed with the continuous curvilinear capsulorhexis (CCC) technique and femtosecond laser capsulotomy (FLC) in ex vivo porcine lens capsule specimens. METHODS Fresh porcine eyes were included in the study (CCC group, n = 50; FLC group, n = 30). The capsule openings were stretched with universal testing equipment until they ruptured. The rupture force and circumference stretching ratio were evaluated. The morphologic profile of the cut capsule edges was evaluated using scanning electron microscopy (SEM). RESULTS The average rupture force was higher in the CCC group (median: 155 mN; interquartile range [IQR]: 129 to 201 mN; range: 71 to 294 mN) than in the FLC group (median: 119 mN; IQR: 108 to 128 mN; range: 91 to 142 mN) (P < .01, Mann-Whitney U test). The average circumference stretching ratio in the CCC group was greater (median: 150%; IQR: 146% to 156%; range: 136% to 161%) than in the FLC group (median: 148%; IQR: 145% to 150%; range: 141% to 154%) (P = .0468, Mann-Whitney U test). When less than 71 mN, no capsular tear occurred in either group. When less than 91 mN, no capsular tear occurred in the FLC group, whereas at 91 mN, the probability of capsular tears was 9% for the CCC group. SEM examination found that the CCC group had smooth edges, whereas those of the FLC group were gently serrated. CONCLUSIONS According to the current results in a porcine eye model, FLC had less average resistance to capsule tear than CCC, but the weakest openings were seen in the CCC group.

Material Properties of the Mandibular Trabecular Bone

The present paper introduces a numerical simulation aided, experimental method for the measurement of Youngs modulus of the trabecular substance in the human mandible. Compression tests were performed on fresh cadaveric samples containing trabecular bone covered with cortical layer, thus avoiding the destruction caused by the sterilization, preservation, and storage and the underestimation of the stiffness resulting from the individual failure of the trabeculae cut on the surfaces. The elastic modulus of the spongiosa was determined by the numerical simulation of each compression test using a specimen specific finite element model of each sample. The received mandibular trabecular bone Youngs modulus values ranged from 6.9 to 199.5 MPa.

Evaluation of the Mechanical Properties of the Anterior Lens Capsule Following Femtosecond Laser Capsulotomy at Different Pulse Energy Settings

PURPOSE To evaluate and compare the mechanical properties of anterior capsule opening performed with femtosecond laser capsulotomy at different energy settings in ex vivo porcine anterior lens capsule specimens. METHODS Twenty-five fresh porcine eyes per group were included in the study. Femtosecond laser capsulotomy was performed with three different pulse energy levels: 2 µJ (low energy group), 5 µJ (intermediate energy group), and 10 µJ (high energy group). The capsule openings were stretched with universal testing equipment until they ruptured. The morphologic profile of the cut capsule edges was evaluated using scanning electron microscopy. RESULTS The high energy group had significantly lower rupture force (108 ± 14 mN) compared to the intermediate energy group (118 ± 10 mN) (P < .05) and low energy group (119 ± 11 mN) (P < .05), but the difference between the intermediate energy and low energy groups was not significant (P = .9479). The high energy group had significantly lower circumference stretching ratio (144% ± 3%) compared to the intermediate energy group (148% ± 3%) (P < .05) and low energy group (148% ± 3%) (P < .05), but the difference between the intermediate energy group and low energy group was not significant (P = .9985). Scanning electron microscopy images showed that the edge was only serrated with low and intermediate energy, but additional signs of collagen melting and denaturation were observed at high energy. CONCLUSIONS Anterior capsule openings created at a high energy level were slightly weaker and less extensible than those created at low or intermediate levels, possibly due to the increased thermal effect of photo-disruption.

Non-invasive in vivo time-dependent strain measurement method in human abdominal aortic aneurysms: Towards a novel approach to rupture risk estimation

We aim to introduce a novel, inverse method for in vivo material parameter identification of human abdominal aortic aneurysms (AAA), which could overcome one of the greatest sources of uncertainty in patient-specific simulations, and could also serve as a rapid, patient-calibrated, novel measure of aneurysm rupture risk. As an initial step, the determination of the kinematic fields is presented here. Images of the AAA lumen, acquired in 10 discrete time-steps through a stabilized cardiac cycle by electrocardiogram-gated computer tomography angiography, are used to approximate the in vivo, time dependent kinematic fields of the arterial wall using a novel, incompressible Kirchhoff-Love shell element implemented into the isogeometric analysis framework. Defining a smoothing parametric surface via 2D bicubic spline fitting in the spatial, and by harmonic regression in the temporal domain, we are able to adequately mitigate the measurement inaccuracy. The ill-posedness of the problem requires certain assumptions on the displacement. In our case, based on numerical fluid structure interaction simulation observations, we hypothesized the incremental displacement vector of the reference surface to coincide with its corrected normal; hence the periodic movement was assured. Finally, we present two examples: an AAA and an undilated calcificated aorta. Strains in the diseased part were compared to those in a healthy arterial section of the same patient and found to have significant differences in both specimens. In the case of AAAs, high spatial gradients surrounding the dilated part indicate abrupt changes in material properties, a phenomenon less significant for the atherosclerotic case.

Numerical analysis of loose and bonded granular materials

Abstract A quasi-static numerical algorithm is introduced for the analysis of quasi-static state-changing processes of granular materials. The paper presents the application possibilities of the model. Both sand-like and cemented materials can be analysed; rearrangements, instabilities, local slips, crack opening, etc., can easily be followed.

Evaluation of the mechanical behaviour and estimation of the elastic properties of porcine zonular fibres

The mechanical behaviour of zonular fibres greatly affects the accommodation process in mammalian eyes. This paper introduces a detailed measurement procedure for the purpose of obtaining the force-displacement diagram necessary to evaluate the mechanical properties of porcine zonular fibres in situ. It is a complex technique, keeping the integrity of the zonular bundles between the crystalline lens and the ciliary muscle cells. We present a brief description of the measurement procedure both in theory and in practice, along with the force-displacement diagrams acquired from a porcine sample group. The strengths of this newly developed method are the unequivocal force transmission between the sample and the transducer, and the intact connection between the ciliary body and the crystalline lens via zonular fibres. With the aid of these measurements, we define an estimated material model for the zonular apparatus both analytically and using the finite element method. The two different evaluation methods show close agreement in the calculated Youngs modulus for the zonular fibres. The range of the calculated elastic modulus is 200-250 kPa. This new measuring method is adaptable to human specimens. Despite its complexity, the entire procedure and the evaluation part are reproducible. The constitutive model aims to shed light on the mechanics of the accommodation process.

Numerical Evaluation of Internal Variables of Granular Materials

Abstract We have developed a quasi-static numerical algorythm to evaluate the different suggested variables for granular materials. With the help of numerical shear and compression tests we measured the changing of the variables suggested by Satake, and Cundall & Strack. We found that only a few of them are characteristic to the internal behaviour of the assemblies.

Novel, Inverse Biomechanical Methods Supporting Abdominal Aortic Aneurysm Management

Clinical treatment of abdominal aortic aneurysms is decided mainly on the maximal dilatation diameter. Biomechanics based criteria calculated by numerical fluid structure interaction simulations on patient specific geometries are superior to that approach, but are extremely time and resource consuming and require high expertise, furthermore they fail to incorporate the real, inhomogenous material parameters. Our method, summarized in this paper, handles these shortcomings by introducing a novel, indirect, in-vivo, patient-specific inverse material parameter identification method.

1D nemlineáris feladatok végeselemes vizsgálata explicit időintegrálással@@@Solution of 1D finite element problems with explicit time integration

Dolgozatunk egydimenzios nemlinearis feladatok vegeselemes vizsgalataval foglalkozik. A megoldast explicit időintegralassal, a centralis differenciak modszerevel allitottuk elő. A vizsgalathoz szukseges sajat keszitesű szoftver elsősorban a nemlinearis viselkedesű huzott-nyomott rud mechanikai vizsgalatara szolgal, kepes kezelni a fizikai es a geometriai nemlinearitast is. Az explicit modszerek numerikusan instabilak, igy a helyes időlepes megvalasztasa kulcskerdes. Ebben nyujtanak segitseget a parameteres futtatasok, ahol a kritikus allapothoz tartozo időlepest alapul veve, es azt valtoztatva ertekeltuk az eredmenyeket, megallapitva az optimalis időlepes nagysagat. Az ellenőrző szamitasokat prizmatikus, linearisan rugalmas rudon vegeztuk, előszor időben allando koncentralt erő, illetve onsuly majd szinuszosan valtozo teher hatasat vizsgalva. Harmadik esetkent a nemlinearisan rugalmas anyagu rud viselkedeset elemeztuk rudvegi koncentralt erőre. | The study deals with the examination of 1D nonlinear finite element problems. The software achieves the solution with explicit time integration, the central differences method. The tool is used for the mechanical examination of the drawn-pressed beam with nonlinear behaviour. The paper includes a guide to adjust the correct time step, for the explicit method, and three fundamental groups of results. In the first case the beam was linear elastic, and the load was constant. The second group contain the results with fluctuating load. In the third group the beam is nonlinear elastic.