Evaluation of brain injury criteria based on reliability analysis.

PURPOSE: Among the proposed brain injury metrics, Brain Injury Criteria (BrIC) is a promising tool for performing safety assessment of vehicles in the future. In this paper, the available risk curves of BrIC were re-evaluated with the use of reliability analysis and new risk curves were constructed for different injury types based on literature data of tissue-level tolerances. Moreover, the comparison of different injury metrics and their corresponding risk curves were performed. METHODS: Tissue-level uncertainties of the effect and resistance were considered by random variables. The variability of the tissue-level predictors was quantified by the finite element reconstruction of 100 frontal crash tests which were performed in Simulated Injury Monitor environment. The applied tests were scaled to given BrIC magnitudes and the injury probabilities were calculated by Monte Carlo simulations. New risk curves were fitted to the observed results using Weibull and Lognormal distribution functions. RESULTS: The available risk curves of diffuse axonal injury (DAI) could be slightly improved, and combined AIS 4+ risk curves were obtained by considering subdural hematoma and contusion as well. The performance of several injury metrics and their risk curves were evaluated based on the observed correlations with the tissue-level predictors. CONCLUSIONS: The cumulative strain damage measure and the BrIC provide the highest correlation (R² = 0.61) and the most reliable risk curve for the evaluation of DAI. Although the observed correlation is smaller for other injury types, the BrIC and the associated reliability analysis-based risk curves seem to provide the best available method for estimating the brain injury risk for frontal crash tests.

Engineering optimization of decompressive craniectomy based on finite element simulations.

PURPOSE: The optimal execution of decompressive craniectomy in terms of the size and location of the skull opening is not straightforward. Our main goals are twofold: (1) constructing a design optimization method which can be applied to determine optimal skull opening for individual patient-specific cases and (2) performing a large-scale parametric optimization study to give some guidance in general about the optimal skull opening in case of oedematous brain tissue. METHODS: A large number of virtual experiments performed by finite element simulations were applied to determine tendencies of tissue behaviour during surgery. The multiobjective optimization is performed by Goal Programming and Physical Programming methods. RESULTS: Our results show that the postoperative pressure has an approximately linear dependence on the preoperative pressure and the skull opening area, while the damaged brain volume could have a more complex nonlinear dependence on the input data. Based on the averaged results of the parametric optimization study, the optimal skull opening has been determined in the function of the preoperative pressure and the relative importance of the pressure reduction. These results show that the optimal size of the unilateral skull opening is usually between 130-180 cm² and these openings are more beneficial than the currently analysed bifrontal openings. CONCLUSIONS: The optimal skull opening is patient-specific and depends on several input data. The presented methodology can be applied to optimize surgery based on these input parameters for different injury types. Based on the results of large-scale parametric study generally applicable approximate results have been provided.

The probability of traumatic brain injuries based on tissue-level reliability analysis.

PURPOSE: Motor vehicle crashes are one of the leading causes of traumatic brain injuries. Restraint systems of cars are evaluated by crash tests based on human tolerance data, however, the reliability of data currently used has been questioned several times in the literature due to the neglect of certain types of effects, injury types and uncertainties. Our main goal was to re-evaluate the currently applied risk curve by taking the previously neglected effects into account. METHODS: In this paper, the probability of traumatic brain injury was determined by reliability analysis where different types of uncertainties are taken into account. The tissue-level response of the human brain in the case of frontal crashes was calculated by finite element analyses and the injury probability is determined by Monte Carlo simulations. Sensitivity analysis was also performed to identify which effects have considerable contribution to the injury risk. RESULTS: Our results indicate a significantly larger injury risk than it is predicted by current safety standards. Accordingly, a new risk curve was constructed which follows a lognormal distribution with the following parameters: µLN = 6.5445 and SLN = 1.1993. Sensitivity analysis confirmed that this difference primarily can be attributed to the rotational effects and tissue-level uncertainties. CONCLUSIONS: Results of the tissue-level reliability analysis enhance the belief that rotational effects are the primary cause of brain injuries. Accordingly, the use of a solely translational acceleration based injury metric contains several uncertainties which can lead to relatively high injury probabilities even if relatively small translational effects occur.

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.

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.

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.

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 Young's 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.

Material Properties of the Mandibular Trabecular Bone.

The present paper introduces a numerical simulation aided, experimental method for the measurement of Young's 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 Young's modulus values ranged from 6.9 to 199.5 MPa.

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

INTRODUCTION: This 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. METHODS: Flow 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. RESULTS: All 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. CONCLUSION: Aneurysm 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.

Laboratory tests for strength paramaters of brain aneurysms.

The presentation is focused on the analysis of biophysical properties of cerebral aneurysms, diagnosed and delineated in living human individuals. An aneurysm is a bulging out of a part of the wall of a blood vessel. The aim of our research was to delineate flow patterns inside the aneurysm and its parent artery, to estimate stresses at critical points of the aneurysm wall, to model the haemodynamic effect of different surgical and endovascular tools in order to define the optimal one in a particular case, and to estimate the likelihood of a later aneurysm rupture. For this reason we carried out a lot of different laboratory tests to analyse the mechanical parameters of the aneurysm wall. We made a comparative study of some material models reported in the literature to describe the mechanical response of arteries. These are models for incompressible materials. For this reason we perform uniaxial and biaxial measurements to have appropriate parameters for the models of underlying material.

[A biomechanical study of the mechanical stress transmission of dental implants using finite element analysis. Part II. Experiments]. / Fogászati implantátumok mechanikai feszültség-atadásának biomechanikai vizsgálata végeselemes analízissel. II. Rész vizsgálatok.

In the first part of this two-part study the possible uses of finite element analysis in studying the stress transmission of dental implants were reviewed. In the present second part our own experiments are presented. In the first series the effect of the geometric parameters and load types on mechanical stresses arising around cylindrical implants in bone were studied with 2D and 3D finite element analysis. In the second series the stress transmissions of various implant geometries were compared and an attempt was made to determine the optimal implant shape from the point of view of stress transmission.