Finite Element Head Model Research Articles

Helmet liner evaluation to mitigate head response from primary blast exposure

Head injury resulting from blast loading, including mild traumatic brain injury, has been identified as an important blast-related injury in modern conflict zones. A study was undertaken to investigate potential protective ballistic helmet liner materials to mitigate primary blast injury using a detailed sagittal plane head finite element model, developed and validated against previous studies of head kinematics resulting from blast exposure. Five measures reflecting the potential for brain injury that were investigated included intracranial pressure, brain tissue strain, head acceleration (linear and rotational) and the head injury criterion. In simulations, these measures provided consistent predictions for typical blast loading scenarios. Considering mitigation, various characteristics of foam material response were investigated and a factor analysis was performed which showed that the four most significant were the interaction effects between modulus and hysteretic response, stress–strain response, damping factor and density. Candidate materials were then identified using the predicted optimal material values. Polymeric foam was found to meet the density and modulus requirements; however, for all significant parameters, higher strength foams, such as aluminum foam, were found to provide the highest reduction in the potential for injury when compared against the unprotected head.

A computational study on brain tissue under blast: primary and tertiary blast injuries

In this paper, a biomechanical study of a human head model exposed to blast shock waves followed by a blunt impact with the surface of the enclosing walls of a confined space is carried out. Under blast, the head may experience primary blast injury (PBI) due to exposure to the shockwaves and tertiary blast injury (TeBI) due to a possible blunt impact. We examine the brain response data in a deformable finite element head model in terms of the inflicted stress/pressure, velocity, and acceleration on the brain for several blast scenarios with different intensities. The data will be compared for open space and confined spaces. Following the initial impact of the shock front in the confined space, one can see the fluctuations in biomechanical data due to wave reflections. Although the severity of the PBI and TeBI is dependent on the situation, for the cases studied here, PBI is considerably more pronounced than TeBI in confined spaces.

Development of a finite element head model for the study of impact head injury.

This study is aimed at developing a high quality, validated finite element (FE) human head model for traumatic brain injuries (TBI) prediction and prevention during vehicle collisions. The geometry of the FE model was based on computed tomography (CT) and magnetic resonance imaging (MRI) scans of a volunteer close to the anthropometry of a 50th percentile male. The material and structural properties were selected based on a synthesis of current knowledge of the constitutive models for each tissue. The cerebrospinal fluid (CSF) was simulated explicitly as a hydrostatic fluid by using a surface-based fluid modeling method. The model was validated in the loading condition observed in frontal impact vehicle collision. These validations include the intracranial pressure (ICP), brain motion, impact force and intracranial acceleration response, maximum von Mises stress in the brain, and maximum principal stress in the skull. Overall results obtained in the validation indicated improved biofidelity relative to previous FE models, and the change in the maximum von Mises in the brain is mainly caused by the improvement of the CSF simulation. The model may be used for improving the current injury criteria of the brain and anthropometric test devices.

Exploratory study on the methodology of fast imaging of unilateral stroke lesions by electrical impedance asymmetry in human heads.

Stroke has a high mortality and disability rate and should be rapidly diagnosed to improve prognosis. Diagnosing stroke is not a problem for hospitals with CT, MRI, and other imaging devices but is difficult for community hospitals without these devices. Based on the mechanism that the electrical impedance of the two hemispheres of a normal human head is basically symmetrical and a stroke can alter this symmetry, a fast electrical impedance imaging method called symmetrical electrical impedance tomography (SEIT) is proposed. In this technique, electrical impedance tomography (EIT) data measured from the undamaged craniocerebral hemisphere (CCH) is regarded as reference data for the remaining EIT data measured from the other CCH for difference imaging to identify the differences in resistivity distribution between the two CCHs. The results of SEIT imaging based on simulation data from the 2D human head finite element model and that from the physical phantom of human head verified this method in detection of unilateral stroke.

Bicycle helmet modelling and validation under linear and tangential impacts

In the context of head protection against traumatic brain injuries, this work attempts to understand the degree of protection offered by a commercial bicycle helmet under both linear and oblique impact conditions. In accordance with EN 1078 standard, an experimental program has been carried out on an existing helmet by performing 90 normative impacts. In parallel, a finite element model (FEM) of this helmet has been developed and implemented under the LS-DYNA® crash code to numerically reproduce the 90 experimental standard impact tests. Experimental oblique impacts on this commercial helmet have also been performed in order to validate it and to be able to assess its protection capability in case of tangential impact. Finally, the bicycle helmet FEM has been coupled to a Hybrid III dummy head FEM and oblique impacts have been reproduced numerically leading to a realistic behaviour of the helmet model in terms of rotational accelerations.

A computational modelling study of transcranial direct current stimulation montages used in depression

Transcranial direct current stimulation (tDCS) is a neuromodulatory technique which involves passing a mild electric current to the brain through electrodes placed on the scalp. Several clinical studies suggest that tDCS may have clinically meaningful efficacy in the treatment of depression. The objective of this study was to simulate and compare the effects of several tDCS montages either used in clinical trials or proposed, for the treatment of depression, in different high-resolution anatomically-accurate head models. Detailed segmented finite element head models of two subjects were presented, and a total of eleven tDCS electrode montages were simulated. Sensitivity analysis on the effects of changing the size of the anode, rotating both electrodes and displacing the anode was also conducted on selected montages. The F3–F8 and F3–F4 montages have been used in clinical trials reporting significant antidepressant effects and both result in relatively high electric fields in dorsolateral prefrontal cortices. Other montages using a fronto-extracephalic or fronto-occipital approach result in greater stimulation of central structures (e.g. anterior cingulate cortex) which may be advantageous in treating depression, but their efficacy has yet to be tested in randomised controlled trials. Results from sensitivity analysis suggest that electrode position and size may be adjusted slightly to accommodate other priorities, such as skin discomfort and damage.

Assessment of electric field distribution in anisotropic cortical and subcortical regions under the influence of tDCS

The focus of this study is to estimate the contribution of regional anisotropic conductivity on the spatial distribution of an induced electric field across gray matter (GM), white matter (WM), and subcortical regions under transcranial direct current stimulation (tDCS). The assessment was conducted using a passive high-resolution finite element head model with inhomogeneous and variable anisotropic conductivities derived from the diffusion tensor data. Electric field distribution was evaluated across different cortical as well as subcortical regions under four bicephalic electrode configurations. Results indicate that regional tissue heterogeneity and anisotropy cause the pattern of induced fields to vary in orientation and strength when compared to the isotropic scenario. Different electrode montages resulted in distinct distribution patterns with noticeable variations in field strengths. The effect of anisotropy is highly montage dependent and directional conductivity has a more profound effect in defining the strength of the induced field. The inclusion of anisotropy in the GM and subcortical regions has a significant effect on the strength and spatial distribution of the induced electric field. Under the (C3-Fp2) montage, the inclusion of GM and subcortical anisotropy increased the average percentage difference in the electric field strength of brain from 5% to 34% (WM anisotropy only). In terms of patterns distribution, the topographic errors increased from 9.9% to 40% (WM anisotropy only) across the brain.

Parametric Comparisons of Intracranial Mechanical Responses from Three Validated Finite Element Models of the Human Head

A number of human head finite element (FE) models have been developed from different research groups over the years to study the mechanisms of traumatic brain injury. These models can vary substantially in model features and parameters, making it important to evaluate whether simulation results from one model are readily comparable with another, and whether response-based injury thresholds established from a specific model can be generalized when a different model is employed. The purpose of this study is to parametrically compare regional brain mechanical responses from three validated head FE models to test the hypothesis that regional brain responses are dependent on the specific head model employed as well as the region of interest (ROI). The Dartmouth Scaled and Normalized Model (DSNM), the Simulated Injury Monitor (SIMon), and the Wayne State University Head Injury Model (WSUHIM) were selected for comparisons. For model input, 144 unique kinematic conditions were created to represent the range of head impacts sustained by male collegiate hockey players during play. These impacts encompass the 50th, 95th, and 99th percentile peak linear and rotational accelerations at 16 impact locations around the head. Five mechanical variables (strain, strain rate, strain × strain rate, stress, and pressure) in seven ROIs reported from the FE models were compared using Generalized Estimating Equation statistical models. Highly significant differences existed among FE models for nearly all output variables and ROIs. The WSUHIM produced substantially higher peak values for almost all output variables regardless of the ROI compared to the DSNM and SIMon models (p < 0.05). DSNM also produced significantly different stress and pressure compared with SIMon for all ROIs (p < 0.05), but such differences were not consistent across ROIs for other variables. Regardless of FE model, most output variables were highly correlated with linear and rotational peak accelerations. The significant disparities in regional brain responses across head models regardless of the output variables strongly suggest that model-predicted brain responses from one study should not be extended to other studies in which a different model is utilized. Consequently, response-based injury tolerance thresholds from a specific model should not be generalized to other studies either in which a different model is used. However, the similar relationships between regional responses and the linear/rotational peak accelerations suggest that each FE model can be used independently to assess regional brain responses to impact simulations in order to perform statistical correlations with medical images and/or well-selected experiments with documented injury findings.

Development of a Finite Element Human Head Model Partially Validated With Thirty Five Experimental Cases

This study is aimed to develop a high quality, extensively validated finite element (FE) human head model for enhanced head injury prediction and prevention. The geometry of the model was based on computed tomography (CT) and magnetic resonance imaging scans of an adult male who has the average height and weight of an American. A feature-based multiblock technique was adopted to develop hexahedral brain meshes including the cerebrum, cerebellum, brainstem, corpus callosum, ventricles, and thalamus. Conventional meshing methods were used to create the bridging veins, cerebrospinal fluid, skull, facial bones, flesh, skin, and membranes-including falx, tentorium, pia, arachnoid, and dura. The head model has 270,552 elements in total. Thirty five loading cases were selected from a range of experimental head impacts to check the robustness of the model predictions based on responses including the brain pressure, relative skull-brain motion, skull response, and facial response. The brain pressure was validated against intracranial pressure data reported by Nahum et al. (1977, "Intracranial Pressure Dynamics During Head Impact," Proc. 21st Stapp Car Crash Conference, SAE Technical Paper No. 770922) and Trosseille et al. (1992, "Development of a F.E.M. of the Human Head According to a Specific Test Protocol," Proc. 36th Stapp Car Crash Conference, SAE Technical Paper No. 922527). The brain motion was validated against brain displacements under sagittal, coronal, and horizontal blunt impacts performed by Hardy et al. (2001, "Investigation of Head Injury Mechanisms Using Neutral Density Technology and High-Speed Biplanar X-Ray," Stapp Car Crash Journal, 45, pp. 337-368; and 2007, "A Study of the Response of the Human Cadaver Head to Impact," Stapp Car Crash Journal, 51, pp. 17-80). The facial bone responses were validated under nasal impact (Nyquist et al. 1986, "Facial Impact Tolerance and Response," Proc. 30th Stapp Car Crash Conference, SAE Technical Paper No. 861896), zygoma and maxilla impact (Allsop et al. 1988, "Facial Impact Response - A Comparison of the Hybrid III Dummy and Human Cadaver," Proc. 32nd Stapp Car Crash Conference, SAE Technical Paper No. 881719)]. The skull bones were validated under frontal angled impact, vertical impact, and occipital impact (Yoganandan et al. 1995, "Biomechanics of Skull Fracture," J Neurotrauma, 12(4), pp. 659-668) and frontal horizontal impact (Hodgson et al. 1970, "Fracture Behavior of the Skull Frontal Bone Against Cylindrical Surfaces," 14th Stapp Car Crash Conference, SAE International, Warrendale, PA). The FE head model was further used to study injury mechanisms and tolerances for brain contusion (Nahum et al. 1976, "An Experimental Model for Closed Head Impact Injury," 20th Stapp Car Crash Conference, SAE International, Warrendale, PA). Studies from 35 loading cases demonstrated that the FE head model could predict head responses which were comparable to experimental measurements in terms of pattern, peak values, or time histories. Furthermore, tissue-level injury tolerances were proposed. A maximum principal strain of 0.42% was adopted for skull cortical layer fracture and maximum principal stress of 20 MPa was used for skull diploë layer fracture. Additionally, a plastic strain threshold of 1.2% was used for facial bone fracture. For brain contusion, 277 kPa of brain pressure was calculated from reconstruction of one contusion case.

Development/global validation of a 6-month-old pediatric head finite element model and application in investigation of drop-induced infant head injury

Drop is a frequent cause for infant head injury. To date, finite element (FE) modeling was gradually used to investigate child head dynamic response under drop impact conditions, however, two shortages still exist on this topic: (1) due to ethical reasons, none of developed 6-month-old (6MO) head FE model was found to be quantitatively validated against child cadaver tests at similar age group; (2) drop height and impact surface stiffness effects on infant head responses were not comprehensively investigated. In this study, motivated by the recently published material properties of soft tissues (skull and suture, etc.) and reported pediatric head global cadaver tests, a 6MO child head FE model was developed and simulated results compared with the child cadaver experimental data under compression and drop conditions. Comparison of results indicated that the FE model showed a fairly good biofidelic behavior in most dynamic responses. The validated FE model was further used to investigate effects of different drop heights and impact surface stiffness on the head dynamic responses. Numerical results show that the pediatric head mechanical parameters (peak acceleration, HIC, maximal vonMises stress and maximal first principal strain of skull) keep increasing with the increase in drop height, and exhibit “logarithmic function” shapes at “fast–slow” trends with increase in impact surface stiffness. Based on above analysis, the regressions were conducted to describe the relationship between drop height and impact surface stiffness and head global injury predictors (head peak acceleration, HIC, etc.). This paper provides a fundamental study of child head injury mechanism and protection under drop conditions.

OP 11. Optimized tDCS electrode configurations for five targets determined via an inverse FE modeling approach

Introduction Transcranial direct current stimulation (tDCS) has shown potential in improving brain function in both healthy subjects and patients suffering from a wide range of neuropathologies. Unfortunately, the effects are too small and short-lived for tDCS to be used as a clinical therapy. Increasing the effect size of tDCS could possibly be achieved by better targeting the current, both in direction and amplitude. Volume conduction modeling has shown that the areas with the highest electric fields strengths do not, as is often assumed, lie beneath the electrodes (Datta et al., 2009). Attempts at optimization have been published for point and ring electrodes (Im et al., 2008; Dmochowski et al., 2011), but not for the square patches used in most labs. In order to find for these electrodes, configurations that do result in maximum stimulation at the target area, we propose an inverse modeling approach. Objectives We simulate tDCS for ∼7000 configurations and determine which of these lead to optimal electric fields, both in strength and direction, at the five most target locations in tDCS research: motor cortex (M1), dorsolateral prefrontal cortex, inferior frontal gyrus, occipital cortex and cerebellum. Methods A detailed finite element (FE) head model was made by automatic segmentation of MR images aided by manual corrections. The model contains over 4 million elements and 11 tissue types. Special attention was given to the skull, the main barrier for the tDCS current, by including the spongiosa layer and skull holes. Brain anisotropy was derived from DTI measurements. On the skin surface, a grid of 89 points was placed consisting of the standard 10–10 EEG system and extra points on the cheeks and neck. For each combination of 2 points, we placed 5×5cm electrode patches onto the model, centred on the two points, and simulated 1mA tDCS. At each target, we placed a cylindrical volume in the brain and selected the configurations leading to highest mean field strength or optimal direction in the target volume. Results For all targets, the optimized configurations did not include the commonly used configurations. Often, highest field strengths were found in configurations that are near-perpendicular to the standard configurations. Optimization based on either strength or direction of the field lead to completely different configurations. Conclusion The optimized configurations found in this study suggest that improved results of tDCS can be expected. We found different optimized configurations by looking at either strength or direction of the field. Comparing these optimized configurations experimentally will not only verify our modeling approach, but also provide valuable information on the mechanisms behind tDCS. This approach can be used to optimize stimulation for any target location.

Computational modeling of human head under blast in confined and open spaces: primary blast injury

In this paper, a computational modeling for biomechanical analysis of primary blast injuries is presented. The responses of the brain in terms of mechanical parameters under different blast spaces including open, semi-confined, and confined environments are studied. In the study, the effect of direct and indirect blast waves from the neighboring walls in the confined environments will be taken into consideration. A 50th percentile finite element head model is exposed to blast waves of different intensities. In the open space, the head experiences a sudden intracranial pressure (ICP) change, which vanishes in a matter of a few milliseconds. The situation is similar in semi-confined space, but in the confined space, the reflections from the walls will create a number of subsequent peaks in ICP with a longer duration. The analysis procedure is based on a simultaneous interaction simulation of the deformable head and its components with the blast wave propagations. It is concluded that compared with the open and semi-confined space settings, the walls in the confined space scenario enhance the risk of primary blast injuries considerably because of indirect blast waves transferring a larger amount of damaging energy to the head.

Anisotropic composite human skull model and skull fracture validation against temporo-parietal skull fracture

A composite material model for skull, taking into account damage is implemented in the Strasbourg University finite element head model (SUFEHM) in order to enhance the existing skull mechanical constitutive law. The skull behavior is validated in terms of fracture patterns and contact forces by reconstructing 15 experimental cases. The new SUFEHM skull model is capable of reproducing skull fracture precisely. The composite skull model is validated not only for maximum forces, but also for lateral impact against actual force time curves from PMHS for the first time. Skull strain energy is found to be a pertinent parameter to predict the skull fracture and based on statistical (binary logistical regression) analysis it is observed that 50% risk of skull fracture occurred at skull strain energy of 544.0mJ.

The Progress of Finite Element Analysis on Internal Combustion Engine Cylinder Head Assembly

The cylinder head is one of the most important and complex parts in the internal combustion engine. In this paper, in view of three-dimensional finite element applications covered in the cylinder head, it mainly discusses the development trend both at home and abroad from the cylinder head finite element model establishment, cylinder head boundary conditions determination and cylinder head finite element computation, which application of three-dimensional finite element in study of cylinder head on temperature field simulation, the stress analysis, the machinery load and hot load, as well as the coupling analysis of multi-physical fields and multi-parts.

Surface material effects on fall-induced paediatric head injuries: a combined approach of testing, modelling and optimisation

The mechanism and tolerance of paediatric head injuries are not well established mainly due to the limited cadaveric tests available. Weber's studies [Experimental studies of skull fractures in infants, Z Rechtsmed. 92 (1984), pp. 87–94; Biomechanical fragility of the infant skull, Z Rechtsmed. 94 (1985), pp. 93–101] in mid 1980s contained the most extensive paediatric cadaver test data under various fall conditions in the literature. However, the limited injury measurements and the unknown material properties of the impact surfaces in Weber's tests limited their application in paediatric fall protection. In the present study, drop tests, inverse finite element modelling and optimisation were first conducted to quantify the material characteristics of four impact surfaces (carpet, vinyl, foam and blanket) used in the Weber's tests. With the impact surface material determined, five cadaver tests from Weber's studies were reconstructed using a parametric paediatric head finite element model. Results showed that the simulated strain and stress distributions on the skulls correlated well with the fracture patterns reported in the cadaver tests. The impact surface material properties developed in this study and the methods of using the parametric paediatric head model to reconstruct the cadaver tests provided valuable information for future ground surface designs for child fall protection and development of paediatric head injury criteria.

Development of a 3-year-old child head–neck finite element model and derivation of novel head injury criterion

Children are increasingly transported in cars or other modes of road transportation. With this increased travel comes the higher risk of children becoming involved in a vehicle accident as an occupant. Among the injuries included in the Abbreviated Injury Scale (AIS), the head is one of the most frequently injured parts of the body. For a 3-year-old child (3-YOC), the most common injury observed during a road accident is skull fracture. This study proposed to develop a 3-YOC head finite element model (FEM) based on the geometrical three-dimensional reconstruction of two-dimensional slices obtained using X-ray computed tomography scanning geometries. In order to reproduce the skull fracture and brain injury with the head FEM, 13 domestic accident reconstructions involving fall cases of 3-YOC were collected from the paediatric emergency departments of different hospitals. After the numerical reconstructions of these fall cases, some mechanical parameters were extracted and then correlated with the observed injuries.

Finite element head model simulation and head injury prediction

Head injury is one of the most frequent causes of death and impairment sustained by pedestrians, cyclists, motorcyclists and vehicle occupants in road accidents, and accounts for approximately half...

Methodology to determine skull bone and brain responses from ballistic helmet-to-head contact loading using experiments and finite element analysis

The objective of the study was to obtain helmet-to-head contact forces from experiments, use a human head finite element model to determine regional responses, and compare outputs to skull fracture and brain injury thresholds. Tests were conducted using two types of helmets (A and B) fitted to a head-form. Seven load cells were used on the head-form back face to measure helmet-to-head contact forces. Projectiles were fired in frontal, left, right, and rear directions. Three tests were conducted with each helmet in each direction. Individual and summated force- and impulse-histories were obtained. Force-histories were inputted to the human head–helmet finite element model. Pulse durations were approximately 4ms. One-third force and impulse were from the central load cell. 0.2% strain and 40MPa stress limits were not exceeded for helmet-A. For helmet-B, strains exceeded in left, right, and rear; pressures exceeded in bilateral directions; volume of elements exceeding 0.2% strains correlated with the central load cell forces. For helmet-A, volumes exceeding brain pressure threshold were: 5–93%. All elements crossed the pressure limit for helmet-B. For both helmets, no brain elements exceeded peak principal strain limit. These findings advance our understanding of skull and brain biomechanics from helmet–head contact forces.

In-depth real-world bicycle accident reconstructions

Use of bicycles on a large scale, encouraged in the context to develop an eco-friendly environment, is facing today a range of barriers. One of these barriers identified by researchers and governments is observed to include ‘road safety’. Hence, it is necessary to set up a protection system for bicyclists especially for the cephalic segment. Currently only few studies are available concerning the head impact loading in case of real accidents. Therefore, the objective of this work is to improve the knowledge of bicyclist head trauma by following a methodology to reconstruct real-world accidents. A step is to identify the initial condition of head impact in case of real accidents. Head impact velocity and head impact area are extracted and implemented in the last generation of head injury prediction tool to simulate the head trauma by impacting directly the Strasbourg University Finite Element Head Model (SUFEHM) on the vehicle structures. The present study can be divided into three activities, i.e. obtain real bicyclist accidents' data issued from in-depth accident investigation databases, reconstruct cyclist body kinematics to obtain the initial conditions of the head just before the impact, and simulate head impact to evaluate the head loading during impact and the injury risk. A total of 24 bicyclists’ accident cases with head injuries have been selected from both French and German accident databases. For each accident case, body kinematics has been simulated using Madymo® software. Two human multi-body models were used: 8 accident cases have been reconstructed by IFSTTAR using its owned developed human model and 18 accident cases have been reconstructed by Unistra using the human pedestrian TNO model. The results show that head is impacted more often on top parietal zone, and the mean impact velocity is 6.8 ± 2.7 m/s with 5.5 ± 3.0 m/s and 3.4 ± 2.1 m/s for normal and tangential components, respectively. Among these real accidents, 19 cases were selected for finite element computations by coupling the human head model and a windscreen model whose properties were extracted from literature. All reconstructed head impact gave results in accordance with the damage actually incurred to the victims.

The validation and application of a finite element human head model for frontal skull fracture analysis

Traumatic head injuries can result from vehicular accidents, sports, falls or assaults. The current advances in computational methods and the detailed finite element models of the human head provide a significant opportunity for biomechanical study of human head injuries. The biomechanical characteristics of the human head through head impact scenarios can be studied in detail by using the finite element models. Skull fracture is one of the most frequent occurring types of head injuries. The purpose of this study is to analyse the experimental head impacts on cadavers by means of the Strasbourg University Finite Element Head Model (SUFEHM). The results of the numerical model and experimental data are compared for validation purpose. The finite element model has also been applied to predict the skull bone fracture in frontal impacts. The head model includes the scalp, the facial bone, the skull, the cerebral spinal fluid, the meninges, the cerebrum and the cerebellum. The model is used to simulate the experimental frontal head impact tests using a cylindrical padded impactor. Results of the computational simulation shows that the model correlated well with a number of experimental data and a global fracture pattern has been predicted well by the model. Therefore the presented numerical model could be used for reconstruction of head impacts in different impact conditions also the forensic application of the head model would provide a tool for investigation of the causes and mechanism of head injuries.