The aim of this study is to evaluate all the 58 available NFL cases and compare various predictors for mild traumatic brain injuries using a detailed and extensively validated finite element model of the human head. Global injury measures such as magnitude in angular and translational acceleration, change in angular velocity, head impact power (HIP) and HIC were also investigated with regard to their ability to predict the intracranial pressure and strains associated with injury. The brain material properties were modeled using a hyperelastic and viscoelastic constitutive law. Also, three different stiffness parameters, encompassing a range of published brain tissue properties, were tested. 8 tissue injury predictors were evaluated for 6 different regions, covering the entire cerebrum, as well as for the whole brain. In addition, 10 head kinematics based predictors were evaluated both for correlation with injury as well as with strain and pressure. When evaluating the results, a statistical correlation between strain, strain rate, product of strain and strain rate, Cumulative Strain Damage Measure (CSDM), strain energy density, maximum pressure, magnitude of minimum pressure, as well as von Mises effective stress, with injury was found when looking into specific regions of the brain. However, the maximal pressure in the gray matter showed a higher correlation with injury than other evaluated measures. On the other hand, it was possible, through the reconstruction of a motocross accident, to re-create the injury pattern in the brain of the injured rider using maximal principal strain. It was also found that a simple linear combination of peak change in rotational velocity and HIC showed a high correlation (R=0.98) with the maximum principal strain in the brain, in addition to being a significant predictor of injury. When applying the rotational and translational kinematics separately for one of the cases, it was found that the translational kinematics contribute very little to the intracranial distortional strains while the rotational kinematics contributes insignificantly to the pressure response. This study underlines that the strain based brain tissue injury predictors are very sensitive to the choice of stiffness for the brain tissue.

Predictors for traumatic brain injuries evaluated through accident reconstructions.

A new 3 dimensional finite element representation of the human head complex has been constructed for simulating the transient occurrences of simple pedestrian accidents. This paper describes the development, features and validation of that model. When constructing the model, emphasis was placed on element quality and ease of mesh generation. As such, a number of variations of the model were created. The model was validated against a series of cadaveric impact tests. A parametric study (a High/Low study) was performed to investigate the effect of the bulk and shear modulus of the brain and cerebrospinal fluid (CSF). The influence of different mesh densities on the models and the use of different element formulations for the skull were also investigated. It was found that the short-term shear modulus of the neural tissue had the predominant effect on intracranial frontal pressure, and on the predicted Von-Mises response. The bulk modulus of the fluid had a significant effect on the contre-coup pressure when the CSF was modelled using a coupled node definition. Differences of intracranial pressure were reported that show the sensitivity of the method by which the skull is modelled. By simulating an identical impact scenario with a range of different finite element models it has been possible to investigate the influence of model topologies. We can conclude that careful modelling of the CSF (depth/volume) and skull thickness (including cortical/ trabecular ratio) is necessary if the correct intracranial pressure distribution is to be predicted, and so further forms of validation are required to improve the finite element models' injury prediction capabilities.

The creation of three-dimensional finite element models for simulating head impact biomechanics

Traumatic brain injury (TBI) occurs when local mechanical load exceeds certain tolerance levels for brain tissue. Extensive research has been done previously for brain matter experiencing compression at quasistatic loading; however, limited data is available to model TBI under dynamic impact conditions. In this research, an experimental setup was developed to perform unconfined compression tests and stress relaxation tests at strain rates≤90/s. The brain tissue showed a stiffer response with increasing strain rates, showing that hyperelastic models are not adequate. Specifically, the compressive nominal stress at 30% strain was 8.83±1.94, 12.8±3.10 and 16.0±1.41 kPa (mean±SD) at strain rates of 30, 60 and 90/s, respectively. Relaxation tests were also conducted at 10%- 50% strain with the average rise time of 10 ms, which can be used to derive time dependent parameters. Numerical simulations were performed using one-term Ogden model with initial shear modulusµ o =6.06±1.44, 9.44±2.427 and 12.64±1.227 kPa (mean±SD) at strain rates of 30, 60 and 90/s, respectively. A separate set of bonded and lubricated tests were also performed under the same test conditions to estimate the friction coefficient µ, by adopting combined experimental-computational approach. The values ofµwere 0.1±0.03 and 0.15±0.07 (mean±SD) at 30 and 90/s strain rates, respectively, indicating that pure slip conditions cannot be achieved in unconfined compression tests even under fully lubricated test conditions. The material parameters obtained in this study will help to develop biofidelic human brainfinite element models, which can subsequently be used to predict brain injuries under impact conditions. c

http://www.maths.nuigalway.ie/~destrade/Publis/destrade_90.pdf

Mechanical characterization of brain tissue in compression at dynamic strain rates.

This study is aimed to develop a next-generation, 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 CT and MRI scans of an adult male. A new feature-based multi-block 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 (CSF), skull, facial bones, flesh, skin, and membranes - including falx, tentorium, pia, arachnoid, and dura. The head model has 270,552 elements in total. A total of 49 loading cases were selected from a range of experimental and real world head impacts to check the robustness of the model predictions based on responses including the brain pressure, relative skull-brain motion, intracranial strain, skull response, facial response, and bridging vein elongation. The brain pressure was validated against intracranial pressure data reported by Nahum et al. (1977) and Trosseille et al. (1992). The brain motion was validated against brain displacements under sagittal, coronal, and horizontal blunt impacts performed by Hardy et al. (2001, 2007). The facial bone responses were validated under nasal impact (Nyquist et al., 1986), zygoma and maxilla impact (Allsop et al., 1988). The skull bones were validated under frontal angled impact, vertical impact, and occipital impact (Yoganandan et al., 1995) and frontal horizontal impact (Hodgson et al., 1970). The FE head model was further used to study injury mechanisms and tolerances for brain contusion (Nahum et al., 1976), bridging vein rupture (Depreitere et al., 2006), and brain strains for real-world brain injury cases (Franklyn et al. 2005). Studies from 49 loading cases demonstrated that the FE head model had good biofidelity in predicting head responses under various impact scenarios. 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 diploe layer fracture. Additionally, a plastic strain threshold of 1.2% was used for facial bone fracture. Average of 17% of engineering tensile strain indicates bridging vein rupture. For brain contusion, 277 kPa of brain pressure was calculated from reconstruction of one contusion case. Lastly, the high strains predicted by the FE head model match the trend of brain injuries reported in four real-world cases. Language: en

Development of a finite element human head model partially validated with thirty five experimental cases.

High-speed biplane x-ray and neutral density targets were used to examine brain displacement and deformation during impact Relative motion, maximum principal strain, maximum shear strain, and intracranial pressure were measured in thirty-five impacts using eight human cadaver head and neck specimens The effect of a helmet was evaluated During impact, local brain tissue tends to keep its position and shape with respect to the inertial frame, resulting in relative motion between the brain and skull and deformation of the brain The local brain motions tend to follow looping patterns Similar patterns are observed for impact in different planes, with some degree of posterior-anterior and right-left symmetry Peak coup pressure and pressure rate increase with increasing linear acceleration, but coup pressure pulse duration decreases Peak average maximum principal strain and maximum shear are on the order of 009 for CFC 60 Hz data for these tests Peak average maximum principal strain and maximum shear increase with increasing linear acceleration, coup pressure, and coup pressure rate Linear and angular acceleration of the head are reduced with use of a helmet, but strain increases These results can be used for the validation of finite element models of the human head

A Study of the Response of the Human Cadaver Head to Impact

In order to create a useful computational tool that will aid in the understanding and perhaps prevention of head injury, it is important to know the quantitative influence of the constitutive properties, geometry and model formulations of the intracranial contents upon the mechanics of a head impact event. The University College Dublin Brain Trauma Model (UCDBTM) [1] has been refined and validated against a series of cadaver tests and the influence of different model formulations has been investigated. In total six different model configurations were constructed: (i) the baseline model, (ii) a refined baseline model which explicitly differentiates between grey and white neural tissue, (iii) a model with three elements through the thickness of the cerebrospinal fluid (CSF) layer, (iv) a model simulating a sliding boundary, (v) a projection mesh model (which also distinguishes between neural tissue) and (vi) a morphed model. These models have been compared against cadaver tests of Trosseille [2] an...

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Influence of FE model variability in predicting brain motion and intracranial pressure changes in head impact simulations

A two-dimensional plane strain finite element representation of the head was constructed and incorporated a single layered skull, cerebrospinal fluid layer, and the brain The model that was taken in an off-centre, mid-sagittal plane in an anterior-posterior direction was used to investigate the dynamic response of the human head when subject to direct translational impact events The physiological consequences of modelling a viscoelastic brain instead of an elastic brain were established The influence of the stiffness of the brain matter upon the response of the head and the effect of different impact sites were also modelled The foramen magnum was shown to have a direct influence upon the magnitude of the strains predicted to develop within the intracranial contents The first resonant natural frequency of the head was predicted by the model as being 126 Hz The mode shape associated with this first resonant frequency indicated that the brain mass would rotate around a point that was located 

/pdf/a-two-dimensional-analysis-of-the-biomechanics-of-frontal-2jzxez6fvy.pdf

TJ Horgan

Papers

The creation of three-dimensional finite element models for simulating head impact biomechanics

Influence of FE model variability in predicting brain motion and intracranial pressure changes in head impact simulations

A two-dimensional analysis of the biomechanics of frontal and occipital head impact injuries

Biomechanics of frontal and occipital head impact injuries: a two-dimensional study