Svein Kleiven

Consequences of head size following trauma to the human head.

The objective of the present study was to evaluate whether variation of human head size results in different outcome regarding intracranial responses following a direct impact. Finite Element models representing different head sizes and with various element mesh densities were created. Frontal impacts towards padded surfaces as well as inertial loads were analyzed. The variation in intracranial stresses and intracranial pressures for different sizes of the geometry and for various element meshes were investigated. A significant correlation was found between experiment and simulation with regard to intracranial pressure characteristics. The maximal effective stresses in the brain increased more than a fourfold, from 3.6kPa for the smallest head size to 16.3kPa for the largest head size using the same acceleration impulse. When simulating a frontal impact towards a padding, the head injury criterion (HIC) value varies from the highest level of 2433 at a head mass of 2.34kg to the lowest level of 1376 at a head mass of 5.98kg, contradicting the increase in maximal intracranial stresses with head size. The conclusion is that the size dependence of the intracranial stresses associated with injury, is not predicted by the HIC. It is suggested that variations in head size should be considered when developing new head injury criteria.

Evaluation of head injury criteria using a finite element model validated against experiments on localized brain motion, intracerebral acceleration, and intracranial pressure

Abstract The objective of the present study was to analyze the effect of different load directions and durations following impact using a finite element (FE) model of the human head. A detailed FE model of the human head was developed and validated against available cadaver experiment data for three impact directions (frontal, occipital, and lateral). Loads corresponding to the same impact power were imposed in different directions. Furthermore, the head injury criterion (HIC), the recently proposed head impact power (HIP) criterion, as well as peak angular acceleration, and change in angular and translational velocity were evaluated with respect to the strain in the central nervous system (CNS) tissue. A significant correlation was found between experiments and simulations with regard to intracranial pressure data for a short-duration impulse and intracerebral acceleration characteristics for a long-duration impulse with a high-angular component. However, a poor correlation with the simulations was found for the intracranial pressures for the long-duration impulse. This is thought to be a result of air introduced to the intracranial cavity during experimental testing. Smaller relative motion between the brain and skull results from lateral impact than from a frontal or occipital blow for both the experiments and FE simulations. It was found that the influence of impact direction had a substantial effect on the intracranial response. When evaluating the global kinematic injury measures for the rotational pulses, the change in angular velocity corresponded best with the intracranial strains found in the FE model. For the translational impulse, on the other hand, the HIC and the HIP showed the best correlation with the strain levels found in the model.

Influence of Impact Direction on the Human Head in Prediction of Subdural Hematoma

The objective of the present study was to analyze the effect of different loading directions following impact, and to evaluate existing global head injury criteria. Detailed and parameterized models of the adult human head were created by using the Finite Element Method (FEM). Loads corresponding to the same impact power were imposed in different directions. Furthermore, the Head Injury Criterion (HIC) and the recently proposed Head Impact Power (HIP) criterion were evaluated with respect to the relative motion between the skull and the brain, as well as the strain in the bridging veins. It was found that the influence of impact direction had a substantial effect on the intracranial response. The largest relative skull-brain motion and strain in the bridging veins occurred with the anterior-posterior (AP) and posterior-anterior (PA) rotational impulses. HIC was unable to predict consequences of a pure rotational impulse while HIP needed individual scaling coefficients for the different terms to account for difference in load direction. When using the proposed scaling procedure, a better prediction of subdural hematoma (SDH) was obtained. It is thus suggested that an evaluation of the synergistic terms is necessary to further improve the injury prediction. These variations should be considered when developing new head injury criteria.

The epidemiology of head injuries in Sweden from 1987 to 2000

The purpose of the present study was to evaluate the variability in the annual head injury incidence rate in Sweden from 1987 to 2000. It was hypothesized that the annual incidence rate would decrease over time due to a variety of primary preventive strategies that have been introduced in Swedish society. We used the Hospital Discharge Register at the National Board for Health and Welfare and head injury codes 800-804, and 850-854 from ICD9 system and S2.0-S2.9, and S6.0-S6.9 codes from ICD-10 system. We evaluated the patterns of age, gender, external cause of injury (E-code), type of injury, length of hospital stay, and trends over time. Head injuries due to transportation collision were reduced over the 14-year period analysis. Falls persisted as the dominant cause of head injury. Overall, men had 2.1 times the incidence of head injury compared to women. There was a decline in younger ages experiencing a head injury over this interval, while the number of head injuries among elderly people increased over time. Concussion was about three times more frequent than fractures. Hematoma and diffuse or focal contusions had a much lower incidence rate than concussion. Concussions and fractures decreased over time. Diffuse or focal injuries showed a steady rate of occurrence over the study interval while hematoma increased. Although length of hospital stay varied widely from zero to more than 50 days, 73.6% of hospital days were confined to two days or less. The incidence rate is stable over this time frame. While head injuries attributable to transportation accidents decreased, falls made up an increasing proportion of head injuries. Since we observed an increase in head injuries among elderly, primary prevention strategies may need to be targeted at this age group, and at preventing falls.

Can sulci protect the brain from traumatic injury

The influence of sulci in dynamic finite element simulations of the human head has been investigated. First, a detailed 3D FE model was constructed based on an MRI scan of a human head. A second model with a smoothed brain surface was created based on the same MRI scan as the first FE model. These models were validated against experimental data to confirm their human-like dynamic responses during impact. The validated FE models were subjected to several acceleration impulses and the maximum principle strain and strain rate in the brain were analyzed. The results suggested that the inclusion of sulci should be considered for future FE head models as it alters the strain and strain distribution in an FE model.

Multi-scale mechanics of traumatic brain injury: predicting axonal strains from head loads

The length scales involved in the development of diffuse axonal injury typically range from the head level (i.e., mechanical loading) to the cellular level. The parts of the brain that are vulnerable to this type of injury are mainly the brainstem and the corpus callosum, which are regions with highly anisotropically oriented axons. Within these parts, discrete axonal injuries occur mainly where the axons have to deviate from their main course due to the presence of an inclusion. The aim of this study is to predict axonal strains as a result of a mechanical load at the macroscopic head level. For this, a multi-scale finite element approach is adopted, in which a macro-level head model and a micro-level critical volume element are coupled. The results show that the axonal strains cannot be trivially correlated to the tissue strain without taking into account the axonal orientations, which indicates that the heterogeneities at the cellular level play an important role in brain injury and reliable predictions thereof. In addition to the multi-scale approach, it is shown that a novel anisotropic equivalent strain measure can be used to assess these micro-scale effects from head-level simulations only.

Micromechanics of diffuse axonal injury : influence of axonal orientation and anisotropy

Multiple length scales are involved in the development of traumatic brain injury, where the global mechanics of the head level are responsible for local physiological impairment of brain cells. In this study, a relation between the mechanical state at the tissue level and the cellular level is established. A model has been developed that is based on pathological observations of local axonal injury. The model contains axons surrounding an obstacle (e.g., a blood vessel or a brain soma). The axons, which are described by an anisotropic fiber-reinforced material model, have several physically different orientations. The results of the simulations reveal axonal strains being higher than the applied maximum principal tissue strain. For anisotropic brain tissue with a relatively stiff inclusion, the relative logarithmic strain increase is above 60%. Furthermore, it is concluded that individual axons oriented away from the main axonal direction at a specific site can be subjected to even higher axonal strains in a stress-driven process, e.g., invoked by inertial forces in the brain. These axons can have a logarithmic strain of about 2.5 times the maximum logarithmic strain of the axons in the main axonal direction over the complete range of loading directions. The results indicate that cellular level heterogeneities have an important influence on the axonal strain, leading to an orientation and location-dependent sensitivity of the tissue to mechanical loads. Therefore, these effects should be accounted for in injury assessments relying on finite element head models.

Sports helmets now and in the future

The paper reports on a symposium on sports helmets and presents a synthesis of information and opinion from a range of presenters and disciplines. A review of the literature shows that helmets play an important role in head injury prevention and control. Helmets have been shown to be very efficacious and effective in a range of sports and in preventing specific head injury risks, especially moderate to severe head injury. The symposium emphasised the importance of helmet standards and the need for further development. There are calls for helmets that address the needs of competitive (elite) athletes separate to helmets for recreational athletes. Deficiencies in the evidence base for head injury risks and helmet efficacy and effectiveness were identified in some sports. Issues in designing helmets that are suitable to prevent severe head injuries and concussion were discussed and explained from biomechanical and engineering perspectives. The need to evaluate helmet performance in oblique impacts and incorporate this into standards was covered in a number of presentations. There are emerging opportunities with in-helmet technology to improve impact performance or to measure impact exposure. In-helmet technology as it matures may provide critical information on the severity of the impact, the location of the injured athlete, for example, snowboarder, and assist in the retrieval and immediate, as well as the long-term medical management of the athlete. It was identified that athletes, families and sports organisations can benefit from access to information on helmet performance. The importance of selecting the appropriate-sized helmet and ensuring that the helmet and visor were adjusted and restrained optimally was emphasised. The translation pathway from the science to new and better helmets is the development of appropriate helmet standards and the requirement for only helmets to be used that are certified to those standards.

The influence of anisotropy on brain injury prediction

Traumatic Brain Injury (TBI) occurs when a mechanical insult produces damage to the brain and disrupts its normal function. Numerical head models are often used as tools to analyze TBIs and to measure injury based on mechanical parameters. However, the reliability of such models depends on the incorporation of an appropriate level of structural detail and accurate representation of the material behavior. Since recent studies have shown that several brain regions are characterized by a marked anisotropy, constitutive equations should account for the orientation-dependence within the brain. Nevertheless, in most of the current models brain tissue is considered as completely isotropic. To study the influence of the anisotropy on the mechanical response of the brain, a head model that incorporates the orientation of neural fibers is used and compared with a fully isotropic model. A simulation of a concussive impact based on a sport accident illustrates that significantly lowered strains in the axonal direction as well as increased maximum principal strains are detected for anisotropic regions of the brain. Thus, the orientation-dependence strongly affects the response of the brain tissue. When anisotropy of the whole brain is taken into account, deformation spreads out and white matter is particularly affected. The introduction of local axonal orientations and fiber distribution into the material model is crucial to reliably address the strains occurring during an impact and should be considered in numerical head models for potentially more accurate predictions of brain injury.

Injury tolerances for oblique impact helmet testing

The most frequently sustained severe injuries in motorcycle crashes are injuries to the head, and many of these are caused by rotational force. Rotational force is most commonly the result of oblique impacts to the head. Good testing methods for evaluating the effects of such impacts are currently lacking. There is also a need for improving our understanding of the effects of oblique impacts on the human head. Helmet standards currently in use today do not measure rotational effects in test dummy heads. However rotational force to the head results in large shear strains arising in the brain, which has been proposed as a cause of traumatic brain injuries like diffuse axonal injuries (DAI). This paper investigates a number of well-defined impacts, simulated using a detailed finite element (FE) model of the human head, an FE model of the Hybrid III dummy head and an FE model of a helmet. The same simulations were performed on both the FE human head model and the FE Hybrid III head model, both fitted with helmets. Simulations on both these heads were performed to describe the relationship between load levels in the FE Hybrid III head model and strains in the brain tissue in the FE human head model. In this study, the change in rotational velocity and the head injury criterion (HIC) value were chosen as appropriate measurements. It was concluded that both rotational and translational effects are important when predicting the strain levels in the human brain.