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.

A parameterized, or scalable, finite element (FE) model of the human head was developed and validated against the available cadaver experiment data for three impact directions (frontal, occipital and lateral). The brain material properties were modeled using a hyperelastic and viscoelastic constitutive law. The interface between the skull and the brain was modeled in three different ways ranging from purely tied (no-slip) to sliding (free-slip). Two sliding contact definitions were compared with the tied condition. Also, three different stiffness parameters, encompassing the range of published brain tissue properties, were tested. The model using the tied contact definition correlated well with the experimental results for the coup and contrecoup pressures in a frontal impact while the sliding interface models did not. Relative motion between the skull and the brain in lowseverity impacts appears to be relatively insensitive to the contact definitions. It is shown that a range of shear stiffness properties for the brain can be used to model the pressure experiments, while relative motion is a more complex measure that is highly sensitive to the brain tissue properties. 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. The material properties of brain tissue are important to the characteristics of relative brain-skull motion. The results suggest that significantly lower values of the shear properties of the human brain than currently used in most three-dimensional (3D) FE models today are needed to predict the localized brain response of an impact to the human head.

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.

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 3.6 kPa for the smallest head size to 16.3 kPa 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 2.34 kg to the lowest level of 1376 at a head mass of 5.98 kg, 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.

Detailed and parameterized models of the adult human head were created using the finite element method. Different sizes of the brain and the subdural space were generated, and the models were impacted toward padded surfaces in the frontal, temporal, and occipital direction. The present results show for the first time that, by reducing the brain size and thereby increasing the volume in the subdural space in the finite element model, a significant increase in relative motion was found between the skull and brain which correlated with the reduction of brain size.