Prevention of Head Injuries Focusing Specifically on Oblique Impacts

The massive number of injuries sustained in traffic accidents is a growing problem worldwide, especially in developing countries. In 1998, more than one million people were killed in traffic accidents worldwide, while about ten times as many people were injured. Injuries to the central nervous system and in particular to the head are especially critical to human life. This thesis contains five research papers looking at head injuries and head protection, proposing new and more efficient ways of protecting the head, especially in traffic accidents. In order to define the national dimensions of the patterns of injuries incurred in motorcycle and moped accidents in Sweden, a statistical survey was performed on data spanning a 13-year period (Paper A). In Sweden, 27,100 individuals received in-patient care for motorcycle and moped accident injuries between 1987 and 1999. The motorcycle and moped injury rate reduced in the second half of the study period, so too were the total number of days of treatment per year. Males had eight times the incidence of injuries of females. Head injuries were the single most frequent diagnosis, followed by fractures of the lower limbs. Concussion was the most frequent head injury. These statistics clearly show the need for better head injury prevention systems. According to the statistics, the most common type of impact to the head in motorcycle and moped accidents is an oblique impact. Oblique impacts generate rotations of the head, which are a common cause of the most severe head injuries. Therefore a new test rig was constructed to reproduce oblique impacts to a helmeted dummy head, simulating those occurring in real life accidents (Paper B). The new test rig was shown to provide useful data at speeds of up to 50 km/h and with impact angles varying from purely tangential to purely radial. This innovative test rig appears to provide an accurate method for measuring accelerations in oblique impacts to helmets. When testing the performances of motorcycle helmets, discrepancies are usually seen in the test results. In order to evaluate these discrepancies, the finite element method (FEM) was used for simulations of a few oblique helmet impacts (Paper C). Among the parameters studied, the coefficients of friction between the impacting surface and the helmet and between the head and the helmet had the most significant influence on the rotational accelerations. Additionally, a thinner and consequently also weaker shell and a weaker liner, provided better protection for the impacts studied. Since there are no generally accepted global injury thresholds for oblique impacts to the human head, a study was designed to propose new injury tolerances accounting for both translations and rotations of the head (Paper D). In that study, FE models of (a) a human head, (b) a Hybrid III dummy head, and (c) the experimental helmet were used. Different criteria were proposed for different impact scenarios. Both the translational and the rotational effects were found to be important when proposing a predictor equation for the strain levels experienced by the human brain in simulated impacts to the head. In order to reduce the level of head injuries in society and to better understand helmet impacts from different aspect, a ballistic impact was also studied (Paper E). The effects of different helmet shell stiffness and different angles of impacts were simulated. In this study, the same FE head model from Paper D was used, however here it was protected with a model of a composite ballistic helmet. It was concluded that the helmet shell should be stiff enough to prevent the inside of the shell from striking the skull, and that the strains arising in the brain tissue were higher for some oblique impacts than for purely radial ones.