FIELD AND ANALYTIC OBSERVATIONS OF IMPACT BRAIN INJURY IN FATALLY INJURED PEDESTRIANS

To develop more effective head protection against impact injury, maxim um levels of mechanical impact or injury tolerance criteria, or both, should be specified for particular levels of injury and for particular structures of the brain, By using a development of an existing very s implified model of the head-vehicle impact for pedestrians we were abl e to make estimates of the peak angular acceleration and change in ang ular velocity of head impacts for fatally injured pedestrians, This mo del also enabled us to examine the relationship between the parameters of the impact, and the critical strain curves for brain injury propos ed by Margulies and Thibault (1992). It was found that the offset of t he impact from the center of mass of the head was a major influence, a nd, in addition, in impacts with a combined head/vehicle stiffness abo ve 130 kN/m, the head impact velocity and change in head angular veloc ity were important, whereas for impacts with lower stiffness, the stif fness of the impact structure and hence, peak angular acceleration, we re the major influences, Transformed into the frequency domain, the 13 0 kN/m region corresponds roughly to a harmonic of the natural frequen cy of the brain and skull, and the change in behavior may be related t o decoupling of the skull and brain at impact, In 12 cases of lateral head impact, all but one case with visible injury in the corpus callos um were found to lie close to or above the 10% critical strain curve, Despite the very wide error limits around each data point, there is su fficient consistency between the field observations of brain injury an d the analytic findings to suggest that the 10% critical strain curve represents a threshold for brain injury, expressed in terms of peak an gular acceleration and change in angular velocity.