Field and analytic observations of impact brain injury in fatally injured pedestrians.

Abstract

To develop more effective head protection against impact injury, maximum 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 simplified model of the head-vehicle impact for pedestrians we were able to make estimates of the peak angular acceleration and change in angular velocity of head impacts for fatally injured pedestrians. This model also enabled us to examine the relationship between the parameters of the impact, and the critical strain curves for brain injury proposed by Margulies and Thibault (1992). It was found that the offset of the impact from the center of mass of the head was a major influence, and, in addition, in impacts with a combined head/vehicle stiffness above 130 kN/m, the head impact velocity and change in head angular velocity were important, whereas for impacts with lower stiffness, the stiffness of the impact structure and hence, peak angular acceleration, were the major influences. Transformed into the frequency domain, the 130 kN/m region corresponds roughly to a harmonic of the natural frequency of the brain and skull, and the change in behavior may be related to 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 callosum 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 sufficient consistency between the field observations of brain injury and the analytic findings to suggest that the 10% critical strain curve represents a threshold for brain injury, expressed in terms of peak angular acceleration and change in angular velocity.