A method of estimating linear and angular accelerations in head impacts to pedestrians

In order to investigate the relationship between impact to the head and brain injury, we have developed a method, using information obtained from reconstruction of the collisions, of estimating the peak linear and angular accelerations of the head for pedestrian impacts on a vehicle. This information includes the location of the impact on the head, the impact velocity of the head, and the stiffness of the struck surface. In developing the method we assumed that the velocity of the head on striking the vehicle was the same as the velocity of the vehicle itself, that the force vector was normal to the surface of the skull, that the force-deflection curve characterising the combined response of the impacted surfaces was linear, and that the kinetic energy of the head immediately prior to impact was converted into strain energy in deforming the head and the vehicle structure. Only the loading phase of the impact was considered, there was no assumption of an elastic unloading phase. Using cadavers, the validity of these assumptions and hence the usefulness of the method were tested by comparing the estimates of peak linear acceleration with the results of 18 pedestrian-vehicle impact reconstructions. On average, the method underestimated the experimental values by about 15%, with a range of +/- 35%. The results from the application of this method are currently being used to study the relationship between the magnitude and direction of the impact to the head and the distribution and severity of the brain injury resulting from actual collisions.