Dynamic biomechanics of the human head in lateral impacts.

The biomechanical responses of human head (translational head CG accelerations, rotational head accelerations, and HIC) under lateral impact to the parietal-temporal region were investigated in the current study. Free drop tests were conducted at impact velocities ranging from 2.44 to 7.70 m/s with a 40 durometer, a 90 durometer flat padding, and a 90 durometer cylinder. Specimens were isolated from PMHS subjects at the level of occipital condyles, and the intracranial substance was replaced with brain simulant (Sylgard 527). Three tri-axial accelerometers were instrumented at the anterior, posterior, and vertex of the specimen, and a pyramid nine accelerometer package (pNAP) was used at the contra-lateral site. Biomechanical responses were computed by transforming accelerations measured at each location to the head CG. The results indicated significant "hoop effect" from skull deformation. Translational head CG accelerations were accurately measured by transforming the pNAP, the vertex accelerations, or the average of anterior/posterior acceleration to the CG. The material stiffness and structural rigidity of the padding changed the biomechanical responses of the head with stiffer padding resulting in higher head accelerations. At the skull fracture, HIC values were more than 2-3x higher than the frontal skull fracture threshold (HIC=1000), emphasizing the differences between frontal and lateral impact. Rotational head accelerations up to 42.1 krad/s(2) were observed before skull fracture, indicating possible severe brain injury without skull fracture in lateral head impact. These data will help to establish injury criteria and threshold in lateral impacts for improved automotive protection and help clinicians understand the biomechanics of lateral head impact from improved diagnosis.