Injury pattern and the biomechanical assessment of skull fracture risk in blows with a rubber mallet

Acts of violence often result in blunt force trauma. On behalf of the court, forensic experts not only have to assess injuries in terms of morphology or severity, but they also have to give statements regarding the risk of potential fatal injuries. We describe a case of domestic violence where a man hit the head of his wife using a rubber mallet. His wife sustained extensive and multiple parietal scalp lacerations and avulsions with exposure of the cranial bone. A CCT revealed neither skull fractures nor intracranial injuries. Biomechanical measurements were performed using a bowling ball and an animal head representing head surrogates. In comparison to steel hammer impacts we hypothesized longer contact durations and lower maximum contact forces in blows with a rubber mallet. However, contact durations in the magnitude of 3ms do not confirm our hypothesis. The contact forces calculated based on mallet and animal head accelerations were between 2.97kN and 4.68kN. These force values are rather below the parietal fracture thresholds, explaining the absence of parietal skull fractures in the case presented. Moreover, the relatively low Young's modulus comes along with a deformation of the rubber mallet and a decrease in contact pressure. In summary, contact times could be reliably estimated, while contact forces remained largely uncertain.