Consequences of nonpenetrating projectile impact on a protected head: Study of rear effects of protections

Background: Police and armed forces have helmets that can now stop handgun bullets and even a certain category of rifle bullets. The trend is to incre ase the ballistic limits of helmets, but injuries caused by nonpenetrating impacts are not well understood. The helmet defeats the projectile and crea tes a local cone of deformation that impacts the head a second time, The te rm "rear effects" describes the behind-armor blunt trauma caused by the non penetrating impact. Methods: To analyze rear effects on the skull, an experimental study was as sociated with parametric simulations on a three-dimensional finite element model. Transfer of energy throughout the head was tested on 30 human skulls filled with a silicone gel. The magnitude of contact forces on the skull s urface and the pressure levels in the skull were recorded during a referenc e impact. Results: A biomedical approach by pathologic findings and radiographs showe d very localized fractures. The protection brought by the diploe in the mul tilayered bone was confirmed and characterized by numerical simulations. Conclusion: This first step toward a better understanding of the rear effec t phenomenon in relation to its consequences on brain tissue will lead to t he design of more efficient protections.