A penetrating hea injury, or open head injury, is a head injury in which the dura mater, the outer layer of the meninges, is breached. Penetrating injury can be caused by high-velocity projectiles or objects of lower velocity such as knives, or bone fragments from a skull fracture that are driven into the brain. A perforating head injury is one in which the object passes through the head and leaves an exit wound. Head injuries caused by penetrating trauma are serious medical emergencies and may cause permanent disability or death.