Head injuries can be divided into two basic categories: open and closed. An open head injury occurs when an outside force to the skull is great enough to fracture the bone; it refers to the state of the skull, not the state of the brain. Because of this, a separate term may be used to describe the state of the brain, i.e. a closed head injury with traumatic brain injury. 

A closed head injury does not involve a fracture, but can be as or more serious than an open injury. Because the skull is not broken, brain swelling and clots can occur. The inside of the skull is rough; this can contribute to closed brain injuries if the impact causes the brain to shift and rub against the abrasive surface of the inside of the skull. Both open and closed head injuries seriously impact the brain and can result in paralysis, loss of consciousness, comas, and death.