There are two kinds of Traumatic Brain Injuries or TBI, closed brain injuries and penetrating brain injuries. A closed brain injury happens when the brain forcefully hits the inside of the skull but there is no damage to the skull. While a concussion is a fairly common kind of closed brain injury, it is not the only kind. Concussions are also called mild TBI. But, just because they are labeled mild, experts stress it doesn’t mean they are any less dangerous. Penetrating brain injuries occur when the skull has been cracked, broken, or penetrated in some manner.