Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
Penetrating trauma is an injury that occurs primarily by an object piercing the skin or entering a tissue of the body.
The severity of the injury is determined largely by the velocity of the object that enters the body. High-velocity objects are usually projectiles such as bullets from high-powered or assault rifles. Bullets from handguns and shotguns are classed as medium-velocity projectiles. Low-velocity items, such as knives, are usually propelled by a person's hand, and usually do damage only to the area that is directly contacted by the object.
In addition to causing damage to the tissues they contact, medium and high velocity projectiles cause a cavitation injury: as the object enters the body, it creates a pressure wave which forces tissue out of the way, creating a cavity that can be much larger than the object itself. The tissues soon move back into place, eliminating the cavity, but the cavitation frequently does considerable damage first. Cavitation can be especially damaging when it affects delicate tissues such as the brain, as occurs in penetrating head trauma.