Fractures occur when a bone cannot withstand physical force exerted on it. There are three main types of fractures: stress fractures which are hairline breaks often invisible on x-rays; compound fractures where the bone protrudes through the skin; and greenstick fractures where the bone cracks but does not break all the way through. Stress fractures are caused by overuse while compound fractures often result from high-energy injuries. Treatment depends on the type of fracture but generally involves rest, braces, immobilization and sometimes surgery to repair the bone and prevent infection in compound fractures.