A break in the normal structure of a bone.

Bone fractures most often result from an accidental injury, although they may be caused by repeated stress from such activities as walking, dancing, or marching, or by medical conditions that weaken the bones. They are commonly divided into two major types: closed fractures, in which the ends of the bones remain in place under the skin, and open fractures, in which they protrude through the skin. Children's fractures are often less severe than those of adults and heal more quickly because their bones are softer and more resilient. (It is possible for a child's bone to bend more than 45 degrees without breaking.) Cracks, hairline, and "greenstick" fractures, in which the bone splits on one side, are common. Other types of fractures include the buckle fracture, in which the bone has been compressed and part of it is raised or...