Fracture, Femur. OverviewA thigh bone (femur) fracture is a break in the upper bone of the leg. Because the femur is the longest, strongest bone in the body, unless the bone is diseased, it takes great force to break it. Femur fracture most commonly occurs after a motor vehicle accident, a collision playing a sport, a fall from a high place, or as the result of a gunshot wound and underlying tumor (neoplasm).