Whenever a bone breaks or cracks, the injury is called a fracture. The leg has three bones that can fracture — the femur (the thighbone) and the tibia and fibula in the lower leg. When a fracture involves the knobby end portions of bones that are part of the hip, knee and ankle joints, the fracture is more complicated. This article describes only fractures of the straight shafts of the three long leg bones.
The femur is very strong, so it takes a lot of force to fracture this bone in healthy people. The femur usually fractures during high-impact trauma, especially in automobile accidents, industrial accidents, falls from high places or gunshot wounds to the thigh. If a low-impact bump or fall causes a femur to fracture, this may be a sign that the femur has been weakened by an illness, such as osteoporosis or cancer.
Femur fractures have the potential to cause dangerous, sometimes life-threatening complications, such as significant bleeding inside the thigh, with blood loss of one quart or more. A femur fracture also may cause blood clots to form within the large veins of the thigh. If these clots break free and travel through the bloodstream, they may eventually lodge in the lungs, creating a life-threatening condition called a pulmonary embolism.
Among children, femur fractures tend to happen because of a fall from a high place, such as a tree or the top of a slide. In adults, these injuries usually are related to motor vehicle accidents (either as a passenger or pedestrian) or to on-the-job trauma. The number of femur fractures caused by gunshot wounds has risen significantly in recent years.
The tibia (shinbone) is the larger of the two bones of the lower leg. Like femur fractures, tibia fractures often occur because of direct, high-impact trauma, especially during motor vehicle accidents. However, the tibia also can fracture from a low impact, even in healthy people, if the lower leg is bent or twisted at just the right angle.
Of all the body's long bones, the tibia is the most likely to be fractured and the most likely to break through the skin when it fractures. This greatly increases the risk of bacterial contamination and infection at the fracture site. It also may prevent normal healing. The sharp ends of a broken tibia can cut into nearby nerves and blood vessels and cause serious damage to soft tissues inside the lower leg.
In 75% to 85% of patients with tibia fractures, the fibula (the thin bone at the outer side of the lower leg) is fractured as well.
The fibula runs parallel to the tibia on the outside of the lower leg, but is smaller. The fibula usually fractures at the same time as the tibia. When only the fibula fractures, it is usually because of a direct blow to the side of the leg or an extreme sideways bend at the ankle or knee.
When only a fibula fractures, it usually does not cause long-term complications. Rarely, when the segments of broken bone are separated significantly by the injury, one of the nerves to the foot may be injured, causing foot drop, a condition in which the foot hangs limp at the ankle and drags on the ground during walking.