Types

There are several different mechanisms that may cause injury and many different combinations:

Inversion

The ankle is turned over so that the outer side almost touches the ground. This is common in sports when cutting or changing direction. Another time this may happen is if your right foot is trapped and your body weight goes to the left. If the bone isn't broken then the ligaments may be stretched or torn and this is an ankle sprain. If the bone does break there may be a pull-off fracture of the end of the fibula, an oblique fracture of the medial malleolus, or both.

Related Document: A Patient's Guide to Ankle Sprain and Instability

Eversion

The foot may be planted or pushed sideways so that the inner side of the ankle is near the ground. This may happen on a slippery surface or if the right foot is trapped and the body weight goes to the right. There will be a rupture of the ligaments on the inner side of the ankle or a pull-off fracture of the medial malleolus. On the outer side there will be an oblique (bending) fracture of the fibula with injury to the ligaments that go between the tibia and fibula.

Outward rotation

This occurs when the right foot is planted or trapped and the body weight rotates inwards to the left. The talus rotates outwards relative to the two other bones and springs them apart. There may be a pull-off fracture on the medial side. However, the hallmark of this mechanism of injury is a spiral fracture of the fibula and/or a tear of the ligaments between the tibia and fibula.

When the ligaments between the tibia and fibula are torn the injury is called diastasis. This injury may be difficult to detect if there is no other bony injury. The X-ray may show a widening of the gap between tibia and fibula, but it may be difficult to see. A spiral fracture of the fibula is always accompanied by diastasis.

Inward rotation

This is less common as a mechanism of injury. The foot can be trapped and the weight of the upper body rotate outwards. The rotation of the talus in the ankle mortise again causes tearing of the ligaments between the tibial and fibula and may cause a spiral fracture of the fibula.

Compression

High energy injuries to the ankle may be caused by motor vehicle accidents, falls from a height and some sports. Pure compression is rare; usually it is combined with rotation, inversion or eversion. However, compression results in the fracture to the weight bearing joint surface of the end of the tibia. This Pilon fracture is named from the French word for hammer and has the worst outcome of all ankle fractures.

Although some of the most severe ankle fractures occur as a result of MVAs they are more common in two other classes of situation. One would be sports with a higher incidence of sudden changes in direction, landing from a jump, or collisions. The second situation would be slip-and-fall injuries particularly in the elderly. Some ankle fractures are fragility fractures and occur in part because the bone is osteoporotic and cannot stand up to normal stresses.