Tillaux ankle fracture occurs in teens and affects the closing (but not fully closed yet) growth plate. The front (anterior) and outside (lateral) portion of the lower tibia (lower leg bone) is broken off (called avulsion) while still attached to the anterior inferior tibial-fibula ligament.
The fragment is avulsed when the ankle is turned out (supinated) and an external rotational force is applied through the leg to the ankle. This injury is rarely seen in adults, because the ligament gives way instead of avulsing the tibial fragment from its epiphyseal attachment, which is what occurs in teens. The bone fracture and ligament injury are known as a Tillaux lesion.
Between ages 12 and 15 the growth plate starts to fuse with the bone. But it does so in a rather uneven, asymmetric fashion. First the middle portion hardens and stops growing, and then the medial or inside edge completes its growth phase. The lateral (outside) edge is the last to solidify. The area that is still open is usually where the fractures occur.
If the lateral force is strong enough, not only does the bone fracture but the fracture also displaces (separates). This displacement leaves a gap or “step-off” when the bones shift. When this happens, the bones no longer line up, which means the two sides of the ankle joint are no longer evenly matched. Such a situation has to be treated or the uneven weight-bearing surface will be painful. Down the road, arthritis will develop causing further problems.
Fortunately, since this fracture occurs in adolescents with near full growth (almost skeletally mature), there is minimal risk of deformity otherwise caused by growth plate injury in younger individuals. Surgery is still advised if the gap is more than two millimeters wide.