An avulsion fracture is break to the bone where a ligament connects to the bone. A common place for this to occur is on metatarsal-5, the bone in the outermost (lateral) aspect of the foot.
When the foot turns inwards medially (inversion) too fast, too far and with too much force a protective reflex arc is triggered. Pulling the foot back straight to correct it. As this occurs the Fibularis muscle and tendon contracts to correct the foot from rolling completely in. If the foot is hindered or prevented form pulling back to its correct position the reflex contraction can pull the end of the bone clean off. This usually occurs in the area between the diaphysis and epiphysis, an area known as the metaphysis. This area is highly avascular and osteoblast activity is low. An avulsion fracture in this area providing there is no displacement (lineal rearrangement of the bones) typically takes 16 weeks to heal.
An avulsion fracture of this and is common when the ankle twists in a fall, seen in dancers and footballers as wells athletes and gymnasts.