Avulsion fracture occurs at the site of ligament or tendon attachment to bone. With excessive tension the ligament/ tendon "pulls away" from its attachment to the bone. Inevitably, a portion of bone comes with it.
The portion of bone that has pulled away from the parent bone is what we typically see radiographically. For example, it you nailed a rope into a wall and pulled on the rope hard enough it would pull out of the wall. In addition, we might suspect that a piece of the wall would come out with the rope. This is analogous to avulsion fracture.
With ligament avulsion, we must realize that there are TWO structures involved:
The ligament or tendon. This structure is typically radiographically invisible and should be evaluated ultrasonographically.
The bone, which has fractured. This is generally visible both radiographically and ultrasonographically.
Once some stability has been reestablished to the area the bone fragment heals back to its parent bone within about 90 days. The associated ligament or tendon, on the other hand, has less blood supply and therefore can take much longer to fully heal. It is persistent inflammation within the affected ligament (called desmitis) or tendon (called tendonitis) that becomes the performance-limiting problem for most horses.
Please click HERE to learn more about ligament and tendon injuries in the horse.