An avulsion fracture is an injury to the bone. Basically it is a fracture that occurs at the location where a tendon or ligament attaches to the bone. This tendon or the ligament pulls off a piece of the bone along with it instead of ligament tear. An avulsion fracture can occur anywhere in the body, most commonly in the knee joint.
When a muscular contraction force is stronger than the force holding the bone together, in that situation this type of injury occurs.
In ACL, PCL, ligament injury, the ligament is torn. Whereas in ACL, PCL avulsion injury, the ligament is intact but the ligament attached with the bone is fractured and pulled off. The commonest site of bone pulling is seen at the tibial attachment site.