Avascular necrosis (osteonecrosis or AVN) 

Avascular necrosis (osteonecrosis or AVN) is a condition in adolescents and young adults where the blood supply to the hip joint is interrupted and the bone begins to die. AVN can lead to tiny breaks in the bone and may cause the ball of the hip joint to collapse. Although the cause of AVN is unknown, it is commonly associated with conditions such as previous chemotherapy, steroid use and sickle cell disease.