Avascular necrosis (AVN), also called osteonecrosis, occurs when the blood supply to the femoral head (ball of the hip joint) is disrupted. Without blood flow, the bone tissue dies and eventually collapses, leading to severe arthritis and loss of joint function.

Risk factors include long-term corticosteroid use, excessive alcohol consumption, sickle cell disease, certain autoimmune conditions, previous hip fracture or dislocation, and radiation therapy. In some cases, no cause is identified (idiopathic AVN).

Early detection is critical — if caught before the femoral head collapses, joint-preserving treatments may be possible. Once collapse occurs, hip replacement is usually the most effective treatment. MRI is the gold standard for early diagnosis.