(I) Stage I (ultrastructural variant stage): X-ray film shows disorganized and broken structure of bone trabeculae in the bearing system of the femoral head, appearing hairy edge of the femoral head, with or without limited mild pain clinically. (II) Stage II (sensitized stage): X-ray film shows that small cystic degeneration shadow will appear inside the femoral head, and the density around the cystic area is uneven. The trabecular structure of the bone is disturbed, sparse or blurred, and fine collapse may also appear, with clinically significant pain and mild limitation of movement. (III) Stage III (necrotic stage): X-rays show morphological changes of the femoral head, including incomplete, wormy or flattened edges, loss of trabecular structures, uneven bone density, widening or narrowing of the acetabulum and femoral head space, and formation of superfluous bones, with pain, intermittent claudication, restricted joint movement, and varying degrees of shortening of the affected limb. (IV) Stage IV (disabling stage): The morphology and structure of the femoral head are obviously changed, with large irregular collapse or flattening, and structural variation of bone trabeculae. The gap between the acetabulum and the femoral head disappears, etc. Clinical manifestations are pain, functional impairment, stiffness and inability to walk, dislocation or subluxation, and limitation of functional activities of the involved knee joint.