Differentiation of lesions with similar X-ray changes or MRI changes should be noted. I. Differential diagnosis of diseases with similar X-ray changes 1. Intermediate and advanced osteoarthritis: it may be confused when the joint space becomes narrow and subchondral cystic changes appear, but its CT shows sclerosis with cystic changes and MRI changes are mainly low signal, which can be differentiated accordingly. 2, acetabular dysplasia secondary to osteoarthritis: the femoral head is not fully wrapped, the acetabular line is in the upper part of the femoral head, the joint space is narrowed and disappears, osteosclerosis, cystic changes, and similar changes appear in the corresponding area of the acetabulum, which can be easily distinguished from ONFH. 3, ankylosing spondylitis involving the hip joint: common in adolescent males, mostly bilateral sacroiliac joint involvement, which is characterized by HLA-B27 positive, the femoral head remains round, but the joint space becomes narrow, disappears or even fuses, so it is not difficult to distinguish. Some patients with long-term application of corticosteroids can be combined with ONFH, the head of the femur can appear collapse but often not serious. 4, rheumatoid arthritis: mostly seen in women, the head of the femur remains round, but the joint space becomes narrower and disappears. The joint surface of the femoral head and acetabular bone erosion are common, and it is not difficult to differentiate. The differential diagnosis of diseases with similar MRI changes 1. temporary osteoporosis (ITOH): seen in middle-aged male and female patients with temporary painful bone marrow edema. x-ray shows reduced bone mass in the femoral head, neck and even rotor. mri shows uniform low signal in the T1-weighted phase and high signal in the T2-weighted phase, ranging to the femoral neck and rotor, without banded low signal, which can be differentiated from ONFH. This disease can be healed within 3-6 months. 2, subchondral insufficiency fracture: mostly seen in elderly patients over 60 years old, without obvious history of trauma, showing sudden onset of hip pain, inability to walk, and restricted joint movement. x-ray shows slightly flattened external upper femoral head, T1 and T2-weighted phase of MRI shows subchondral low signal line, surrounding bone marrow edema, T2 lipid suppression phase shows lamellar high signal. 3, pigmentation choroidal nodular synovitis: mostly in the knee joint, hip joint involvement is rare. CT and X-ray may show cortical bone erosion of the femoral head, neck or acetabulum, and mild to moderate narrowing of the joint space. MRI shows extensive synovial hypertrophy with uniform distribution of low or moderate signal. 4, femoral head contusion: Most often seen in middle-aged patients with a history of hip trauma, manifesting as hip pain and claudication. mri is located within the femoral head with moderate intensity signal in T1-weighted phase and high signal in T2-weighted phase, more medially. 5. Synovial herniation pit: This is a benign lesion of synovial tissue proliferation invading the cortex of the femoral neck, and MRI shows a small round lesion with low signal in T1-weighted phase and high signal in T2-weighted phase.