TABLE 2.

Differential diagnosis of ORN of the hip (66).

Disease	Age predilection	Sex predilection	Etiology	Unilateral or bilateral	Acetabular involvement	Diagnosis elements
ORN of the hip	Adults and the elderly	No gender differences	Radiation	Bilateral	Yes	History of radiation therapy; MRI: acetabulum, pubis, femoral head, and upper femur show long T1 and T2 signals.
Osteoarthritis	Middle-aged and older	No gender differences	Degeneration	Bilateral	Yes	CT: sclerotic bone and cystic change; MRI: crescent sign
Secondary acetabular dysplasia	Children and youth	Female	Genetic factors	Bilateral	Yes	X-rays: hip joint dislocation, hip joint space narrowing, and features of secondary osteoarthritis
Ankylosing spondylitis involving the hip	Teenagers	Male	Genetic and environmental factors	Bilateral	Yes	HLA-B27(+), sacroiliac joint erosions, and iliac subchondral sclerosis
Idiopathic transient osteoporosis of the hip	Middle-aged and youth	No gender differences	None	Unilateral	No	MRI: low signal intensity on T1WI, high signal intensity on T2WI, extending from the femoral head to the intertrochanteric region
Chondroblastoma of the femoral head	Children and teenagers	Male	Unclear	Unilateral	No	MRI: high signal intensity on T2WI; CT: irregular bone dissolution
Subchondral insufficiency fracture	Elderly	Female	Osteoporosis	Unilateral	No	X-rays: flattening of the femoral head; MRI: subchondral low signal intensity on T1WI and T2WI, with bone marrow edema
Pigmented villonodular synovitis	Young adults	No gender differences	None	Unilateral	Yes	X-rays and CT: hip joint space narrowing; MRI: extensive thickening of the joint lining or an extensive mass, possibly with destructive bone changes
Bone infarction	Unclear	Unclear	Unclear	Bilateral	No	MRI: high signal intensity on T2WI, characteristic double-line sign, which consists of a hyperintense inner ring and a hypointense outer ring