Developmental dysplasia of the hip (DDH) is a developmental abnormality of the hip joint that is underdeveloped at birth and continues to progress after birth. Clinical instability of the hip is an early symptom of DDH, and if not treated in infancy, it can develop into joint dislocation or degenerative changes of the hip joint and cause dysfunction in adulthood. In terms of X-ray diagnosis, the difference between DDH and congenital dislocation of hip (CDH) is mainly that the joint deformity of DDH is not obvious, and the abnormalities of various indicators are relatively mild, so the diagnosis is easily missed if not carefully observed on X-ray. Qiu Huaxing, Department of Radiology, Shaoxing Maternal and Child Health Hospital Children with DDH may gradually develop subluxation of the femoral head, cartilage degeneration in the weight-bearing area, focal necrosis of the femoral head and severe osteoarthritis due to poor coverage of the femoral head by the acetabulum caused by defective development of the acetabulum. The main pathological changes are the deficiency of the acetabulum superiorly and anteriorly, the shallowing of the acetabulum, and the outward shift of the center of the hip joint, resulting in inadequate inclusion and coverage of the femoral head by the acetabulum. Early diagnosis and treatment of DDH can significantly reduce the emergence of late treatment complications of DDH, reduce the degree of deformity of secondary arthropathy, improve the success rate of late surgical treatment, and ultimately improve the overall treatment level of DDH. Statistics show that more than half of the cases of middle-aged and elderly degenerative hip osteoarthritis come from failure to receive timely diagnosis and effective treatment of hip dysplasia in infancy and childhood . Therefore, the early diagnosis and treatment of DDH is very important. Regarding the causes of DDH, some scholars believe that in the second half of pregnancy, due to certain conditions such as low amniotic fluid and fetal malposition, there is a continuous “mechanical change” in the uterus that is unfavorable to the stability of the fetal hip joint, and the acetabulum is relatively shallow at this time, which is extremely unfavorable to hip stability. At the same time, this “mechanical change” also causes the originally shallow acetabulum to develop and become more shallow as the gestational age increases, leading to the development of DDH. The occurrence of DDH depends on three anatomical factors that maintain hip stability: the diameter of the acetabulum, the ratio of the acetabular depth to the length of the femoral head and the circumferential ligament, and the muscles surrounding the hip joint. On X-ray, the position of the femoral head in Perking’s square, the continuity of Shenton’s line, and the symmetry of Rosen’s line reflect the hip stability, while the acetabular index reflects the diameter and depth of the acetabulum. These indices can objectively reflect the stability and development of the hip joint. Clinically, hip instability, as an early symptom of DDH, is mainly manifested by asymmetry of skin folds on the hip, inner thigh or iliac fossa, deepening of folds on the affected side, or even shortening of the affected limb, limited abduction, late walking, and limping. In recent years, the use of ultrasound to screen newborns and infants before 3 months of age for DDH has become more common. The only drawback is that ultrasound is susceptible to the skill level of the examiner. MRI is able to distinguish the structures of the joint and obtain high-contrast and high-resolution images of muscles, ligaments, cartilage, synovium and other structures, and it can also evaluate the morphology of the hip joint very effectively from a three-dimensional spatial perspective using multi-directional imaging and three-dimensional reconstruction imaging. The main disadvantages are the long examination time, the need for the child to be examined under anesthesia or sedation, the high price of the examination, and its unsuitability for screening or multiple follow-up visits in younger infants and children. The acetabular index, Rosen’s line, Perking’s square, and Shenton’s line can be used to visualize DDH by taking an orthopantomogram of the hip joint plus a Von Rosen’s position. The acetabular index is the most important indicator of acetabular development. The greater the acetabular index, the worse the acetabular development and the greater the possibility of DDH. X-ray examination is a simple, reliable and definitive diagnosis method.