Many parents encountered some doubts during the online consultation, and the uploaded pelvic x-ray was interpreted differently by different experts, or even with completely opposite views, leaving parents at a loss. In summary, parents raised two very meaningful propositions: (1) How to diagnose and treat acetabular dysplasia? (2) How should the acetabular index measured on the pelvic X-ray of infants and children be viewed? These two seemingly simple propositions are not so simple even for a pediatric orthopedic surgeon. The acetabular index refers to a straight line drawn through the apex of the Y-shaped cartilage of the acetabulum bilaterally and extended, and then a straight line from the apex of the Y-shaped cartilage to the most prominent point of the lateral upper edge of the top of the bony acetabulum, and the angle between this line and the horizontal line of the pelvis is the acetabular index. The acetabular index can reflect the degree of acetabular coverage of the femoral head more accurately, so the acetabular index has been widely used to assess the degree of acetabular development, and is one of the most important clinical indicators for the diagnosis and assessment of the efficacy of hip dysplasia. I believe that no pediatric orthopedic surgeon would disagree with this point. But why do experts have different opinions? This raises the question of how to interpret the pelvic X-rays uploaded by parents. First of all, an accurate measurement of the acetabular index depends on whether the orthopantomogram of the pelvis is standard. Because the hip joint is a three-dimensional structure, the pelvic x-ray is a two-dimensional structure. Different positions of the same child at the time of the radiograph can result in different acetabular indices! Therefore, the position of the body and the angle of projection at the time of the radiograph have a great influence on the accurate measurement of the acetabular index. This ultimately leads to different conclusions. Tonnis describes the standard position of the pelvic orthopantomogram: lying naturally with the lower extremities slightly apart, shoulder width apart, and with mild internal rotation of the lower extremities about 20 degrees. The tip about the diagnosis of hip dysplasia gives a good answer: “No advice about the disease is a substitute for a face-to-face diagnosis by a licensed physician”. This is a clear reminder that in addition to various tests and examinations, the diagnosis of any disease requires a face-to-face “diagnosis” and “diagnosis” between the physician and the patient. After looking up the information on my desk, I found the article “Natural History of Hips With Borderline Acetabular Index and Acetabular Dysplasia in Infants” in J Pediatr Orthop, Vol. 22, No. 5, 2002. Dysplasia in Infants” clearly states “The management of developmental hip dislocations has been well described over the past 20 years. However, the management of infants with clinically stable hips but an increased acetabular index on pelvic radiographs remains controversial”. The authors suggest that “for clinically stable hips, many people treat mild-moderate hip dysplasia with an abductor brace, but there is no evidence that this is effective”. Therefore, “the systematic use of abduction braces is not justified. Natural evolution of these cases into normal hips is possible, and therefore some authors recommend no treatment and simple clinical and radiological follow-up”. An important conclusion of the authors is that “clinically and radiologically stable hips do not progress morphologically and functionally to displacement or deterioration”. In practice, it is very important to select the hip to be treated. Our opinion is that hips that are not unstable or displaced, but simply have an increased acetabular index or isolated acetabular dysplasia, do not need to be treated. In these cases, clinical and radiological follow-up is necessary. In cases of unstable hips, with displacement and dislocation on radiographs, are indications for abduction brace treatment”. It is not possible to interpret the stability of the hip from an X-ray, which requires a clinical physical examination, and Barlow’s sign is an important sign to check the stability of the hip. If the acetabular index (acetabular angle) is significantly increased on the standard pelvic X-ray and the clinical examination of Barlow’s sign (+), treatment such as abduction brace is needed; if the acetabular index is increased and the femoral head is displaced or dislocated, active treatment is also needed; if the acetabular index is only increased and the hip joint is stable, active treatment is not needed and most of them can gradually return to normal. A study entitled “Erros in Measurement of Acetabular Index” in J Pediatr Orthop 1995, Vol. 15, No. 6 is worth reading. The authors studied a series of radiographs of the pelvis of four cadaveric 15-day, 2-month, 3-month, and 21-month-old boys, radiographs were taken at 60 KV mAS at 0.8 s, tilted at 5°, 10°, 15°, 20°, 25°, and 30° in the sagittal direction and tilted at 5°, 10°, 15°, 20°, 25°, and 30° in the caudal direction to measure the acetabular index. , analyzing the measurement error. The authors concluded that “there are many potential sources of error in the measurement of the acetabular index. These include observer error, and differences in positioning of the child at the time of the radiographs.” “If the closed-hole ratio is kept at 0.5 to 2, then the error due to hip rotation is ±3°. If flexion and extension of the pelvis is limited to ±10°, then the error caused by flexion and extension of the pelvis is ±3°. The error is ±2° for multiple observations by the same observer and ±3° for observations by different observers. This gives a total error of ±5°. We have not found a satisfactory method to limit the pelvic flexion/extension limit to between ±10°. In some cases, especially when children are in pain, pelvic flexion and extension may reach 20°, and the error can even reach 10°”. The authors further state that “very large errors (10°) can occur even if there is no indication on the radiographs”. The authors warn: “Surgeons should be aware of the possibility of large errors when measuring the acetabular index!” . Finally, I feel that these two articles are authoritative research articles, which conclude that (1) there is a large variation in the acetabular index measured by pelvic x-ray in infants and children, and that the diagnosis of “acetabular dysplasia” should not be made hastily on the basis of an increase in the acetabular index measured by a single pelvic x-ray. We should analyze the position of the child at the time of the X-ray, exclude errors caused by pelvic rotation and torsion, and pay attention to other evaluation indicators, which also need to be combined with clinical practice. (2) The use of an abductor brace to treat “increased acetabular index” should be cautious; if the acetabular index is significantly increased on the pelvic X-ray with femoral head displacement or Barlow’s sign (+) on clinical examination, abductor brace treatment is needed. If the acetabular index is only enlarged on the X-ray, treatment with an abductor brace should be done with caution. Of course, in small infants under 4 months of age, the guidelines can be greatly relaxed with Pavlik slings. For children with a large acetabular index on the pelvic X-ray and a stable hip joint on clinical examination, if it is not possible to determine whether there is acetabular dysplasia, an MRI of the hip joint can be performed. This is because hip X-rays only show the development of the bony acetabulum, ignoring the development of acetabular cartilage. If MRI shows that the acetabular cartilage is well developed and the cartilage at the outer edge of the acetabulum has sufficient thickness, even if the bony acetabular index is large, it does not constitute acetabular dysplasia. On the contrary, if the cartilaginous acetabulum is dysplastic, even if the bony acetabular index is normal, acetabular dysplasia cannot be excluded and needs to be closely observed and reviewed regularly.