Despite the cost and possible requirement of additional sedatives, the advantages of MR imaging are considerable in DDH assessment, including the absence of radiation, possibility of imaging in multiple planes, and higher resolution and contrast between bony and cartilaginous components . The high accuracy of MRI in demonstrating the details of DDH has been described in previous literature by comparing with anatomical preparations in cadavers . More recently, MRI is being used in the diagnosis of the anatomical obstructions to reduction, such as fibrofatty pulvinar tissue, joint effusion, thickened ligamentum teres, inversed and thickened labrum, and iliopsoas muscle atrophy. Most of these studies evaluated one of these characteristics and confirmed the role of MRI in the diagnosis of DDH [20-22]. In these papers, the diagnostic values of MRI are evaluated comprehensively. And the diagnostic capability of MRI was in good agreement with pathological examination or surgical findings.
In the evaluation of 43 affected hips, the fibrofatty pulvinar tissue and joint effusion were easily detected by MRI due to different signal manifestations in T1WI, T2WI and STIR fat suppression sequences for tissues with fat or liquid. T1WI and T2WI showed high signal for fat and liquid signals, while stir fat suppression sequence showed low signal intensity. T1WI and Flair showed low signal for liquid signals, while T2W showed high signal. MRI can show fat and liquid sensitively, and the coincidence rate with pathological diagnosis is 100%. However, Benjamin et al  reported that a small amount of adipose tissue and joint effusion can be completely absorbed once the femoral head is relocated and well covered by the acetabulum.
The labrum is one of the most important structures of the hip joint, which can improve the lateral coverage of the femoral head and deepen the joint. The labrum is typically everted with mild joint dislocation, but it may invaginate into the hip joint together with a capsular fold and inhibit reduction in the case of complete joint dislocation. In order to achieve a concentric reduction, the everted, inverted, and hypertrophied labrum require radial cuts . In this study, the sensitivity and specificity of detection of the affected labrum were 90.3% and 83.3%, respectively, which were consistent with previous reports [24-26].
As another secondary adaptive change, thickened or elongated ligamentum teres often requires surgical excision, especially in grade III DDH. The stress is concentrated on a small area of the acetabular roof and may lead to high rates of avascular necrosis of the femoral head. Therefore, the hypertrophied ligament trees should be removed in the surgery to achieve optimal results. Devitt et al  reported that MRI can be used to rule out partial tears of the ligamentum teres with sufficient sensitivity (91%) and PPV(67%) and accuracy (64%) but relative low specificity (9%) and NPV (31%). In our study, 25 hips with thickened ligamentum teres were identified in the surgery, 23 of them were detectable by MRI. And the sensitivity (92%) and specificity (83.3%), PPV (88.5%), NPV (88.2%) and accuracy (88.4%) for detecting thickened ligamentum teres were much higher in our reports, which may be partly attributed to higher resolution 3.0T MRI used in this study. AVN is one of the most common serious complication of DDH . In the study, bilateral AVN were shown on 2 of 4 patients with elongated ligamentum teres during 3.5 years follow up, which led to the surgery. One of the possible reasons is that ligamentum teres exerts abnormally high pressure on ossific nucleus for a long time before surgery, indicating the early diagnosis by MRI and surgical removal of elongated ligamentum teres will be the key to decrease the occurrence of AVN.
The iliopsoas tendon passes between the acetabulum and the displaced femoral head and may obstruct concentric reduction . The iliopsoas muscle atrophy often occurs on the affected hip as a result of biomechanical changes after DDH. Therefore, complete tenotomy of the iliopsoas muscle is recommended due to its advantages such as lower risk of avascular necrosis of the femoral head and decreased pressure on the hip joint. Keeney et al  reported that the three-dimensional (3D) MRI was quite useful in diagnosing the iliopsoas muscle atrophy and was helpful in predicting the reduction difficulty. In our study, preoperative MRI has a high diagnostic efficiency, and its sensitivity, specificity, positive predictive value, negative predictive value and accuracy are highly consistent with postoperative pathology. The preoperative MRI would be helpful to identify the iliopsoas muscle atrophy.
In our research, there was good consistency between the two senior radiologists of MRI in thickening of the ligamentum teres, inversion of the labrum, atrophy of the iliopsoas muscle, fibrofatty pulvinar tissue and joint effusion (ICC > 0.72), indicating that MRI has good repeatability in the diagnosis of DDH and is suitable for preoperative diagnosis and postoperative evaluation and follow-up.
There were significant differences in the number of boys and girls in the study, with European scholars reporting a male-to-female ratio of about 1:7 and a unilateral incidence of about 90%, with left side more likely to be affected than right side (3:1) . In our group, there were 9 boys and 30 girls, with the incidence of DDH in girls much higher than that in boys. This may have something to do with genetics . In addition, during childbirth, the mother will secrete a large amount of relaxation hormone, in order to achieve the purpose of relaxing ligaments and expanding the birth canal. Girls are more sensitive to this relaxation hormone than boys, and have more loose soft tissue around the hip. If there is resistance during delivery, hip dislocation is more likely to occur, which is also the reason why DDH rates are higher in girls than boys.
DDH is more common in the left hip . In this group of DDH cases, there were 25 left hip lesions and 18 right hip lesions. Due to the insufficient sample size, the difference of prediction accuracy of left and right hip joints was not further studied.
This study has some limitations. It was a retrospective study, and the sample size of this study was small. Given these limitations, prospective and global sample studies may be necessary to confirm the diagnostic capability of MRI.