The principle of the treatment for DDH is to establish a stable, concentric reduction of the hip to enable the subsequent hip development as early as possible, given the well-established correlation between residual dysplasia and the age of reduction. CR plays an essential role during the process of DDH treatment, especially the young children, with high success rate and low complications. Tschauner et al reported a safer, shorter and simpler way for the early DDH diagnosis and treatment using a sonographic hip screening programmer according to the Graf-method. In recent years, more attention has been drawn to the proper intervention strategy for DDH patients who are approaching or older than 18 months old. Normally, treatment can be CR followed by plaster casting, or performing OR as soon as possible once the diagnosis was established, since several studies indicated that older age might indicate poor outcome [5-7]. However, the timing for CR procedure is still a controversial issue among pediatric orthopedists. In our study, 107 pediatric patients (156 hips) with DDH in a single center were evaluated for the effect of CR with respect to different age groups, therefore to identify the risk factors of complications of CR and to discuss the possible indicators for failure of CR, especially in older age patients.
Compared with Group II and III, Group I showed significant difference about IHDI grade, but not Tönnis grade. Moreover, the ossific nucleus was not present in 34% hips. Comparing IHDI classification to Tönnis classification, Both Miao and Brandon et al [13, 14] concluded that IHDI classification is more flexibly and better reflect the severity of the condition, especially for those cases without ossific nucleus of the femoral head.
For postoperative clinical attributes, the difference between Group I and Group II、III were statistically significant among post-op AI, which revealed that the older the child, the lower chance for the normalization of AI. The decrease of AI is a sign of gradual normalization of acetabular morphological structures under the condition of concentric reduction of the affected hip. Shin et al  considered that an AI > 32° and CEA < 14° at the age of three years could serve as a guideline for osteotomy. Consistently, our results showed that if the post-op AI > 26.4°, CR was more likely to fail (84.1%). Pre-op AI also manifested with an obvious tendency to be fail if the value larger than 38.7° (68.8%). The ROC curve also showed that the age predictor for CR failure was the initially treatment age > 12.5months (65%).
Treatment of DDH hinges heavily on the timing of diagnosis and treatment. The earlier the concentric reduction is achieved, the better outcomes it will be, due to the fact that the pathomorphology cannot proceed to extremely severe stages of luxation. The accompanying pathoanatomic obstacles greatly reduces the chance of a successful CR. Hence, early ultrasound screening using the Graf method in some countries is recommended to timely detect hip immaturity and pathologies and to provide the optimal approach. Several studies reported older age at the time of CR showing a higher rate of complications or further corrective surgeries [5-7], while others not[8, 9, 17]. RAD in group III (older age) was found to be significantly high, compared with Group I and II. Moreover, the result of univariable logistic regression models identified that age≥18months was the single significant risk factor for the occurrence of RAD (OR: 4.000; p=0.012), which indicates the higher chance of RAD with the age of hip reduction increases. Other researches have indicated that in the case of lateral hip subluxation, the pressure on the femoral head becomes concentrated along the medial aspect of the head as the hip hinges along the edge of the acetabulum. The acetabular growth cartilage fills the acetabular floor and arrests its lateral growth, forming a progressively shallower and more oblique acetabulum [18, 19]. Therefore, we conclude that, for the dislocation patients, the risk of RAD must be brought for attention for children older than 18 months, which might in turn require FS to correct DDH.
Although in our study age does not play as a significant risk factor for AVN and re-dislocation, Similar to our results, it was also reported in previous studies [9, 17, 22] that age was not the risk factor of AVN after CR, while other studies gave the opposite conclusion [11, 20, 21]. The rate of AVN (15.4%) in this study falls into the range of the previously reported studies (10%-33%) [23-26]. The most common cause of AVN is the immobilization in a position that places excessive pressure on the femoral head. Thus, Ramsey et al.  recommended creating a “safe zone” to prevent AVN. In certain situation, an adductor tenotomy will increase the safe zone by allowing for a wider range of abduction, especially for patients with high Tönnis grade. Madhu et al  collected data from nine studies and found out the most critical element of AVN was extreme abduction angle, whereas the ossification of the femoral head was not associated with AVN, which is consistent with our result and other studies [7, 29]. AVN is not associated with age nor other factors (sex, side, ossific nucleus etc.) in our cohort, but the IHDI IV was found to be a risk factor for both AVN and re-dislocation in univariable logistic regression analysis (OR: 2.524, p=0.033; OR: 4.211, p=0.004l, respectively). For severe patients, CR is difficult to perform when extreme abduction is needed to stable reduction, which AVN might occur. The incidence of re-dislocation after CR is 14.7% in this study, which is similar to Sankar’s study (9%) . Except from IHDI IV, the walking experience is also a risk factor for re-dislocation (OR: 2.524, p=0.033). As the time proceeds, especially after the patient is capable of independent walk, a series of pathological changes of the affected hip will make CR more difficult, which, certainly, lowers the efficiency of CR [30, 31]. This is consistent with results in our study, namely, walking ability should be an important factor to evaluate at the time of treatment.
We also want to mention that a number of limitations exist in this study. First, a longer follow-up until early adulthood is more comprehensive, which may change the AVN and FS rate within the cohort, therefore affect the risk factors identified. Second, all the included cases had successful CRs at the initial attempt, which might bring a selection bias to the study. Third, the study was retrospective. More randomized controlled trials or large-scale case-control studies are required for further validation.