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. It has drawn more attention, in recent years, with various studies and researchers focusing on the topic that how to make a proper intervention strategy for DDH patients who are approaching or older than 18 months old, treatment could be CR followed by plaster casting, or performing OR as soon as possible once the diagnosis was established, since several articles indicated that older age might indicate poor outcome [5–7]. It is still a controversial issue among pediatric orthopedists. This study enrolled 107 children (156 hips) with DDH in a single center from 2011 to 2013 in order to evaluate the effect of CR among different age groups, to identify the risk factors of complications of CR and to discuss the possible indicators for failure of CR, especially in controversial age abovementioned.
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 [12, 13] concluded that IHDI classification can be applied more flexibly which can better reflect the severity of the conditions, especially for those cases without ossific nucleus of the femoral head.
Postoperatively, for all the measurements, 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 the potential for the normalization of AI. The decrease of AI indicated 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. Correspondingly, 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 predictor of failure DDH treated by CR was the initially age > 12.5 months (65%).
Several articles 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, 15]. RAD in group III was found to be significantly high, compared with Group I and II. Moreover, the result of univariable logistic regression manifested that age ≥ 18months was the only risk factor for the happening of RAD (OR: 4.000; p = 0.012). That is to say the prevalence of RAD increases with the age of hip reduction. 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 [16, 17]. Therefore, we thought that, for the dislocation patients, RAD was a complication that must be carefully considered for children older than 18 months, which might require FS to correct.
Although there was no influence of age at initiation of outcomes on AVN and re-dislocation in our study, some researches granted age as risk factor of AVN[11, 18, 19]. Similar to our results, age was not found to be the risk factor of AVN after CR also reported in other literatures [9, 15, 20]. The rate of AVN (18.6%) in this present study was similar to previously reported studies (10%-33%) [21–24]. The most common cause 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 nine articles and analyzed the data, found out the most critical element of AVN was extreme abduction angle, and the ossification of the femoral head was not associated with AVN, which was similar to this our result and other studies [7, 27]. AVN was not associated with age or other factors (sex, side, ossific nucleus etc.) in our cohort, but the IHDI IV was found to be the risk factor for AVN and re-dislocation resulting from univariable logistic regression (OR: 2.524, p = 0.033; OR: 4.211, p = 0.004). For severe patients, CR was difficult to perform when extreme abduction was warranted to stable reduction, which AVN might occur. The incidence of re-dislocation after CR was 23.1% in this study, which was similar to Sankar’s study . Except from IHDI IV, the walking experience was also a risk factor about re-dislocation (OR: 2.524, p = 0.033). As the time went on, especially after independent walking, a series of pathological changes of the affected hip would make CR more difficult, which, certainly, lowered the efficiency of CR [28, 29]. This is consistent with results in our present study, namely, walking ability should be an important evaluation at the time of treatment.
This study has a number of limitations. First, a longer follow-up until adult is necessary, which may lead to different results of AVN and FS rate. Second, all the included cases had successful CRs at the initial attempt, which might bring to a selection bias. Third, the study was retrospective and more randomized controlled trials or large-scale case-control studies are required for further validation.