Complete concentric reduction of the femoral head and acetabulum is a necessary condition for the normal development of the hip joint. Stable CR with an MDP of <4 mm can obtain good long-term results.[25, 26] We couldn’t compare the MDP statistically between the groups because there were no scales on the intraoperative radiography. However, the average MJS on postoperative MRI in OR group is close to 2mm which is apparently less than that in CR group. The factors that affect the quality of CR include abnormal bony, cartilaginous, and soft tissue structure. The CR standard for safety and stability in our study combined the effects of these pathological factors. For the first time, this study compared the results of OR and CR under the similar degree of DDH reducibility and excluded the influence of different baseline factors before reduction.
The shaping ability of the acetabulum is an important internal factor for normal development of the hip joint after CR or OR. The AI is an important index for the morphological development of the acetabulum and an important reference for the prognosis of reoperation intervention and long-term osteoarthritis. Generally, the AI of normal infants decreases continuously before the age of 4, which is the peak of development. After the age of 4, development enters the platform stage.[23, 24] The key period of acetabulum shaping is 2 years after DDH reduction, and the shaping ability is obviously weakened by the age of 4 years.[28, 29] Therefore, CR should be performed in patients aged <2 years to make full use of the self-shaping ability of the acetabulum.[2-4]. The average age at the final follow-up in our study was >48 months, allowing us to fully compare acetabulum shaping between the groups. It is traditionally believed that fully concentric reduction can be restored early and to the greatest extent through OR, which may benefit acetabular remodelling. However, the current results indicated that the improvement of AI, incidence of RAD, CEA, and RMI were not different between CR and OR when the reducibility was similar; therefore, our results demonstrate that a safe and stable CR is acceptable in children aged 6–24 months.
Re-dislocation is a common complication of CR or OR. The reported re-dislocation rates vary between 0% and 18%.[4, 30, 31] In a multicentre, prospective study, a re-dislocation rate of 9% after CR was reported. In the CR group of our study, the subluxation rate was 3.77%. This may be related to our emphasis on the stability of reduction (safe zone > 30˚) and shows that the stability was important for the maintenance of reduction. Although OR can maximize the stability after reduction, up to 14% of the patients may present with re-dislocation.[33, 34] There was no re-dislocation in our OR group, which may be related to the small sample size and biased sample selection.
Previous studies have suggested that OR increases the risk of osteonecrosis,[31, 35] but other studies do not support this conclusion.[10, 36] Here, there was no significant difference in the presence of AVN between the two groups. Because of the lack of follow-up to bone maturity, the Kalamchi–MacEwen method could not be used for accurate typing of AVN.
An increase of coxa magna after DDH reduction is a manifestation of developmental disorders. If the increase in the width of the epiphyseal nucleus of the femoral head is >15% of the normal side, it may eventually lead to adverse consequences.[22, 37] Our study indicated that overgrowth of the femoral head was related to OR. Imatani et al. observed that even if children receiving OR had concentric and matched reduction, excessive growth of the femoral head cartilage occurred several months postoperatively and resulted in a poorer prognosis. Excessive growth after OR may be related to the abnormal mechanical stress caused by acetabular dysplasia or subluxation and/or induction of synovitis in the hip joint that may increase the vascularization of the femoral head, activate chondrocytes, and lead to overgrowth of the femoral head.[38, 39] Furthermore, overgrowth of the femoral head may be related to changes in microcirculation within the proximal femur after OR; however, more studies are needed to confirm this.
The orientation of the proximal femur epiphysis (OPFE) is an important reference for the morphological development of the proximal femur and serves as the basis for the diagnosis and early surgical intervention of Kalamchi–MacEwen type II developmental disturbance.[33, 40] The AA can be used as an index to measure OPFE. In children with successful CR, the AA reached the level found in normal children (approximately 73˚) by the age of 5. Here, the increased AA in the OR group [Fig.3] indicated that OR interfered with the development of the OPFE, which has not been reported previously. Special attention should be paid to long-term follow-up for patients with DDH who undergo OR to determine the incidence of type II development disorder during the bone maturation period.
Our study has the following limitations. First, a retrospective study inevitably presents selective bias. Second, we only compared the AI and IHDI classification of the two groups before reduction; thus, other pathological factors may have hindered reduction. Third, with short-term follow-up, the hip joint did not reach the mature developmental state, making it impossible to accurately compare AVN and the reoperation rate of the two groups. Fourth, bilateral DDH is an important factor for poor prognosis; however, here, there were few bilateral cases, and stratified analysis was not possible. Therefore, it was not clear whether there were differences in the outcomes of CR or OR in bilateral DDH. Fifth, OR can increase stability after reduction and maintain the reduction with a relatively small abduction angle, but this effect was not specifically evaluated.
In summary, although OR can achieve maximum concentric reduction, it had a potential risk leading to development disturbance of the proximal femur, due to the invasiveness of OR and the vulnerability of the femoral head in infants and young children. In children aged 6–24 months with DDH, if a stable and safe CR can be obtained but with a widening joint space, an OR may not benefit acetabular remodelling more than a CR procedure. Therefore, CR should be attempted for these patients firstly, although further research is warranted to determine the long-term outcomes of CR and OR.