Acetabular index is the best predictor of late residual acetabular dysplasia after closed reduction in developmental dysplasia of the hip

Purpose Our objective was to find the best predictor of late residual acetabular dysplasia in developmental dysplasia of the hip (DDH) after closed reduction (CR) and discuss the indications for secondary surgery. Methods We retrospectively reviewed the records of 89 patients with DDH (mean age 16.1 ± 4.6 months; 99 hips) who were treated by CR. Hips were divided into three groups according to final outcomes: satisfactory, unsatisfactory and operation. The changes in the acetabular index (AI), centre-edge angle of Wiberg (CEA), Reimer’s index (RI) and centre-head distance discrepancy (CHDD) over time among groups were compared. The power of predictors for late residual acetabular dysplasia of AI, CEA, RI and CHDD at different time points was analysed by logistic regression analysis. Receiver operating characteristics (ROC) curve analysis was used to determine cutoff values and corresponding sensitivity, specificity and diagnostic accuracy for these parameters. Results Both AI and CEA improved in all groups of patients following CR. In the satisfactory group, AI progressively decreased until seven to eight years, while CEA increased until nine to ten years ( P < 0.05). In the unsatisfactory group, AI and CEA ceased to improve three and two years after CR, respectively ( P < 0.05). CEA and RI were significantly better in the satisfactory group compared with the unsatisfactory group at all time points ( P < 0.05). Following CR, both RI and CHDD remained stable over time in all groups. Final outcome following CR could be predicted by AI, CEA and RI at all time points ( P < 0.01). Cutoff values of AI, CEA and RI were 28.4°, 13.9° and 34.5%, respectively, at one year and 25°, 20° and 27%, respectively, at two to four years post-CR. A total of 80–88% of hips had an unsatisfactory outcome if AI > 28.4° and >25 at one and two to four years following CR, respectively. However, if CEA was less than or RI was larger than the cutoff values at each time point, only 40–60% of hips had an unsatisfactory outcome. Mean sensitivity (0.889), specificity (0.933) and diagnostic accuracy (92.1%) of AI to predict an unsatisfactory outcome were significantly better compared with CEA (0.731; 0.904; 78.2%) and RI (0.8; 0.655; 70.8%) ( P < 0.05). Conclusions Satisfactory and unsatisfactory hips show different patterns of acetabular development after reduction. AI, CEA and RI are all predictors of final radiographic outcomes in DDH treated by CR, although AI showed the best results. AI continues to improve until seven years after CR in hips with satisfactory outcomes, while it ceases to improve three to four years after CR in hips with unsatisfactory outcomes. According to our results, surgery is indicated if AI >28° 1 year following CR or AI >25° two to four years after CR. CEA and RI should be used as a secondary index to aid in the selection of patients requiring surgery.