Developmental dysplasia of the hip (DDH) is a common musculoskeletal disorder in pediatric orthopedics, its prevalence in the population ranges from 0.1% to 3.4% (1). The pathological process includes hip dysplasia, subluxation, and complete dislocation (2). Osteoarthritis will develop more rapid and have to be treated by arthroplasty eventually if DDH is not identified timely and not treated in early childhood (3, 4). However, there is a substantial likelihood of residual acetabular dysplasia (RAD) even after systematic therapy in infancy and early childhood, and acetabuloplasty is needed to protect the hip in the later stage (5–8). There is still no uniform standard for the surgical timing and indications of RAD, so if some predictors can be identified to accurately assess and forecast the developmental prognosis after treatment, it can help to guide the timing of correction or avoid unnecessary surgery.

Previous researchers have proposed the use of acetabular index (AI), center edge angle (CEA), Reimer's index (RI), and center-head distance discrepancy (CHDD) as predictors of surgical indications (4, 9–16). However, all of these indices are measured in radiographs and only reflect bony acetabular development, not cartilaginous acetabular development, which represents the true potential of acetabular development (17–20). Therefore, some cartilaginous indicators based on MRI or hip arthrography have been proposed to be used as early warning indicators of RAD, such as cartilaginous acetabular index (CAI), cartilaginous centre-edge angle (CCE), labral acetabular index (L-AI) [It is also called Acetabular Cartilaginous Angle (ACA) by Zamzam et al. (20)], labral centre-edge angle (L-CEA) [It is also called the center-edge of the acetabular limbus angle (CEALA) by Satsuma et al.] (Figure 1). This meta-analysis summarized common cartilaginous predictors for assessing developmental prognosis after early treatment of DDH. And the most commonly used indicators by clinicians and researchers—CAI and L-AI (We define them as CAI group), CCE and L-CEA (We define them as CCE group), were combined separately to compare their diagnostic and prognostic performance.

This systematic review and meta-analysis was registered with PROSPERO (CRD42022338332) and was conducted based on the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines (21).

Even after successful early treatment, coverage of acetabulum on the femoral head may not be able to recover in children with DDH. Residual acetabular dysplasia (RAD) is a manifestation after early treatment of DDH and a common cause of secondary osteoarthritis (17, 35). David et al. (36) reported that 4% of children with early successful Pavlik harness treatment still had dysplasia. Malvitz and Weinstein (37) reported 54% of patients who underwent successful closed reduction remained dysplasia at long-term follow-up (mean, 30 years). Terjesen et al. (38) reported 38% of patients who underwent closed reduction remained dysplasia at skeletal maturity and 23% had undergone total hip replacement at a mean age of 43.7 years.

Previous scholars used bony indicators to evaluate the diagnosis of RAD and the determination of surgical indications (4, 9–16). However, many studies have shown that cartilage coverage represents the true potential of acetabular development (17–20). Therefore, using only bone indicators to evaluate RAD will lead to many unnecessary operations when acetabular coverage still had the potential for spontaneous improvement (39). Increasingly researchers are aware of this problem, so other indicators representing cartilage coverage have been proposed, and we have summarized these indicators in Table 1. It can be seen that research on cartilage indicators has emerged in recent years, and most of them have been carried out in Asia. All of the 9 studies included have adopted a case-control design. These studies basically selected children with DDH who had undergone closed or open reduction in the early stage for the follow-up and finally obtained the prediction results of a certain cartilage index for these patients. It is worth mentioning that most studies (6/9) selected Severin classification as the gold standard for judging the maturity of children, which is consistent with our meta-analysis because studies have confirmed the strong correlation between Severin classification and hip degenerative diseases (37, 38). Three studies did not select Severin classification as the gold standard, of which Satsuma 2016 and Johnson 2022 were included in our quantitative synthesis. The reference standards for these two studies were “the center edge angle of Wiberg at age 18 years” and “the 90th percentile of normal boney AI values”. Although we believe that both of them have the ability to distinguish between illness and health, we have marked their applicability to this study as “unclear” in the “Assessment of Risk Bias of included studies”, which has a minor impact on the overall applicability (Figure 3A). In addition, about the question “when does the potential for acetabular remodeling persist after early treatment”, the minimum age reported in the literature is 2 years old (40) or 2 years after treatment (15), and the maximum age is 11 years old (41). The follow-up time of all included studies exceeded the minimum acetabular remodeling time.

Six of the nine qualitative synthetic studies searched CAI, CCE, L-AI, and L-CEA, so we performed quantitative synthesis to compare who had better diagnostic and prognostic efficacy. There are two points worth noting. First of all, CAI and CCE can be measured by MRI or arthrography, which are two different imaging methods. Secondly, from the perspective of the anatomical structure of the acetabulum, some studies have only measured the angle of the cartilage part, that is, CAI and CCE (18, 29, 31, 34); other studies on the measurement of the angle include both the cartilage part and the labrum part, named L-AI and L-CEA (28, 33) (Figure 1). Although the two methods have different angles, they belong to the same type of measurement in principle, so we merge them into the same group and redefine them as the CAI group and CCE group. Different imaging modalities and different measurements may be important reasons for the heterogeneity caused by non-threshold effects. However, due to the small number of studies included in the two groups (both 4 studies), we did not perform subgroup analysis or regression analysis. Apart from the above two reasons, we found some other reasons that may cause heterogeneity: (1) different population distribution; (2) different gold standard selection. The specific effects of these factors will be revealed by adding more studies to a meta-analysis in the future.

Current studies have compared the prognostic performance of CAI and CCE grossly. Takeuchi et al. (18) found that CCE was more reliable than CAI in predicting the future development of acetabular. However, as two different measurement methods, there is no meta-analysis to explore which way has better diagnostic and prognostic efficacy. We innovatively combine the cartilage index (such as CAI) and the labrum index (such as L-AI) and then merge 6 original articles (503 patients) to compare the performance of the two measurement methods. From the results of this study, it can be seen that compared with the CCE group, the CAI group had higher sensitivity, DOR, and AUC in the prognosis assessment of DDH, indicating that the CAI group had better accuracy in RAD diagnosis and prompt.

The AUC of the two groups exceeded 0.80, indicating that both of them have good diagnostic accuracy. AUC values of 0.50 to 0.70, 0.70 to 0.90, and ≥0.90 represent low, moderate, and high diagnostic accuracy, respectively (42). As Onaç et al. (8) concluded that a series of indicators measured in MRI including CAI had a good correlation with persistent dysplasia, and these indicators could be used to predict the RAD of DDH patients after early treatment. However, when it came to specific clinical applications, Fagan plot analysis showed that in the case of a predicted probability of 50%, the CAI group and the CCE group had a 75% and 76% probability of correctly detecting positive results, which was, acetabular dysplasia. It can be seen that it is doubtful to use them as a single surgical indication, because it may cause overtreatment in more than 20% of cases. Therefore, for the prediction of RAD and the determination of surgical indications, it may be more accurate to use a combination of multiple indicators for a more comprehensive assessment.

This study also has some limitations. First of all, the studies included in the quantitative synthesis did not avoid the case-control design, and the threshold of the index was not predetermined but was calculated by the receiver operating characteristic (ROC) curve, which led to the low quality of the included studies in “patient selection” and “index test”. Secondly, the number of literature included in this meta-analysis is small, and we have not been able to perform subgroup analysis or meta-regression to explore possible sources of heterogeneity. Therefore, in the future, we need to include more studies to update this meta-analysis, and also expect more large cohort studies to reveal the clinical application of CAI and CCE in the prognosis of DDH.

In summary, the current research results provide some cartilage acetabular coverage indicators to predict RAD after early treatment, with CAI/L-AI and CCE/L-CEA most commonly used. Overall, these two groups have good diagnostic accuracy, the CAI group has a little edge over the CCE group in terms of accuracy. They serve as a valuable resource for acetabular development and the execution of subsequent orthopedic surgery, however, there is still more research needed to determine whether they can be used as independent indications for secondary orthopedic surgery.