Prevalence of Developmental Dysplasia of the Hip in Japanese Patients with Adolescent Idiopathic Scoliosis: Comparison of Conventional and Age-Adjusted Criteria

Background/Objectives: The prevalence of developmental dysplasia of the hip (DDH) in adolescent idiopathic scoliosis (AIS) remains unclear, partly because of differences in diagnostic criteria and measurement accuracy. Additionally, spinopelvic alignment and skeletal maturation may affect radiographic assessment of acetabular morphology in patients with AIS. This study aimed to clarify the prevalence of DDH in Japanese patients with AIS using standardized radiographic assessment and to compare conventional and age-adjusted diagnostic criteria for DDH. Methods: This cross-sectional study included 602 Japanese patients aged 10–18 years with AIS. Patients with inadequate radiographs, including those with pelvic rotation and lateral inclination, were excluded to improve measurement accuracy. DDH was defined using two criteria: (1) conventional (lateral center-edge angle <20°) and (2) age-adjusted thresholds (<15° for <15 years and <18° for ≥15 years). Radiographic parameters were compared between patients with and without DDH. A multivariate logistic regression analysis was performed to identify factors independently associated with DDH. Results: The prevalence of DDH in AIS was 5.6% (34/602) using the conventional criterion and 1.5% (9/602) using the age-adjusted criteria. Patients with DDH showed significantly lower acetabular coverage, with a lower lateral center-edge angle and acetabular head index and higher Sharp and Tönnis angles than those without DDH (all p < 0.01). No significant difference in the main curve Cobb angle was observed between the groups. A younger age was independently associated with DDH, whereas sex and the main curve Cobb angle were not. Conclusions: The prevalence of DDH in Japanese patients with AIS varies substantially depending on the diagnostic criteria. Standardized radiographic evaluation with exclusion of pelvic rotation provides a reliable estimate. These findings highlight the importance of evaluating acetabular morphology on standing whole-spine radiographs in patients with AIS while considering skeletal maturation and spinopelvic alignment.