Acetabular index and acetabular depth ratio in newborns and infants aged 6 months or less with the healthy development of hips: A retrospective cross-sectional study.

Radiographic assessment of the hip may render critical in the diagnosis of developmental dysplasia of the hip (DDH) in newborns and infants aged ≤6 months. There is no complete dataset on the acetabular index (AI) and acetabular depth ratio (ADR) values in this age group. The objective of this study was to assess the AI and ADR values in newborns and infants aged ≤6 months with healthy development. A retrospective analysis was performed on pelvic radiographs of newborns and infants (≤6 months) between August 2020 and September 2021. There were 3000 children with pelvic radiographic imaging. Normal sonographic findings and radiographs without any structural deformity of the hip were inclusion criteria. A total of 1132 newborns and infants (2264 hips) were analyzed. Measurements of AI and ADR (ischium and pubic bone as landmarks for acetabular depth ratio A [ADR-A] and acetabular depth ratio B [ADR-B]) were performed. Correlation and intraclass correlation coefficient (ICC) values were calculated. Left-sided AI values were significantly higher than the right-sided AI values, except in infants aged 4 to ≤5 months (P < .05). ADR-B values differed significantly between male and female newborns and infants both in terms of the side of the hip measured and age (P < .05). AI values were fairly correlated with age (r = -0.286 for left and r = -0.254 for right) in the negative direction and with ADR-A (r = 0.449 and r = 0.469 for left and right) and ADR-B (r = 0.545 and r = 0.592 for left and right) in the positive direction. Inter-observer ICC was 0.845 to 0.989 (excellent) for AI, 0.534 and 0.904 (moderate to excellent) for ADR-A, and -0.014 and 0.774 (slightly good to good) for ADR-B. Intra-observer ICC was 0.811 to 0.996 (excellent) for AI, 0.575 to 0.98 (moderate to excellent) for ADR-A, and 0.023 to 0.954 (slightly good to excellent) for ADR-B. This study features the first complete data set of AI and ADR measurements, which are essential for pelvic radiographic imaging of hip dysplasia, in newborns and infants aged ≤6 months.