Normal Percentile Reference Curves and Correlation of Acetabular Index and Acetabular Depth Ratio in Children

Radiographic surveillance of the hip is vital in the diagnosis and treatment of developmental dysplasia of the hip (DDH) in children. The acetabular index (AI) and the acetabular depth ratio (ADR) are radiographic parameters for evaluation of acetabular morphology. Normal reference curves for these parameters that allow for serial evaluation of acetabular development in a manner that is independent of age are necessary and clinically useful. The purpose of this study was (1) to establish normal values of AI and ADR in the normally developing pediatric hip up to age 14, (2) to generate percentile reference curves of both parameters, (3) to determine the extent of correlation between AI and ADR, and (4) to assess intrarater and interrater reliability of AI measurement.

We identified 1734 patients who underwent anterior-posterior pelvic radiography between 2004 and 2014. A total of 1152 patients (age range, 0.15 to 13.97 y; 2304 hips) were identified as radiographically normal in the radiology report, signed by the attending pediatric radiologist on the basis of the absence of structural deformity of the hip and previously established reference values for DDH assessment. A review of the medical records confirmed that patients had no diagnosis of DDH or any other orthopaedic hip pathology. The AI and ADR were measured in all radiographs. Normal values and fully parametric percentile curves were generated from birth to skeletal maturity. Correlation between AI and ADR was assessed using linear regression analysis.

Normal AI decreased, and ADR increased, with age. Percentile curves were generated for AI and ADR. Using the provided equations, measured values can be converted to age-appropriate percentile and Z-score. The 2 parameters exhibited strong correlation (Pearson correlation=-0.789, P<0.001). For every unit increase in ADR, AI decreased by 0.94 degrees.

We present updated normative values of AI that expand up to age 14, and novel reference values for ADR. The reference curves allow for the easy conversion of measured values to percentile and Z-score. Using the presented method during surveillance of the pathologic hip, change in acetabular development can now be assessed in a manner that is independent of age and the natural development of the acetabulum.

Level IV-case series.