Equivalence of Anteroposterior and Frog-Leg Lateral Radiographs in Measuring Acetabular Index in Children

Background Developmental dysplasia of the hip (DDH) is one of the most common pediatric musculoskeletal conditions, with an incidence of 1 in 1,000 live births. The acetabular index (AI), measured on anteroposterior (AP) radiographs, is the primary radiographic tool for DDH diagnosis and monitoring. However, its accuracy is critically dependent on pelvic positioning; suboptimal studies frequently require repeat imaging, resulting in additional radiation exposure for the child. Objective To determine whether frog-leg lateral (FL) radiographs are interchangeable with AP radiographs for AI measurement in children being evaluated for suspected DDH. Methods In this retrospective, IRB-approved study, bilateral AP and FL hip radiographs from 100 pediatric patients (aged 6–24 months) were evaluated at a single tertiary pediatric center. Only AP studies meeting strict pelvic positioning criteria (rotation index 0.5–2.0; tilt index 0.9–1.4) were included. Four readers, including three pediatric radiologists and a pediatric orthopedic surgeon, independently measured bilateral AI on each view on two separate sessions, with a 4-week washout between sessions. Interchangeability was assessed using the Individual Equivalence Index (IEI) with a 3° margin. Reliability was assessed with intraclass correlation coefficients (ICC). Results The √IEI was 2.07°, with a one-sided 95% upper confidence bound of 2.34°, both below the 3° interchangeability margin. Intra-reader ICCs ranged from 0.845 to 0.898 across both views. Three of four readers showed slightly higher repeatability on the FL view. Inter-reader ICC was 0.783 (95% CI: 0.720–0.833) for AP and 0.807 (95% CI: 0.744–0.854) for FL. Conclusions AI measurements from FL radiographs are interchangeable with those from AP radiographs. When the AP view is suboptimally positioned, the FL view may be used for AI measurement, potentially eliminating repeat imaging and reducing radiation exposure in this pediatric population.