Inter-rater and Intrarater Reliabilities of the Identification of a “Gothic Arch” in the Acetabulum of Children With Cerebral Palsy

BACKGROUND: Progressive hip displacement in children with cerebral palsy (CP) is monitored by measuring migration percentage (MP) on anteroposterior (AP) pelvis radiographs. Accurate measurement of MP requires the lateral margin of the ossified acetabulum to be identified for the placement of Perkin’s line. It has been suggested that when there is an erosion of the acetabular rim, described as a gothic arch, the midpoint of the arch be used for the placement of Perkin’s line. However, this requires that there be agreement on what constitutes a gothic arch. The purpose of this study was to evaluate the inter-rater and intrarater reliabilities of identifying a gothic arch on pelvis radiographs. METHODS: An online survey with 100 AP pelvis images (200 hips) of children with CP was sent to international experts. Participants were asked to identify which hip(s) had a gothic arch (left, right, both, and neither). The Fleiss κ statistic for inter-rater reliability was calculated. Eight weeks later, the images were shuffled and redistributed to calculate intrarater reliability. RESULTS: The initial survey was completed by 10 participants with 9 participants completing the second survey. The average inter-rater κ value was 0.18 [95% confidence interval (CI), 0.14-0.23] and 0.19 (95% CI, 0.14-0.24) for the 2 surveys, respectively. Among the pediatric orthopaedic surgeons subgroup, the κ values were 0.06 (95% CI, 0.02-0.1) and 0.08 (95% CI, 0.03-0.13). The average intrarater reliability κ value was 0.61 (95% CI, 0.2-1), ranging from 0.32 to 0.86. CONCLUSIONS: There were poor inter-rater and moderate intrarater reliabilities in identifying a gothic arch on AP pelvis radiographs in children with CP. Further characterization and clarification of what constitutes a gothic arch are required. The lack of agreement on the identification of a gothic arch may negatively impact the measurement of MP and referrals to a pediatric orthopaedic surgeon.