Variations in AP and Frog-Leg Pelvic Radiographs in a Pediatric Population

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Abstract
Introduction: It is common for pediatric patients with a history of hip dysplasia to undergo anteroposterior (AP) and frog-leg (FL) lateral radiographs to monitor development of the hip. Because of sensitivity of pediatric tissues to ionizing radiation, we sought to document the relationship between AP and FL radiographs and to determine which image was better to assess acetabular dysplasia. Methods: An Institutional Review Board–approved, retrospective review was conducted on 33 patients screened for hip dysplasia at a single institution. We sought to determine whether either film was more likely to be qualitatively better in position and whether there were differences in acetabular measures between the 2 radiographs. Standard measurements on AP and FL radiographs were made by 5 observers on 2 different occasions to assess interobserver and intraobserver variability. Results: The mean age was 23+1 months, and 80% were female; none of the children had hip dysplasia. There was no statistical significance in the quotient of pelvic rotation (AP 1.2+0.1 degrees and FL 1.1+0.1 degrees; P=0.84). There was a statistical difference in obturator height in mm (AP 16.0+0.3 and FL 17.8+0.2; P=7.2E−10). The mean intraobserver variability for the acetabular index was 1.8+0.2 and 1.7+0.2 degrees for AP and FL alignments, respectively (P=0.58), and the mean interobserver variability was 1.7+0.1 and 1.7+0.1 degrees, respectively (P=0.75). The measurements of linear variability between the AP and FL showed no statistical significant difference with the exception of the left Hilgenreiner distance in mm (AP 1.00+0.08 and FL 0.81+0.08; P=0.02). Discussion: These results demonstrate that there is little clinical difference in variability of positioning and rotational variables between AP and FL radiographs for hip dysplasia. These results indicate that either image could be used assess to acetabular dysplasia thus decreasing risks of ionizing radiation. Level of Evidence: Level III.

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