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Background  The acetabular index (AI) is the most commonly used parameter for diagnosing hip dysplasia. Pelvic malposition can result in misinterpretation of AI measurement especially in younger children. We aimed to investigate the correlation between pelvic orientation and acetabular index (AI) by using digital reconstructed radiographs (DRRs) and identify reliable parameters predictive of pelvic orientation on plain radiographs.    Methods  We retrospectively identified 33 children (52 hips) who received dual source CT examinations. Virtual pelvic models were reconstructed after scanning. After orientating in the standard neutral position, the models were rotated and tilted around corresponding axes. DRRs were generated at every 3° during the process. The acetabular index, the horizontal diameter (Dh) and vertical diameter (Dv) of bilateral obturator foramina, the vertical distance (h) between upper border of pubic symphysis, and Hilgenreiner’s line were measured on each DRR by two independent observers. Rotation index (Rr = right Dh/left Dh), tilt index (Rt = h/Dv), intra-observer error, and inter-observer error of AI were calculated.    Results  For tilt and rotation up to 12.0°, AI increased with anterior tilt and decreased with posterior tilt. And for rotation, it increased on the side toward which the pelvis rotated and decreased on the opposite side. AI varied dramatically if angulation exceeded 6.0°. Malposition below this limit demonstrated the intra- and inter-observer errors were ± 2.0° and ± 3.0° respectively and caused no significant effect on AI measurement.    Conclusions  For children up to age 6 years, an acceptable pelvic plain radiograph can be determined when Rt is approximately between 0.9 and 1.4 and Rr between 0.7 and 1.5. For the first time, we have identified parameters derived from a group of subjects which can predict this degree of malposition. The parameters obturator diameters (Dh), obturator height (Dv), and distance (h) between symphysis and Hilgengreiner’s line can be feasibly measured on X-ray and employed in clinical practice to assess the acceptability of the pediatric pelvic radiograph prior to measurement of the AI.

How to judge pelvic malposition when assessing acetabular index in children? Three simple parameters can determine acceptability