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.

中文翻译：

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评估儿童髋臼指数时如何判断骨盆位置不良？三个简单的参数可以确定可接受性。

背景髋臼指数（AI）是诊断髋关节发育不良的最常用参数。骨盆位置不正确会导致对AI测量的误解，尤其是在年幼儿童中。我们的目的是通过使用数字重建X射线照片（DRR）来研究骨盆方向与髋臼指数（AI）之间的相关性，并在平片上确定可预测骨盆方向的可靠参数。方法我们回顾性鉴定了接受双源CT检查的33名儿童（52髋）。扫描后重建虚拟骨盆模型。定向到标准中立位置后，将模型旋转并围绕相应的轴倾斜。在此过程中，每3°产生一次DRR。髋臼指数 由两个独立的观察者在每个DRR上测量双侧闭孔器孔的水平直径（Dh）和垂直直径（Dv），耻骨联合上缘之间的垂直距离（h）和Hilgenreiner线。计算AI的旋转指数（Rr =右Dh /左Dh），倾斜指数（Rt = h / Dv），观察者内误差和观察者间误差。结果对于倾斜和旋转直至12.0°，AI随前倾而增加，而随后倾而下降。对于旋转，它在骨盆旋转的一侧增加，而在相反的一侧减少。如果角度超过6.0°，则AI会发生巨大变化。低于此极限的位置不当表明观察者内和观察者间误差分别为±2.0°和±3.0°，并且对AI测量没有明显影响。结论对于6岁以下的儿童，当Rt大约在0.9和1.4之间且Rr在0.7和1.5之间时，可以确定可接受的骨盆平片。首次，我们从一组对象中识别出可以预测这种错位程度的参数。可以在X射线上测量闭孔直径（Dh），闭孔高度（Dv）和共骨与Hilgengreiner线之间的距离（h）的参数，并在临床实践中用于评估小儿骨盆X线片的可接受性AI。