Objectives: The objective of this study was to investigate the predictive value of asymmetric joint space widening on simple radiographs for the presence of hip joint effusion in children with unilateral hip pain.

Methods: Sixty-eight patients aged 6 to 17 years with both simple radiograph and magnetic resonance imaging of both hips were retrospectively reviewed. We evaluated the predictive effects of radiographic surrogates for joint space widening on the presence of high-grade hip joint effusion. We also sought to investigate a predictive model for the presence of hip joint effusion with the patients' clinical and radiological data.

Results: Radiographic values showed high specificity but low sensitivity for the presence of high-grade joint effusion. On multivariate analysis, sex and the difference ratio of medial and superior gaps were independent predictive factors for unilateral high-grade hip joint effusion.

Conclusions: For children 6 years and older with asymmetric hip joint space widening on simple radiographs, further evaluations for the presence of hip joint effusion should be considered because of its high specificity. The predictability can improve if both medial and superior gaps, and sex are included in the evaluation. In girls, hip joint effusion seems to widen the joint space more easily than in boys, which may be owing to the more ligamentous laxity of and less musculature around the hip in girls. However, the risk of joint effusion should not be underestimated even if a child presents symmetry on the simple radiograph because of its low sensitivity and the possibility of bilateral involvements or early stage of diseases.