Anterior dynamic ultrasound of the infant hip: evaluation of investigator dependence

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Anterior dynamic ultrasound of the infant hip has been shown to be a valuable tool for detection of developmental dysplasia of the infant hip, but to what extent the results from this method depends on the investigator is unknown. To study to what extent the proportion of positive findings at anterior dynamic ultrasound investigation of infant hips varies with the observer. Information on all ultrasound investigations of infant hips at a county hospital between January 2004 and August 2007 was evaluated. Proportion of findings for different investigators were calculated and in a multivariable logistic regression analysis, risk of treatment by investigating orthopedic surgeon and radiologist were adjusted for each other as well as for calendar period and infant sex. There was a striking variation in proportion of positive findings depending on the investigator. The proportion of infants treated varied with orthopedic surgeon from 4.2 to 53.9% (95% CI: 2.27 to 7.13 and 43.0 to 64.6, respectively) and the corresponding variation for radiologists ranged from 7.4 (95% CI: 4.29 to 11.8) to 23.9% (95% CI: 17.2 to 31.8). In the adjusted analysis, variation between different investigators remained essentially unaltered. Although the true proportions of positive findings in our study are unknown, our findings are unlikely to be explained by selection bias or chance. Thus, our study suggests that anterior dynamic ultrasound of the infant hip is highly dependent on the investigator and further studies of the implications of our results are warranted.