Our aim in this study was to analyze the footprint measurements of flatfoot in a population of preschool-aged children. Three footprint measurements, the Clarke's angle (CA), Chippaux-Smirak index (CSI), and Staheli arch index (AI), were used for comparison with clinical diagnosis. A total of 2,638 static footprints of children aged from 3 to 6 years were recorded. The clinical diagnosis as a gold standard compared with the results of the CA, CSI, and AI and displayed in a receiver operating characteristic (ROC) curve. In order to illustrate the diagnostic accuracy in clinical settings, their likelihood ratios were calculated given their cutoff points, and their pretest/posttest probabilities were plotted as the Fagan nomogram. The optimal cutoff points for CA, CSI, and AI were 14.04°, 62.70%, and 107.42%, respectively, and all of them showed high sensitivity. The areas under curves were 0.91, 0.95, and 0.92, respectively. The positive predictive values were 0.84, 0.91, and 0.85, and the negative predictive values were 0.82, 0.85, and 0.85, respectively. The positive likelihood ratio values for CA, CSI, and AI were 4.09, 7.52, and 4.61, and the negative likelihood ratio values were 0.18, 0.14, and 0.13, respectively. In conclusion, this study demonstrated that footprint analysis methods are suitable for diagnosing flatfoot in preschool-aged children, and that the most appropriate cutoffs are as follows: CA ≤ 14.04°, CSI > 62.70%, and AI > 107.42%. The CSI had a predictive probability of more than 90% and is recommended in screening for flatfoot in preschool-aged children.
