Comparing Validity and Diagnostic Accuracy of Clarke's Angle and Foot Posture Index-6 to Determine Flexible Flatfoot in Adolescents: A Cross-Sectional Investigation

Clinically, the foot posture index-6 (FPI-6) and Clarke's angle (CA) are widely utilized to assess static foot posture; however, due to scarcity of scientific evidence, clinicians continue to debate the validity and diagnostic accuracy of these measures in children and adolescents. This study aimed to evaluate and compare the validity and diagnostic accuracy of the FPI-6 and CA in determining adolescents' flexible flatfoot between ages of 12 and 18 years, considering radiographic investigation as a reference standard measure. Cross-sectional study. Governmental hospitals. A sample of 460 people (920 feet) with flexible flatfoot between the ages of 12 and 18 (15.23 ± 0.84 years) was enrolled in the study. The findings of the investigated measures were compared to the reference standard radiographic measure and plotted on the receiver operating characteristic (ROC) curve. The area under the ROC curve (AUC) was determined as a measure of FPI-6 and CA diagnostic accuracy. Intra-rater reliability, sensitivity, specificity, predictive values, and likelihood ratios of FPI-6 and CA were calculated and compared. The post-test probability of flatfoot was determined using the Fagan nomogram. CA had a substantially higher intra-rater reliability (ICC=0.99) than the FPI-6 (ICC=0.96), with p-value of 0.001. CA has greater sensitivity (98%) than FPI-6, as well as a superior specificity (99%), positive predictive value (98%), negative predictive value (98.9%), positive likelihood ratio (97), and a lower negative likelihood ratio (0.02). CA had such an area under ROC curve of 0.98 with a 95% confidence interval of 0.95-1.00, while FPI-6 had an area under ROC curve of 0.80 with a 95% confidence interval of (0.77-0.85). FPI-6 and Clarke's angle are both valid and diagnostically accurate clinical tests for flatfoot detection, with Clarke's angle outperforming FPI-6 in adolescents aged 12 to 18 years with a normal BMI.