Validity and diagnostic accuracy of foot posture Index-6 using radiographic findings as the gold standard to determine paediatric flexible flatfoot between ages of 6-18 years: A cross-sectional study

Background: Diagnosis of flexible flatfoot is usually based upon radiographic or clinical measures. Radiographic measures pose a potential risk of radiation exposure especially in Paediatric population. Clinicians need a valid, accurate, easily used, noninvasive and cost effective measure to evaluate static foot posture clinically. Although, foot posture index-6 (FPI-6) are commonly used in clinical practice, its validity and diagnostic accuracy in evaluation of paediatric flexible flatfoot have not been fully proven yet.

Objectives: To investigate validity and diagnostic accuracy of FPI-6 to determine Paediatric flexible flatfoot between ages of 6-18 years using radiographic findings as the gold standard measure.

Study design: Cross-sectional study.

Methods: A cross-sectional study conducted on 612 participants (1224 feet) with flexible flatfoot aged 6-18 years (mean age ± standard deviation of 12.36 ± 3.39 years). The results of FPI-6 were compared to the gold standard radiographic measures and displayed on the receiver operating characteristic curve. Intra-rater reliability, sensitivity, specificity, predictive values and likelihood ratios were calculated. Posttest probability was calculated from Fagan nomogram.

Results: FPI-6 demonstrate high intra-rater reliability (ICC = 0.96) with p value < 0.001. FPI-6 showed a sensitivity of 83.7%, a specificity of 80.4, a positive predictive value of 64.7, a negative predictive value of 92, a positive likelihood ratio of 4.62 and a negative likelihood ratio of 0.20. FPI-6 shows moderate diagnostic accuracy [AUC = 0.82; 95%CI (0.78-0.85)].

Conclusion: FPI-6 is valid with moderate diagnostic accuracy to determine paediatric flexible flatfoot between ages of 6-18 years.