Reliability and Radiographic Correlation of the Foot Posture Index-6: A Multi-Rater Analysis in Symptomatic and Asymptomatic Individuals.

Abstract

Background/Objectives: The foot posture index (FPI-6) is a practical clinical tool for evaluating standing foot posture using six specific criteria. Although widely used, its reliability and correlation with radiographic parameters remain uncertain. This study aimed to assess the inter-rater reliability of the FPI-6, in both asymptomatic individuals and patients with foot and ankle symptoms, and to examine its correlation with radiographic measurements. Methods: We included 40 asymptomatic male volunteers (group A) and 60 symptomatic patients (group B). Four raters independently assessed the FPI-6 scores, and inter-rater reliability was evaluated using the intraclass correlation coefficient. Radiographic parameters included the talocalcaneal angle (TCA) on anteroposterior (AP) and lateral views, talonavicular coverage angle (TNCA), AP talo-first metatarsal angle (TMA), hindfoot alignment angle (HAA), calcaneal pitch angle (CPA), and Meary's angle (MA). Correlations between the FPI-6 and radiographic measurements were analyzed using Pearson's correlation (r). Results: The FPI-6 showed good to excellent inter-rater reliability in both groups, with higher consistency in group B and among experienced raters. The total FPI-6 score significantly correlated with TNCA (r = 0.665), AP TMA (r = 0.453), lateral TCA (r = 0.369), MA (r = 0.570), and HAA (r = -0.773) (all p < 0.001). Group B demonstrated overall stronger correlations between the FPI-6 and radiographic measurements compared to group A (TNCA: 0.664 vs. 0.258; AP TMA: 0.542 vs. 0.139; lateral TCA: 0.492 vs. -0.101; MA: 0.544 vs. 0.172; and HAA: -0.712 vs. -0.374). Conclusions: With careful application, the FPI-6 is a reliable and valid tool for clinical assessment of foot posture, especially in settings without immediate access to radiographs.