17-Italian Foot Function Index with numerical rating scale: Development, reliability, and validity of a modified version of the original Foot Function Index

Background: Clinical research quantifies symptoms and signs of pain. Objective: To develop a brief outcome measure to assess foot and ankle conditions, the psychometric properties of a modified version of the original Foot Function Index (FFI) were examined. Methods: Eighty-six subjects with musculoskeletal foot and ankle disorders were enrolled. The internal consistency and test-retest reliability were evaluated by using Cronbach's α and intraclass correlation coefficient (ICC). Criterion validity was tested by Pearson's correlation coefficient between 17 items of the Italian FFI (17-IFFI) and the Lower Extremity Functional Scale (LEFS). The responsiveness was calculated using the receiver operating characteristic curve (ROC). Results: Cronbach's Alpha was 0.95 (95% CI: 0.92, 0.99). The intra-interviewer and inter-interviewer ICC values were, respectively, 0.92 (95% CI: 0.88-10 0.96) and 0.90 (95% CI: 0.89-0.94). Correlations between the 17-IFFI scores and the LEFS scores were -0.564 and -0.456 at the initial and at the end of the treatment, respectively. The ROC analysis revealed an area under the curve of 0.732 (95% CI: 0.61-0.82) for the 17-IFFI and 0.633 (95% CI: 0.52-0.71) for the LEFS score. Conclusions: The 17-IFFI is a reliable and valid scale and we recommend its application to evaluate the effectiveness of a treatment in patients with musculoskeletal foot and ankle disorders.