Cross-Cultural Adaptation and Validation of the Foot Function Index for Use in German-Speaking Patients with Foot Complaints

Background: The purpose of this study was to cross-culturally adapt the Foot Function Index (FFI) for use in German-speaking patients with foot complaints. Materials and Methods: The FFI was adapted for the German language according to the recommended forward/backward translation protocol. The following metric properties were assessed in 53 consecutive patients (mean age, 57.2 years; 39 women) undergoing foot surgery at our department: feasibility, reliability (intraclass correlation coefficient ICC), internal consistency (Cronbach's alpha, CA), and construct validity (correlation with the Short Form (SF-36), a VAS assessing pain, a VAS assessing function, and the University of California at Los Angeles (UCLA) activity scale. Results: The German FFI (FFI-D) comprised 18 items separated into a pain and a disability subscale. Completion of the FFI-D was feasible. The reliability and the internal consistency were both excellent with an ICC of 0.98 and a CA of 0.97 for the total score. We found moderate to high correlations between the FFI-D and the physical related SF-36 domains (r = -0.43 to -0.80), the VAS pain (r = 0.81), the VAS function (r = 0.77), and the UCLA (r = -0.52). Correlation coefficients between the FFI-D and the mental health related SF-36 domains were significantly lower (r = -0.08 to -0.32; p < 0.01), indicating divergent validity. Conclusion: The German version of the FFI is a reliable and valid questionnaire for the self-assessment of pain and disability in German-speaking patients with foot complaints.