Evaluation of the validity of the Foot Function Index in measuring outcomes in patients with foot and ankle disorders

BACKGROUND: There is uncertainty regarding which outcomes tools should be used to report the results of treatment for patients with foot and ankle disorders. This study evaluates the validity of the Foot Function Index (FFI) by examining its level of correlation to the Medical Outcomes Study Short Form-36 (SF-36). The SF-36 is an extensively validated outcomes tool that has been used as a benchmark in examining the validity of several orthopaedic outcomes tools. METHODS: Seventy-three patients were recruited at a tertiary referral foot and ankle practice. Patients completed packets which included informed consent forms, the FFI, and the SF-36 questionnaires. The questionnaires were scored and Pearson correlation coefficients were determined between the three domains of the FFI and the eight SF-36 sub-scales, as well as the two SF-36 summary scales. RESULTS: Sixty-nine patients completed an adequate number of items to be included in the study. The mean age of the patient sample was 46 (range 16 to 82) years and 44 were women (64%). Twenty-one patients (30%) had conditions affecting the forefoot, while 48 patients (70%) had conditions affecting the ankle or hindfoot. All three FFI domains had moderate to high levels of correlation to many of the SF-36 scales. The Disability domain of the FFI had the most consistent level of correlation to the SF-36 with Pearson coefficients in the range of -0.23 to -0.69. The Activity Limitation (r=-0.28 to -0.64) and Pain domains (r=-0.10 to -0.61) also demonstrated moderate levels of correlation to several of the SF-36 scales. CONCLUSIONS: The consistently moderate to high levels of correlation of the FFI to the SF-36 seen in this study support the FFI as a valid measure of health status. This suggests that the FFI is a reasonable method to monitor patient outcomes. Future studies should focus on determining if the FFI improves responsiveness to clinical change when used in combination with generic instruments like the SF-36.