Reliability and validity of the Taiwan Chinese version of the foot function index

Background/Purpose: To test the reliability and validity of the Taiwan Chinese version of the Foot Function Index (FFI) among patients with plantar fasciitis and ankle/foot fracture. Methods: Fifty plantar fasciitis patients and 29 ankle/foot fracture patients volunteered for the cross-sectional survey and 24 were re-retested later. The response distribution was compared between the two groups. The internal consistency and test-retest reliability were evaluated by using Cronbach's alpha and intraclass correlation coefficient (ICC). Criterion validity was examined for correlations with the Taiwan version of the Medical Outcomes Study Short Form-36 (SF-36) questionnaire. The influence of covariates (diagnosis, age, gender, employment) on the scores of total FFI and subscales was explored by multivariate general linear model analysis. Results: Two items in the pain subscale had many answers in the non-applicable category and were removed from analysis. The internal consistency of the 21-item FFI was high (Cronbach's alpha = 0.94) and the test-retest reliability was satisfactory (ICC = 0.82). The activity limitation subscale had relatively lower Cronbach's alpha (0.75) and the items in this subscale had a high proportion of floor scores. There was a moderate correlation between the FFI total and subscale scores to the physical component summary scores rather than to the mental component summary scores of the SF-36. Multivariate general linear model analysis showed that the cases of plantar fasciitis had higher pain scores and lower activity limitation scores, although their FFI total scores were comparable. Conclusion: The adapted Taiwan Chinese version of the FFI is reliable and valid and can be applied among traumatic and non-traumatic foot disorders. Nevertheless, its clinical application may be limited by a relatively high proportion of non-applicable answers and significant floor effect in some items.