Reliability and validity of the Taiwan Chinese version of the Foot Function Index

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.

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.

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.

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.