Translation and validation of the Danish Foot Function Index (FFI-DK) J. E. Jorgensen 1 , J. Andreasen 2 , M. S. Rathleff 2 1 Private Practice Clinic, Dronninglund, Denmark, 2 Department of Occupational and Physiotherapy, Aalborg University Hospital, Aalborg, Denmark Corresponding author: Jens Erik Jorgensen, MScPT, Klinik for Fysioterapi, Roerholtvej 3A, Dronninglund 9330, Denmark. Tel: +45 30 23 95 85, +45 98 84 28 00, Fax: +45 98 84 44 13, E-mail: jeja26@gmail.com Accepted for publication 30 August 2014 The objective of this study was to translate the Foot Func- tion Index (FFI) for use in Danish-speaking patients with foot complaints. The FFI consists of 23 items scored on a numeric rating scale from 0 to 10. The 23 items are grouped into three subscales: pain (nine items), activity limitation (five items), and disability (nine items). The Danish FFI was developed according to the recommended forward/backward translation protocol. The data analy- sis included reliability [intraclass correlation coefficient (ICC) 2.1] and internal consistency (Cronbach’s alpha). Excellent internal consistency was shown for the three subscales: pain (0.99), disability (0.98), and activity limi- tation (0.98), as for the total score (0.97). The test-retest reliability was excellent: pain subscale: ICC 0.98 [95% confidence interval (CI): 0.97–0.99]; activity limitation subscale: ICC: 0.95 (95% CI: 0.91–0.98); disability subscale: ICC 0.97 (95% CI: 0.95–0.98); total score: ICC: 0.95 (95% CI: 0.91 to 0.98). The mean difference between test and retest was below 1 point and P > 0.08. Bland– Altman plots showed no significant or clinically relevant differences from test to retest in any of the subscales or in the total score. The Danish version of the FFI was found to be valid and reliable and therefore acceptable for use in the Danish population. Approximately 25% of the adult population will, at any given time, report foot pain (Hawke & Burns, 2009; Mølgaard et al., 2010) and almost 10% of adolescents between 12 and 19 years of age report having foot pain (Rathleff et al., 2013). Thirty-three percent of athletes sustain a foot or ankle injury each year with the highest incidence in ballet and running (Sobhani et al., 2013). Almost 40% of runners will sustain a foot-related injury each year with 8% relating to fascia plantaris (van Gent et al., 2007; Hespanhol Junior et al., 2013). Among all foot-related injuries, plantar fasciitis is one of the most common foot conditions and has a lifetime prevalence of 10% (Kindred et al., 2011). Collectively, this highlights that foot pain is prevalent in both the general population and among those involved in sports. Foot pain can be very disabling and underlines the need of having an instrument capable of measuring the condi- tion and evaluating the effect of treatment (Haywood, 2006, 2007). A systematic review by Hunt and Hurwit (2013) showed the FFI to be the fifth most used patient- reported outcome measure within the orthopedic literature on foot and ankle research. It was originally developed for foot complaints in patients with rheumatoid arthritis (RA) but is also valid and reliable in other foot conditions such as plantar fasciitis, posterior tibial tendinitis, and ankle instability (Bal et al., 2006; Grondal et al., 2006; SooHoo et al., 2006; Budiman-Mak et al., 2013). The FFI consist of 23 items grouped into three subscales: pain (nine items), activity limitation (five items), and disability (nine items). The FFI aligns with the intentions of the International Classification of Functioning, Disability and Health (ICF, 2010). The FFI has been translated into several languages (Kuyvenhoven et al., 2002; Naal et al., 2008; Wu et al., 2008; Martinelli et al., 2014; Paez-Moguer et al., 2014) and has been revised in several versions over the years (Naal et al., 2010). The FFI has been found to be a feasible instrument, easily calculated, easy to use across sectors and practitioners, and requires less than 10 min to answer (Lau et al., 2005; Naal et al., 2010; Budiman-Mak et al., 2013). However, the FFI does not exist in Danish. The aim of the study was therefore to translate and undertake a cross-cultural adaptation of the FFI to a Danish context including a face validation by cognitive interviewing. Methods Ethical approval The study was approved by The Danish Data Protection Agency (j.nr 2009-41-4040). The ethics committee in the region of North Jutland, Denmark, stated that no approval was necessary as the study included no intervention (j.nr N-20090051). Informed oral consent was obtained from all participants in the study. The study was in compliance with the Declaration of Helsinki. Scand J Med Sci Sports 2015: 25: e408–e413 doi: 10.1111/sms.12331 © 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd e408