Intrinsic risk factors for acute ankle injuries among male soccer players: a prospective cohort study A. H. Engebretsen 1 , G. Myklebust 1 , I. Holme 1 , L. Engebretsen 1,2 , R. Bahr 1 1 Oslo Sports Trauma Research Center, Norwegian School of Sports Sciences, N-0806 Oslo, Norway, 2 Department of Orthopaedic Surgery, Ulleval University Hospital and Medical School, University of Oslo, N-0407 Oslo, Norway Corresponding author: A. H. Engebretsen, MD, Oslo Sports Trauma Research Center, Norwegian School of Sports Science, PO Box 4014, Ulleva ˚l Stadion, N-0806 Oslo, Norway. Tel: 147 23 26 23 57, Fax: 23 26 23 07, E-mail: anders. engebretsen@nih.no Accepted for publication 12 April 2009 This prospective cohort study was conducted to identify risk factors for acute ankle injuries among male soccer players. A total of 508 players representing 31 amateur teams were tested during the 2004 pre-season through a questionnaire on previous injury and function score (foot and ankle out- come score; FAOS), functional tests (balance tests on the floor and a balance mat) and a clinical examination of the ankle. Generalized estimating equations were used in uni- variate analyses to identify candidate risk factors, and factors with a P-value o0.10 were then examined in a multivariate model. During the season, 56 acute ankle injuries, affecting 46 legs (43 players), were registered. Univariate analyses identified a history of previous acute ankle injuries [odds ratio (OR) per previous injury: 1.25, 95% confidence interval (CI) 1.09–1.43] and the FAOS sub- score ‘‘Pain’’ (OR for a 10-point difference in score: 0.81, 95% CI 0.62–1.04) as candidate risk factors. In a multi- variate analysis, only the number of previous acute ankle injuries proved to be a significant (adjusted OR per previous injury: 1.23; 95% CI 1.06–1.41, P 5 0.005) predictor of new injuries. Function scores, functional tests and clinical ex- amination could not independently identify players at an increased risk in this study. The ankle joint is one of the most common injury locations in sports in general and soccer in particular. The injury incidence ranges from 1.7 to 4.5 injuries per 1000 playing hours, accounting for 11–25% of all acute injuries (Ekstrand & Tropp, 1990; A ´ rnason et al., 1996; Juma, 1998; Hawkins & Fuller, 1999; Andersen et al., 2004; Junge et al., 2004). An ankle sprain may leave an athlete out of play for several weeks, and in many cases full recovery takes much longer. Injuries to the ankle are therefore a concern. To possibly prevent new injuries, the specific intrinsic and extrinsic risk factors for the injury type in question must be known (Meeuwisse, 1994). Regarding intrinsic risk factors, it has been suggested that previous injury, especially when rehabilitation is inadequate, places an athlete at an increased risk of suffering an injury to the ankle (Ekstrand & Gill- quist, 1983; Tropp et al., 1985; A ´ rnason et al., 2004; Kofotolis et al., 2007). Several other potential risk factors have been tested and suggested as possible predictors of increased risk, however, with limited data on male soccer players. These include a slow reaction time (Taimela et al., 1990; A ´ rnason et al., 2004), personality factors (Taerk, 1977; Lysens et al., 1989; Taimela et al., 1990; Junge et al., 2000; A ´ rna- son et al., 2004), age (Backous et al., 1988; Linden- feld et al., 1994; Ostenberg & Roos, 2000), general joint laxity (Baumhauer et al., 1995; Ostenberg & Roos, 2000; Beynnon et al., 2001), ankle joint laxity (Beynnon et al., 2001) and balance tests (Trojian & McKeag, 2006). Regarding body size measures such as height, weight and body mass index (BMI), the literature is also inconclusive (Backous et al., 1988; Baumhauer et al., 1995; Beynnon et al., 2001; Tyler et al., 2006). Some risk factors have been tested further in intervention studies, and balance training (Tropp et al., 1985) and orthoses (Tropp et al., 1985; Surve et al., 1994) have resulted in significantly fewer ankle sprains, indicating that reduced neuro- muscular control is an important risk factor for ankle injuries. To examine the contribution of the various risk factors of injuries and etiology and to explore their interrelationship, it is necessary to include all in a multivariate analysis (Meeuwisse, 1994). Even though a large number of risk factor studies have been carried out, only a few of them have included multivariate analyses. We therefore planned the pre- sent prospective cohort study on soccer players to screen for several potential risk factors for ankle Scand J Med Sci Sports 2009 & 2009 John Wiley & Sons A/S doi: 10.1111/j.1600-0838.2009.00971.x 1