Rainfall, evaporation and the risk of non-contact anterior cruciate ligament injury in the Australian Football League

Objective: To determine if weather conditions affect the risk of anterior cruciate ligament (ACL) tear in Australian Football. Design: Prospective observational analytic study of football matches. Setting: The Australian Football League (AFL), a professional competition. Participants: All players in 2280 matches from 1992-1998. Main outcome measures: Surgically-proven ACL injury, not involving a direct contact mechanism, during a match; rainfall; water evaporation. Results: 59 ACL injuries not involving direct contact occurred during the study period, more commonly in cities north of Melbourne (chi(2) = 17.0; df = 1; P < 0.001). Senior grade matches (relative risk [RR], 3.03; 95% confidence interval [CI], 1.52-6.03), high water evaporation in the month before the match (RR, 2.80; 95% CI, 1.53-5.10) and low rainfall in the year before the match (RR, 1.93; 95% CI, 1.12-3.34) were significantly associated with these injuries. Conclusion: Low water evaporation and high rainfall significantly lower the risk of ACL injuries in AFL footballers. The likely mechanism is a softening of the ground, which lowers shoe-surface traction. Consistent extra watering and covering of grounds during periods of high water evaporation may lower the rate of ACL injuries.