Rye grass is associated with fewer non-contact anterior cruciate ligament injuries than bermuda grass.

OBJECTIVE To assess the contribution of ground variables including grass type to the rate of anterior cruciate ligament (ACL) injury in the Australian Football League (AFL), specifically which factors are primarily responsible for previously observed warm season and early season biases for ACL injuries. METHODS Grass types used at the major AFL venues from 1992 to 2004 were established by consultation with ground managers, and ground hardness and other weather variables were measured prospectively. RESULTS There were 115 ACL injuries occurring in matches during the survey time period, 88 with a non-contact mechanism. In multivariate analysis, use of bermuda (couch) grass as opposed to rye grass, higher grade of match, and earlier stage of the season were independent risk factors for non-contact ACL injury. Ground hardness readings did not show a significant association with ACL injury risk, whereas weather variables of high evaporation and low prior rainfall showed univariate association with injury risk but could not be entered into a logistic regression equation. DISCUSSION Rye grass appears to offer protection against ACL injury compared with bermuda (couch) grass fields. The likely mechanism is reduced "trapping" of football boots by less thatch. Grass species as a single consideration cannot fully explain the ACL early season bias, but is probably responsible for the warm season bias seen in the AFL. Weather variables previously identified as predictors are probably markers for predominance of bermuda over rye grass in mixed fields.