Research

In the Australian Football League (AFL), ACL injuries account for 12% of all missed playing time,¹ which costs the competition over $1 million annually in injury payments alone. The rate of ACL injury in the AFL competition has increased over recent years.²

The exact numbers and costs of ACL injuries in amateur football are unknown. One report that tried to estimate the cost of sports injuries in Australia³ suggested that the cost of ACL injuries in all grades and codes of football was at least 10% of the nation's entire bill for sports injuries. It estimated 5000 football-related ACL injuries in Australia annually, directly costing $5000 each (not including lost work time and the increased susceptibility to later knee problems caused by these injuries).⁴

Recent studies have noted that there have been relatively more ACL injuries in the early months of the AFL season, in non-Victorian games, and in recent seasons of dry weather.^2,5 Our aim was to examine the strength of the relationship of weather conditions to ACL injury.

The occurrence of ACL injuries was determined by an ongoing injury surveillance program, which has attempted to determine the cause of every game missed through injury of listed players. The accuracy of this survey in determining the cause of missed games reached 100% in 1997, and has been greater than 90% in every year.¹ In addition, a separate register for ACL injuries is held by the AFL. ACL injuries in the AFL are managed surgically and have the diagnosis proven at the time of simultaneous arthroscopy and reconstruction.

The mechanism of injury was recorded by team doctors and divided into the categories of direct contact (where the player's knee or leg was contacted by another player) and no direct contact (which was subdivided into indirect contact and no contact).

Daily weather variables were measured prospectively (but obtained by us retrospectively) by the Bureau of Meteorology at central locations in each city studied. No differentiation was made between rainfall in the differing suburbs of any city, except for Waverley Park (30 km from the centre of Melbourne), for which rainfall data were taken from the nearest centre (Moorabbin). Maximum and minimum temperature of the day of the match, rainfall, water evaporation and maximum wind gust were the raw weather variables considered. Water evaporation is a meteorological variable measuring the change of surface water into water vapour -- it is affected by temperature, sunshine, humidity and wind.⁶ Composite rainfall and evaporation variables were created (measuring the totals for the previous 7, 14, 28, 90 and 365 days), which were considered to have had potentially more effect on ground conditions than the weather only on the day of the match.

Statistical analysis was carried out using SPSS for Windows.⁷ Chi-square and t tests (after variances assessed using Levene's test) were performed in the initial stages of the analysis. Multivariate analysis was performed using a logistic regression forward stepwise technique, with a significance of 0.05 to enter the equation. Continuous variables in the logistic regression were redefined into binary variables based on group median values, to calculate risk ratios adjusted for confounding.

Overall, games played north of Melbourne had a significantly higher rate of all ACL injuries (Table). However, injuries involving contact were not significantly associated with being north of Melbourne, although indirect-contact injuries were more common north of Melbourne.

For the 59 ACL injuries that did not involve direct contact, the most predictive rainfall and evaporation variables for ACL injury were 28-day evaporation (t = - 3.8; df = 59; P < 0.001) and 365-day rainfall (t = 1.34; df = 2278; P = 0.18). Other significant associations were higher grade of match (t = 4.8; df = 59; P < 0.001), lower minimum temperature (t = - 3.7; df = 59; P < 0.001) and month (fewer injuries in winter) (t = 3.1; df = 2278; P = 0.002). Non-significant associations were night games (t = - 1.7; df = 59; P = 0.09) and wind speed (t = - 0.18; df = 2278; P = 0.86).

Although 365-day rainfall was not significant according to the t test, this variable was entered into the logistic regression equation as significant. This is because of the confounding effect of evaporation -- venues with high evaporation also tend to have high rainfall. In a multivariate analysis including evaporation, variance in rainfall is more significant than when rainfall is considered alone. In contrast, night games and minimum temperature had less significance in the logistic regression model.

All variables were considered in a logistic regression model with match grade, 28-day evaporation and 365-day rainfall qualifying to be entered into the equation. Senior grade matches had a 3.03-times risk of injury compared with reserve grade matches (95% confidence interval [CI], 1.52-6.03). Matches with high evaporation in the previous 28 days (> 48 mm) had a 2.80-times greater risk of injury (95% CI, 1.52-6.03). Matches with low rainfall (< 449 mm) in the previous year had a 1.93-times greater risk of injury (95% CI, 1.12-6.03).

Figures 1 and 2 illustrate a strong association between the rate of ACL injuries not involving direct contact and 28-day evaporation and 365-day rainfall, respectively, for the matches in Melbourne over the study period.

The most promising extrinsic risk factors postulated relate to the shoe-surface interface. It has been hypothesised for many years that increasing traction between a football boot and the playing surface would cause an increase in the rate of knee injuries.¹⁵ A recent study showed that American football boots, with a greater number of cleats and higher torsional resistance, were prospectively associated with an increased number of ACL injuries.¹⁶

Our results show that high water evaporation in the month before and low rainfall in the year before an AFL match confer an increased risk of ACL injury. This relationship is strong for most injuries not involving contact, but may not apply when the mechanism of injury involves a direct blow to the knee. The mechanism of this association is almost certainly through ground-related factors (either soil moisture content or amount and/or quality of grass). Increased speed of the game on dry grounds may also be relevant. These results are in keeping with the theory that excessive shoe-surface traction is a risk factor for ACL injury. Friction and torsional resistance from football boots has been shown to be higher in dry conditions on natural grass compared with wet conditions.¹⁷ In a recent review of non-contact ACL injuries in American Football, it was noted that almost all injuries on natural grass occurred in dry conditions,¹⁸ but that review did not measure or control for conditions on days when injury did not occur.

We found that the long term effects of rainfall and water evaporation (over a period of months) are more relevant than the amount of rainfall and evaporation on the day or in the days leading up to a match.

Further studies are required to determine the exact mechanism by which this relationship occurs and what the effects of intervention would be. The AFL has embarked on a study, beginning with the 1998 season, where matches have ground hardness readings taken with a Penetrometer, a device used in horse racing to measure track hardness. Penetrometer results are reliable and correlate with the speed of races.¹⁹ The preliminary results of the AFL study confirm that low rainfall and high evaporation are associated with hard (low) Penetrometer readings (< 4.7 cm).²⁰

We did not study differences between ground conditions in coastal and inland locations, as (except for Canberra) all the major AFL venues are currently in coastal cities. Further study could include inland cities, which may experience different ground conditions due to much colder and drier winters.

Irrespective of their mechanism of action, the effects of high water evaporation and low rainfall could be reversed on football grounds by consistently watering grounds during times of lower rainfall and covering them during times of increased sunshine. If grounds were prepared in this way to simulate the ground conditions typically experienced in a Melbourne winter, the number of ACL injuries would almost certainly be reduced. This is unlikely to lead to an increase in injuries of other types, as overall injury rates are consistently higher outside Victoria than in Victoria.^1,5 In rugby league in the United Kingdom, injury rates increased across the board when the season was changed from winter to summer,^21,22 suggesting harder grounds may be a universal risk factor for football injuries.

In AFL, the main competing concern would be whether games played on consistently softer grounds would be a lesser spectacle for the attending and television public. In amateur football, similar reductions in ACL injuries could probably be achieved, without the standard of spectacle being an issue. The cost of manipulating ground conditions might seem initially prohibitive in this environment, but when the massive cost of ACL injury is considered, great overall savings could be made.

In conclusion, low water evaporation and high rainfall significantly lower the risk of non-contact ACL injuries in AFL footballers. The mechanism is likely to be a softer surface, with lower shoe-surface traction and consequently less force transferred to the knee in movements such as pivoting. Consistent extra watering and covering of grounds during times of high water evaporation (sunny, windy periods with no rain) is likely to lower the rate of these devastating and costly injuries.

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Australian Football League Medical Officers Association, Melbourne, VIC.
Hugh Seward, MB BS, FACSP, President;
Jeanne McGivern, MB BS, FRCS, Club Medical Officer;
Simon Hood, BAppSci(PE), Research Officer.

Reprints: Dr J Orchard, South Sydney Sports Medicine, 111 Anzac Parade, Kensington, NSW 2033.
Email: johnorchardATmsn.com.au

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