Dear Editor

I would like to congratulate Ekstrand et al. [1] for their study comparing the injury incidence on new generation artificial turf to natural grass. At the very least, their findings are reassuring that it is relatively safe to continue to play football (soccer) on new generation artificial surfaces in cool climates, subject to ongoing injury surveillance. As an author who has published multiple articles comparing injury incidence on different surfaces in other (non-soccer) varieties of football [2-4], I would like to make a few comments on the topic in general, which are relevant to the discussion:

1. It appears that most surfaces (other than perhaps artificial surfaces when indoors at constant temperature) can vary substantially in terms of characteristics and therefore contribution to injury, depending on weather. For example, Astroturf outdoors in hot weather appears to be a riskier surface for American football injury than natural grass in the same weather, but the relationship may be reversed in cold weather [4].

2. The new generation artificial surfaces do not appear to be as hard as Astroturf and this appears to translate to higher acceptance by players. However, excessive shoe-surface traction is probably a greater injury concern than even excessive hardness, with little published data about the traction generated on the new generation surfaces with typical player footwear choices in various weather conditions. There is much greater shoe-surface traction developed on Astroturf as the ambient temperature rises [5] and at this stage it is a reasonable assumption that new generation artificial surfaces may have a similar relationship.

3. If an outdoor new generation artificial surface was found to generate excessive shoe-surface traction in, say, hot weather, there may be fewer measures at the disposal of the ground managers to reduce this traction compared to natural grass. It is clear that there are circumstances under which natural grass can create the opportunity for excessive shoe-surface traction (particularly when high lateral growth species such as bermudagrass or Kentucky bluegrass are used and there has been low recent rainfall and/or traffic)[3]. However, the traffic of football matches will naturally de-thatch the grass and, if this does not have the desired effect, vertical mowing and/or oversowing with ryegrass can be used [2]. Whatever the risk of artificial surfaces compared to natural grass, it is less amenable to manipulation in the direction of reduced shoe-surface traction (and therefore possibly injury risk), which is of particular relevance in hot weather (when traction may naturally be higher).

4. Whilst it is good to read that this recent study was done without sponsorship from artificial turf companies, the nature of their industry is very much like the pharmaceutical industry. There may be a tendency to conduct industry-sponsored biomechanical and/or injury research and publish only when the findings are suitable. I would caution readers against any assumption, based on the recent Ekstrand et al. paper [1] and any industry promotional material, that new generation artificial surfaces have been proven to be as safe as natural grass for football in all climatic conditions.

If the authors have additional data (such as weather conditions or natural grass types used on playing fields) from this recent study [1], it would be interesting to learn about these.

References

1. Ekstrand J, Timpka T, Hägglund M. Risk of injury in elite football played on artificial turf versus natural grass: a prospective two-cohort study. British Journal of Sports Medicine. 2006;40:975 - 980.

2. Orchard J, Chivers I, Aldous D, Bennell K, Seward H. Ryegrass is associated with fewer non-contact anterior cruciate ligament injuries than bermudagrass. British Journal of Sports Medicine. 2005;39:704-709.

3. Orchard J. Is there a relationship between ground and climatic conditions and injuries in football? Sports Medicine. 2002;32(7):419-432.

4. Orchard J, Powell J. Risk of knee and ankle sprains under various weather conditions in American football. Medicine & Science in Sports & Exercise. 2003;35(7):1118-1123.

5. Torg JS, Stilwell G, Rogers K. The effect of ambient temperature on the shoe-surface interface release coefficient. American Journal of Sports Medicine. 1996;24(1):79-82.