Early Release Of Injury Research On Grass vs. Turf.

Dr. Don Kirkendall

Most medical journals will publish findings online ahead of the actual print version of the journal becomes available. Subscribers get an email listing new papers weeks ahead of the print version.

These prerelease emails from the orthopaedic corner of sports medicine are largely surgical (“Fixation of the Achilles Tendon insertion using suture button technology”), biomechanical (“Tibial plateau geometry influences lower extremity biomechanics during landing”) or biochemical (“Failed healing of rotator cuff repair correlates with altered collagenase and gelatinase in supraspinatus and subscapularis tendons”).

But occasionally, something catches my eye -ankle and knee injuries in National Football League games: A Field Turf vs. grass comparison. Keep in mind that soccer is often played on artificial turf.

History lesson: artificial turf has been around since 1966 when the Astrodome opened in Houston. The surface was a carpet-like material over a pad over an asphalt base. If it sounds hard, it was.

Manufacturer claims of fewer injuries were refuted as early as 1969. Research &Development departments refined the materials and construction to what we see today, sometimes referred to as “3rd generation” artificial turf.

Modern artificial turf consists of long polyethelene fibers that have been woven into a mat with the spaces between fibers filled with a combination of rubber and sand. The most widely recognized brand is Field Turf. The first NFL game on Field Turf was in 2000 in Seattle. As of 2009, 14 of the 32 teams play their home matches on a 3rd generation surface and 18 of 32 have grass.

Full disclosure here. As a player, I don’t care for artificial turf. Yes, the ball has a consistent and predictable roll. But the traction causes me to catch the outside of my shoe and stumble. I never wear any type of studded boot on artificial turf, the added traction just make things worse.

Plus, I don’t like how much heat radiates off the turf during hotter months or how hard it feels in the cold. Finally, I don’t care what anyone says, on passes that are longer than they are high, the first bounce seems more like a skip that comes off the ground faster than expected and is hard to judge and control. Tactically, if the ‘home’ team knows how to play the skip, the opponent who is used to grass is at a distinct disadvantage. To me, at least.

The NFL has been conducting an injury surveillance program since 1980 and is updated almost daily by team physicians or trainers. Identifying an injury is the easy part. The hard part is in the details like player characteristics (height, weight, injury history, shoe type, position, time lost, etc.), and game day details (time of injury, place on the field, pass/run/punt/kickoff/change of possession, weather, open/closed air stadium, roof open/closed/partially open, type of surface, etc.). This particular paper was focused on knee and ankle injuries that occurred during games.

The study covered the 2000 through the 2009 seasons and compared injuries on grass vs. injuries on Field Turf (the most common brand used in the NFL). There were 6612 team-games (team-game = 1 team playing 1 game; remember each game has 2 teams playing!). The number of team games rose from 20 in 2000 to 214 in 2009.
The basic results were: (1) 22% more knee sprains (any knee ligament injury) occurred during games on Field Turf, (2) 67% more ACL sprains, (3) 22% more ankle sprains, and (4) 31% more eversion ankle sprains (an inversion sprain is what most people experience, where the sole of the foot rolls inward. An eversion sprain is the opposite injury).
The nature of the data prevented the authors from making any comments on the mechanism of injury (the “why” of the injury).
Other reports from football: high school players had 60% more injuries overall on artificial turf and twice as many lower extremity injuries, especially knee sprains in linemen and special team players.
Back to the NFL, the ACL sprain rate has been reported to be twice as high on artificial turf, but the rate on non-contact ACL injuries (the most common mechanism) was five times higher on grass. Overall injury rates on artificial turf are lower in open vs. domed stadiums. In the preseason, injury rates on artificial turf were nearly three times higher than injury rates on grass.
You might think the data is pretty conclusive, but don’t forget about confounding variables like shoe type, stud configuration, brand of turf, environment, style of play, and how those and others interact over the course of a long season.
But that’s the NFL. What about “our football?” The NCAA injury statistics show little difference in injury rates in college soccer, men or women. One item of note was that 8% of match concussions were due to head-ground contact with artificial turf and that players with those concussions took longer to recover. Plus, 12.5% of ACL injuries were due to ground contact. Those numbers are not 8% or 12.5% MORE injuries, just a higher frequency of those injury mechanisms.
Both FIFA and UEFA have studied this problem. In general, they report overall injury rates differ little, but do find some specific differences like more lacerations in men playing on artificial turf. Men also had more foot, ankle, ligament, and cartilage injuries on artificial turf. Females, on the other hand, had lower rates of ligament and cartilage injury on artificial turf.
Muscle-tendon injury was lower on artificial turf in another project. Injury rates on 16-year old and younger players were not different between surfaces. In Iceland, overuse injuries were more common on artificial turf than grass.
And unless things have changed recently, the only artificial turf approved by FIFA for international competition is Field Turf.
Confused? You should be. That’s the nature of the research process. One project is done one way and someone else sees a hole in that project and conducts their own study. Once published, others may question how previous studies defined their terms, or analyzed the data, or neglected selected interactions or confounding variables (e.g. in researching this article, I’d never thought of a variable that quantifies how much a stadium roof was open).
For example, I’ve wondered about how to catalogue the data according to home or away. When you are the visiting team, you are on a different surface. The team that trains on grass and plays on grass may have more injuries away on turf because the players aren’t used to the turf.
Conversely, the team that trains and plays on artificial turf at home may have more injuries away on grass for the same reason. The result may well be no difference in overall injuries between the two surfaces.
All in all, the jury is still out on whether the injury rate or the patterns of injuries on artificial turn vs. grass truly are different. Wish I had the answer.

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