Artificial turf technology has advanced significantly, and some research suggests newer surfaces are as safe as grass, if not safer. But in other reports, including a high-profile NFL study, turf has been associated with higher rates of lower extremity injury.
By P.K. Daniel
Twenty years ago Chicago Bears wide receiver Wendell Davis attempted to leap for a pass against the Philadelphia Eagles. But the AstroTurf in Philadelphia’s Veterans Stadium grabbed hold of his shoes, cementing him to the carpet. Davis ruptured both of his patellar tendons on what was a noncontact career-ending play.
Yet, artificial turf has served many purposes since its introduction nearly a half-century ago. It’s cost-effective. It survives indoors. It’s consistent. It’s aesthetically pleasing. It’s an alternative to bodies slamming into the frozen tundra. There also have been many turf technology advances over the years. The benefits, however, have not come without considerable debate, particularly regarding risks of lower extremity injury.
The risk-reward analysis and controversy continues today with the advancement of third-generation turf that’s touted as comparable to grass–“looks like grass … feels like grass … plays like grass!” Previous research supported the assertion that newer-generation artificial turf was a great improvement over the older-generation, injury-causing surface that’s been described as glorified carpet laid over a pad, laid over a slab of concrete.
“This turf is so much better than the turf we played on,” said former National Football League (NFL) quarterback and CBS in-studio analyst Boomer Esiason last season on The NFL Today show. “I played at the Vet, Three Rivers Stadium, Riverfront, the Houston Astrodome. They were terrible. This is a much better brand of product.”
The original turf systems of the 1960s, sometimes referred to as first-generation turf, were short-pile fibers without infill. The next wave of artificial turf, generally called second-generation turf, featured sand infills. Third-generation systems, which are most widely used today, have simulated blades of grass with infills made of polyethylene fibers over a mix of sand and rubber particles. There is even a fourth-generation turf, which is an unfilled and typically low-cost version.
Calhoun, GA-based FieldTurf has popularized newer-generation artificial turf, which is used by 21 of 32 NFL teams. High schools and colleges are also part of FieldTurf’s business. The company reports that it has 60% of the market for artificial turf, which frequently covers multiservice fields that act as a football field one day and a soccer field the next.
“That’s where artificial surfaces have an advantage,” said researcher Mike Meyers, PhD, who is an assistant dean of the Department of Business, Education, and Wellness at the College of Western Idaho in Nampa.
While there is literature that reports there may not be a difference between general injury rates for grass and infill turf surfaces, recent studies have shown that artificial surfaces have higher injury rates than natural grass for specific lower extremity injuries.
Although FieldTurf is generally considered better than AstroTurf, the original artificial playing surface, a 10-year study published in the American Journal of Sports Medicine (AJSM) last fall suggests it’s not as safe as grass, or as advertised. The NFL’s Injury and Safety Panel found players suffered a 22% higher incidence of knee sprains and ankle sprains, including a 67% higher rate of anterior cruciate ligament sprains and a 31% higher rate of eversion ankle sprains, on FieldTurf than on grass.1`
“Right now, the numbers seem to indicate that there’s a higher incidence of noncontact ACL injuries on artificial turf,” said Neal ElAttrache, MD, a sports medicine surgeon at the Kerlan-Jobe Orthopaedic Clinic in Los Angeles. ElAttrache also is the team physician for the Los Angeles Dodgers, and an orthopedic consultant to several professional sports teams and the Professional Golfers Association Tour.
“If you’ve got ideal conditions, I can’t think of any situation where Field Turf would be better,” ElAttrache said.
But the turf surface in and of itself isn’t necessarily responsible for an increase in lower extremity injuries. There are many variables. The sports that are typically played on these surfaces involve complex loading, running, jumping, and cutting patterns that are influenced by a multitude of factors, including player technique, shoe-surface interface, and weather, said Jason Dragoo, MD, an assistant professor of orthopaedic surgery at Stanford University School of Medicine in Palo Alto, CA.
“The etiology of the injuries that occur in these sports cannot be explained by the surface alone,” Dragoo said.
Elliott Hershman, MD, lead author of the NFL study, also acknowledged other possible factors contributing to increased knee and ankle injuries on FieldTurf.
“These injuries could be happening for myriad reasons, and we need to further explore and initiate research into exactly why this is happening,” Hershman said in a news release.
The study did not draw any conclusions related to the cause of the injuries analyzed and the authors concluded that the findings warranted additional analysis.1 (Hershman did not respond to multiple interview requests from LER.)
Meyers agreed that additional analysis is warranted. He also suggested that, ideally in such a study, players should have equal exposure to the control condition (grass) and the experimental condition (artificial turf) during the same time period. In the NFL study, however, more than 4000 games were played on grass compared with 1356 played on FieldTurf, Meyers said.
“[That is] a great discrepancy, which is not fair to either surface,” Meyers said. “When you don’t have players playing [to the same extent] on both surfaces, it’s not fair to either surface. [The authors] have some quality control statistical challenges. There’s no doubt about it.”
Taking grass to task
Meyers, meanwhile, conducted two studies—in high school2 and college football3 players—that showed no statistically significant differences between the surfaces with regard to most injury categories. Published in 2004 in AJSM, the five-year study of high school athletes found no significant difference between surfaces for lower extremity injuries, but that ACL injuries (with or without associated tissue damage) were twice as likely to have occurred on natural grass as on FieldTurf. In that study, 62.5% of team games were played on grass.
In the three-year study of college football players that appeared in the April 2010 issue of AJSM, 50.5% of team games were played on natural grass. That study found that artificial turf was associated with significantly lower incidence of minor, substantial, and severe injuries than natural grass; there was no significant effect of playing surface on traumatic knee injuries, but there was a significantly higher rate of distal tibiofibular ligament sprains on grass than on FieldTurf.
“I’ve never seen it where [natural grass] is safer,” Meyers said. “I’ve seen where it’s equal. And that’s in ideal, mint-condition weather, brand-new surface, untouched by anybody.”
But Mark Drakos, MD, an orthopedic surgeon at the Hospital for Special Surgery in New York City and former assistant team physician for the New York Giants, disagreed. He said the NFL study adjusted the data according to each player’s exposure to grass, turf, or both.
“They quantified it per exposure to the playing surface,” he said. “That is absolutely accounted for. Is each player exposed to the same exact number of FieldTurf versus grass [games]? No, but the injuries were recorded per exposure to the surface rather in the total. There’s a reason that it’s a lead article in the [AJSM]. You can argue that there were a number of other variables that weren’t considered—weather, shoes, those types of things. But from a per-
exposure risk on the NFL level, I think the data are pretty convincing.”
The NFL data are collected through its Injury Surveillance System. Teams’ athletic trainers and physicians record data about a player’s injury, including the severity of injury, activity at the time of the injury, and time lost as a result.
“They have to report these injuries whether they’re contact or noncontact, which plays a role,” Drakos said.
Drakos also pointed to the relationship between shoe and surface.
“The shoe-surface interface plays more of a role in noncontact injuries,” said Drakos, who was the lead author of a 2010 study in the Journal of Biomechanical Engineering that looked at the effect of the shoe-surface interface in the development of ACL strain.4
The National College Athletic Association also has an Injury Surveillance System, but it includes only about 10% of its member schools. In the August 2012 issue of Knee, Dragoo, who studied the 2004-2009 college football seasons, reported that the rate of complete ACL tears on artificial surfaces was 1.39 times higher than the injury rate on grass.5 In addition to the Knee paper, Dragoo’s research appeared in the May 2012 issue of AJSM.6
Although Meyers found rates of noncontact injury related to shoe-surface interface were similar for both natural and artificial surfaces,2,3 Dragoo found noncontact ACL injuries occurred more frequently on artificial turf surfaces than on natural grass.5,6
Echoing Drakos and ElAttrache, Dragoo said, “Although third-generation turf more closely resembles the properties of real grass, it is unclear if it is any safer than grass. Recent reports, such as our paper in AJSM and NFL data, show higher rates of ACL injury on turf.”
Meanwhile, Drakos pointed out that, while the NFL numbers were sent to a third-party statistician for validation, FieldTurf’s partial funding of Meyers’ studies suggests a conflict of interest.
“The best study that’s out there is the NFL study with regard to injuries on these different surfaces,” Drakos said.
While stressing that further investigation is required, researchers in the November 2012 issue of The Physician and Sportsmedicine suggested it could be inferred that high school and college football are not indicative of the torque and violence of the NFL game, in which players carry more mass and generate more power.7 Drakos said it’s not known if the results of the NFL’s 10-year study are generalizable to other levels of football. However, Dragoo said the results are more generalizable within a sport than between sports.
“Using football as an example, while NFL players are competing and practicing at a faster pace and higher skill level than college or high school athletes, the movements and patterns are similar, if not the same, so surface-dependent injury incidences and risks are likely to be comparable,” Dragoo said. “Generalizing across sports is difficult because both movements and equipment differ, introducing even more variability. Differences in injury risks and rates between men and women are well-documented, but if the men and women are participating in the same sport [for example, soccer], surface-related findings should be relatively generalizable.”
On the pitch
As noted earlier, artificial turf is a multiservice surface—something that makes it particularly attractive to high schools and universities. Besides football, it’s also used for soccer, baseball, softball, field hockey, lacrosse, and rugby. Although a 2009 review found that playing on artificial surface versus natural grass was a risk factor for noncontact ACL injuries8 in soccer, in which most ACL tears are noncontact in nature, other studies have shown no significant difference.
In a 2006 study of elite soccer players that appeared in the British Journal of Sports Medicine (BJSM), Swedish researchers found no evidence of a greater risk of injury on artificial turf compared with natural grass.9 A 2010 study in BJSM that compared the risk for acute injuries between grass and artificial turf in Norwegian male professional soccer players also showed no significant differences in injury rate or pattern.10 And a June 2012 study in the Scandinavian Journal of Medicine and Science in Sports that investigated the risk of acute injuries among male and female youth soccer players also concluded there was no difference in the overall risk, and a lower risk of ankle injuries, on third-generation artificial turf compared with grass.11
However, Chris Hughes, MBBS, MSc, associate editor of the Clinical Journal of Sport Medicine (CJSM), referenced a 2011 review article in Sports Medicine by New Zealand researchers that showed an increased incidence of ankle injury associated with playing on artificial turf, though injury risk for knee injuries was inconsistent.12 Hughes, who is a team physician for English professional soccer team Leyton Orient FC, wrote in the CJSM Blog: “For me, the jury’s still out on the issue of injury risk with the newer artificial playing surfaces, but the traditionalist in me still thinks that soccer was meant to be played on a natural surface.”
NFL players seconded Hughes’ opinion. The NFL Players Association has surveyed its active players about playing surfaces every other year since 1994. Players generally prefer FieldTurf over previous artificial surfaces. However, players overall prefer natural grass over artificial turf by a significant margin. In the most recent survey available of all 32 teams, conducted from September through November 2010, 82.4% of the 1619 players surveyed said they thought artificial infill turf was more likely to contribute to injury compared with 15.9% who identified grass.13
Meyers chalked up those assertions to tradition.
“Typically, they’re still mired in the artificial turf problems of the seventies and eighties, and they’re not looking at the statistics and the research,” he said.
Footwear and the shoe-surface interface play a significant role in injury risk, and researchers seem to agree that the relationship requires further study. Feng Wei, PhD, a research scientist with Northwestern University Feinberg School of Medicine in Chicago, referenced a 1974 study in the Journal of Sports Medicine by Joseph Torg, MD, a clinical adjunct professor of Orthopaedic Surgery and Sports Medicine at Temple University in Philadelphia, on the subject of shoe-surface interface.14
“Their studies for the first time showed the important role that the interaction between a shoe and the playing surface has in noncontact sports injuries,” Wei said.
Additionally, a 1996 AJSM study of more than 3000 high school football players showed a relationship between cleat design, amount of rotational traction, and risk of ACL injury. The edge cleat design—with longer irregular cleats at the periphery and many smaller cleats on the interior—produced significantly higher rotational traction and was associated with an ACL injury rate 3.4 times higher than all other designs (flat, screw-in, and pivot disk) combined.15
“I think it is well-believed that traction from shoe-surface interface is necessary for high-level performance in many sports,” Wei said. “One of the selling points of those artificial turfs is that they may provide higher traction that could benefit players and teams, and improve the competition of games. On the other hand, studies have also shown that excessive traction from the shoe-surface interface may cause injuries to the lower extremity.”
Finding a balance between performance benefit and injury risk is necessary, Wei said.
“To do so, of course, more studies need to be done as football shoe and surface designs continue to update,” he said.
Dragoo noted that cleats affect the shoe-surface interface through factors such as friction, rotation, impact attenuation, and pressure distribution. Researchers from the University of Dundee in Scotland showed in a 2011 study in Foot & Ankle Surgery that studded boots resulted in a normal pressure distribution whereas bladed boots caused unnatural increased lateral loading.16
“Stud size and configuration has also been investigated; shorter, plain distributed studs appear best for artificial turf,” said Dragoo. “However, the bulk of these studies are laboratory investigations and the direct relationship to both overall and specific injury risk/rate remains poorly understood.”
Drakos has studied the effect of synthetic playing surfaces and the shoe-surface interface on lower extremity injuries in athletes, including the aforementioned 2010 study in the Journal of Biomechanical Engineering in which researchers used cadavers to test four different shoe-surface interfaces.
The study concluded the cleat and natural grass combination produced less strain in the ACL than other shoe-surface combinations, possibly because of less traction generated on grass. The researchers theorized that the favorable biomechanical properties of the cleat-grass interface may result in fewer noncontact ACL injuries than turf shoes on grass or either shoe on artificial turf.
This difference may be due to the rotational traction that occurs between the shoe and surface during certain injury mechanisms, but solutions may be approaching the red zone.
“The turf conditions versus the shoe conditions are definitely improving, and improving rapidly,” ElAttrache said. “And so, with that kind of science and technology improving, I think you’re going to ultimately see conditions as close as possible to good, natural grass surfaces within a few years. It’s going to happen, and it’s going to happen pretty soon.”
P.K. Daniel is a freelance sportswriter and editor based in San Diego, CA.
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