Introduction

Since its introduction in the 1960s, artificial turf has become increasingly common at sporting venues, ranging from professional stadiums and arenas to local community athletic fields. There are currently more than 3500 artificial turf fields in use in the United States, and over two-thirds of National Football League (NFL) stadiums have artificial turf installed. Artificial turf’s appeal is its greater versatility and durability, easier maintenance, and established long-term cost savings relative to natural grass playing surfaces. Despite these advantages, there is concern among athletes, parents, physicians, trainers, and coaching staff about the perceived increased risk of musculoskeletal injuries (particularly noncontact injuries, such as anterior cruciate ligament [ACL] injuries) with play on artificial surfaces.

In response to these concerns, manufacturers have evolved the characteristics of these surfaces in an attempt to better mimic the properties of natural grass fields, which has been arbitrarily defined as the benchmark safe playing surface. There have been three generations of artificial playing surfaces. First-generation artificial turf consisted of a densely woven fiber carpet over of a thin layer of underpadding. Second-generation artificial turf, introduced in the late-1970s and 1980s, was composed of longer, thicker fibers (22–25 mm) with sand infill, and were mounted on a rubberized base to reduce stiffness. Third-generation artificial turf was introduced in the 1990s and early-2000s and is the current standard across professional sporting leagues worldwide. These modern surfaces are composed of longer fibers (50–60 mm) with a rubberized infill, and are marketed as substantially safer relative to earlier generations and even natural grass surfaces.

While widely adopted at all levels of play, there remains persistent skepticism in the sporting community concerning the safety of artificial playing surfaces compared to natural grass surfaces. Multiple athlete surveys suggest a persistent fear of increased injury risk on artificial surfaces, even among professional athletes. Fear of injury prompted a group of elite international soccer players to file suit against Fédération Internationale de Football Association (FIFA) and the Canadian Soccer Association over plans to use artificial playing surfaces in the 2015 Women’s World Cup. While the case was ultimately dropped, widespread media reporting garnered significant public attention and support for players.

Introduction

Since its introduction in the 1960s, artificial turf has become increasingly common at sporting venues, ranging from professional stadiums and arenas to local community athletic fields. There are currently more than 3500 artificial turf fields in use in the United States, and over two-thirds of National Football League (NFL) stadiums have artificial turf installed. Artificial turf’s appeal is its greater versatility and durability, easier maintenance, and established long-term cost savings relative to natural grass playing surfaces. Despite these advantages, there is concern among athletes, parents, physicians, trainers, and coaching staff about the perceived increased risk of musculoskeletal injuries (particularly noncontact injuries, such as anterior cruciate ligament [ACL] injuries) with play on artificial surfaces.

In response to these concerns, manufacturers have evolved the characteristics of these surfaces in an attempt to better mimic the properties of natural grass fields, which has been arbitrarily defined as the benchmark safe playing surface. There have been three generations of artificial playing surfaces. First-generation artificial turf consisted of a densely woven fiber carpet over of a thin layer of underpadding. Second-generation artificial turf, introduced in the late-1970s and 1980s, was composed of longer, thicker fibers (22–25 mm) with sand infill, and were mounted on a rubberized base to reduce stiffness. Third-generation artificial turf was introduced in the 1990s and early-2000s and is the current standard across professional sporting leagues worldwide. These modern surfaces are composed of longer fibers (50–60 mm) with a rubberized infill, and are marketed as substantially safer relative to earlier generations and even natural grass surfaces.

While widely adopted at all levels of play, there remains persistent skepticism in the sporting community concerning the safety of artificial playing surfaces compared to natural grass surfaces. Multiple athlete surveys suggest a persistent fear of increased injury risk on artificial surfaces, even among professional athletes. Fear of injury prompted a group of elite international soccer players to file suit against Fédération Internationale de Football Association (FIFA) and the Canadian Soccer Association over plans to use artificial playing surfaces in the 2015 Women’s World Cup. While the case was ultimately dropped, widespread media reporting garnered significant public attention and support for players.

Biomechanics/Surface Characteristics

ACL injuries occur most commonly with cutting, pivoting, landing, and changing direction. The loading of the knee joint at foot strike during quick deceleration, coupled with tibial rotational forces and a valgus-directed force generated by the athlete’s rapid change in direction, produce high strain on the ACL. Frictional force at the shoe–surface interface plays a crucial role in whether or not injury occurs. The concern, expressed for many decades by multiple authors, is that artificial playing surfaces generate increased frictional force at the shoe-surface interface and thus increase risk of both contact and noncontact injuries.

Torg and Quedenfeld were the first to propose the concept of the release coefficient (torque required to release an engaged shoe–surface interface). In their foundational work, they calculated the release coefficient of several different surfaces with varying types of footwear. This resulted in changes to athletic footwear design in the hopes of reducing knee injuries. In subsequent years, other authors have identified rotational stiffness at the shoe–surface interface as a separate independent risk factor for ACL injury. In each of these studies, artificial turf resulted in generally increased frictional forces.

Biomechanical data can be difficult to interpret, and at best these prior studies provide only indirect evidence of a potentially increased risk of ACL injuries on artificial playing surfaces. Further attempts to refine testing models have been proposed, such as accounting for sliding between the shoe and playing surface. However, no clear standard has emerged, and different authors have continued utilizing different testing apparatuses and protocols. It is also worthwhile to note that most studies have examined only forces generated at the level of the foot, without direct measurement of forces applied to the ACL. While one recent study has shown that forces across the ACL can be adequately measured, to date we are not aware of any critical re-evaluation of prior studies using such an updated testing protocol.

A variety of other potential risk factors and environmental confounders have also been proposed, with limited studies attempting to quantify their influence on injury rates (or lack thereof). Ground conditions, such as moisture levels and ambient temperature, have been proposed as risk factors for noncontact injury, with harder, drier surfaces potentially generating greater frictional force across the shoe–surface interface. Others, however, have questioned the strength of this finding. The cleat distribution at the ball and heel of athletic shoes has been shown to correlate with torque generation. Larger, stronger players may be capable of generating frictional forces with cutting and pivoting that exceeds proportional increases in ACL strength. Differences in foot loading patterns by athletes on different surfaces during cutting have been demonstrated, though no definitive link to knee injuries has been established. It has even been suggested that different species of grass or portable sod may play a role in elevating or decreasing ACL injury rates.

In summary, there is moderate evidence that synthetic playing surfaces (to include modern third-generation surfaces) result in increased frictional force at the shoe–surface interface. The importance of this finding as related to ACL injuries is uncertain. Environmental risk factors seem to play some role in this process, but the precise evidence for their role in ACL injuries is confusing and mixed . Consequently, researchers have expanded clinical surveillance of athletes to define precisely what, if anything, the increased use of artificial playing surfaces has done to athlete ACL injury rates.

Clinical

Injury surveillance has improved in both scope and quality since the early-1990s. Much of this improvement is due to the establishment of large, standardized injury surveillance systems in specific sports or groups of athletes. The National Collegiate Athletic Association (NCAA) Injury Surveillance System began in 1982, and the Australian Football League has conducted continuous surveillance since 1992. The NFL, FIFA, the International Olympic Committee, and a large American high school athletics cohort have all established standardized surveillance methodologies, including injury monitoring on artificial playing surfaces. Much of the current data on ACL injuries in athletes come from these injury surveillance systems.

While studies comparing injury rates on artificial turf to natural grass have been reported since shortly after the introduction of Astroturf, early studies aggregated multiple injuries by anatomic location (e.g., lower extremity injuries) or simply reported a single overall injury rate. The first studies specifically examining ACL injury rates were not published until the 1990s, with data extracted from the proprietary NFL injury surveillance system. Powell and Schootman identified 114 ACL injuries occurring in games between 1980 and 1989, with a nonsignificant trend toward more injuries on artificial turf. Scranton et al. reported on 61 ACL injuries in both games and practices between 1989 and 1993 and found an ACL injury rate on artificial turf that was nearly twice as high as natural grass. In both studies, the only artificial turf examined was first-generation Astroturf.

With the adoption of modern third-generation turf in the late-1990s and 2000s, an increasing number of ACL-specific studies appeared in the sports medicine literature. This was especially true at the collegiate level. Five studies were published between 2007 and 2013 reporting ACL injury rates in NCAA soccer and football by playing surface type. Most of these studies found no significant difference in ACL injury rates, with cohort sizes between 22 and 83 ACL injuries. Only one study identified a statistically significant increased rate of ACL injury on artificial turf. This study, however, was the largest to date, and statistical methods were especially rigorous. Dragoo et al. identified 318 ACL injuries in NCAA football players occurring in both games and practice, and calculated an overall injury rate ratio of 1.4, which was significant. This suggests that other studies may simply have been underpowered to detect a difference.

Studies at other levels of play show a similar pattern of results. Bjorneboe et al. followed a single professional Norwegian soccer team over four seasons of play, collecting information on 14 ACL injuries. They identified no significant difference between playing surfaces, despite a large imbalance of total injuries on each (11 ACL injuries on grass, 3 injuries on artificial turf). Meyers and Barnhill prospectively collected injury data on eight high school football teams over five seasons. Paradoxically, ACL injuries were significantly more likely to occur on natural grass. This study was limited by the overall small number of injuries (15 total) and exposures (240 team games). In contrast, Hershman et al. examined data on 260 ACL injuries in NFL players, finding a significantly increased rate of ACL injuries on artificial turf.

A recent systematic review of Level II evidence identified a total of four studies reporting a significantly increased rate of ACL injuries on artificial turf. All studies examined football players. In contrast, all studies of soccer players at any level of competition reported trends toward decreased rates of ACL injuries on artificial turf. The review also examined reporting of various environmental factors, such as field moisture, player position, type of play when injury occurred, and type of footwear. However, these environmental variables were sparsely reported, and no clear trends could be identified on the effects of these factors. The authors concluded that there was an apparent increased risk of ACL injury on artificial playing surfaces in football, but not soccer. The reasons for this discrepancy are not clear, but it may be due to the larger body mass of players in football, as well as the increased rate of contact-type injuries. The review was limited by the lack of meta-analysis, owing to variable reporting measures used between studies.