Association Between Playing Surface and Anterior Cruciate Ligament (ACL) Injury Risk in the National Football League: A Systematic Review and Exploratory Meta-Analysis

In professional sports, anterior cruciate ligament (ACL) ruptures are among the most consequential injuries, especially in American football. This review set out to determine whether National Football League (NFL) players are more likely to sustain ACL injuries on artificial turf than on natural grass. To answer this, studies were gathered from PubMed, and the review process followed PRISMA 2020 guidelines to ensure clear and reliable reporting. Ten studies met the inclusion criteria: eight on NFL epidemiology or clinical outcomes, one systematic review covering multiple sports, and one biomechanical study. For all studies with multiple populations, data extraction and analysis were limited to the NFL stratum, comparing ACL injury outcomes between artificial turf and natural grass within this group of professional athletes. When looking at the total number of ACL injuries across NFL studies, the counts were either close between artificial turf and natural grass or slightly higher on grass. But after adjusting for how often each surface was used, the rate of ACL injuries was the same or higher on artificial turf. This pattern held whether exposure was expressed per game or per team-game, and whether the comparison was reported as an injury rate, an incidence-density ratio, or an odds ratio; no individual comparison reached statistical significance. Through an exploratory random-effects meta-analysis of four recent NFL studies, calculated through the Hartung-Knapp method, we found a small increase in injury rates on turf, with a pooled estimate modestly above the null. This trend remained steady even when each study was left out one by one. However, the difference wasn't statistically significant, and because these studies often overlapped in the seasons they covered, the authors view these results as an early indication rather than a definitive conclusion. Other findings back up this tendency: knee injuries overall, and noncontact knee injuries in particular, occurred at higher rates on synthetic turf, and season-ending surgeries, usually ACL reconstructions, were also more common after injuries sustained on turf. Previous pooled studies show that the increased risk of ACL injury is specific to American football, while biomechanical evidence suggests that natural grass reduces the force transmitted through the cleat. None of the NFL studies show a statistically significant increase in ACL injuries on artificial turf when considered individually. However, after accounting for exposure, the direction of the data, results from pooled analyses, higher rates of season-ending surgery, and biomechanical findings all suggest that ACL injuries could be more common on synthetic surfaces. A clearer understanding of these trends will be essential for guiding future decisions on field surfaces and supporting the long-term health and safety of NFL athletes.