What factors influence concussion risk in American football? Bayesian analysis of 8 seasons of National Football League data.

Importance: Professional American football players are concerned that artificial turf playing surfaces and extended seasons increase health risks, but existing research is limited by weak methodology and limited sample size. Objective: To examine how playing surface, time of season, and game temperature relate to concussion risk in the National Football League (NFL). Design: Retrospective cohort study of 8 years of NFL regular season games. Setting: Database of NFL game data and weekly injury reports. Participants: All NFL players who participated in a regular season game from the 2012-2019. Exposures: NFL game participation. Main Outcomes and Measures: The primary outcome was the number of concussions within a game. Bayesian negative binomial regression models were fit to relate how playing surface (artificial turf vs. natural grass), game temperature, and week of the season independently related to concussion risk, and any interactions between these factors. Results: 1096 concussions were identified in the 1830 games were included in this study. There was a 98% probability that concussion risk was reduced on grass surface (median Incidence rate ratio (IRR) = 0.534), 99% probability that concussion risk was lower at higher temperatures (IRR=0.979 for each 1oC), and 91% probability that concussion risk increased with each week of the season (IRR=1.019). There was an 84% probability for a surface x temperature interaction (IRR=1.013) and 74% probability for a surface x week interaction (IRR=1.018). If the model was fitted without interaction terms, the probability of each main effect was >99%. Conclusions and Relevance: Concussion risk is increased on artificial turf compared to natural grass, and this is exacerbated in cold weather. Concussion risk is also increased later in the season (regardless of temperature) on both surfaces, but the apparent protective effect of natural grass is diminished in later weeks. The complex interplay between these factors may explain seemingly contradictory findings in the literature and must be accounted for in sports injury epidemiology. These findings suggest that player safety concerns regarding artificial turf and extended playing seasons are warranted and should be considered by the NFL. The risk factors should be explored at non-professional levels, and in other sports.