Interaction of surface type, temperature, and week of season on diagnosed concussion risk American football: Bayesian analysis of 8 Seasons of National Football League Data

Background Artificial turf fields and environmental conditions may influence sports concussion risk, but existing research is limited by uncontrolled confounding factors, limited sample size, and the assumption that risk factors are independent of one another. The purpose of this study was to examine how playing surface, time of season, and game temperature relate to diagnosed concussion risk in the National Football League (NFL). Methods This retrospective cohort study examined data from the 2012-2019 NFL regular season. Bayesian negative binomial regression models were fit to relate how playing surface, game temperature, and week of the season independently related to diagnosed concussion risk and any interactions among these factors. Results 1096 diagnosed concussions were identified in 1830 games. There was a >99% probability that concussion risk was reduced on grass surface (median Incidence rate ratio (IRR) = 0.78 [95% credible interval: 0.68, 0.89], >99% probability that concussion risk was lower at higher temperatures (IRR=0.85 [0.76,0.95] for each 7.9°C), and >91% probability that concussion risk increased with each week of the season (IRR=1.02 [1.00,1.04]). There was an >84% probability for a surface × temperature interaction (IRR=1.01 [0.96, 1.28]) and >75% probability for a surface × week interaction (IRR=1.02 [0.99, 1.05]). Conclusions Diagnosed concussion risk is increased on artificial turf compared to natural grass, and this is exacerbated in cold weather and, independently, later in the season. The complex interplay between these factors necessitates accounting for multiple factors and their interactions when investigating sports injury risk factors and devising mitigation methods. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This study did not receive any funding ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: We performed a retrospective cohort analysis of eight NFL seasons (2012-13 through 2019-20) of concussion data from the PBS Frontline Concussion Watch and Football Outsiders injury databases. The Football Outsiders database includes data from all weekly NFL injury reports during this time period, including concussion and non-concussion injuries from weeks 1-16 of the regular season. This concussion dataset was combined with data from the Frontline database, which provided an independently collected list of concussions incurred by an NFL player from 2012-2015. Sources: I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines and uploaded the relevant EQUATOR Network research reporting checklist(s) and other pertinent material as supplementary files, if applicable. Yes All data produced in the present study will be available upon acceptance into a peer-review journal.