Biomechanics of subdural hemorrhage in American football: review of the literature in response to rise in incidence.

The number of catastrophic head injuries recorded during the 2011 football season was the highest since data collection began in 1984--the vast majority of these cases were secondary to subdural hemorrhage (SDH). The incidence of catastrophic head injury continues to rise: the average yearly incidence from 2008 to 2012 was 238% that of the average yearly incidence from 1998 to 2002. Greater than 95% of the football players who suffered catastrophic head injury during this period were age 18 or younger. Currently, the helmet industry utilizes a standard based on data obtained at Wayne State University approximately 50 years ago that seeks to limit severity index--a surrogate marker of translational acceleration. In this manuscript, we utilize a focused review of the literature to better characterize the biomechanical factors associated with SDH following collisions in American football and discuss these data in the context of current helmet standard. Review of the literature indicates the rotational acceleration (RA) threshold above which the risk of SDH becomes appreciable is approximately 5,000 rad/s(2). This value is not infrequently surmounted in typical high school football games. In contrast, translational accelerations (TAs) experienced during even elite-level impacts in football are not of sufficient magnitude to result in SDH. This information raises important questions about the current helmet standard--in which the sole objective is limitation of TA. Further studies will be necessary to better define whether helmet constructs and quality assurance standards designed to limit RA will also help to decrease the risk of catastrophic head injury in American football.