Rotational head kinematics in football impacts: An injury risk function for concussion
Rowson S., Duma S., Bekcwith G., Chu J., Greenwald RM., Crisco JJ. 2011 October; Ann Biomed Eng; ahead of print
The mechanism that leads to a head injury could cause different outcomes. Researchers have suggested that a rotational head injury will have a worse outcome then a head injury obtained with a linear acceleration/deceleration force (Holbourn, 1943; Ommaya and Gennarelli, 1974; Gaetz, 2004). The purpose of this study was to characterize the rotational kinematics (e.g., acceleration) of the head associated with concussive and subconcussive impacts using an acceleration data set collected from athletes wearing helmets. Researchers followed 335 NCAA Division I Football players with accelerometers placed in their helmets, which measured the impacts sustained during games and practices. The impacts were measured using the HIT System (314 athletes) or the 6DOF device (21 athletes; used due to the large size of the helmets for the lineman). There were minimal differences among measurements between the 2 systems. Across 3 football seasons (2007 to 2009), 57 concussive impacts were recorded among 300,977 head impacts. Based on the HITS System, subconcussive impacts had an average rotational acceleration of 1,230 rad/s2 (most subconcussive impacts were between 682 and 1506 rad/s2), and concussive impacts had an average rotational acceleration of 5,022 rad/s2 (most concussive impacts were between 4026 and 7688 rad/s2). If the measurement recorded a reading ~7,000 rad/s2 the athlete was at a 75% chance for a concussion, and at ~7,500 rad/s2 the athlete was over 90% chance of risk for a concussion. Most of the concussions sustained happened in the sagittal plane (front to rear impact locations; 33 [58%]), 7 (12%) resulted in a coronal plane (side impact locations), and 17 (30%) of concussions were obtained by an impact to the top of the head.
This study took advantage of a large data set of head acceleration recorded during subconcussive and concussive impacts to determine a possible injury threshold. However, it should be noted that there were a small number of impacts with accelerations at or above ‘concussive levels’ that did not result in an injury, which suggests other factors could influence an athlete’s tolerance to neuronal stress (e.g., genetic variations, muscle activation, duration of impact). It would be interesting to see future studies quantify head impact exposure or assess the cumulative effect of subconcussive impacts (does having more subconcusive impacts in a season increase the risk of a concussion at lower impact accelerations). This data is useful for analyzing the mechanism of brain injury and tolerance to mechanical stimuli. This information is necessary to improve prevention and evaluation of brain injuries as well as treatment protocols. Future research should continue to improve methodology for obtaining concussion threshold as well as determine the effect of cumulative subconcussive impacts. Though an injury threshold would most likely be different for each athlete do you believe that knowing an injury threshold would help prevent, evaluate, and/or treat a concussion? How do you think it would help, or why would it not help your clinical practice?
Written by: Jane McDevitt MS, ATC, CSCS
Reviewed by: Jeff Driban