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
Head Impact Biomechanics Don’t Relate to Concussion Severity
If (in a perfect world) we actually knew each athlete’s injury threshold, and were aware of the rotational accelerations they were receiving in real-time, I think that could absolutely translate into better care for our athletes. Primarily, it would reduce underreporting because the clinician would know the athlete was hurt at the time of injury. Even if the data was only available retrospectively, clinicians may be able to identify athletes who didn’t report their symptoms before their injury was exacerbated. As far as prevention, perhaps the data could be used as a coaching tool in practice to educate players on their technique. The evolution of this technology could really have an impact on the care we, as clinicians provide. Great post!
This technology is exciting and has given the Sports Medicine community an inside look into the forces that cause concussion, but I am not sure what would change in clinical practice if we could determine a "concussion threshold" for each athlete.
I am not convinced that this technology can help us prevent or treat concussions. I do believe that it can help us diagnose them, or at least alert us to individuals who are playing at a higher risk for injury. What really needs to happen in order to prevent a concussion from occurring?
My simple answer to that is protect the brain. This can be done by equipment advancement or altering the brutality of some collision sports. I used to be a huge advocate for advancing helmet technology, but now I am not so sure that is the best answer. As equipment advances, an increase in speed and intensity is also seen. Take football for example. Compared to todays helmets, were there more or less head injuries when a leather helmet was worn? Could advanced technology actually be the cause of more injuries?
Erica, great comment! Studies have suggested that linear acceleration is correlated to the intracranial pressure response and tend to occur more focally. Rotational kinematics are correlated to the strain response of the brain, and typically produce more diffuse brain damage. The authors suggest that with increased understanding of injury risk related to single biomechanical parameters, and propose that with this model with combined measures of biochemical markers that measure tissue response to injury, strain, and pressure in the brain could be used to assess the injury. They caution that these predictors are going to be model specific as each model would find a different parameter that best predicts injury. The researchers also believe that with an increased understanding of the kinematics associated with injury engineering analysis can be used to evaluated and influence product design to reduce brain injuries. This method would take into consideration the increase speed and intensity that continues with the sport.