Head Impact Density: A Model To Explain the Elusive Concussion Threshold

Abstract

Concussion is a heterogeneous injury occurring throughout a range of impact magnitudes. Consequently, research focusing on a single or set of variables at the time of injury to understand concussive biomechanics has been thwarted by low injury prediction sensitivity. The current study examined the role of Impact Density in estimating concussive injury risk. Head impact data were collected across seven high school football seasons with the Head Impact Telemetry System (HIT System). Over the study period, 29 concussions were included for data analysis. The linear acceleration of the concussive impact was matched to a Control athlete, along with impacts in the 24 h before. Linear and rotational acceleration for the 19 impacts leading into the final event and the cumulative accelerations over time were evaluated. Analyses indicated no difference in impact counts within the final 24 h, or impact magnitudes for linear and rotational acceleration among the final 20 impacts (p > 0.05). A novel metric, Impact Density, was calculated from the final 20 impacts by summing the acceleration magnitude divided by time from the previous impact. Analyses indicated the Concussed athletes incurred a significantly higher linear (concussed: 255.4g/sec (standard error of the mean [SEM] = 40.1), controls:145.4g/sec (SEM = 23.8), p = 0.016), and rotational (Concussed:10311.3 rad/s/s/s (SEM = 1883.7), Controls: 6083.8 rad/s/s/s (SEM = 1115.9), p = 0.029) Impact Density than the Control athletes. Similar to other investigations, there was no difference in individual linear or rotational impact magnitude in the 20 impacts before and including the injury. The measure of Impact Density, however, revealed differences between the Concussed and Control athletes. These data suggest that the biomechanical threshold for concussion fluctuates downwardly with a greater impact magnitude and number with a return to pre-impact levels with time, suggesting physiological vulnerability to repeated head impacts. The current results highlight that time between impacts, not just impact magnitude, influences risk for concussion.

American Medical Society for Sports Medicine position statement on concussion in sport

“…Neither a device nor a specific threshold measure of force or angular acceleration can be used to diagnose concussion 38 48. Some athletes experience high forces with no clinical symptoms of concussion, and some athletes sustain a concussion at much lower impact forces, making current impact measures a poor predictor of SRC 49. The number, location, density and individual thresholds of head impacts may be important parameters.…”

A novel repetitive head impact exposure measurement tool differentiates player position in National Football League

“…Neurobiological damage may be present in the absence of clinical symptoms, and without a sufficient recovery there is a risk of cumulative injury 6,8 . The density to which sub-concussive impacts are experienced in succession may also collectively lead to concussion 35 .…”

“…The degree of alteration associates with both the intensity of impact and number of impacts received 7,13,14,[30][31][32] . Moreover, taking time for brain tissue to recover from either frequent or intense impacts has been implicated in recurrent concussion and brain health 8,[33][34][35] . Frequency, magnitude, and interval characterize RHI and are important and relevant to long-term psychiatric sequelae.…”

Proceedings from the Ice Hockey Summit III: Action on Concussion

“…Emerging research in objective, involuntary neurofunctional metrics and biomarkers are bridging gaps between human and animal research. SRC biomechanics provide a rational research foundation to test hockey SRC prevention strategies [89][90][91][92][93].…”