Risk of Chronic Traumatic Encephalopathy in Football Players Depends on Number and Intensity of Head Impacts
Daniel H. Daneshvar, MD, PhD, and colleagues used helmet accelerometer data to show that duration of play, cumulative g-force and cumulative rotational force — and not the number of concussions — significantly influence the risk of chronic traumatic encephalopathy and its severity in players of American football.
Helmets fitted with accelerometers are now available that record the number of hits to the head and the force and direction of those hits during American football play.
In this study, a positional exposure matrix was derived from 34 studies of football helmet data, then was used to study the relationship between lifetime exposure to repetitive head impacts and the risk of chronic traumatic encephalopathy (CTE).
Of 631 brains donated by former football players, 180 did not have CTE, 163 (26%) had CTE stage I or II and 288 (46%) had CTE stage III or IV.
The number of years of play and cumulative exposure to repetitive head impacts were associated with CTE status (yes/no) and CTE severity, but the lifetime number of concussions was not.
These findings, if validated, could be used to identify changes to policy and game play that might limit CTE risk by decreasing cumulative exposure.
Chronic traumatic encephalopathy (CTE) is known to be associated with repetitive head impacts. For players of American football, helmet accelerometers are now available that record both the number of hits to the head and the force and direction of those hits.
Daniel H. Daneshvar, MD, PhD, chief of the Division of Brain Injury Rehabilitation at Spaulding Rehabilitation Hospital and director of training in the Rehabilitation Outcomes Center, Jesse Mez, MD, MS, of Boston University, and colleagues have found the number and force of head impacts — not the number of concussions — drives the risk of CTE in football players. Their report appears in Nature Communications.
The researchers studied the donated brains of 631 men who had played tackle football. According to informants, the highest level of play was semi-professional or professional for 49%, college for 33%, high school for 15%, and pre–high school for 3%. The average age at first exposure to football was 11, and the average duration of play was 13 years. The median number of reported concussions was 20 (range, 7–92).
Neuropathologists blinded to each donor's history determined 180 brains did not have CTE, 163 (26%) had CTE stage I or II, and 288 (46%) had CTE stage III or IV.
For the first time in a study of this type, the researchers created a positional exposure matrix, just as public health researchers do to estimate occupational exposures retrospectively. Derived from 34 studies of football helmet accelerometer data, the matrix was used to estimate each subject's lifetime number of head impacts and the associated cumulative linear and rotational forces.
Effects of Concussions and Playing Position
The number of concussions was not associated with CTE status (yes/no) or CTE severity across the entire cohort or athletes with CTE. Similarly, the position played at the highest level was not associated with CTE status or severity.
Effects of Cumulative Exposure
Duration of play, number of head impacts, and cumulative force were significantly associated with CTE status and severity after adjustment for age at death:
Every additional year of play — Overall, associated with 15% increased odds of being diagnosed with CTE; among those with CTE, associated with 14% increased odds of being diagnosed with severe CTE
Every additional estimated 1,000 head impacts — Overall, 21% increased odds of CTE; among those with CTE, 13% increased odds of severe CTE
Every additional estimated 10,000 g of cumulative linear acceleration to the head — Overall, 20% increased odds of CTE; among those with CTE, 19% increased odds of severe CTE
Every additional estimated 1,000,000 rad/sec2 of cumulative rotational acceleration to the head — Overall, 22% increased odds of CTE; among those with CTE, 20% increased odds of severe CTE
Implications for Players, Coaches, and Leagues
The medical and lay literature about head impacts in football tends to focus on concussions, but this study shows that repeated non-concussive impacts should be emphasized.
The findings, if validated, could be used to identify changes to policy and gameplay that might limit CTE risk by decreasing cumulative exposure to head impacts, such as by limiting the duration of exposure, the number of exposures, and the magnitude of those exposures.
- 15% greater odds of developing chronic traumatic encephalopathy for every additional year of playing American football
- 21% greater odds of developing chronic traumatic encephalopathy for every additional 1,000 head impacts while playing American football