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Repetitive head-impact exposure in American tackle football was linked with chronic traumatic encephalopathy (CTE), a model based on helmet accelerometer data found.

Cumulative repetitive head impacts were associated with CTE status, CTE severity, and pathologic burden (all P<0.001) among brain donors who played football an average of 12.5 years, according to Dan Daneshvar, MD, PhD, of Harvard Medical School in Boston, and colleagues.

Concussion counts alone were not associated with CTE risk (P=0.23), Daneshvar and co-authors reported in .

The findings support earlier data suggesting that head hits, not concussions, are tied to CTE, the researchers noted.

"These results provide added evidence that repeated non-concussive head injuries are a major driver of CTE pathology rather than symptomatic concussions, as the medical and lay literature often suggests," said co-author Jesse Mez, MD, MS, of Boston University, in a statement.

The results also may mean that CTE risk could be reduced by changing how football is practiced and played, Daneshvar observed. "If we cut both the number of head impacts and the force of those hits in practice and games, we could lower the odds that athletes develop CTE," he said.

CTE is a delayed neurodegenerative disorder that can be diagnosed only at autopsy. Over 97% of published CTE cases have been reported in people with , mostly in contact sports.

Daneshvar and co-authors evaluated data from 631 brain donors who played American tackle football for an average of 12.5 years, and who died at a mean age of 59.7 years. All were male former football players from the and Framingham Heart Study (FHS) brain banks. Most donors (82%) had played at the college level or above.

UNITE donors all had a history of repetitive head impact exposure; recruitment began in 2008 and the study included donors through 2020. The community-based FHS sample, which started in 1997, included athletes with fewer years of American football play.

In the total sample, 180 athletes did not have CTE, 163 had low-stage CTE (stage I or II), and 288 had high-stage CTE (stage III or IV). For athletes with CTE, there was no association between the number of reported concussions and CTE severity (P=0.49) or cumulative pathology burden (P=0.95).

The researchers developed a positional-exposure matrix (PEM) -- similar to a job-exposure matrix to assess occupational risks -- to characterize repetitive head impact exposure for each donor. They used 34 published helmet sensor studies of findings for specific field positions and levels of play to quantify a player's lifetime exposure to repetitive head hits, including linear accelerations (measured in g-force) and rotational accelerations (measured in rad/s²).

Mean duration of play was 9.5 years for athletes without CTE, 11.6 years for those with low-stage CTE, and 15.0 years for those with high-stage CTE.

The PEM estimated the mean number of cumulative number of head impacts, as well as the cumulative linear and rotational accelerations associated with those impacts, respectively, as:

• 4,515 hits, 88,972 total g-force, and 6.57 × 10⁶ total rad/s² for players without CTE
• 5,553 hits, 107,650 total g-force, and 8.32 × 10⁶ total rad/s² for players with low-stage CTE
• 7,641 hits, 148,777 total g-force, and 12.26 × 10⁶ total rad/s² for players with high-stage CTE

Models that incorporated cumulative linear and rotational accelerations classified CTE status and CTE severity better than ones with duration of play or cumulative number of head impacts alone.

"The finding that estimated lifetime force was related to CTE in football players likely holds true for other contact sports, military exposure, or domestic violence," Daneshvar said.

The study had several limitations, the researchers acknowledged. It relied on a convenience sample of brain donors who tended to have more exposure to repetitive head impacts than other football players. Cumulative exposures were not observed directly, but were extrapolated from helmet accelerometer studies of other recent players. In addition, accelerometers may move independently from the head and may not accurately reflect acceleration experienced by the brain.

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