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The cumulative number and force of head hits that American football players experienced were associated with chronic traumatic encephalopathy (CTE), an analysis of autopsy data showed.

Players who spent more time at certain positions with higher levels of exposure to linear and rotational acceleration may face a greater risk of CTE, said Daniel Daneshvar, MD, PhD, of Massachusetts General Hospital, who co-authored research presented at the 2022 Alzheimer's Association International Conference.

Defensive backs appeared to face the highest risk due to the kinds of hits they face on the field, Daneshvar said. "We can now potentially target interventions to reduce people's risk by reducing the number of hits and the force of hits," he told MedPage Today.

CTE is associated with a , including those sustained in contact or collision sports like American football and boxing, and is . Previous research in football players also has linked dementia in CTE with repetitive head impact exposure.

In their analysis, Daneshvar and colleagues studied 656 former American football players who died at an average age of about 60. Most (69%) had CTE pathology and all had donated their brains to the Veterans Affairs-Boston University-Concussion Legacy Foundation brain bank.

Players' next of kin provided details about how long and which positions players played in youth, high school, college, and professional football.

From a literature search of studies that used helmet accelerometers, the researchers calculated the mean frequency of hits, linear acceleration, and rotational acceleration for 1 year of play at each level-position combination. These values were combined with each player's career exposure to determine the cumulative frequency of head impacts, cumulative linear acceleration, and cumulative rotational acceleration.

Each cumulative measure was significantly associated with the presence and severity of CTE (P<0.001 for all). A risk score based on cumulative hits (determined by time on the field) and cumulative force (determined by position) was the best predictor of CTE, Daneshvar said.

Cumulative rotational acceleration, which had an area under the receiver operating curve (AUC) of 0.765, and cumulative linear acceleration (AUC 0.758) performed significantly better than years of play (AUC 0.716) in classifying CTE pathology. Results were similar for CTE severity.

The findings suggest head impact intensity is an important factor in developing CTE pathology, the researchers said.

Strategies to reduce head impacts include delaying the debut of tackle football in school-age players and reducing the number of hits and level of force during practice, which can account for two-thirds of impacts, Daneshvar noted.

This analysis is impressive but has significant limitations, observed Keisuke Kawata, PhD, of Indiana University School of Public Health in Bloomington, who wasn't involved with the research.

"The validity is definitely low when it comes to this type of study" since it relies on helmet impact data that's often flawed, Kawata told MedPage Today. "But it's a good start."

Linemen aren't necessarily the most at risk and some wide receivers sustain more hits, Kawata said. "The team's playing style, coaching style and philosophy, and individual aggressiveness play a big role in influencing head impact exposure," he pointed out.

Going forward, "this present research neither will nor should influence rules and policy surrounding football since it needs to be validated and replicated in different methods and designs," Kawata noted.

"On the other hand, there are many studies testing ways to mitigate overexposure to head impacts, whether it is by reducing the duration of summer camp or eliminating two-a-day practices," he added.

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