As football preseason gets underway, new research suggests that it’s the intensity of the hits players sustain, rather than the total number of blows to the head or their cumulative years of play, that better predicts who will develop a devastating brain disease.
A team of Boston scientists found that players who developed chronic traumatic encephalopathy, or CTE, a condition that silently destroys the minds of athletes after years of repetitive hits, were more likely to have received the hardest collisions over time, rather than necessarily the most hits.
CTE can only be diagnosed after death, so scientists relied on a combination of brain tissue from former players, as well as interviews with players’ family members and helmet sensor data from people who played more recently, to conduct their research.
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The helmet sensor data was used to estimate the intensity of hits, or G-force, the deceased players may have cumulatively sustained, depending on their position and how long they played.
Dr. Jesse Mez, an associate professor of neurology at Boston University School of Medicine, and the senior author of the study, said a single hit in football can range from 10 Gs [G-force] up to 180 Gs, and that typically hits of 60 Gs to 90 Gs produce active concussion symptoms.
“We found that for each additional 10,000 Gs, the odds of developing CTE increased by 20 percent,” Mez said.
Using Mez’s calculations, a player who sustained more than 100 hard-hit concussions, or at least 55 of the most intense hits, would raise their chances of developing CTE by 20 percent.
The research has not been published but was scheduled to be presented Wednesday at the Alzheimer’s Association International Conference in San Diego.
To be sure, the number of years played, as well as repeated head injuries, remain significant risk factors for developing CTE, Mez said. But his team’s research found that measuring the cumulative force of those hits was about 6 percent more accurate in predicting CTE than simply calculating years played or total blows to the head.
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“We have used duration of play to show this, but duration of play could be confounded by a lot of things,” Mez said.
“Some may say it was the steroids they were taking, or the chemicals on the grass, that it wasn’t necessarily the hits,” he said. “This [research] does not tell us definitely, but it does take us closer.”
Other brain specialists not involved in the study said it was a clever approach that adds a vital, missing piece to the CTE puzzle.
CTE is diagnosed through microscopic exams of brain tissue after death.
“Right now, we know there are individuals likely playing at increased risk, but we don’t know who these individuals are,” said Dr. James Noble, an associate professor of neurology at Columbia University Irving Medical Center.
“I practice in a field of substantial clinical uncertainty. None of the diagnostic tools we have are perfect,” he said. “So anything that provides a more precise estimate of lifetime exposure to repetitive head impacts could be useful in framing personalized risks for neurological problems, in both current and former athletes.”
A number of well-known former football players have been publicly identified as having the disease after their deaths, including Ken Stabler, Bubba Smith, Dave Duerson, and former Patriots Mosi Tatupu, Junior Seau, and Aaron Hernandez.
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The hallmarks of CTE in life include erratic behavior, disturbed moods, and impaired thinking. In death, it is diagnosed by abnormal, distinct clusters in the brain of a protein called tau.
Mez’s team studied the donated brains of 656 people who played football. For each donor, they tabulated the years and position of play at each level — youth sports, high school, college, and professional — through interviews with family members.
The average age of the deceased players was about 60; most played before helmet sensors were created.
So the researchers combed through recent studies that reported the intensity of various hits gleaned through modern helmet sensors to calculate the mean frequency and intensity of hits for one year of play for each combination of levels and positions.
The data was used to calculate the estimated frequency and intensity of hits to the head that each deceased player experienced over the course of their career.
The team then compared the findings for each player with the level of tau in their brains and found those who experienced the most punishing blows in life were more likely to display increased tau brain clusters in death.
“This cumulative measure takes into account the years they played and the number of hits they sustained, as well as how hard those hits are,” Mez said. “If you look at the [number of] hits only, the strength of the relationship is not as strong.”
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Mez’s team looked to see whether the risk for CTE might be higher for some player positions, and did not find such a link. Their goal, he said, is to find a reliable method for measuring risk of CTE in life before irreversible brain damage sets in.
“The better we are able to predict the pathology, the better we can ultimately create policies or predictions about risk and give people information, so they can make appropriate choices, and we can make players more safe,” he said.
Over the past two decades, there’s been a growing awareness about potential brain injuries from contact sports, and CTE has been diagnosed in former hockey and soccer players, as well.
That rising toll prompted the National Football League in 2014 to settle a $765 million class-action lawsuit with retired players for allegedly covering up the dangers of concussions.
The mounting evidence of devastating brain injuries has also spurred many NFL rule changes, such as the types of hits allowed, to better protect players from head injuries.
Still, scientists say, players continue to face the risk of serious brain damage, and the urgency remains to find a simple test, scan, or other measurement that can help provide more informed guidance to players.
“If we had a biomarker that was sensitive and specific enough to diagnose during life, then we would have a much easier job directing and relating something happening to people in life with what is happening in their brain,” said Dr. C. Dirk Keene, professor of neuropathology at the University of Washington School of Medicine.
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“That is the holy grail for CTE right now,” he said.
Kay Lazar can be reached at kay.lazar@globe.com Follow her @GlobeKayLazar.