And in some respects, hockey players may be at greater risk than athletes in other sports, said Douglas H. Smith, director of the University of Pennsylvania's Center for Brain Injury and Repair.
Concussions occur when the head comes to a sudden stop and the brain keeps moving inside it - a phenomenon that is all too prevalent in a fast-moving game with a hard playing surface surrounded by rigid walls.
"You have a skimpy helmet and a bunch of hard surfaces," Smith said. "It's not a good combination. And you're on ice."
The true scope of the problem is hard to grasp. Many past studies have relied on players or team officials to report the number of concussions. Both groups may underreport the incidence of these head injuries, either because they don't want players to come out of a game, or because they do not realize what symptoms constitute a concussion, physicians say.
A more rigorous approach was used in a 2010 study published in the journal Neurosurgical Focus. Physicians observed junior-hockey games in Canada, in which the players' average age was 18, and evaluated players with suspected concussions on the spot.
A concussion was diagnosed by a physician in 19 of the 52 games observed - or 36.5 percent of the games - whereas a previous study of NHL games found concussions in just 3 percent of games.
The authors also looked at concussions as a function of "athlete exposures," defined as one player playing one game.
The 2010 study found 21.52 concussions per 1,000 athlete exposures - seven times the highest rate that had previously been reported for ice hockey.
Of the hits that caused concussions in the study, 80 percent occurred on purpose, said lead author Paul Echlin, a sports-medicine physician in Canada whose coauthors included Boston University physician Robert Cantu, one of the top sports-concussion specialists in the United States.
In an interview, Echlin said he used to be a team physician for junior-level hockey but stopped because he felt that in the culture of the sport, there was too little concern for the gravity of the injury.
"I just couldn't accept it anymore," said Echlin, who is a physician at the Elliott Sports Medicine Clinic in Burlington, Ontario.
He has since devoted himself to education, helping to set up an online library - www.sportconcussionlibrary.com - for parents seeking information.
Contact staff writer Tom Avril at 215-854-2430 or email@example.com.