NFL seasons can rise and fall on the health of what the dictionary calls a "short band of tough, flexible, fibrous connective tissue" in the middle of a star player's knee.
Sometimes, that anterior cruciate ligament (ACL) gets torn by the sheer brute force of the game.
"I had three ACLs in college," former Vikings linebacker Michael Mauti said. "The second one on my left knee, a guy chop-blocked me. I had a brace on. Nothing would have saved me."
But many times — more so in recent years — there is no contact when a player's ACL snaps. Vikings quarterback Sam Bradford fell into this growing category twice with the Rams in 2013 and 2014.
"We know the percentages of a recurrent tear can range somewhere between 5 and 25 percent [higher], so it's not uncommon," said Dr. Bert Mandelbaum, a Santa Monica-based orthopedic surgeon and ACL injury prevention researcher at the Kerlan-Jobe Institute. "It's one of the things we focus on trying to minimize."
Bradford was fortunate not to suffer a third tear in the season opener. But another noncontact injury occurred when he twisted the knee badly enough that he had to miss last week's game at Pittsburgh and was still limited in Thursday's practice.
Mandelbaum isn't familiar with Bradford's latest injury, and he won't venture a guess as to when his swelling will subside or when there's enough stability in the knee to play on it.
But Mandelbaum has studied Bradford's first two injuries. They were part of an NFL award-winning video analysis of noncontact ACL injuries that Mandelbaum helped create.