What do the anterior cruciate ligament injuries that new Eagles quarterback Sam Bradford and linebacker Kiko Alonso suffered last season entail?

The ACL is a small ligament in the center of the knee that has a significant importance, especially for athletes.

"Its function is to prevent the lower leg from shifting forward and rotating," said Steven Cohen, a board-certified sports medicine surgeon at the Rothman Institute. "It's primarily responsible for helping execute movements like cutting and pivoting, and also landing or taking off for jumps."

When you tear the ligament, the knee can become loose or unstable, and you lose the ability to change directions. A tear in the ACL also damages other structures of the knee, such as cartilage around the meniscus, putting the knee at an even greater risk for injury.

"When we reconstruct the ACL" in surgery, the typically recommended treatment, "we would like to think that we make it as good as the day you were born, but it's not perfect," Cohen said. "Once you've injured the ACL, you have a 6 to 14 percent risk of retearing the ligament on either knee.

"If you tore your ACL playing football, and you go back and continue to make those same cutting and pivoting movements, you're definitely going to increase your risk of tearing your ACL again," Cohen said.

Recovery from an ACL injury varies.

"You'll hear of an athlete getting back within six or seven months," he said. "But studies on return to play from the NFL generally say it takes about nine months for a full recovery." - Kelly O'Shea