Estimated number of ACL (the full name of this key knee ligament is anterior cruciate ligament) injuries that occur each year in the United States. Women are four to eight times more likely to suffer a torn ACL. The ACL tears more often than any other ligament because of the lack of muscle support for twisting or rotational movements around the joint.
Mariano Rivera, of the New York Yankees, tore an ACL while shagging balls during batting practice before a game against the Kansas City Royals in May.
Photo: Ed Zurga, Getty Images