What happens when the ACL is torn Most people who rupture their ACL can recall the exact moment at which they felt it pop. This usually happens when changing direction quickly in pivoting or cutting sports like soccer, landing from a jump in sports such as basketball, or falling while skiing. The four "classic" symptoms that people may feel when they tear their ACL are:

• They hear a "pop" from inside the knee
  
• They feel the knee give away at the time of injury
  
• They develop a swollen knee immediately, or within a few hours
  
• The pain is bad enough that they can not continue play that day.

When the ACL ligament is stretched too far and ruptures, the blood vessels inside the ligament rupture as well, and this blood fills the knee joint, causing the knee to swell. While the ACL injury itself is usually not terribly painful, the swelling that results from blood inside the knee prevents most people from returning to the game or the athletic activity they were involved in at the time of injury. Examining an athlete immediately after they have had a knee injury can sometimes be difficult and it is often not possible to tell what structures, if any, have been injured. This is because the quadriceps and hamstring muscles "guard" the knee, preventing the examiner from testing the ligaments accurately. A better examination can usually be obtained after the swelling has gone down and the pain from the injury has subsided. Although accurate diagnosis of the exact injury can be difficult, it is relatively certain that an athlete who develops a swollen knee immediately after an injury should not return to play and should seek medical evaluation.