An ACL tear can result from violent movement of the leg against resistance (e.g. two football players kicking the ball from opposite directions) or from falling on an extended leg. ACL tears often occur in combination with a meniscus, collateral ligament or cartilage injury.
Swelling of the knee is usually immediate and extensive. Normal bending and extension of the knee is no longer possible and usually painful.
When the swelling subsides and it is possible move the leg normally and walk again, you will notice that the knee joint is unstable in certain situations.