Now that you have seen how a normal knee functions, let's look at how an ACL can tear and how this affects the motion and function of your knee.
The ACL can be injured or torn in a number of ways. The most common way is a sudden pivoting or cutting maneuver done during a sporting activity. At the time of the injury a "pop" or "snap" can sometimes be felt or heard.
When the ACL is torn, the shinbone is able to slide forward. This causes the knee ot "give-out" or be unstable. In an abnormal position, other structures in the knee can be damaged, especially when the knee bends. For example, your meniscus can tear, due to the additional force against it. If your meniscus tears or becomes damaged, the pressure between the shinbone and thighbone is increased. This increase in pressure will speed up the wear and tear on the articular cartilage and lead to arthritis.