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. When this happens, other structures in the knee can be damaged, especially when the knee bends. For example, the meniscus may tear, due to the additional force against it. So, when the knee bends, the thighbone pushes against this torn area and causes pain. Squatting becomes especially painful. Furthermore, a meniscal tear can cause symptoms of your knee giving out, clicking or catching and sometimes locking. This occurs because the torn piece of the meniscus can actually move around inside the joint. When the torn piece of the meniscus moves, it often becomes caught between the bones of the joint, the thighbone and shinbone.
When tissue is damaged, blood provides the necessary nutrients for it to heal properly. Unfortunately, most of the meniscus has no blood supply and cannot usually heal itself when it is damaged.