If you have a sudden (acute) anterior cruciate ligament (ACL) injury, you generally know when it happens. You may feel or hear a pop and the knee may give out, causing you to fall. The knee swells and often is too painful or unstable to continue any activity.

An ACL injury can cause small or medium tears of the ligament, a complete tear of the ligament (rupture), a separation of the ligament from the upper or lower leg bone (avulsion), or a separation of the ligament and part of the bone from the rest of the bone (avulsion fracture). When any of these occur, the lower leg bone moves abnormally forward on the upper bone, with a sense of the knee giving out or buckling.

When the ACL ligament tears, the blood vessels around the ligament tear and blood fills the knee joint, causing swelling. If you see a health professional immediately, he or she may not be able to examine the knee thoroughly because of the swelling.

Other parts of the knee can be injured at the same time. These may include one of the pads that act as cushions between the two leg bones (menisci), another knee ligament (medial collateral ligament or lateral collateral ligament), or the dense tissue that covers the ends of bones (cartilage). The bones of the knee joint may also be broken (fractured).

Diagnosis may not be done at the time of injury. Sometimes people think the injury is not serious, especially if the knee gets better in a few days. In this case, or if the diagnosis is missed during the initial examination, the injury may develop into a long-lasting and recurrent (chronic) ACL deficiency, in which the knee moves abnormally and gives way occasionally. This can potentially cause progressive damage to the joint, including osteoarthritis. However, not everyone with an ACL injury develops a chronic ACL deficiency.

The course of an ACL injury depends on:

• The condition of the ACL before this injury, including prior injuries, partial tears, ACL deficiency, and degenerative changes due to age.
• The general condition and health of all of your knee structures prior to this injury.
• The amount of damage or injury to the ACL. Injuries to the ACL are usually grouped into grade I, II, or III sprains (tears) according to the amount of damage.
• Additional injuries to the knee joint, such as to the cartilage or menisci, or to bones in the knee.
• Your age, how active you are, and how committed you are to treatment and rehabilitation.
• The time of diagnosis. If the ACL diagnosis is not made soon after the injury, the knee may be further damaged with use.