An anterior cruciate ligament (ACL) injury can involve a small or medium tear 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 may move abnormally on the upper bone, with a sense of the knee giving out.

An ACL injury may develop into chronic (long-lasting and recurrent) ACL deficiency: this is also called ACL insufficiency or an ACL-deficient knee. The ability of the ACL to control knee movement decreases, resulting in more sliding of the bones. The knee becomes more and more unstable—it begins to buckle or give out, sometimes with pain and swelling. This can result in further injury, loss of strength (weakness), and instability. This abnormal sliding also can damage cartilage and trap and damage the pads that cushion the knee joints (menisci) and can lead to premature osteoarthritis.

ACL surgery is often done for chronic ACL deficiency, unless the knee is so damaged that surgery won't help. When possible, starting a rehabilitation program before surgery can help speed rehabilitation after surgery.

If you have chronic ACL deficiency and you are willing and able to stop activities that require a lot of knee stability, rehabilitation may make your knee stable enough to perform daily activities, and you may not need surgery.