If you have a sudden (acute) anterior cruciate ligament (ACL) injury, you typically 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 for you 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.

How an anterior cruciate ligament (ACL) injury is treated and how it heals depends on:

• The condition of the ACL before the injury. This includes prior injuries, partial tears, ACL deficiency, and changes due to age.
• The general condition and health of the rest of your knee before this injury.
• The amount of damage or injury to the ACL. Injuries are usually grouped into grade I, II, or III sprains (tears) according to the amount of damage.
• Other 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.

An ACL injury may develop into long-lasting and recurrent (chronic) ACL deficiency that leads to an unstable knee-the knee buckles or gives out, sometimes with pain and swelling. This can occur if you had an ACL injury in the past and didn't know it or if your ACL has not been treated or has been treated unsuccessfully. ACL deficiency can cause damage to the joint, including osteoarthritis. But not everyone with an ACL injury gets ACL deficiency.

People with minor ACL injuries usually begin treatment with a physical rehabilitation (rehab) program. Rehab exercises build strength and flexibility in the muscles on the front of the thigh (quadriceps) and strengthen and tighten the muscles in the back of the thigh (hamstrings). Most people return to their normal activities after a few weeks of rehab.

More serious ACL injuries may need several months of rehab or surgery followed by several months of rehab to regain your knee strength, knee stability, and range of motion.

Not all ACL injuries require surgery. But whether you have surgery or not, you need to start strengthening your knee and regaining motion soon after you injure it. This prepares you for your rehab program if you choose not to have surgery. It also helps prepare the knee for surgery if you choose to have it.