Symptoms

Symptoms of a severe and sudden (acute) anterior cruciate ligament (ACL) injury include:

• Feeling or hearing a "pop" in the knee at the time of injury.
• Sudden instability in the knee. (The knee feels wobbly, buckles or gives out.) This may happen after a jump or change in direction or after a direct blow to the side of the knee.
• Pain on the outside and back of the knee.
• Knee swelling within the first few hours of the injury. This may be a sign of bleeding inside the joint. Swelling that occurs suddenly is usually a sign of a serious knee injury.
• Limited knee movement because of swelling and/or pain.

After an acute injury, you will almost always have to stop the activity you are doing, but you may be able to walk.

Other health problems can cause symptoms like those of an ACL injury. They include a bone break or injuries to the knee cushions (menisci) or to other ligaments in the knee.

For more information on knee injuries, see the topics:

• Knee Problems and Injuries.
• Patellar Tracking Disorder.

Chronic ACL deficiency

The main symptom of chronic (long-lasting and recurrent) ACL deficiency is an unstable knee joint. The knee buckles or gives out, sometimes with pain and swelling. This happens more often over time. But not everyone with an ACL injury develops a chronic ACL deficiency.

What Happens

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.