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) 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 (hemarthrosis). 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 engaged in but may be able to walk.

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.

Other conditions with symptoms similar to ACL knee pain include injuries to other knee structures, such as:

• An injury to the cartilage lining the knee joint.
• An injury to the knee cushions (menisci). About 70% of people with an ACL injury also have a meniscus tear.¹
• An injury to the knee ligaments that connect the upper leg bone to the lower leg bone along the inner side of the knee joint (medial collateral ligament) and the outer side of the knee joint (lateral collateral ligament).
• A break (fracture) in the bones of the knee joint.

For more information on knee injuries, see the topics Knee Problems and Injuries and Patellar Tracking Disorder.

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