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:
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.