Exams and Tests
An anterior cruciate ligament (ACL) injury is diagnosed through a medical history and a physical exam. A doctor who specializes in knee injuries (for example, an orthopedic surgeon or sports medicine specialist) will usually be able to accurately diagnose an ACL injury after:
- Taking your medical history. You will be asked how you injured your knee, about your symptoms at the time of injury, whether you have had any other knee injuries, and general questions about your health.
Checking your knees for stability, strength, range of movement, swelling, and tenderness. Tests for stability include a Lachman test and a pivot shift test. The Lachman test compares the degree of looseness (laxity) in your knees.
- Looking at an X-ray, which is usually done for any knee injury if there is pain, swelling, or you cannot put your weight on the leg. Although an ACL injury cannot be directly diagnosed by an X-ray, an X-ray can show whether a bone is broken, any bone fragments are in the knee, the ACL is torn from the bone (avulsion), or blood is present in the knee (effusion).
If you see your doctor soon after your injury, the pain and the degree of swelling and muscle tenseness may make it difficult for your doctor to accurately diagnose the condition.
More imaging tests
Other tests that may help your doctor see how badly the knee is injured include:
- An MRI. It can identify an ACL tear or other problems, such as meniscus tears or other ligament injuries.
- A CT scan. It can be done to see any small breaks in the bones.
Looking at fluid in the knee
If your knee looks red, feels warm to the touch, or is very swollen, a knee joint aspiration (arthrocentesis) may be done. This involves removing fluid from the knee joint with a needle. It is done to:
- Help relieve pain and pressure. This may make the physical exam easier and make you more comfortable.
- Check joint fluid for possible infection or inflammation.
- Look for blood, which may mean there is a tear.
- Look for drops of fat, which may mean there is a broken bone.
Local anesthetic may be injected to reduce pain and make the knee easier to examine.