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. 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 is red, hot, or 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.

Other tests

• Arthrometric testing: In this test, your doctor uses a tool to measure the looseness of your knee. This test is especially useful in people whose pain or size makes a physical exam difficult. An arthrometer has two sensor pads and a pressure handle that allows your doctor to put force on the knee.
• Arthroscopy: This can be used to diagnose an ACL injury and as a method of surgery. It involves inserting tools through one or more small incisions in the knee, allowing your doctor to examine the structures inside the knee joint, including the ACL.

Before arthroscopy, you and your doctor will decide what will be done if certain conditions are found. For example, you may decide in advance that if a complete tear of the ACL is found, it will be reconstructed during the arthroscopy. Or if a more severe condition is found, you and your doctor may agree to discuss the condition rather than proceeding with surgery at that time.