An anterior cruciate ligament (ACL) injury is diagnosed through a medical history and a physical examination. A health professional 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, it can determine 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 health professional immediately after your injury, the pain and the degree of swelling and muscle tenseness may make it difficult for your health professional to accurately diagnose the condition.

Other tests that may help determine how badly the knee is injured include:

• Magnetic resonance imaging (MRI), which can identify other problems, such as meniscus tears or other ligament injuries.
• Computed tomography (CT), which is usually done to see any small breaks in the bones.

If your knee is red, hot, or very swollen, a knee joint aspiration (arthrocentesis) may be done, which involves removing fluid from the knee joint with a syringe (needle). This is done to:

• Help relieve pain and pressure, which may make the physical exam easier and make you more comfortable.
• Check joint fluid for possible infection or inflammation.
• Identify blood, which may indicate a tear.
• Identify drops of fat, which may indicate a broken bone.

Fluid removed from the knee joint may be tested to identify blood and fatty droplets from a hidden fracture. Local anesthetic may be injected to reduce pain and make the knee easier to examine. If the ACL is torn, fluid drained from the knee may contain a lot of blood.

If chronic ACL deficiency is suspected, arthrometric testing of the knee may also be done. In this test, your health professional will use an instrument to measure the looseness of your knee. This test is especially useful in people whose pain or physical size makes a physical exam difficult. An arthrometer has two sensor pads and a pressure handle that allows your health professional to put force on the knee. The instrument is strapped to your lower leg so that the sensor pads are placed on the knee cap and the small bump just below it (tibial tubercle). Your health professional then measures pressure by pulling or pushing on the pressure handle.