ACL Injury
Investigations
To diagnose a torn ACL, your doctor first wants to know as much as possible about the injury, such as whether you heard or felt your knee pop, whether your knee swelled up afterward and if you were able to continue being physically active.
Swelling that occurs shortly after the injury usually means there's blood in the joint from torn blood vessels in the damaged ligament. Your doctor may decide to draw the blood out with a needle and syringe. This can reduce pain and make it easier to examine the knee joint.
Your doctor examines your knee in a variety of positions to assess whether or not your ACL is torn. Two common exams are:
- Lachman's test: In this test you lie on your back on the exam table with your injured leg bent at a 30-degree angle and your foot flat on the table. Your doctor then moves the lower portion of your injured leg forward from the knee. If your leg moves freely without reaching a firm endpoint, you have a tear in your ACL.
- Pivot shift test: For this test, your injured leg is extended, and your doctor rotates your foot at the same time he or she applies pressure to the outside of your knee and bends your knee. Signs of instability in your shinbone suggest an ACL tear.
Often the diagnosis can be made on the basis of the physical exam alone, but you may need X-rays to rule out a bone fracture. If your doctor has questions about the cause or extent of your injury, he or she may order a magnetic resonance imaging (MRI) scan, a painless procedure that uses magnetic fields to create an image of the soft tissues of your body. An MRI can show the extent of ACL injury and whether other knee ligaments or joint cartilage also are injured.