In diagnosing an ACL sprain, your doctor will want to know exactly how you hurt your knee. He or she will ask about:
The type of movement that caused the injury (knee twist, sudden stop, pivot, direct contact, hyperextension)
Whether you felt a "pop" inside your knee when the injury happened
How long it took for swelling to appear
Whether severe knee pain sidelined you immediately after the injury
Whether your knee immediately felt unsteady and could not bear weight
Also, if you are an athlete who hurt your knee while you were training or competing in a sport, your doctor may want to contact your coach or trainer to get an eyewitness account of your injury.
The doctor will examine both your knees, comparing your injured knee with your uninjured one. He or she will check your injured knee for signs of swelling, deformity, tenderness, fluid inside the knee joint, and discoloration. The doctor also may check your knee's range of motion if it's not too painful or too swollen, and will pull against the ligaments to check their strength. To do this, the doctor will ask you to bend your knee and he or she will gently pull forward on your lower leg. If your ACL ligament is torn, when your lower leg is moved it will create the appearance of an "underbite" or a protruding "lower lip" of the knee. The more your lower leg can be displaced forward from its normal position, the greater the amount of ACL damage and the more unstable your knee.
If the physical examination suggests that you have a significant ACL injury, your doctor may order a magnetic resonance imaging (MRI) scan of your knee joint or perform camera-guided surgery (arthroscopy) to inspect the damage to your ACL. For diagnosing partial ACL tears, arthroscopy is usually more efficient than MRI.