Your MCL, which is short for medial collateral ligament, is one of the four major ligaments in your knee. The thick membranous band is located on the inside of your knee joint, attaching at your femur and tibia. The MCL prevents forces that cause your knee to collapse inward. The MCL is commonly sprained or torn during athletic activities, when there is a valgus stress, or a blow from the outside of your knee. Football and basketball players are susceptible to MCL tears. You can take steps to determine whether it's likely you've torn your MCL, but an MRI is needed for a true diagnosis.
Consider how you injured your knee and compare it to how MCL's become injured. Most MCL tears are caused by a blow to the outside of your knee, which causes your knee joint to collapse toward the center of your body, placing stress on your MCL that causes it to rupture.
Analyze your symptoms. According to MedlinePlus, a torn MCL is usually associated with significant knee swelling, a locking or catching as you extend and flex your knee joint, pain or tenderness to the touch at the inside of your knee -- and your knee may give way when you're walking.
Decrease pain and swelling to allow diagnosis testing. If your knee is too swollen, you will be unable to determine whether your MCL is injured. Decrease swelling by applying ice, taking non-steroidal anti-inflammatory drugs and elevating your knee above your heart.
Have a partner complete the valgus stress test on your knee. Sit on a table with both legs extended, the leg of your injured knee slightly hanging off one side. Have your partner rest their hip against your knee, then place their closest hand on the inside of your knee to keep it steady against their hip. They then reach across and place their other hand on the inside of your ankle, pulling your ankle toward them. If your knee joint feels flimsy and has excessive laxity, or is allowed to open further than normal, the MCL is torn or significantly sprained.
Visit your medical professional. According to the University of California San Francisco's Health Center, your doctor is likely to order x-rays to check for bone injury, or magnetic resonance imaging, MRI, to diagnose your MCL tear and determine its grade of injury.