When one knee ligament suffers a serious sprain, there is a good chance that other parts of the knee also have been injured. For example, because the MCL helps to protect the ACL from certain types of extreme knee forces, the ACL can become vulnerable to injury when the MCL is torn. In more than half of moderate or severe MCL sprains, the ACL also is sprained.

Knee sprains are very common in the United States. ACL sprains alone affect 100,000 to 250,000 Americans each year. MCL injuries probably are even more common than ACL sprains, but many are so mild that they don't result in a visit to a doctor.

More than any other group, competitive athletes have a very high risk of knee sprains and other types of knee problems. In U.S. high schools, the knee is the most frequently injured joint among athletes who compete in football, soccer or wrestling. The knee is the most frequently injured joint in college wrestlers, and almost 40% of these injuries involve a torn ligament. Among alpine skiers, knee trauma accounts for 20% to 36% of all skiing injuries and typically involves some degree of damage to the ACL.

Symptoms

Symptoms of a knee sprain vary depending on the specific ligament that is torn:

ACL sprain

• Feeling a pop inside your knee at the moment of injury

• Significant knee swelling within a few hours after injury

• Severe knee pain that prevents you from continued participation in your sport

• Black and blue discoloration around the knee

• Knee instability - the feeling that your injured knee will buckle or give out if you try to stand

PCL sprain

• Mild knee swelling, with or without knee instability

• Mild difficulty in moving the knee

• Mild pain at the back of the knee that worsens when you kneel

MCL sprain

• Knee pain and swelling

• Knee buckling toward the outside

• An area of tenderness over the torn MCL (at the inner side of the knee)

LCL sprain

• Knee pain and swelling

• Knee buckling toward the inside

• An area of tenderness over the torn LCL (at the outer side of the knee)