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About.com Health's Disease and Condition content is reviewed by Kate Grossman, MD

What is the medial collateral ligament?
The medial collateral ligament (MCL) is one of four ligaments that are critical to the stability of the knee joint. A ligament is made of tough fibrous material and functions to control excessive motion by limiting joint mobility. The four major stabilizing ligaments of the knee are the anterior and posterior cruciate ligaments (ACL and PCL, respectively), and the medial and lateral collateral ligaments (MCL and LCL, respectively).

The medial collateral ligament spans the distance from the end of the femur (thigh bone) to the top of the tibia (shin bone) and is on the inside of the knee joint. The medial collateral ligament resists widening of the inside of the joint, or prevents "opening-up" of the knee.

How is the medial collateral ligament injured?
Because the medial collateral ligament resists widening of the inside of the knee joint, the MCL is usually injured when the outside of the knee joint is struck. This action causes the outside of the knee to buckle, and the inside to widen. When the medial collateral ligament is stretched too far, it is susceptible to tearing and injury. This is the injury seen by the action of "clipping" in a football game.

An injury to the medial collateral ligament may occur as an isolated injury, or it may be part of a complex injury to the knee. Other ligaments, most commonly the anterior cruciate ligament (ACL), or the meniscus (cartilage), may be torn along with a medial collateral ligament injury.

What are the symptoms of a medial collateral ligament injury?
The most common symptom following a medial collateral ligament injury is pain directly over the ligament. Swelling over the torn ligament may appear, and bruising and generalized joint swelling are common 1 to 2 days after the injury. In more severe injuries, patients may complain that the knee is unstable, or feel as though their knee may 'give out' or buckle.

Symptoms of a medial collateral ligament injury tend to correlate with the extent of the injury. MCL injuries are graded on a scale of I to III.

• Grade I MCL Tear
  This is an incomplete tear of the MCL. The tendon is still in continuity, and the symptoms are usually minimal. Patients usually complain of pain with pressure on the MCL, and may be able to return to their sport very quickly. Most athletes miss 1-2 weeks of play.

• Grade II MCL Tear
  Grade II injuries are also considered incomplete tears of the MCL. These patients may complain of instability when attempting to cut or pivot. The pain and swelling is more significant, and usually a period of 3-4 weeks of rest is necessary.

• Grade III MCL Tear
  A grade III injury is a complete tear of the MCL. Patients have significant pain and swelling, and often have difficulty bending the knee. Instability, or giving out, is a common finding with grade III MCL tears. A knee brace or a knee immobilizer is usually needed for comfort, and healing may take 6 weeks or longer.

Read more about the treatment of MCL injuries...