A ligament is a piece of tissue that connects one bone to another. The medial cruciate ligament is one of four ligaments that stabilise the knee joint. The other three are the posterior cruciate ligament, the anterior cruciate ligament, and the lateral collateral ligament. 'Medial' is another word for inside, and this ligament runs from the inside of the shin bone (the tibia) to the inside of the thigh bone (the femur).
Injury to the MCL occurs most often in contact sports, and is classified as one of three grades:
Grade 1 is when the MCL is over-stretched, commonly know as a sprain.
Grade 2 is when the MCL is partially torn.
Grade 3 is a complete tearing of the MCL, causing looseness of the knee joint.
Nearly all MCL injuries are treated with a hinged knee brace, and repair surgery is very rare.
Source: http://www.knee1.com