Definition: A medial collateral ligament (MCL) is a ligament on the inner side of a joint that resists a valgus stress to the joint. Many joints have an MCL (elbows, fingers) but most often people refer to the knee.
The MCL of the knee is one of four ligaments 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 MCL prevents knee injury by stabilizing the joint and preventing the knee from buckling inwards. The ligament attaches to the femur (thigh bone) and the tibia (shin bone) and runs across the inside (medial aspect) of the knee. People who injure the MCL of the knee are often said to have an MCL tear.