The medial collateral ligament (MCL) is the ligament that is located on the inner part of the knee joint. It runs from the femur (thighbone) to the top of the tibia (shinbone) and helps in stabilizing the knee. Medial collateral ligament (MCL) injury can result in a stretch, partial tear, or complete tear of the ligament. Injuries to the MCL commonly occur because of a pressure or stress on the outside part of the knee.