The medial collateral ligament, commonly referred to as the MCL, is a thick and strong ligament located along the inner side of the knee. The MCL stretches from the femur (thighbone) to the tibia (shinbone) and helps to stabilize the medial (inner) part of the knee. While several other ligaments and tendons, such as the hamstring tendons, provide additional support, the MCL is the most important structure to prevent the knee from collapsing into a knock-kneed stance.