The medial collateral ligament (MCL), one of four knee ligaments, connects the thigh bone (femur) to the lower leg bone (tibia) along the inside of the knee joint. It stabilizes the knee joint and stops the knee from bending sideways.

MCL injuries often occur during twisting, bending, or quick changes of direction, such as might happen if a knee is hit from the side while it is locked in position.

Minor injuries are treated with rest, ice, compression, and elevation (RICE). Knee movement may also be restricted for a period of time with a knee immobilizer. Severe MCL tears may require surgery.