Medial collateral ligament (MCL) injuries stretch or tear a ligament in the knee. According to UK Health Care, the medial collateral ligament keeps the thigh bone and leg bone in alignment along the side of the knee. This is the structure that prevents knees from buckling. Tearing the MCL is a common sports injury.

Your MCL, which is short for medial collateral ligament, is one of the four major ligaments in your knee. The thick membranous band is located on the inside of your knee joint, attaching at your femur and tibia. The MCL prevents forces that cause your knee to collapse inward.

Your medial collateral ligament, or MCL in common shorthand, is a ligament located in your knee. The MCL is a common injury site for athletes, particularly those involved in contact sports. An MCL injury can be mild or severe.