A band of tissue runs along the inner edge of the knee known as the medial collateral ligament, or MCL. It assists in connecting the thigh and shin bones together, keeping the knee stable when moving. When the outer knee suffers a heavy impact, the MCL, which sits on the inner-side of the knee, can overstretch or tear. Athletes participating in football, hockey, and other high impact sports may commonly injure their MCL this way. Patients can also tear their MCL if an impact suddenly pushes the knees to the side, or if they twist or bend out too far.