MCL stands for medial collateral ligament, which is one of the ligaments in the knee joint. It is a tough, flexible band of tissue that connects the bottom of the thighbone (femur) to the top of the shinbone (tibia) on the inner side of the knee joint. The MCL helps keep the knee from moving side to side and allows the knee joint to move while remaining stable. MCL injuries happen when excessive pressure is put on the knee joint, causing a torn ligament. An MCL injury can be a stretch, partial tear, or complete tear of the ligament. The tear might be partial (through a part of the MCL) or complete (all the way through the MCL) . MCL injuries can occur by themselves or with other injuries such as an ACL or meniscus tear. Symptoms of an MCL injury include pain, swelling, and instability in the knee joint. Treatment for MCL injuries depends on the severity of the injury and can range from rest and physical therapy to surgery.