The medial collateral ligament (MCL) is a band of tissue located on the inner side of the knee. It connects the thigh bone, or femur, to the shin bone, known as the tibia. The MCL helps stabilize the knee joint and prevents excessive side-to-side movement. Injuries to the MCL are common in sports, often resulting from a direct blow to the outer side of the knee. Symptoms of an MCL injury may include pain, swelling, and difficulty bending the knee. Treatment typically involves rest, ice, and physical therapy to restore strength and flexibility.