The medial collateral ligament (MCL) is a band of tissue that runs along the inner edge of the knee. An MCL sprain occurs when there is a direct force applied to the outside of the knee, pushing the knee inward. MCL sprains are common in athletes, especially if they participate in direct contact sports like soccer or football. The severity of an MCL sprain can range from a mild sprain or partial tear to a complete grade 3 rupture. Symptoms of an MCL sprain include pain on the inner side of the knee, swelling, tenderness, and the feeling of the knee "giving way".

MCL sprains can usually be diagnosed clinically via physical exam, which may include a special test called a valgus stress test. X-rays may be ordered to reveal an avulsion fracture, and an MRI may be ordered to verify the extent of tearing that has occurred to the MCL and diagnose any additional associated injuries. Treatment for MCL sprains depends on the severity of the injury. Most MCL sprains will heal with conservative treatment, which involves non-weight bearing with the use of crutches until the patient is able to walk without a limp or pain, and physical therapy. For severe MCL sprains, surgical reconstruction of the MCL may be required.