A medial collateral ligament (MCL) injury is a sprain or tear to the MCL, which is a band of tissue on the inside of the knee that connects the thighbone to the bone of the lower leg. The MCL keeps the knee from bending inward, and it can become injured when the knee is pushed outside its typical range of motion due to a twisting motion or a blow to the outside of the knee. MCL sprains are common in athletes, especially those who participate in direct contact sports like soccer or football. Symptoms of an MCL sprain include pain on the inner side of the knee, swelling, tenderness, and a feeling of the knee "giving way". In some cases, a pop or snap may be heard or felt at the time of injury. MCL injuries are categorized based on the severity of the injury, ranging from a mild sprain or partial tear to a complete grade 3 rupture. Treatment for an MCL sprain depends on the severity of the injury, but most MCL injuries can be treated at home with rest, ice, compression, and elevation (RICE) and physical therapy exercises to increase range of motion and strengthen the knee. In some cases, surgical reconstruction of the MCL may be required.