A full or partial tear in the medial collateral ligament (MCL), one of four ligaments that are critical to the stability of the knee joint. The MCL spans the distance from the end of the femur (thigh bone) to the top of the tibia (shin bone) and is on the inside of the knee joint.

The ligament is usually injured when the outside of the knee joint is struck. This force causes the outside of the knee to buckle, and the inside to widen. When the MCL is stretched too far, it is susceptible to tearing and injury.

Pain directly over the ligament is the most common symptom following an MCL injury. Swelling may appear over the torn ligament, and bruising and generalized joint swelling are common one to two days after the injury. In more severe injuries, patients may complain that the knee feels unstable.

Treatment depends on the severity of the injury. After allowing the pain to subside, therapy will focus on mobility, followed by strengthening the knee to return to sports and activities. Bracing can often be useful for treatment of MCL injuries. Surgery is usually not necessary.