Anterior cruciate ligament tear. Computer tomography (CT) scan of the knee joint of a patient with a tear of the anterior cruciate ligament (ACL). This ligament is one of the four major ligaments holding the knee joint together, joining the femur (thigh bone, top) and the tibia (bottom). Injuries to this ligament are a relatively common sporting injury, and surgery is needed to restore full function to the knee. Here, the injured area is the cloudy area (centre right) between the ends of the two bones (dark).