In brief The anterior cruciate ligament passes diagonally across a large joint space and thus is not surrounded by vascularized tissue, which jeopardizes its ability to heal. Its main function is to control anterior motion of the tibia relative to the femur. Injuries to this ligament usually result from a combination of rotation and flexion and almost always cause immediate disability. X-rays, aspiration, and arthroscopy may be necessary for the differential diagnosis. The decision for or against surgical treatment is controversial, but it should be based on factors including the patient's age, the demands put on the knee, the amount of laxity, and the surgical techniques available.