The medial collateral ligament (MCL), or tibial collateral ligament (TCL), is one of the four major ligaments of the knee. It is on the medial (inner) side of the knee joint in humans and other primates. Its primary function is to resist outward turning forces on the knee. Structure It is a broad, flat, membranous band, situated slightly posterior on the medial side of the knee joint. It is attached proximally to the medial epicondyle of the femur immediately below the adductor tubercle; below to the medial condyle of the tibia and medial surface of its body. It resists forces that would push the knee medially, which would otherwise produce valgus deformity. The fibers of the posterior part of the ligament are short and incline backward as they descend; they are inserted into the tibia above the groove for the semimembranosus muscle. The anterior part of the ligament is a flattened band, about 10 centimeters long, which inclines forward as it descends. It is inserted into the medial surface of the body of the tibia about 2.5 centimeters below the level of the condyle. Crossing on top of the lower part of the MCL is the pes anserinus, the joined tendons of the sartorius, gracilis, and semitendinosus muscles; a bursa is interposed between the two. The MCL's deep surface covers the inferior medial genicular vessels and nerve and the anterior portion of the tendon of the semimembranosus muscle, with which it is connected by a few fibers; it is intimately adherent to the medial meniscus. Development Embryologically and phylogenically, the ligament represents the distal portion of the tendon of adductor magnus muscle. In lower animals, adductor magnus inserts into the tibia. Because of this, the ligament occasionally contains muscle fibres. This is an atavistic variation. Clinical significance Injury An MCL injury can be very painful and is caused by a valgus stress to a slightly bent knee, often when landing, bending or on high impact. It may be difficult to apply pressure on the injured leg for at least a few days. It can be caused by a direct blow to lateral side of knee. The most common knee structure damaged in skiing is the medial collateral ligament, although the carve turn has diminished the incidence somewhat. MCL strains and tears are also fairly common in American football. The center and the guards are the most common victims of this