Injuries of the Semilunar Menisci
The semilunar menisci are two crescentic plates of white fibro-cartilage, which lie upon the upper end of the tibia, and serve to deepen the articular surface for the condyles of the femur. Each cartilage is firmly attached to the tibia by its anterior and posterior ends, and, through the medium of the coronary ligaments, is loosely attached along its peripheral, convex edge to the head of the tibia, the medial meniscus being connected also to the capsular ligament of the joint. The tendon of the popliteus muscle intervenes between the lateral meniscus and the capsule. The central, concave edges of the menisci are thin and unattached.
The cartilages enjoy a limited range of movement within the joint, passing backwards during flexion, and forwards again when the limb is extended; under normal conditions the lateral moves more freely than the medial. While the limb is partly flexed, a slight degree of rotation of the leg at the knee is possible, and during this movement the cartilages glide from side to side, and the tibia rotates below them.
Any abnormal laxity of the ligaments of the joint may render the cartilages unduly mobile, so that they are liable to be displaced from comparatively slight causes, and when so displaced it is not uncommon for one or other to be torn by being nipped between the femur and the tibia. It is convenient to consider these “internal derangements of the knee-joint” separately, according to whether the meniscus is merely abnormally mobile, or is actually torn.
Mobile Meniscus—Displacement of Medial Semilunar Cartilage ([Fig. 86]).—The medial meniscus exhibits undue mobility much more frequently than the lateral, and the condition is usually met with in adult males who engage in athletics, or who follow an employment which entails working in a kneeling or squatting position for long periods, with the toes turned outwards—for example, coal-miners. The tibial collateral ligament, and through it the coronary ligament, are thus gradually stretched, so that the cartilage becomes less securely anchored, and is rendered liable to be displaced towards the centre of the joint during some sudden movement which combines flexion of the knee with medial rotation of the femur upon the tibia, as, for example, in rising quickly from a squatting position, or turning rapidly and pushing off with the foot, in the course of some game such as football or tennis. It may occur also from tripping on a loose stone or slipping off the kerbstone.
Fig. 86.—Diagram of Longitudinal Tear of Posterior End of Right Medial Semilunar Meniscus.
What actually happens when the meniscus is displaced would appear to be, that the combined flexion and abduction of the knee opens up the medial side of the joint by separating the medial condyles of the femur and tibia, and that the medial meniscus in its movement backward during flexion slips under the femoral condyle and is caught between it and the tibia. It may even slip past the condyle and into the intercondyloid notch, and come to lie against the cruciate ligaments.
The mechanism by which this lesion is produced doubtless explains the greater frequency with which the left knee is affected, as most sudden movements are made from right to left, thus throwing the strain upon the left knee.