Joints and Ligaments

Comparative Anatomy. In some Mammals, e.g., Bradypus and Ornithorhynchus, the knee is divided into three parts, two con dylotibial and one trochleo-patellar, by synovial folds. In a typical Mammal the external semilunar cartilage is attached by its pos terior horn to the internal condyle of the femur only. In the Monkeys and anthropoid Apes this cartilage is circular. The semi lunar cartilages first appear in the Amphibia, and, according to B. Sutton, are derived from muscles which are drawn into the joint. When only one kind of movement (hinge) is allowed, as in the fruit bat, the cartilages are not found. In most Mammals the superior tibio-fibular joint communicates with the knee.

The tibio-fibular articulations resemble the radio-ulnar in posi tion but are much less movable. The superior in Man is usually cut off from the knee and is a gliding joint ; the middle is the interosseous membrane, while the lower is of a syndesmosis or fibrous half joint.

Ankle Joint.

This is a hinge, the astragalus being received into a lateral arch formed by the lower ends of the tibia and fibula. Backward dislocation is prevented by the articular surface of the astragalus being broader in front than behind. The anterior and posterior parts of the capsule are feeble, but the lateral ligaments are very strong, the external consisting of three separate fasciculi which bind the fibula to the astragalus and calcaneum. To avoid confusion it is best to speak of the movements of the angle as dorsal and plantar flexion.

The tarsal joints resemble the carpal in being gliding articula tions. There are two between the astragalus and calcaneum, and at these, inversion and eversion of the foot largely occur. The inner arch of the foot is maintained by a very important ligament called the calcaneo-navicular or spring ligament; it connects the sustentaculum tali of the calcaneum with the navicular, and upon it the head of the astragalus rests. When it becomes stretched, flat-foot results. The tarsal bones are connected by dorsal, plantar and interosseous ligaments. The long and short calcaneo-cuboid are plantar ligaments of special importance, and maintain the outer arch of the foot.

The joints of the foot closely resemble those of the hand, except that the tarso-metatarsal joint of the great toe is not saddle shaped.

Comparative Anatomy. The anterior fasciculus of the external lateral ligament of the ankle is only found in Man, and is prob ably an adaptation to the erect position. In animals with a long foot, such as the Ungulates and the Kangaroo, the lateral liga ments of the ankle are in the form of an X, to give greater pro tection against lateral movement. In certain marsupials a fibro cartilage is developed between the external malleolus and the astragalus, and its origin from the deeper fibres of the external lateral ligament of the ankle can be traced. These animals have a rotatory movement of the fibula on its long axis, in addition to the hinge movement of the ankle.

BIBLIOGRAPHY.-For

further details of joints see R. Fick, Handbuch der Gelenke (Jena, 1904) ; H. Morris, Anatomy of the Joints (1879) Quain's, Gray's and Cunningham's Text-books of Anatomy; J. Bland

Sutton, Ligaments, their Nature and Morphology (1902) ; F. G. Par sons, "Hunterian Lectures on the Joints of Mammals," Journ. Anat. and Phys., xxxiv. 41 and 3c1 ; P. Poirier and A. Charpy, Traite d'ana tomie humaine, 4th ed., vol. i. (bibl.) (Paris, 1926) ; G. A. Piersol, Human Anatomy, 6th ed., vol. i. (bibl.) (Philadelphia, 1918).

(F. G. P.) The affection of the joints of the human body by specific dis eases is dealt with under various headings (see RHEUMATISM, etc.) ; in the present article the more direct forms of ailment are discussed. In most cases, disease starts either in the synovial lining or in the bone—rarely in the articular cartilage or ligaments. Often the disease begins after an injury. There are three princi pal types of injury : (I) sprain or strain, in which the ligamentous and tendinous structures are stretched or lacerated ; (2) contusion, in which the opposing bones are driven forcibly together; (3) dis location, in which the articular surfaces are separated from one another.

Sprains and Strains.

A sprain or strain of a joint means that as the result of violence the ligaments holding the bones together have been suddenly stretched or even torn. Since on the inner aspect the ligaments are lined by a synovial membrane, this is necessarily damaged. Small blood-vessels are also torn, and the joint may become full of blood. If bleeding does not take place the swelling is not immediate, but synovitis having been set up, serous effusion comes on sooner or later. Of ten local heat and pain accompany synovitis. In a healthy individual the effects of a sprain may quickly pass off, but in a rheumatic or gouty person obstinate chronic synovitis may remain, and in one with a tubercu lous history may start serious disease of the joint. A sprain should, therefore, be treated with rest and prolonged supervision. In a person of health and vigour a sprained joint should be at once bandaged. Wide pieces of adhesive strapping, layer on layer, form a useful support and may be the only treatment needed. If the sprain is followed by much synovitis a plaster of paris or leather splint may be applied, complete rest being secured for the limb. Later on, blistering or even "firing" may be found advisable. Synovitis.—When a joint has been injured, slight or severe inflammation occurs in the damaged tissue. As a rule, it is the synovial membrane lining the fibrous capsule of the joint which first and chiefly suffers. Synovitis may, however, be due to causes other than mechanical injury, as when the interior of the joint is attacked by the micro-organisms of pyaemia (blood-poi soning), typhoid fever, pneumonia, rheumatism, gonorrhoea or syphilis. Under judicious treatment the synovitis generally clears up, but it may linger on and cause the formation of adhesions which may temporarily stiffen the joint ; or it may, especially in tuberculous, pneumonic, septic or pyaemic infections, involve the cartilages, ligaments and bones in such serious changes as to destroy the joint, and possibly call for amputation.