The joint, in anatomy, an articulation or connection between bones. Many of the immovable joints are so close in their union that the two bones practically become one, as in the dovetailed sutures of the skull. In the make-up of a movable joint there are the two bones, and covering the surface of each a layer of tissue of rubbery consistency called cartilage; binding the bones together are firm inelastic bands of tissue called ligaments. Over the surface of the cartilage and the inner surfaces of the ligaments there is a thin, smooth layer of tissue, the synovial membrane, which is kept constantly moist with an oily fluid that prevents friction.
Joints are classified by the variety of motions they allow. Movable diarthroses joints include ball-and-socket, gliding, and hingelike. A ball-and-socket joint, as in the hip or shoulder, is the freest of all. Immovable synarthroses joints include grooved, socketed, and sutural. Amphia-thrpses joints are a mixed type.
Kinds of Movable Joints
The major kinds of movable joints are ball-and-socket joints, hinge joints, pivot joints, gliding joints, and angular joints. In a ball-and-socket joint the rounded end of one bone fits into the cup-shaped cavity of another bone. Examples are the shoulder joint and the hip joint. A ball-and-socket joint has more freedom of movement than any other kind of joint. The shoulder joint, for instance, permits a person to swing his arms in a complete circle.
Elbows and finger joints are hinge joints. Hinge joints can move in one plane only, backward and forward, like a door on hinges. A pivot joint allows rotating movement. The bones of the lower arm, the radius and ulna, are connected at a pivot joint. The radius can pivot over the ulna, permitting movements such as those required when using a screwdriver.
The vertebrae, or bones of the spine, are connected by gliding joints. In this type of joint the bones slide over one another. Gliding joints make possible such movements as bending over. The wrist is an angular joint, in which the concave surface of one bone fits into the convex surface of another bone. Such a joint arrangement permits movement in two directions.
Some joints allow a combination of movements. For example, the ankle and the elbow, primarily hinge joints allowing movement back and forth, permit other motions as well.
Diseases of Joints
Joint diseases are distinguished by names that indicate the principal structure involved and the causes of disease. Where the cause is a specific disorder, as tuberculosis, rheumatism, syphilis, or gonorrhea, the name of that malady is given to the inflammation. The only structure of a joint that is apt to be involved alone is the synovial membrane, and inflammation of this structure is called synovitis. Inflammation of all the joint structures is called arthritis, and, where the -bone is principally involved, the name osteoarthritis is used.
This condition results from injuries (sprains, bruises, and wounds), injurious overuse, or poisons circulating in the blood. The joint is painful when at rest, moves with even greater pain, and may be red outside. Blood serum is poured out and the synovial sac is distended. (This condition constitutes “water on the knee.”1) The general symptoms and severity depend on the cause of the trouble. Absolute rest of the joint and pressure by a snug bandage hasten recovery. When such injuries are repeated, or when the inflammation is slight but persistent, painting with iodine tincture may be of value. When the condition of inflammation reaches the suppurative stage, all the structures of the joint are involved, and the condition is considered under the term arthritis.
This is due to the extension of disease of contiguous bone, or it may result from wounds or in “the course of various diseases. The joint becomes swollen, red, and very painful, and the general symptoms are severe. When the poison is sufficiently virulent, suppurative arthritis results and the pus collects in the synovial sac. (See inflammation.) In this condition the joint becomes “boggy,” and the poisoning of the entire system is so severe as to warrant the most radical measures for relief. Opening the joint and allowing the escape of the pus, with thorough cleansing, may be sufficient, or the limb may have to be sacrificed.
This is an inflammation of a bursa, a serous lined sac or space situated in a part of the body where there is likely to be increased friction from the movement of adjacent parts. The commonest localities are the shoulder, elbow, knee, hip, and the Achilles tendon. Symptoms are pain upon movement of a joint or muscle, and swelling, occasionally with an evidence of fluid within the bursa.
This is a deformity of the legs, consisting in the angular projection of the knee inward, and is sometimes called in-knee. It arises in children learning to walk who are affected with rickets. The deformity is due to the faulty growth of the bones which enter into the joint. In early life, correction of it may be secured by splints and braces; but when the bones become less pliable, the femur, the tibia, or both, may have to be severed by chiseling and the bones held in the normal line by plaster casts until union has taken place.
The peculiar form of arthritis thus named occurs in the course of loco-motor ataxia, the knee joint usually being the one affected. The joint swells painlessly, the structures are worn away, and the function of the joint is lost.
These are wrenches resulting in greater or less stretching or laceration of ligaments, hemorrhage in and around the joint, and sometimes the displacement of tendons. The part usually swells at once, and movement causes severe pain. An injury of this nature may closely resemble a joint fracture. Absolute rest of the joint must be insisted upon, as synovitis may follow. Alternating hot and cold applications, if started early, lessen the damage. The joint should then be snugly bandaged until swelling and tenderness disappear.
Wounds of Joints
Injuries of this nature may cause damage directly to the structures or may infect the joint and produce arthritis. Penetration into the synovial sac is indicated by an escape of the viscid fluid.
These are rarely seen except in the knee joint, where they are ordinarily due to a small portion of cartilage being bitten off between the bones; this piece then floats around in the synovial sac and causes trouble when the bones lock together on it. When this occurs, the patient falls to the ground and suffers severe pain. The synovial membrane is injured and is apt to be mildly inflamed. Floating cartilages may sometimes be felt by the examining physician through the skin. They can be held in place if the joint is kept in the same position until a surgical opening has been made. Ordinarily, however, a more drastic procedure is called for, particularly when attacks become so frequent as to be unbearable. The joint has to be opened and carefully searched.