Point of movement or articulation in any animal with a skeleton. In vertebrates, it is the point where two bones meet. Some joints allow no motion (the sutures between the bones of the skull), others allow a very small motion (the sacroiliac joints in the lower vertebral column), but most allow a relatively free motion. Of these, some allow a gliding motion (one vertebra of the spine on another), some have a hinge action (elbow and knee, see biceps and triceps), and others allow motion in all directions (hip and shoulder joints) by means of a ball-and-socket arrangement.
Movement at a joint is usually produced by muscles, which are attached to the bones by tendons. The ends of the bones at a moving joint are covered with cartilage for greater elasticity and smoothness (to make movement easy), and enclosed in an envelope (capsule) of tough white fibrous tissue lined with a membrane which secretes a lubricating and cushioning synovial fluid. The joint is further strengthened by ligaments, which connect bone to bone. These joints are known as synovial joints. In invertebrates with an exoskeleton, the joints are places where the exoskeleton is replaced by a more flexible outer covering, the arthrodial membrane, which allows the limb (or other body part) to bend at that point.
Classes of human joints The study of anatomical joints or articulations is called arthrology. Three classes of joints are recognized in the human skeleton and each class may include two or more types. The three classes are fibrous joints, cartilaginous joints, and synovial joints. In a fibrous joint, the two bones are held together by bands of dense fibrous tissue extending from one bare bony surface to the other. The sutures between adjacent bones in the skull are a type of fibrous joint at which no active movement occurs. Cartilaginous joints are differentiated into primary (or synchondroses) and secondary (or symphyses). In primary cartilaginous joints, the two pieces of bone are united by a continuous plate of hyaline cartilage which is a relic of the embryonic cartilage model of the skeleton. Thus the commonest synchondroses are the epiphyseal (or growth) cartilages between the diaphysis (main shaft) and epiphyses (bony ends) of a long bone. Like the sutures of the skull, the epiphyseal synchondroses are immobile, but are obliterated by the fusion of the related bones before the age of 25 years (whereas the sutures are not completely obliterated until very old age). Secondary cartilaginous joints only occur in the median plane (mid-line) of the body and involve the union of two bones by a disc of fibrocartilage. Such joints occur in the vertebral column, where thick plates of fibrocartilage separate the flat surfaces of the vertebrae.
Synovial joints have evolved specifically to permit free movement, hence their preponderance in the limbs. In all synovial joints the articulating bony surfaces are covered with cartilages, the convexities of the one cartilaginous cap fitting more or less closely into the concavities of the other. In some cases these articular cartilages are separated by interarticular plates of cartilage lying between them, and in this case the opposing faces of the interarticular and of the articular cartilages fit one another; examples of such interarticular plates are the semi-lunar cartilages of the knee joint. The joint is enclosed by a synovial membrane forming a closed sac and containing a viscid lubricating secretion termed synovial fluid. Outside the synovial membrane is a sleeve of fibrous tissue, the capsular ligament. Sacs known as bursae, formed from synovial membrane, are present where joints are subjected to pressure, for instance the prepatellar bursa of the knee joint. Inflammation of the bursae is bursitis. The shape of the surfaces forming the joint varies greatly and may be spheroidal, cylindrical, or pulley-shaped.
Types of synovial joint There are six main forms of synovial joint in the human frame. (1) Ball-and-socket. In this type of joint a spheroidal surface furnished by one bone works within a cup furnished by another; this will allow motion of the former bone in any direction, its extent being dependent on the concavity of the cup. In the case of the hip joint, the cup is deep and the extent of movement is sacrificed to obtain additional strength. The shoulder, in contrast, allows a wide range of movement, since the cup is shallow. (2) Gliding. In such a joint the articular surfaces are flat, and the bones slide against each other. Examples occur in the tarsal joint of the foot, the intercarpal joints of the wrist, and in one movement of the jaw joint. (3) Hinge. A joint in which a nearly cylindrical head fits into a corresponding socket, but the movement is restricted to a direction perpendicular to the axis of the cylinder: examples are the elbow, knee, ankle, and finger joints. Attachments or bony processes prevent a backward dislocation. (4) Double hinge or saddle.
Here the articular surface of each bone is concave in one direction and convex in a direction at right angles to this, for example the carpo-etacarpal joint at the base of the thumb. (5) Condyloid. This joint is similar to the saddle and allows flexion, extension, and lateral movement, but no rotation, however it is usually a weaker form of joint; examples occur in the wrist and metacarpo-phalangeal joints in the fingers. (6) Pivot. A joint in which one bone furnishes a pivot on which another turns, or turns itself on its own axis resting on another bone; an example of the former is the atlanto-axial joint between the first and second cervical (neck) vertebrae, in which the odontoid peg of the axis (second vertebra) passes through a ringlike portion of the atlas (first vertebra); this arrangement permits the head to be turned or shaken through a considerable angle. (The occipito-atlantal joint of the first cervical vertebra, used in nodding the head, is of a different type.) The case of the rotation of a bone on its own axis is illustrated by the radius (at the elbow), which has a shallow cup adjacent to the humerus (upper-arm bone) and a concave surface at its lower end which articulates with the ulna. In pronation (when the hand is turned palm downwards), the radius turns on its own axis at its upper end and glides round the ulna at its lower end.
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