joint(redirected from AC joint)
Also found in: Dictionary, Thesaurus, Medical, Legal, Financial.
joint,in geology, fracture in rocks along which no appreciable movement has occurred (see faultfault,
in geology, fracture in the earth's crust in which the rock on one side of the fracture has measurable movement in relation to the rock on the other side. Faults on other planets and satellites of the solar system also have been recognized.
..... Click the link for more information. ). Nearly vertical, or sheet, joints that result from shrinkage during cooling are commonly found in igneous rocks. Similar joints occur in thick beds of sandstone and gneiss, with the sheets resembling the structure of a sliced onion. The prismatic joints of the Palisades of New Jersey and Devil's Tower, Wyoming, are examples of joints caused by contraction during the cooling of fine-grained igneous rock masses. Deep-seated igneous rocks often have joints approximately parallel to the surface, suggesting that they formed by expansion of the rock mass as overlying rocks were eroded away. Some joints in sedimentary rocks may have formed as the result of contraction during compaction and drying of the sediment. In some cases, jointing of the rock may result from the action of the same forces that cause foldsfold,
in geology, bent or deformed arrangement of stratified rocks. These rocks may be of sedimentary or volcanic origin. Although stratified rocks are normally deposited on the earth's surface in horizontal layers (see stratification), they are often found inclined or curved
..... Click the link for more information. and faults. In relatively undisturbed sedimentary rocks, such joints are often in two vertical sets perpendicular to one another. Commonly, streams develop along zones of weakness caused by joints in rocks, and thus the regional pattern of joint orientation often exerts a strong control on the development of drainage patterns.
joint,in anatomy, juncture between two bones. Some joints are immovable, e.g., those that connect the bones of the skull, which are separated merely by short, tough fibers of cartilage. Movable joints are found for the most part in the limbs. Hinge joints provide a forward and backward motion, as at the elbow and knee. Pivot joints permit rotary movement, like the turning of the head from side to side. Ball-and-socket joints, like those at the hip and shoulder, allow the greatest range of movement, as the rounded end of one bone fits into the hollow or socket of another bone, separated by elastic cartilage. Joints can further be classified as fibrous, cartilaginous, and synovial. Collagen fibers connect fibrous joints. Synovial joints ease movement through the use of a lubricating liquid, supplied by the synovial membrane that lines movable joints. In synovial joints, a cushioning sac known as a bursabursa
, closed fibrous sac lined with a smooth membrane, producing a viscous lubricant known as synovial fluid. Bursas are found in regions where muscles or tendons rub against other muscles, tendons, or bones.
..... Click the link for more information. contains the fluid, which lubricates and nourishes the joint. Those joints which lack synovial fluid are nourished by blood. Holding the joints in place are strong ligamentsligament
, strong band of white fibrous connective tissue that joins bones to other bones or to cartilage in the joint areas. The bundles of collagenous fibers that form ligaments tend to be pliable but not elastic.
..... Click the link for more information. fastened to the bones above and below the joint. Joints are subject to sprainssprain,
stretching or wrenching of the ligaments and tendons of a joint, often with rupture of the tissues but without dislocation. Sprains occur most commonly at the ankle, knee, or wrist joints, causing pain, swelling, and difficulty in moving the involved joint.
..... Click the link for more information. and dislocations, as well as to infections and disorders caused by such diseases as arthritisarthritis,
painful inflammation of a joint or joints of the body, usually producing heat and redness. There are many kinds of arthritis. In its various forms, arthritis disables more people than any other chronic disorder.
..... Click the link for more information. . In recent years, the use of artificial joints has become increasingly common, particularly in hip and knee replacement. Many orthopedic surgeons now perform operations of this sort, using metal or plastic replacement joints in order to relieve pain, or to prevent or correct joint deformity.
The structural component of an animal skeleton where two or more skeletal elements meet, including the supporting structures within and surrounding it. The relative range of motion between the skeletal elements of a joint depends on the type of material between these elements, the shapes of the contacting surfaces, and the configuration of the supporting structures.
In bony skeletal systems, there are three general classes of joints: synarthroses, amphiarthroses, and diarthroses. Synarthroses are joints where bony surfaces are directly connected with fibrous tissue, allowing very little if any motion. Synarthroses may be further classified as sutures, syndesmoses, and gomphoses. Sutures are joined with fibrous tissue, as in the coronal suture where the parietal and frontal bones of the human skull meet. Syndesmoses are connected with ligaments, as are the shafts of the tibia and fibula. The roots of a tooth that are anchored in the jaw bone with fibrous tissue form a gomphosis. Amphiarthroses are joints where bones are directly connected with fibrocartilage or hyaline cartilage and allow only limited motion. An amphiarthrosis joined with fibrocartilage, as found between the two pubic bones of the pelvis, is known as a symphysis; but when hyaline cartilage joins the bones, a synchondrosis is formed, an example being the first sternocostal joint. The greatest range of motion is found in diarthrodial joints, where the articulating surfaces slide and to varying degrees roll against each other. See Ligament
The contacting surfaces of the bones of a diarthrodial joint are covered with articular cartilage, an avascular, highly durable hydrated soft tissue that provides shock absorption and lubrication functions to the joint (see illustration). Articular cartilage is composed mainly of water, proteoglycans, and collagen. The joint is surrounded by a fibrous joint capsule lined with synovium, which produces lubricating synovial fluid and nutrients required by the tissues within the joint. Joint motion is provided by the muscles that are attached to the bone with tendons. Strong flexible ligaments connected across the bones stabilize the joint and may constrain its motion. Different ranges of motion result from several basic types of diarthrodial joints: pivot, gliding, hinge, saddle, condyloid, and ball-and-socket. See Collagen
mortise and tenon
a conventional term used by miners to designate cracks in a rock mass. Joints occur during geological dis-locations (tectonic shoves) or are formed as a result of detonating borehole charges (frequently parallel to the line of distribution of the charges). Joints contribute to rockslides (at times sudden) from the upper area of the mine face during the loading of a detonated body of rock.
a movable junction of bones that enables them to move in relation to one another. The main elements of a joint include the cartilage-covered surfaces of articulating bones, a cavity containing fluid, and a capsule enclosing the cavity. Some joints also have such auxiliary structures as ligaments, disks, menisci, and synovial bursae. The shape of joints has changed in the course of animal evolution and the development of locomotion. In man, the characteristics of joints result from the body’s upright position, which conditions the number of axes of rotation and of degrees of movement.
In simple joints, two bones articulate, whereas in composite joints, several bones articulate. The joint surfaces resemble geometric figures and may be spherical, ellipsoidal, saddle-shaped, or flat. Joints may be movable, for example, the spherical shoulder joint, or immovable, for example, the joint between a rib and the sternum. The range of joint movements is measured in the degrees of the angles formed by the articulating bones. Movements may occur around one, two, or three axes. Uniaxial movement is characteristic of cylindrical and hinged joints, biaxial movement, of ellipsoidal and saddle-shaped joints, and polyaxial movement, of spherical joints. Movements are normally restricted by bony prominences and by the tension of ligaments and of the joint capsule.
Joint injuries may be caused by traumas (dislocation), congenital defects (arthrogryposis), destructive metabolic changes (arthrosis), or inflammatory changes (arthritis). Limited mobility or total immobility of a joint may result from a variety of pathological processes. Joint diseases and methods of treating and preventing them are the concerns of traumatology, orthopedics, and a special branch of clinical medicine called arthrology.
REFERENCESSinel’nikov, R. D. Atlas anatomii cheloveka, vol. 1. Moscow, 1963.
Astapenko, M. G., and E. G. Pikhlak. Bolezni sustavov. Moscow, 1966.
V. IU. GOLIAKHOVSKII
The surface at which two or more mechanical or structural components are united. Whenever parts of a machine or structure are brought together and fastened into position, a joint is formed. See Structural connections
Mechanical joints can be fabricated by a great variety of methods, but all can be classified into two general types, temporary (screw, snap, or clamp, for example), and permanent (brazed, welded, or riveted, for example).