JOINT

in carpentry, junction of two or more members of a framed structure. Joinery, or the making of wooden joints, is one of the principal functions of the carpenter and cabinetmaker. Wood, being a natural material, is not uniform in quality, and moisture, present in the tree during growth, is uneven in cut wood. Wood used for building is subject to movement caused by changes in its moisture content. Though such movement is frequently quite small and accurately predictable, it remains a critical consideration in joint design. Because wood has been used as a building material for centuries throughout the world, the designs of most joints were perfected hundreds of years ago and have changed little since that time. The object of the joint is to fix two members together so that the joint has the greatest possible mechanical strength and is as unobtrusive as possible. Though there are many joints in use, they fall into a few basic groups, many being variations and elaborations on fundamentally simple ideas. Practically all are based on handwork, and with few exceptions most machine-made joints follow the traditional patterns; most joints rely to a considerable extent on a combination of mechanical fit and glue for their strength. Common types of joints include the dovetail, used for joining two flat members together at right angles, as in the sides of a drawer; the dowelled joint, in which dowelling is employed to impart mechanical strength; and the mortise and tenon, used to join a horizontal member with the vertical member of a frame. in anatomy, a structure that separates two or more adjacent elements of the skeleton. Depending on the type of joint, such separated elements may or may not move on one another. Though this discussion is intended to deal with human joints, its content is applicable as well to those of vertebrates in general and mammals in particular. The Latin stem word for joint is art(as in articular, articulation); the Greek stem word is arthro(as in arthritis, arthrology). The international vocabulary of arthrology (the science of joints) uses Latin and Greek terms, but the more important of these have well-known English equivalents that will be used here when possible. in anatomy, a structure connecting two or more adjacent elements of the skeleton. Structurally, there are two types of joints: diarthrosis, containing fluid, and synarthrosis, which function without fluid. Synarthroses are classified as fibrous, cartilaginous, or fibrocartilaginous. The elements of a fibrous joint are connected with collagen fibres. Cartilaginous joints, or synchondroses, are transient, linking bones which become fused after birth; they usually disappear before the age of 25. Fibrocartilaginous joints, or symphyses, are the connections between the body of one bone and the body of another. With two exceptions, all symphyses are located in the spinal column and are termed intervertebral disks. These disks have a hard, elastic outer ring, and a soft, mucoid core. Disks permit the vertebrae of the spine to move and insulate against the jolts of severe movements, such as jumping. A diarthrosis, or synovial joint, consists of a sac (bursa) containing fluid which both nourishes and lubricates the joint, making motion possible. All synovial joints are either ovoid (which is either convex or concave in all directions) or sellar, with concave and convex sides at right angles to each other. The ovoid bone surface is termed male if it is convex, and female if it is concave. The collagen fibres which connect the bones of a joint are ligaments. It was accepted until recently that ligaments held the bones of a joint in place, but it is now believed that it is in fact muscles which keep bones in their joint. Joints are nourished by blood supplied through capillaries, with the exceptions of the moving cartilage parts of a joint, disks, and menisci, which are nourished by synovial fluid. Joint disease is one of the leading causes of invalidism. The disease can be of a short duration or can be chronic and can affect one joint or many. The aged are most susceptible to joint disease, but all ages can be affected. These disorders can be classified in two general categories: inflammatory, causing swelling, pain, redness, and heat; and noninflammatory, in which inflammation is not the principal effect. Arthritis refers to all inflammatory joint diseases, whereas synovitis describes inflammation of the lining of a joint. Spondylitis is the term for inflammation of the spine. Rheumatism refers to a state of discomfort related to any part of the skeletal system, including tendons and ligaments. When joints become infected the illness is known as infectious arthritis. Having symptoms similar to infectious arthritis, but with no known cause, is rheumatoid arthritis. It usually affects matching joints of the body, such as knees and wrists, and leads to a serious incapacity in an estimated one-third of all cases. Diseases referred to as collagen diseases describe a general condition of abnormality of the connective tissues. These include lupus erythematosus, which can affect any organ or structure of the body, causing eruptions on the face, inflammation of the joints and eyes, and kidney disease or afflictions of the central nervous system. Rheumatic fever is frequently defined as a collagen disease. It resembles rheumatoid arthritis but is usually linked to a streptococcal infection. Noninflammatory joint diseases include traumatic joint diseases, which are the result of joint injuries and can take the form of a sprain, fracture, or dislocation. A sprain is an injury to the ligaments, tendons, or muscles resulting from an abrupt, jerking movement. It will normally heal in a matter of days. A more severe stress can cause ligaments, tendons, or muscles to tear, healing by fibrous union. Joint dislocations demand extended immobilization and, in some instances, surgery to heal. Osteoarthritis is a degenerative joint disease of the cartilage. All adults past middle age are to some degree affected. This disease is the most destructive when located in the hip. It is extremely unusual for a single injury to be the cause of osteoarthritis, but repeated stresses can make people vulnerable to this affliction. Congenital and hereditary abnormalities are not always evident at birth. Defects that appear at birth or at an early age are most often structural, whereas those that manifest later in life are usually the result of systemic metabolic deficiencies which lead to structural problems. Joint diseases can have other causes. Nutritional inadequacies can cause rickets and scurvy. Hemophilia causes bleeding into the joints, a damaging condition called hemarthrosis. Bone tissue death not caused by infection, termed aseptic necrosis, can lead to insufficient blood supply to the joint. Acromegaly, extreme activity of the pituitary gland, causes an overabundance of joint cartilage in peripheral joints, leading to osteoarthritic damage. A Charcot joint is characterized by an inability to receive sensory stimulation. This has a severely destructive impact on joints. A heart attack is frequently the cause of shoulder-hand syndrome, or it can be related to neck vertebrae diseases. Its symptoms are pain in the shoulders and swelling in the hands. Evidence of joint disease can be traced as far back as fossils from the Mesozoic Era. Rheumatoid arthritis has been detected in Egyptian mummies. All chronic arthritis was interpreted as gout until recent times. The first man to determine the existence of rheumatism was the Parisian physician Guillaume de Baillou in 1591. For diagnostic purposes, X-rays are used to detect bone problems but not abnormalities of tissue or joints. Synovial fluid is analyzed to aid in the diagnosis of inflammatory diseases. A synovial biopsy assists in determining whether arthritis is infectious or rheumatoid in nature. in geology, a brittle-fracture surface in rocks along which little or no displacement has occurred. Present in nearly all surface rocks, joints extend in various directions, generally more toward the vertical than to the horizontal. Joints may have smooth, clean surfaces, or they may be scarred by slickensides, or striations. Jointing does not extend to a very great depth in the Earth's crust, because at about 12 kilometres (7.5 miles) even rigid rocks tend to flow plastically in response to stress. In unweathered rocks, joints are relatively inconspicuous, but upon weathering they become marked, especially in a soluble rock such as limestone. Solution by water percolating through joints has led to the formation of large caves and underground rivers. Quarrying operations are facilitated by the presence of a well-developed joint system. Sedimentary rocks usually show two sets of joints at right angles to one another, each extending down perpendicular to the bedding; one set extends in the direction of dip and the other in the direction of strike (trend of the line of intersection of the bedding and the horizontal). The distance between joints varies from about two centimetres to a few hundred metres; in alternating strata the degree of jointing may vary from bed to bed and in some cases is related to the compaction of sediments during rock formation. In igneous rocks, jointing is generally quite irregular; but in granite, two vertical sets forming right angles to one another on the top surface and another set of cross joints approximately horizontal occur frequently. (These cross joints are the effect of weathering.) Intrusions of molten rock, when cooled, form sills and dikes, which, in many places, show columnar jointing. Three sets of joints perpendicular to the cooling surfaces intersect each other at angles of about 120. These form polygonal columns of rock that range from about 78 centimetres (3 inches) to about 6 metres (19 feet) in diameter; the size depends on the rate of cooling of the intrusive rockthe faster the cooling, the smaller the columns. The principal cause of jointing in both stratified and igneous rocks is crustal movement, although the specific origin of the movement may not always be apparent. Contraction upon consolidation of sediment, as well as crystallization, also contributes to minor irregular jointing, as does expansion and contraction from the intrusion of hot igneous rocks. Additional reading C.H. Barnett et al., Synovial Joints: Their Structure and Mechanics (1961), a text on the general arthrology of synovial joints that contains early advances; M.A. MacConaill and J.V. Basmajian, Muscles and Movements (1969), which includes a theory of kinematics of muscles and joints and the information about how movements are brought about at individual joints or joint systems as revealed by electromyography; and the Journal of Bone and Joint Surgery (8/year), current research in both basic and applied arthrology from all parts of the English-speaking world. Dhanjoo N. Ghista (ed.), Osteoarthromechanics (1982), discusses bone physiology, human mechanics, and joint anatomy.