Joint
A joint, or articulation, is a junction between bones of the skeleton. The construction of a joint can be viewed on a stability versus mobility continuum.
Stability is increased when the bones fit snugly together and the ligaments and tendons are tight, restricting movement. Movement is increased with a looser construction though stability is decreased. There are three types of joints: fibrous, cartilaginous, and synovial.
Fibrous joints are joints in which two bones are connected by a fibrous connective tissue; the joint space is completely filled with fibrous tissue, allowing no movement. These joints can be found in between the plates of the skull, in the bones of the forearm and other areas.
Cartilaginous joints are bones joined by cartilage. The cartilage fills the joint and very little movement is allowed; they are found between the ribs and sternum, and between vertebrae discs of the back. Synovial joints are the most common joints in the body. The ends of the bones of these joints are capped with smooth cartilage allowing them to slide easily over each other. In some of these joints, an example of which is the knee, accessory cartilaginous structure, which provide extra cushioning and improve the stability of the joint, are found. In a synovial joint, the ends of the bones are enclosed in fibrous joint capsule.
Tendons and ligaments intermix at some point in the capsule. The tautness of the capsule determines the stability and range movement of the joint. Under the capsule is a synovial membrane which secretes a fluid called synovial fluid. This fluid lubricates and cushions the joint, and provides nutrients to the cartilage. There are six types of synovial joints: ball and socket (e.g., hip joint), hinge (e.g., elbow), saddle (e.g., thumb), condyloid (e.g., knee), pivot (e.g., neck), and plane (e.g., wrist). Each type of joint differs in range of movement and stability.
Double-jointedness is caused by a joint constructed to permit an extreme range of motion. This can be due to the lack of ligaments or a difference in bony configuration.
References
Crafts, R.C. 1985. A Textbook of Human Anatomy. John Wiley and Sons, Inc., New York. 906 pp.
Lockhart, R.D., G.F. Hamilton, et. al. 1974. Anatomy of The Human Body. Faber and Faber Limited. London. 697 pp.
Rahlman, J. & J.L. Smith. 1981. Ucla Kinesiology 14 Human Neuromuscular Anatomy. Academic Publishing Service. L A. 490 pp.
Van Amerongen, C. The Way Things Work; Book Of The Body. New York: Simon and Schuster, 1979.
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