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The number of joints depends on if sesamoids are included, age of the human and the definition of joints. However, the number of sesamoids is the same in most people with variations being rare. [5] [6] [7] Joints are mainly classified structurally and functionally.
The joint can be considered a synarthrosis. [13] The gomphosis is the only joint-type in which a bone does not join another bone, as teeth are not technically bone. In modern, more anatomical, joint classification, the gomphosis is simply considered a fibrous joint because the tissue linking the structures is ligamentous.
Synovial joints, joints that are not directly joined, are lubricated by a solution called synovial fluid that is produced by the synovial membranes. This fluid lowers the friction between the articular surfaces and is kept within an articular capsule , binding the joint with its taut tissue.
The synovial cavity/joint is filled with synovial fluid. The joint capsule is made up of an outer layer of fibrous membrane, which keeps the bones together structurally, and an inner layer, the synovial membrane, which seals in the synovial fluid. They are the most common and most movable type of joint in the body of a mammal.
The knee joints and ankle joints are less typical, as they allow a slight degree of rotation or side-to-side movement in certain positions of the limb. The knee is the largest hinge joint in the human body. Hinge and pivot joints are both types of synovial joint. A hinge joint can be considered a modified sellar/saddle joint, with reduced ...
Finally, the joint capsule matures and forms a cavity, with a central meniscus, and an encasement of synovium. [9] This final structure will form several distinct layers of the articular cartilage found in all synovial joints including the deep zone (closest to the bone), middle zone, and superficial zone (closest to the synovial fluid).
In amphiarthroses, the contiguous bony surfaces can be: A symphysis: connected by broad flattened disks of fibrocartilage, of a more or less complex structure, which adhere to the ends of each bone, as in the articulations between the bodies of the vertebrae or the inferior articulation of the two hip bones (aka the pubic symphysis).
A synarthrosis is a type of joint which allows no movement under normal conditions. Sutures and gomphoses are both synarthroses. Joints which allow more movement are called amphiarthroses or diarthroses. Syndesmoses are considered to be amphiarthrotic, because they allow a small amount of movement. [1]