Anatomy and Chemistry Basics
Joints
Structural classification is based on the materials that hold the joint together and whether or not a cavity is present in the joint. There are three structural classes.
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Fibrous joints are held together by fibrous connective tissue. No joint cavity is present. Fibrous joints may be immovable or slightly movable.
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Cartilaginous joints are held together by cartilage (hyaline or fibrocartilage). No joint cavity is present. Cartilaginous joints may be immovable or slightly movable.
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Synovial joints are characterized by a synovial cavity (joint cavity) containing synovial fluid. Synovial joints are freely movable and characterize most joints of the body. Figure 1 lists other features of a synovial joint, including
Figure 1 A synovial joint.
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Articular cartilage (hyaline cartilage), which covers the end of each bone.
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A synovial membrane, which surrounds the synovial cavity. Its areolar connective tissue secretes a lubricating synovial fluid into the synovial cavity.
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A fibrous capsule outside the synovial membrane, which surrounds the joint. It often contains bundles of dense, irregular, connective tissue called ligaments. The ligaments provide strength and flexibility to the joint.
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The articulate capsule is composed of the synovial membrane and fibrous capsule.
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Accessory ligaments lie outside the articular capsule (extracapsular ligaments) or inside the synovial cavity (intracapsular ligaments).
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Functional classification is based on the degree to which the joint permits movement. There are three types:
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A synarthrosis joint permits no movement. Structurally, it may be a fibrous or cartilaginous joint.
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An amphiarthrosis joint permits only slight movement. Structurally, it may be fibrous or cartilaginous joint.
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A diarthrosis joint is a freely movable joint. Structurally, it is always a synovial joint.
