Classification of Joints on the Basis of Structure
There are two ways to classify joints: on the basis of their structure or on the basis of their function. The structural classification divides joints into bony, fibrous, cartilaginous, and synovial joints depending on the material composing the joint and the presence or absence of a cavity in the joint.
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The bones of fibrous joints are held together by fibrous connective tissue. There is no cavity, or space, present between the bones and so most fibrous joints do not move at all, or are only capable of minor movements. There are three types of fibrous joints: sutures, syndesmoses, and gomphoses. Sutures are found only in the skull and possess short fibers of connective tissue that hold the skull bones tightly in place (Figure 1).
Figure 2. Gomphoses are fibrous joints between the teeth and their sockets. (credit: modification of work by Gray’s Anatomy)
Figure 3. Synovial joints are the only joints that have a space or “synovial cavity” in the joint.
Cartilaginous joints are joints in which the bones are connected by cartilage. There are two types of cartilaginous joints: synchondroses and symphyses. In a synchondrosis, the bones are joined by hyaline cartilage. Synchondroses are found in the epiphyseal plates of growing bones in children. In symphyses, hyaline cartilage covers the end of the bone but the connection between bones occurs through fibrocartilage. Symphyses are found at the joints between vertebrae. Either type of cartilaginous joint allows for very little movement.
Synovial joints are the only joints that have a space between the adjoining bones (Figure 3). This space is referred to as the synovial (or joint) cavity and is filled with synovial fluid. Synovial fluid lubricates the joint, reducing friction between the bones and allowing for greater movement. The ends of the bones are covered with articular cartilage, a hyaline cartilage, and the entire joint is surrounded by an articular capsule composed of connective tissue that allows movement of the joint while resisting dislocation. Articular capsules may also possess ligaments that hold the bones together. Synovial joints are capable of the greatest movement of the three structural joint types; however, the more mobile a joint, the weaker the joint. Knees, elbows, and shoulders are examples of synovial joints.
Classification of Joints on the Basis of Function
The functional classification divides joints into three categories: synarthroses, amphiarthroses, and diarthroses:
Synarthroses are a joints that are immovable. This includes sutures, gomphoses, and synchondroses.Amphiarthroses are joints that allow slight movement, including syndesmoses and symphyses.Diarthroses are joints that allow for free movement of the joint, as in synovial joints.