Synovial fluid is a viscous fluid in the cavities of the synovial joints, which include the knees and elbows. The main purpose of synovial fluid is to reduce friction between cartilage layers and bones during joint movement. In a healthy synovial joint, bones slide smoothly around each other within the joint cavity. An articular cartilage layer on the bone ends, and synovial fluid in the joint cavity, provide a buffer to allow this smooth movement and prevent bone damage.
Synovial joints are the only joints in the body where the bones are not connected. Instead, a cavity exists between them. Fibrous connective tissue forms the walls of the cavity, creating an articular capsule. The connective tissue attaches to bones on either side of the joint, just below the articulating surface -- the end of the bone that moves inside the joint cavity.
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