Each mobile joint in your body has the same basic structure. The joint capsule is on the outside; it is made of tough fibrous tissue that adds strength and stability to the joint and connects the articulating bones. A sheet of synovium lines the capsule; its cells secrete synovial fluid. This fluid, along with water, is the chief constituent of the fluid within the joint. Synovial fluid is thick, straw-colored, and found in small amounts in the joint. The thickness is due in large part to an important component called hyaluronic acid. Injecting hyaluronic acid into the knees and other joints (such as the hips and shoulders) can alleviate symptoms in some people with arthritis. Another important substance inside the joint is called cartilage.
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