Why do joints swell?

Orthopaedic surgeons see patients with effusions in joints very frequently. The reasons why effusions occur in joints, and sometimes remain in joints, requires an understanding of the physiology of production of synovial fluid, and the ways in which it enters and leaves joints. Most orthopaedic surgeons do not understand the basic physiology of this common clinical sign, and this article attempts to answer the question of "why do joints swell?".