Inflammation of the joint lining (synovium).
Most of the moveable joints are contained within a 'capsule' which makes the space watertight. The inner lining of the capsule has special cells (synovial cells) that secrete the special lubricating fluid of the joint - the synovial fluid, If the joint becomes inflamed for any reason, it affects these synovial cells, which then produce excess synovial fluid. This causes the joint to swell and become painful.
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