Synovitis is the inflammation of the synovial membrane, located inside the joints, which secretes a fluid, called synovial fluid, that works as a lubricant for the joint. Synovitis can occur with effusion, the accumulation of fluid in a joint. This is a common condition of the knee
, where it swells, becomes painful, and movement is reduced. Synovitis of the knee is a common complaint in cases of rheumatoid polyarthritis.