Synovial fluid lipids and apolipoproteins: a contemporary perspective.

Normal human synovial fluid contains extremely low concentrations of lipoproteins and apolipoproteins, in sharp contrast to those found in plasma. Increased amounts of cholesterol and other lipids have been found in the synovial fluid of a chronic inflammatory joint disorder, rheumatoid arthritis (RA). More recently, apolipoproteins AI, B and E have also been found in increased amounts in RA synovial fluid. Theories have been proposed to account for this increase in the amount of apolipoproteins and for the source of lipids and lipoproteins in normal synovial fluid; however, the mechanisms have not yet been established. Lipoproteins may play dual roles in synovial fluid: A functional one in normal synovial fluid and, as some suggest, a pathologic one in the abnormal synovial fluid of certain arthritic diseases. The recent data prompt the need to define synovial fluid lipids, lipoprotein particle subfractions and their constituent apolipoproteins, as well as their respective roles in synovial fluid.