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An Australian study of structural and protective features of synovial tissue from normal knee joints has been the first to establish a baseline reference for comparison with arthritic joints. It also suggested that an excess of anti-inflammatory cytokines osteoprotogerin (OPG) over receptor activator of nuclear factor κ B ligand (RANKL) and interleukin 1 receptor antagonist (IL1Ra) over interleukin 1 β (IL1 β) help to maintain a normal non-inflammatory state.
OPG concentration was very high and inflammatory RANKL very low in blood vessels and synovial lining cells within synovial membranes (RANKL:OPG ratio 0.13 (SD 0.23, range 0–0.98)). Concentrations of cell adhesion molecules ICAM-1 and VCAM-1 were low and were seen mainly in endothelial cells or fibroblast-like synoviocytes, respectively. ILIRa concentration was high and IL 1 β low (IL1Ra: IL1 β ratio 9.1 (16.6, 0–54.8)).
The cellular picture was variable. Essentially, synovial membranes contained scattered macrophages and monocytes but no plasma cells. The intimal lining layer varied in width and contained mostly CD55+ fibroblast-like synoviocytes, few CD68+ macrophages, and no or minimal inflammatory cells.
Twenty synovial samples banked from knee joint arthroscopies for unexplained knee pain were studied; arthritis had not developed. The subjects were not taking anti-inflammatory drugs. Sections were stained immunohistochemically to determine cellular profile and presence of cytokines and cell adhesion molecules by semiquantitative scoring and digital image analysis.
Normal synovial features of limited scope have been described mostly in small numbers of controls, not always in normal subjects but from necropsy or limbs amputated because of cancer.