MMP9 and rheumatoid arthritis: Despite recent basic research advances and clinical findings which has vastly improved our understanding of the role of pro-inflammatory cytokines and MMPs in RA and OA disease pathogenesis and progression, a detailed understanding of which mechanism(s) contribute to the capacity of TNF-α or IL-6 to produce these changes in human chondrocyte MMP-9 remains to be elucidated.