SPP1 and rheumatoid arthritis: Serum and synovial fluid OPN was significantly higher in RA patients compared to healthy patients and is correlated with markers of bone resorption in RA patients, and their high levels are directly proportional to serum levels of CRP, chemo-monocyte attractant 1 (MCP-1), macrophage inflammatory protein-1 beta (MIP-1β) on monocytes, interleukin-17 (IL-17), and other inflammatory cytokines including TNF-α and IL-1, IL-6, and IL -8.