Profiling of inflammatory mediators in the synovial fluid related to pain in knee osteoarthritis
Abstract Background: Inflammatory mediators in the synovial fluid (SF) play critical roles in the initiation and development of pain in knee osteoarthritis (KOA). However, the expression of inflammatory mediators is controversial and the role of SF inflammatory mediators in neuropathic pain is not clear. Therefore, the aim of this study is to identify the SF inflammatory mediators associated with nociceptive and neuropathic pain in KOA. Methods: The levels of IL-1β, IL-6, TNF-α, macrophage colony-stimulating factor, MMP-3, MMP-13, metalloproteinase with thrombospondin motifs 5, calcitonin gene-related peptide, neuropeptide Y, substance P and bradykinin were measured in 86 patients using enzyme-linked immunosorbent assays. Nociceptive pain was measured using the numeric rating scale (NRS), visual analog scale (VAS) and Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) pain score. Neuropathic pain was measured using the PainDETECT questionnaire. Moreover, knee function was evaluated by the WOMAC score and range of motion (ROM) assessments. Radiological grade was defined using the Kellgren-Lawrence (K-L) grading scale. Results: Pain scores measured using different methods were highly correlated to each other. The worse the pain, the worse the K-L grade and knee function were. The expression of IL-1β and IL-6 was increased in the early stage compared with the late stage. The NRS was positively correlated to age, K-L grade, and the WOMAC score and negatively correlated to ROM and TNF-α expression. The VAS was positively correlated to age, K-L grade, and the WOMAC score but negatively correlated to ROM and the levels of IL-1β, IL-6 and TNF-α. The WOMAC pain score was not correlated to any of the measured inflammatory mediators; it correlated to only ROM. The PainDETECT score correlated to only the WOMAC score. The expression of other inflammatory mediators was not correlated to any of the pain scores. Conclusions: IL-1β, IL-6 and TNF-α play critical roles in pain in the early stage of KOA and correlated to pain. The measured catabolic enzymes and neuropeptides are not correlated to nociceptive and neuropathic pain. New biomarkers related to pain in the late stage need to be further investigated.