osteoarthritis ( oa ) is a chronic degenerative joint disorder characterized by articular cartilage degeneration , subchondral bone abnormalities , and nonspecific synovial inflammation . as a part of the ageing process , oa is the leading cause of physical disability and impaired quality of life in the elderly . of all the severe symptoms caused by oa , the pain sensation is the primary complaint that leads to a significantly reduced quality of life . as oa is a continuing pathological process , there has been a long - standing interest in identifying new specific biological markers for symptomatic severity to facilitate the early diagnosis of joint destruction and to evaluate disease . the establishment of reliable biomarkers could facilitate investigating the etiology and pathogenesis underlying oa and thus determine possible new therapeutic targets and improve clinical outcomes . although many underlying mechanisms , such as bone alterations , contribute to oa - induced pain , inflammation is now believed to play an important role in the pathogenesis of oa - induced pain . macrophage migration inhibitory factor ( mif ) is a pro - inflammatory cytokine produced by macrophages within innate and adaptive immune responses . mif can induce the production of a large number of pro - inflammatory molecules and thus may be involved in the pathophysiology of arthritis by promoting inflammation and angiogenesis [ 79 ] . moreover , liu et al . recently revealed that serum and synovial fluid ( sf ) mif levels were significantly associated with the radiographic severity of oa . therefore , we hypothesized that mif might be implicated in the pathophysiology of oa - induced pain . the present study thus aimed to investigate the possible correlation between mif levels in serum and sf and self - reported pain intensity in oa patients . a total of 226 primary knee oa patients undergoing diagnostic or therapeutic arthroscopy or total knee replacement in our hospital from april 2010 to november 2012 were recruited . knee oa was diagnosed based on the american college of rheumatology published criteria of knee oa . as controls , we enrolled 106 age- and sex - matched volunteers without clinical and radiological evidence of oa . participants were excluded if they had rheumatoid arthritis ( ra ) , post - traumatic arthritis , previous joint infection , crystal deposition arthritis , enteropathic arthritis , hemophilic arthropathy , acute inflammatory disease , malignant tumor , end - stage renal or hepatic disease , or used corticosteroid medication for pain control . the pain severity of each patient was evaluated according to the western ontario mcmaster university osteoarthritis index ( womac ) pain score . the items were scored with the use of a 04 rating scale , where 0 represents no pain and higher scores represent greater pain . the psychometric properties of the womac score , including reliability , validity , and responsiveness , are all well established in an oa population . sf samples were obtained from oa patients before the treatment with hyaluronic acid injection or surgery . sf samples were immediately centrifuged at 3000 rpm for 15 min to remove cell debris , and the supernatant was aliquoted into 1.5-ml micro - centrifuge tubes and frozen at 80c until use . serum was obtained by centrifugation at 3000 rpm and then stored at 80c before measuring . serum and sf mif levels were determined with commercially available enzyme - linked immunosorbent assay ( elisa ) kits ( r&d systems , minneapolis , mn , usa ) according to the manufacturer s protocol . data are presented as mean values  standard deviation , median and the inter - quartile range , or frequency , as appropriate . the statistical significance of differences between groups was analyzed by parametric tests ( unpaired t - test ) , nonparametric tests ( mann - whitney u test ) , or chi square test and wilcoxon signed rank test for paired samples , as appropriate . differences among groups were analyzed by one - way analysis of variance ( anova ) followed by tukey post hoc analysis , kruskal - wallis analysis , or chi square test , as indicated . associations between mif levels and womac pain scores were examined by spearman s rank correlation and expressed as correlation coefficients . to assess the independent predictors of womac pain scores , multivariate stepwise linear regression analysis was used . a total of 226 primary knee oa patients undergoing diagnostic or therapeutic arthroscopy or total knee replacement in our hospital from april 2010 to november 2012 were recruited . knee oa was diagnosed based on the american college of rheumatology published criteria of knee oa . as controls , we enrolled 106 age- and sex - matched volunteers without clinical and radiological evidence of oa . participants were excluded if they had rheumatoid arthritis ( ra ) , post - traumatic arthritis , previous joint infection , crystal deposition arthritis , enteropathic arthritis , hemophilic arthropathy , acute inflammatory disease , malignant tumor , end - stage renal or hepatic disease , or used corticosteroid medication for pain control . the pain severity of each patient was evaluated according to the western ontario mcmaster university osteoarthritis index ( womac ) pain score . the items were scored with the use of a 04 rating scale , where 0 represents no pain and higher scores represent greater pain . the psychometric properties of the womac score , including reliability , validity , and responsiveness , are all well established in an oa population . sf samples were obtained from oa patients before the treatment with hyaluronic acid injection or surgery . sf samples were immediately centrifuged at 3000 rpm for 15 min to remove cell debris , and the supernatant was aliquoted into 1.5-ml micro - centrifuge tubes and frozen at 80c until use . serum was obtained by centrifugation at 3000 rpm and then stored at 80c before measuring . serum and sf mif levels were determined with commercially available enzyme - linked immunosorbent assay ( elisa ) kits ( r&d systems , minneapolis , mn , usa ) according to the manufacturer s protocol . data are presented as mean values  standard deviation , median and the inter - quartile range , or frequency , as appropriate . the statistical significance of differences between groups was analyzed by parametric tests ( unpaired t - test ) , nonparametric tests ( mann - whitney u test ) , or chi square test and wilcoxon signed rank test for paired samples , as appropriate . differences among groups were analyzed by one - way analysis of variance ( anova ) followed by tukey post hoc analysis , kruskal - wallis analysis , or chi square test , as indicated . associations between mif levels and womac pain scores were examined by spearman s rank correlation and expressed as correlation coefficients . to assess the independent predictors of womac pain scores , multivariate stepwise linear regression analysis was used . the baseline clinical and demographic parameters of controls and oa patients are displayed in table 1 . no statistically significant differences were found between the 2 groups in terms of age or sex distribution . in oa patients , clinical and demographic characteristics we demonstrated that there were no significant differences in baseline clinical characteristics among oa patients with various radiographic kl grades ( p>0.05 ) . oa patients had similar serum mif levels compared to controls ( 11.93 [ 5.6818.10 ] vs. 10.06 [ 6.6014.61 ] ng / ml , p>0.05 ; table 1 ) . in oa patients , mif levels in sf were dramatically lower than those in paired serum samples ( 3.39 [ 1.875.89 ] vs. 11.93 [ 5.6818.10 ] ng / ml , p<0.01 ; table 1 ) . the correlation between serum and sf mif levels was not statistically significant ( r=0.127 , p=0.057 ) . in oa patients with various kl grades , there were no significant differences in serum mif levels among the subgroups ( p>0.05 ; table 1 ) . however , oa patients with kl grade 3 and kl grade 4 had significantly higher sf mif levels compared to those with kl grade 2 ( p<0.01 ; table 1 ) . as demonstrated in figure 1 , sf mif levels were significantly correlated with womac pain scores ( r=0.237 , p<0.001 ) . however , there was no significant association between serum mif levels and womac pain scores in oa patients ( r=0.009 , p=0.898 ; figure 2 ) . the analysis revealed that mif levels in sf were still the significant determinants of womac pain scores ( t=2.262 , p=0.009 ; table 2 ) . the baseline clinical and demographic parameters of controls and oa patients are displayed in table 1 . no statistically significant differences were found between the 2 groups in terms of age or sex distribution . in oa patients , clinical and demographic characteristics we demonstrated that there were no significant differences in baseline clinical characteristics among oa patients with various radiographic kl grades ( p>0.05 ) . oa patients had similar serum mif levels compared to controls ( 11.93 [ 5.6818.10 ] vs. 10.06 [ 6.6014.61 ] ng / ml , p>0.05 ; table 1 ) . in oa patients , mif levels in sf were dramatically lower than those in paired serum samples ( 3.39 [ 1.875.89 ] vs. 11.93 [ 5.6818.10 ] ng / ml , p<0.01 ; table 1 ) . the correlation between serum and sf mif levels was not statistically significant ( r=0.127 , p=0.057 ) . in oa patients with various kl grades , there were no significant differences in serum mif levels among the subgroups ( p>0.05 ; table 1 ) . however , oa patients with kl grade 3 and kl grade 4 had significantly higher sf mif levels compared to those with kl grade 2 ( p<0.01 ; table 1 ) . as demonstrated in figure 1 , sf mif levels were significantly correlated with womac pain scores ( r=0.237 , p<0.001 ) . however , there was no significant association between serum mif levels and womac pain scores in oa patients ( r=0.009 , p=0.898 ; figure 2 ) . the analysis revealed that mif levels in sf were still the significant determinants of womac pain scores ( t=2.262 , p=0.009 ; table 2 ) . we investigated the correlation between mif levels in serum and sf and severity of oa - induced pain . we demonstrated that mif levels in sf but not in serum were correlated with the severity of self - reported pain in knee oa patients . these findings indicate that the increased expression of mif in arthritic joints may be responsible for the higher pain intensity of oa . it is now well - known that oa is also a chronic inflammatory process ; therefore , pro - inflammatory cytokines have potential as oa - related biomarkers . mif is a pro - inflammatory cytokine and glucocorticoid - induced immunomodulator , mainly produced by macrophages . serum mif levels have been reported to be elevated in several inflammatory disorders , such as coronary artery disease , metabolic syndrome , and rheumatoid arthritis . in the present study , we demonstrated that oa patients had similar serum mif levels compared with controls , suggesting that systemic circulating levels of mif have no relationship with oa . we also revealed that in oa patients , mif levels in serum were significantly higher than those in paired sf samples , and the correlation between serum and sf mif levels was not statistically significant . these results suggest that mif has limited ability to transfer across the synovial membrane due to its molecular weight or complex structure ; mif levels in sf might reflect the intra - articular expression of mif . these results are different from those of a study by liu et al . , which reported that oa patients had significantly higher serum mif levels compared with controls . as we mentioned above , serum mif level is a sign of inflammatory response in systemic circulation rather than in local arthritic joints . we excluded patients with acute inflammatory disease from our study , but liu et al . the difference in systemic inflammatory state of the subjects might partially explain the conflicting data . these conflicting data may also be attributable to differences in disease advancement , populations , or assays used . in addition , we demonstrated that oa patients with kl grade 3 and kl grade 4 had significantly higher sf mif levels compared to those with kl grade 2 . this result is in accord with the results of liu et al . , which revealed that sf mif levels rose as kl grade increased . the most intriguing finding of our study was that sf mif levels were positively correlated with womac pain scores in oa patients . this correlation was still significant after adjusting for age , sex , and bmi as potential confounders in multivariate linear regression analysis . these results indicate that oa patients with higher sf mif levels may have more severe self - reported pain . increasing evidence shows that the over - expressed inflammatory cytokines in the inflamed joints have an important pathophysiological role in the generation and maintenance of oa - induced pain by acting on nociceptive nerve cells . mif is a potent pro - inflammatory cytokine that can induce the release of several inflammatory cytokines , such as interferon ( ifn)- , interleukin ( il)-1 , 6 , 8 , and tumor necrosis factor ( tnf)- , through activating an inflammatory cascade . therefore , the pro - inflammatory role of mif might explain the relationship between mif levels in sf and pain intensity of oa observed in our study first , the cross - sectional design and relatively small sample size are the major limitations of our study ; therefore , it is necessary to repeat this research in prospective longitudinal studies with larger populations . second , we did not examine the correlation of mif levels with other inflammatory cytokines , such as ifn- , il-1 , and tnf-. further studies examining these correlations could provide more useful information on the potentially pathogenic role of mif in oa - induced pain . mif levels in sf , but not in serum , were independently associated with the severity of self - reported pain in oa patients . if these results are confirmed by further studies , the inhibition of mif signaling pathways may be a novel therapeutic approach for ameliorating oa - induced pain .
backgroundinflammation is considered as one of the main pathogeneses in oa - induced pain . 
 macrophage migration inhibitory factor ( mif ) is a well known pro - inflammatory cytokine . 
 we aimed to determine whether mif levels in serum and synovial fluid ( sf ) are associated with severity of oa - induced pain.material/methodswe recruited 226 patients with knee oa and 106 controls . 
 self - reported pain severity of oa patients was evaluated using the western ontario mcmaster university osteoarthritis ( womac ) pain scores . 
 mif levels were detected using enzyme - linked immunosorbent assay ( elisa).resultsoa patients had similar serum mif levels compared to controls ( 11.93 [ 5.6818.10 ] vs. 10.06 [ 6.6014.61 ] ng / ml , p>0.05 ) . in oa patients , mif levels in 
 sf were dramatically lower compared to paired serum samples ( 3.39 [ 1.875.89 ] vs. 11.93 [ 5.6818.10 ] ng / ml , p<0.01 ) . 
 mif levels in sf were significantly correlated with womac pain scores ( r=0.237 , p<0.001 ) , but mif levels in serum had no significant correlation with womac pain scores ( r=0.009 , p=0.898).conclusionsmif levels in sf , but not in serum , were independently associated with the severity of self - reported pain in oa patients . 
 the inhibition of mif signaling pathways may be a novel therapeutic approach for ameliorating oa - induced pain .