Objective: To investigate osteopontin (OPN) levels in both synovial fluid and articular cartilage of patients with primary knee osteoarthritis (OA) and their relationship with severity of the disease.

Method: Fifty patients aged 48e81 years with knee OA and 10 healthy controls were enrolled in this study. Anteroposterior knee radiographs or/and Mankin score were taken to determine the disease severity of the affected knee. The radiographic grading of OA in the knee was performed by using the Kellgren-Lawrence criteria. OPN levels in synovial fluid were measured using enzyme-linked immunosorbent assay. OPN levels in articular cartilage were assessed by immunohistochemical methods.

Results: Compared to healthy controls, OA patients had higher OPN concentration in synovial fluid (4519.60  1830.37, 95%CI 3999.42e5039.79 vs 1179.70 303.39, 95%CI 1035.53e1438.74 pg/ml, P < 0.001)and articular cartilage(0.6  0.06, 95%CI 0.59e0.62 vs 0.43  0.07, 95%CI 0.38e0.48, P < 0.01). In addition, synovial fluid OPN levels showed a positive correlation with articular cartilage OPN levels (r ? 0.411, 95%CI 0.150e0.619, P ? 0.003). Subsequent analysis showed that synovial fluid OPN levels significantly correlated with severity of disease (Spearman’s r ? 0.581, 95%CI 0.335e0.726, P < 0.001). Furthermore, the articular cartilage levels of OPN also correlated with disease severity (Spearman’s r ? 0.675, 95%CI 0.500e0.808, P < 0.001).

Conclusions: OPN in synovial fluid and articular cartilage is associated with progressive joint damage and is likely to be a useful biomarker for determining disease severity and progression in knee OA.