The purposes of this study were to examine osteopontin levels in both plasma and synovial fluid of patients with primary knee osteoarthritis (OA) and to investigate their relationship with severity of the disease.

Thirty-two patients aged 53–83 years with knee OA and 15 healthy controls were enrolled in this study. Anteroposterior knee radiographs 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. Osteopontin levels in the plasma and synovial fluid were measured using enzyme-linked immunosorbent assay.

The mean plasma osteopontin concentration of the knee OA patients was significantly higher compared with that of healthy controls (168.8±15.6 vs 67.2±7.7 ng/mL, P<0.0001). Osteopontin levels in synovial fluid were significantly higher with respect to paired plasma samples (272.1±15.0 vs 168.8±15.6 ng/mL, P<0.001). In addition, plasma osteopontin levels showed a positive correlation with synovial fluid osteopontin levels (r=0.373, P=0.035). Subsequent analysis showed that plasma osteopontin levels significantly correlated with severity of disease (r=0.592, P<0.001). Furthermore, the synovial fluid levels of osteopontin also correlated with disease severity (r=0.451, P=0.01).

The data suggest that osteopontin in plasma and synovial fluid is related to progressive joint damage in knee OA. Osteopontin may serve as a biochemical marker for determining disease severity and could be predictive of prognosis with respect to the progression of knee OA.