Involvement of BDNF in knee osteoarthritis: the relationship with inflammation and clinical parameters

The aim of this study was to analyze the levels of brain-derived neurotrophic factor (BDNF) in both the plasma and synovial fluid of patients with primary knee osteoarthritis compared with control individuals and to investigate the relationship between BDNF levels and self-reported pain. Twenty-seven patients with knee osteoarthritis (OA) and 19 healthy subjects were enrolled in the study. Anteroposterior knee radiographs were taken to determine the disease severity of the affected knee. Radiographic grading of OA in the knee was performed using the Kellgren-Lawrence criteria. The BDNF levels in the plasma and synovial fluid were measured by enzyme-linked immunosorbent assay. The mean plasma BDNF levels of the knee OA patients were significantly higher than that of the healthy controls (2,378 ± 1,067.2 vs. 1,756 ± 804.3 pg/mL, p < 0.05). BDNF levels in the synovial fluid of OA patients (358.9 ± 178.4 pg/mL) were sixfold lower than in corresponding blood samples (p < 0.0001) and fourfold lower than in the plasma of healthy controls (p < 0.0001). Subsequent analyses showed that the plasma BDNF levels significantly correlated with self-reported pain (Western Ontario and McMaster Universities Osteoarthritis Index) (r s = 0.39, p = 0.04). Furthermore, no correlation was found between the plasma and synovial fluid BDNF concentrations and knee OA severity. The findings of this study suggest that systemic BDNF levels are most likely associated with the mechanism of joint pain in knee OA in the acute stage of joint inflammatory process. Further studies are necessary to address the functional role of BDNF in the modulation of pain to establish new therapeutic implications.