NGF and intervertebral disk degenerative disorder: We observed a positive correlation between the expression levels of NGF and SP and the severity of histological disc degeneration (r = 0.467, p = 0.010 and r = 0.611, p = 0.0006, respectively) (Figure 5), indicating a pathogenetic link between neuroinflammation and cartilage degeneration that is independent of age.