BACKGROUND:The ligamentum flavum is important to connect the rear of the spine. If the flexibility of the ligamentum flavum is decreased due to degeneration, in the role of various factors, it can reduce the effective space of the spinal canal to induce spinal cord and nerve root compression, thereby resulting in neurological symptoms, even paralysis or residual disease. OBJECTIVE:To study the mechanism of over-dose fluoride on the degeneration and ossification of the ligamentum flavum in rats. METHODS:Thirty male Sprague-Dawley rats were randomly divided into two groups: experimental group was given distiled water containing NaF (mass fraction 10-4) and control group was given the same amount of distiled water. After 3 and 6 months, the bone mineral density, Ca2+ and P3+ contents in serum and bone tissue and serum alkaline phosphatase activity were detected, respectively. Meanwhile, the ligamentum flavum specimens were taken for X-ray examination and histopathological observation. RESULTS AND CONCLUSION: In the experimental group, chalk stripes or spots occurred in the lower incisors of five rats at 3 months after treatment and dental fluorosis that was mostly in chalk color was found in al the rats at 6 months after treatment; moreover, the bone density of the lumbar spine significantly increased (P < 0.05). Compared with the control group, the contents of serum Ca2+ were significantly higher, while the contents of P3+were significantly lower in the experimental group at 3 and 6 months after treatment (P < 0.05); the serum alkaline phosphatase activity was also significantly higher in the experimental group than the control group at 3 and 6 months after treatment (P < 0.05). X-ray films, at 6 months after treatment, showed that the bone density of the lumbar vertebra was increased obviously; pathological observation showed that the number of elastic fibers in the ligamentum flavum was reduced further, a large amount of colagen fibers, fibroblasts and calcium deposition were found, and the ossification of the ligamentum flavum was visible at the lamina attachment site. These findings indicate that the over-dose fluorides can lead to the degeneration and ossification of the ligamentum flavum in Sprague-Dawley rats, which may plan an important role in the ossification of the ligamentum flavum.