Osteoporosis and tooth decay are a public health problem. For their treatment, sodium fluoride (NaF) and monofluorophosphate are the drugs containing fluoride (F) most commonly used. Disorganization of bone tissue and presence of inflammatory foci observed when NaF is administered could be the cause of the lack of effectiveness of F in the treatment of osteoporosis. The aim of this work was to evaluate the effect of different F doses on biomechanical and morphometric properties of trabecular and cortical bone. Twenty four Sprague-Dawley rats were divided into 4 groups (n=6/group): NaF20, NaF40, NaF80, which received orally 20, 40 or 80 µmolNaF/100g bw.day for 30 days; and Control that received water. After treatment, tibias and femurs were extracted. Histopathological and histomorphometric parameters were evaluated in the left tibia. In the right tibia, bone mineral density (BMD) and cortical parameters were determined. The femurs were used for biomechanical tests to evaluate trabecular and cortical bone properties. Results are expressed as mean±SEM and differences were considered significant if p<0.05 (*). Comparisons were performed using ANOVA test with Bonferroni's post test.

Trabecular bone volume (%) was significantly decreased by the treatment with F (Control: 23±2; NaF20: 21±2; NaF40: 18±1; NaF80: 14±2*). Coincidently, we observed a significant decrease in the trabecular fracture load (N) (Control: 52±11; NaF20: 24±4; NaF40: 26±6; NaF80: 18±4*) and Young´s Modulus (GPa) (Control: 0.16±0.04; NaF20: 0.08±0.02; NaF40: 0.07±0.02*; NaF80: 0.06±0.02*) of NaF-treated groups. Cortical bone area (mm²) decreased significantly in NaF-treated groups (Control: 4.1±0.19; NaF20: 4.0±0.14; NaF40: 3.8±0.09; NaF80: 3.4±0.07*) and the three point flexural test revealed a significant and a dose-dependent decrease in cortical bone strength (N) (Control: 108±5; NaF20: 100±3; NaF40: 87±2*; NaF80: 84±4*) and stiffness (Control: 312±38; NaF20: 271±49; NaF40: 200±26; NaF80: 173±19*). Apoptosis and inflammation were observed in the trabecular bone of NaF-treated groups. There were no significant differences in BMD between the groups. Finally, there was a significant correlation between the fracture load of cortical bone and bone volume, independently of Young's Modulus. At the trabecular level, fracture load correlated with Young's Modulus and not with bone volume. These results indicate that cortical bone depends on the amounts of tissue but trabecular bone depends on the quality of it.

It is concluded that chronically administered F as a single daily dose, produce significant decrease in biomechanical properties of bone by changing the quality and bone volume; although BMD is not affected.

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