Time-course changes of intraosseous environment and shear resistance mechanical properties of jawbone in SD rats with fluorosis

Background At present, a large number of reports focus on the bones of limbs and trunk, while there are few studies on the effect of fluorosis on jawbone which is the inevitable structural basis for the development and treatment of oral diseases.

Objective To preliminarily investigate the effect of fluoride exposure on the mechanical properties of jawbone by observing the changes in the intraosseous environment and the maximum load against shearing force (LSF_max) of the jawbone in rats with chronic fluoride treatment.

Methods Screening experiment: 48 SD male rats were randomly divided into a control group and three fluoride exposure groups (50, 150, and 250 mg·L⁻¹ fluoride concentration), 12 rats in each group. The fluoride exposure groups were molded by feeding different concentrations of sodium fluoride solution, and the control group drank tap water from Guizhou area. Each group was further divided into 4 subgroups with 3 animals each according to observation time points after 0, 2, 4, and 6 months. The LSF_max of the jawbone was measured with an electronic universal ergometer, the expression of type I collagen (Col1) was shown by Sirius red staining, and the expression of runt-related transcription factor 2 (Runx2) was determined semi-quantitatively by immunohistochemistry at selected time points. Formal experiment: 12 male SD rats were randomly divided into a fluoride exposure group and a control group. The fluoride exposure group were fed with 150 mg·L⁻¹ sodium fluoride solution, and the control group drank tap water from Guizhou. After feeding with fluoride for 5 months, the ergometer was used to measure the LSF_max of the jawbone. Osteoclasts were counted after tartrate resistant acid phosphatase (TRAP) staining. Col1, Runx2, bone morphogenetic protein 2 (BMP-2), alkaline phosphatase (ALP), and cathepsin K (Cath K) were detected semi-quantitatively by immunohistochemistry expression and Sirius red staining. Micro computed tomography (Micro CT) was used to observe the trabecular bone microstructure.

Results Screening experiment: The LSF_max of the control group and the 50 mg·L⁻¹ fluoride exposure group reached the peak value at the 2nd month, and the LSF_max of the 50 mg·L⁻¹ fluoride exposure group reached the valley value at the 4th month. The LSF_max of the 150 mg·L⁻¹ fluoride exposure group at the 4th month was higher than that at the 6th month (P<0.05). There was no significant difference in the LSF_max at each time point in the 250 mg·L⁻¹ fluoride exposure group. At the same time point, there was no statistically significant difference in LSF_max among the groups. The Col1 levels of the 50 mg·L⁻¹, 150 mg·L⁻¹, and 250 mg·L⁻¹ fluoride exposure groups were higher than the time point 0 from the 2nd month (P＜0.05). The Runx2 showed no statistically significant difference by concentration or time. Formal experiment: After feeding with 150 mg·L⁻¹ fluoride for 5 months, the LSF_max of the fluoride exposure group was greater than that of the control group (P<0.05). The expressions of Col1, Runx2, BMP2, ALP, and Cath K in the fluorosis exposure group were higher than those in the control group (P<0.05). There were no statistically significant differences in osteoclast count or indicators of bone trabecular microstructure.

Conclusion Chronic fluoride exposure may increase the shear strength of jaw bone.