[Microscopic observation of the enamel microstructures of SD rats with different degrees of fluorosis]

Objective: To establish a dental fluorosis model of SD rats with various degrees, to observe the microstructures of enamel samples under scanning electron microscope and to clarify the changes of enamel microstructures with various degrees of dental fluorosis, so as to provide clinical reference for the treatment of patients with moderate and severe dental fluorosis. Methods: Thirty male SD rats (6 weeks of age) were randomly divided into 3 groups with 10 rats in each group. The control group was fed with deionized water without fluoride, the low fluoride group was fed with 50 mg/L NaF deionized water and the high fluoride group was fed with 100 mg/L NaF deionized water in order to establish the dental fluorosis model of rats. After feeding for 6 weeks, the rats were sacrificed and the mandibular incisor teeth were collected and recorded. The surface and sagittal plane of each tooth were observed by scanning electron microscopy and the enamel thickness was measured. Results: In the control group, the enamel color was brown yellow. Enamel color discoloration occurred both in low-fluoride group and high-fluoride group. The enamel color in low-fluoride group was mostly yellow and white striped while in high-fluoride group was mostly chalky white. Under electronic microscope, the enamel rods were alternately arranged and their structure was clear and plump in the control group. The enamel rods of moderate fluorosis were arranged in a straight orientation like tips of bamboo shoots. The enamel rods of severe fluorosis, however, became thinner and the tips of rods were broken. In the control group, sagittal images of enamel turned out to be a dense outer structure with clear boundaries among the inner. The structure of the middle layer was reticulated showing a clear boundary with middle and outer layers. The structure of enamel rods in the inner layers was arranged vertically and horizontally. In the moderate fluorosis group, the outer layer of the enamel became thinner and the middle layer disappeared although the boundary between the outer and middle layers was still clear. In the inner layer, the vertically arranged enamel rods seemed still clear, however the horizontal enamel rods disappeared. In the severe fluorosis group, the outer layer could not be traced. The middle layer was exposed to the air and the inner enamel rods contracted. The inner layers of the enamel had gradually become thinner with the development of the dental fluorosis. The thicknesses of inner layers in control, moderate and the severe groups were (180.71±7.01), (157.10±11.04) and (121.10±12.56) μm respectively. As for the thicknesses of the full layers in the above mentioned three groups, the same trend was observed. The thicknesses, in order of the severity of dental fluorosis, were (241.54±7.76), (207.42±14.36) and (143.79±14.60) μm. Conclusions: With the development of dental fluorosis, the outer enamel layers became thinner or disappeared and the inner enamel layers became thinner or lost its normal structure as well. It is highly recommended that the resin penetration could be used for the proper treatment of moderate and severe dental fluorosis and the strong bleaching and the micro-grinding should be used cautiously.