Effect of pH challenge on the micro-hardness of artificially induced de-mineralized enamel treated with resin infiltrant and fluoride varnish

The purpose of this study was to evaluate the micro-hardness of artificially induced de-mineralized enamel after application of resin infiltrant and fluoride varnish and after exposure to pH challenges.
Material and methods: In the current study; sound permanent bovine incisors were used. A total of 40 sound enamel were then embedded in pre-cut metal cylinders. Two layers of acid-resistant nail varnish were applied to cover most of the enamel surfaces, leaving a window of 4×4 mm for demineralization. Each specimen was immersed in 32 ml of a de-mineralizing solution which contains 50 mM acetate buffer solution and 1.28 mM Ca(NO3)2_4H2O, 0.74 mM (NaH2PO4)_2H2O, and 0.03 ppm F at pH 5.0,10 for 24 hours at 37 C. The samples were then divided into two groups according to the material used to treat the de-mineralized enamel, each group consisted of 20 samples. Group 1: The samples of de-mineralized enamel were infiltrated with resin infiltration. Then the micro-hardness was recorded for all resin infiltrated samples before pH cycling challenge. The samples were then submitted to a pH cycling model at 37 C over 7 days. The pH cycling consisted of immersion of the samples in 35.5 ml of de-mineralizing solution: (2.0 mmol/ L Ca, 2.0 mmol/ L P, 0.075 mol/ L acetate buffer, 2.22 ml/mm2 of enamel surface) for 6 hours, alternated with immersion in 17.75 ml of re-mineralizing solution: (1.5 mmol/ L Ca, 0.9 mmol/ L P, 0.15 mol/ L KCl, 0.02 mol/ L cacodylate buffer, pH 7.0, 0.25 mL/mm2) for 18 hours for 5 days. The specimens were then kept for 2 more days in a fresh re-mineralizing solution, which completed 7 days of treatment. The samples were then washed in de-ionized water for 30 seconds among de-mineralizing and re-mineralizing cycles. Group 2: Fluoride varnish (NUPRO®WhiteVarnish, DENTSPLY) was applied as a thin layer by a brush and totally dried, then the micro-hardness was recorded [32] . The samples were then subjected to a pH cycling as in group 1. Then after challenge the micro-hardness measurements were performed as formerly described.
Results: The difference was highly significant between resin infiltrant and fluoride varnish treated enamel. The comparison among micro-hardness values of initial, de-mineralized enamel, resin-infiltrated enamel and resin infiltrated after pH cycling showed that there was a significant difference. The difference between micro-hardness values was highly significant when comparison was accomplished between initial and de-mineralized enamel ,initial and after pH cycling, de-mineralized enamel and resin infiltrated enamel ,de-mineralized enamel and after pH cycling and finally between resin infiltrated enamel and after pH cycling. The difference between micro-hardness values was not significant regarding initial and resin infiltrated enamel.
Conclusion: Under the limitations of the present study, it was concluded that the micro-hardness of resin infiltrated enamel was higher than that of de-mineralized enamel treated with fluoride varnish before and after pH cycling.