Objectives The aims of this study were to investigate an alternative pre-treatment method for resin infiltration using 37% H 3 PO 4 with a brush applicator and to evaluate the penetration effect of the infiltrant for masking the natural white spot lesions (WSLs) in human teeth. Methods Seventy extracted human molars and pre-molars with non-cavitated WSLs were collected. Thirty teeth met criteria of ICDAS code 2, and were sectioned, providing a total of sixty paired halves. For the control group, 15% HCl gel was applied for 120 s, and 37% H 3 PO 4 , gel was gently rubbed with a brush applicator for 30 s to the experimental group. Also, to evaluate the penetration effect of the infiltrant by pre-treatment, the specimens were treated with the infiltrant (Icon ® ). Thicknesses of the removed surfaces and percentages of the infiltrated areas (IA%) were evaluated by CLSM, and micro-morphological changes were observed by SEM. Results The mean thicknesses of removed surface layers were significantly different between the control group (36 ± 7.62 μm) and the experimental group (13 ± 2.76 μm) (p < 0.001). But, the means of IA% were similar in both groups (p > 0.05). In the SEM images, the prism cores were preferentially dissolved in the control group, while the prism peripheries were preferentially dissolved in the experimental group. Conclusions Applying 37% H 3 PO 4 gel with an applicator brush for 30 s could increase the permeability and minimize removal of the surface layer of natural WSLs. Moreover, the effect of resin infiltration was similar to the control group which was pretreated 15% HCl gel for 120 s in vitro study. Clinical significance For resin infiltration, applying 37% H 3 PO 4 gel with a brush applicator can preserve the protective surface layers of the WSLs with reduced application time.
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