Objectives: The popularity of over the counter (OTC) tooth whitening products stems from their availability, ease of use, affordability, and rapid mode of action in yielding desirable effects. Several published studies have reported detrimental effects on hard dental tissues due to bleaching, particularly affecting enamel hardness. Products are advertised to work ranging from few minutes single application to several days through multiple applications. Three commonly used products; Luster Pro Light by Dentovations Inc. (40 min), as well as Crest 3D Whitestrips Professional Effects (3 days) and Opalescence Tooth Whitening Gel (multiple applications) were used. The purpose of this in vitro investigation was to compare the immediate after bleaching effects on hardness of human enamel. Methods: Extracted human incisor teeth were invested into acrylic resin blocks exposing the clinical crowns. The bleaching agents were applied directly on the facial surfaces of human natural teeth according to the manufacturer's instructions. The lingual surfaces were not bleached and served as control. Teeth were sectioned and prepared for nanoindentation. Indents were done in a wet environment using a Berkovich tip at a rate of 10 nm/sec up to 500 nm. A total of 216 indents were performed on enamel tissue to calculate enamel nanohardness (H) and were statistically analyzed ANOVA (Fisher's, P<0.05). Results: MeanSD values in GPa and ANOVA comparison test results are shown below:
Conclusion: Immediate post bleaching enamel nanohardness testing revealed statistically significant differences between the 3 tested materials. The current findings indicate that enamel nanohardness is influenced differentially by various whitening agents; being greater for the Luster Pro Light group, followed by Opalescence gel and Crest Whitestrips.