Qualitative and quantitative effects of hydrogen peroxide on enamel

Abstract

Background: A pleasing smile can be considered part of an attractive appearance and having “whiter” teeth is perceived as an integral part of achieving this. Tooth whitening provides a treatment option that is a conservative, relatively quick and inexpensive. It uses bleaching agents such as hydrogen peroxide (HP) at various concentrations that have been marketed for in-office and at home use.

Aim: The aim of this study was to investigate the effects of 30% HP and 10% carbamide peroxide (CP) on the nanomechanical properties and the protein content of enamel.

Methodology: The effect of bleaching for 30 minutes with 30% HP or eight hours with 10% CP on the mechanical properties of enamel was investigated. This was determined by measuring the hardness (H) and the elastic modulus (E) of twelve sound enamel samples using nanoindentation testing with a Berkovich tip at 25 and 250 mN force, before and after bleaching.

The effect of bleaching for 30 minutes with 30% HP on the protein content of sound enamel was also investigated. Protein was extracted from the enamel of twelve sound permanent teeth by precipitation with trichoracetic acid after dissolution of the inorganic phase and the amount of protein was quantified using the Lowry and the Bradford protein assays.

Results: The mean value for the E of enamel before and after bleaching with 30% HP was 99.08 GPa ± 3.64 and 98.63 GPa ± 6.44 respectively, and the mean value for H was 4.1 ± 0.4 GPa and 3.9 ± 0.7 GPa before and after bleaching respectively. The nanohardness tests showed no statistically significant difference in the mean values of the E and the H of enamel samples following exposure to the bleaching agent (P > 0.01).

From 0.01g of sound enamel, the mean amount of protein detected using the Lowry and the Bradford assays was 5.2 ± 1.2 µg, 2.8 ± 1 µg respectively. After surface treating the enamel with 30% hydrogen peroxide, the mean protein values using the same two protein assays was 2 ± 0.6 µg and 1.1 ± 0.4 µg respectively. These results indicate that bleaching treatment with 30% HP resulted in a significant reduction in detectable protein content (P > 0.01).

Conclusions: Bleaching sound enamel with a 30% HP for 30 minutes and 10% CP for 8 hours does not affect the microhardness of enamel despite altering its protein content. Within the limitations of this study, alteration of the protein content in mature, sound enamel by 30% HP bleaching agent did not appear to alter the mechanical properties of enamel.