Hard and soft toothbrushes: effect on human enamel in vitro

Objective: to evaluate the effect of the stiffness of the toothbrush bristles, with and without a low RDA dentifrice, on human dental enamel in vitro. Methods: Ethical approval (proc.#011292/2006-51) came from the Research Ethics Committee of the Federal University of Alagoas, Brazil. Human enamel blocks (3x2x2mm) were cut from the buccal and lingual surfaces of extracted, for clinical reasons, 3rd molars. The enamel surfaces of the blocks were sequentially polished with sand paper, grit 400 through 1200 and final polishing was achieved with an alumina solution (grain of 1 and 0.3 µm) (Arotec, São Paulo, Brazil), using a polishing machine. The baseline mean surface microhardness (MH) of the blocks were evaluated by averaging the length of 3 indentations (50g load/15seconds) made at the surface of each block. Surface roughness (Ra) of each block was then evaluated using atomic force microscopy. The mean Ra of 5 images (50x50µm) from each block was considered the baseline Ra of the block. The blocks were then randomly assigned for one of the 4 experimental groups (n=6/group): a) soft toothbrush and water; b) soft toothbrush and dentifrice (RDA=60, supplied by Colgate-Palmolive); c) hard toothbrush and water; d) hard toothbrush and dentifrice (RDA=60). Using an automated toothbrushing machine with a 200g load, each block was submitted to 15000 toothbrushing cycles after which the enamel blocks were immediately re-analyzed, as before, for MH and Ra. The difference in mean MH and Ra, before and after toothbrushing, were statistically analyzed using analysis of variance (ANOVA) and the Schèfé's test. Results: There were no significant changes in MH between treatment-groups (range -0.791 to 5.019µm). Regarding the Ra, group c was significantly different from group a (p<0.05, ANOVA and Schèfé's test). Conclusion: we may conclude that the effects of the stiffness of the toothbrush bristles on enamel are minor.