Effect of repeated bleaching by low hydrogen peroxide regimens – with and without MI Paste Plus – on enamel hardness and composition

ObjectivesThe aim of the present study was to test the following hypotheses: 1. In-vitro repeated bleaching by low hydrogen peroxide (HP) regimens –at 2-month interval- will affect enamel hardness and composition. 2. The use of MI Paste Plus, GC Co. (MIPP) during bleaching treatments will reduce these possible enamel changes.MethodsTwo 14-day bleaching regimens; 1 h/day of 10% carbamide peroxide (CP) gel; Opalescence PF (OpPF) and 30 min/day of 9.5% HP strips; Crest Whitestrips Advanced Seal (CWAS) were evaluated. Each bleaching regimen was applied on enamel three times, separated by 2 months of artificial saliva with or without MIPP regimen. Surface treatment groups (n = 10/group) included: (1) Artificial saliva (control) (2) OpPF, (3) CWAS (4) OpPF-MIPP, and (5) CWAS-MIPP. Vicker Hardness (VH) was repeatedly measured for all groups; at baseline and after 2, 12 and 22 weeks (after each bleaching treatment). The final enamel surface of each group was chemically analyzed using Fourier Transform Infrared Spectroscopy (FTIR).ResultsThe median VHs (Kg/mm2) were: (1) 310, 312, 330, 343; (2) 299, 272, 283, 264; (3) 320, 277, 271, 255; (4) 300, 311, 304, 316, and (5) 305, 315, 309, 320, respectively. Friedman repeated measures ANOVA on ranks (Dunnett method) indicated that enamel VH was significantly reduced after the third OpPF treatment and after the second and third CWAS treatments. Within OpPF-TMP and CWAS-TMP groups, no significant differences were found among VHs measured at different time intervals (p < 0.05). FTIR of the final enamel surfaces revealed decreased phosphate group intensity; enamel mineral content, for all groups except OpPF-MIPP group where its intensity increased.SignificanceRepeating of low HP bleaching regimens – at short intervals – may reduce enamel hardness and mineral content. Daily over-night use of MIPP may preserve enamel hardness and mineral content only if the OpPF regimen is used.