In vitro evaluation of single application per session of two-bottle bleaching gels

To evaluate the hydrogen peroxide (HP) permeability into the pulp chamber, bleaching efficacy, physicochemical properties (pH and HP concentration), and enamel surface effects of two-bottle in-office bleaching gels applied in 30-minute sessions.

Sixty sound human premolars and thirty molars were randomly assigned into six groups according to the in-office bleaching protocol: DSP White Clinic 35 %, DSP [WC]; Pola Office, SDI [PO]; Potenza Bianco Pro 35 %, PHS [PB]; Total Blanc One Step Drop 35 %, DFL [TD]; Whiteness HP, FGM [WH]; and Whiteness HP Maxx, FGM [WM]. All gels were applied in three 30-min sessions, with seven-day intervals. HP permeability (μg mL^-1) into the pulp chamber was determined using UV-Vis spectrophotometry. Bleaching efficacy was assessed by digital spectrophotometer (ΔE₀₀, ΔWI_D, ΔL*, Δa*, and Δb*). pH and HP concentration were measured using a pH meter and titration, respectively. Enamel surface analysis was performed by scanning electron microscopy, and microhardness (VHN) and roughness (Ra) were measured. Statistical analysis was performed using two-way ANOVA and Tukey's post-hoc (α = 0.05).

PO exhibited the highest diffusion of HP (p < 0.01), while TD showed the lowest (p < 0.01). All gels demonstrated progressive bleaching over the sessions (p < 0.0001), with no significant differences among groups (p > 0.05). Most gels exhibited initial acidic pH, whereas TD maintained neutral pH; after 30 minutes, all presented unstable pH. HP concentration decreased after 30 min but remained above 30 % active HP in all gels. All gels induced detectable enamel surface alterations, with PO exhibiting the most pronounced irregularities and TD the least morphological changes. Enamel microhardness decreased and surface roughness increased after bleaching, regardless of the gel used (p < 0.001).

A 30-min application per session was sufficient to promote bleaching efficacy and morphological changes in all groups. However, only the TD exhibited significantly lower HP diffusion into the pulp chamber, due to its more neutral initial pH.

The two-bottle gels should still be used according to the manufacturer's recommendations for renewal. Although they all have similar bleaching efficacy and morphological changes, only the TD gel had a lower amount of HP in the pulp, due to its more initial neutral pH, and may allow a single application per session.