Colour assessment of bleaching effect on orthodontically treated teeth

To assess whether the efficacy of external tooth bleaching differs between untreated and orthodontically treated teeth. Three groups of subjects were formed; group Debonded included 24 consecutive orthodontically treated patients evaluated immediately after removal of fixed appliances. The Retention group included 24 consecutive orthodontically treated patients in the phase of retention. The Untreated group consisted of 24 arbitrarily selected undergraduate dental students without history of fixed-appliances orthodontic therapy. Each of the above three groups was further randomized into two subgroups, Bleaching and Placebo. Each subgroup received either a 38% hydrogen peroxide bleaching treatment or a placebo agent, respectively. Tooth colour changes were assessed at seven timepoints: 1 day before intervention; on the day of intervention before and after treatment; and at 7, 14, 21, and 90 days for parameters L*, a*, b*, and ΔΕ in all upper incisors and canines with the use of a reflectance spectrophotometer. Intra-rater agreement was estimated with the Intra-class Correlation Coefficient and method's error was calculated using the Repeatability Coefficient. Regarding tooth colour changes, an analysis of variance was used to assess differences between groups. Bleaching was associated with an increase of the L* value and decrease of b* and a* values in both orthodontically treated and untreated teeth. Parameters ΔE, L*, and b* exhibited statistically significant differences between the Bleaching-Untreated and Bleaching-Retention subgroups. In the Bleaching subgroups, statistically significant differences were found between different teeth for all parameters. Previous exposure to fixed orthodontic appliance influenced the efficacy of external tooth bleaching. The effect of bleaching was higher after orthodontic treatment and with longer period in retention. Canines changed in colour more than incisors, and the effect was reduced over time.