Effect of tooth bleaching on the mechanical properties of enamel

Methods: Extracted human teeth were used. Half of each sample subjected to a bleaching agent and the other half was covered with nail varnish (control). Two bleaching agents were used;16% carbamide peroxide (CP)"Polanight" and 35% hydrogen peroxide (HP) "Polaoffice⁺". Bleaching agents were applied according to the manufacturers’ instructions. CP group was treated for 90 mins daily for 14 days. 35%HP was applied four times on enamel surface for 10 mins each in a single session. Samples were stored in HBSS following bleaching treatment. Nanoindentation was used for the analysis of enamel hardness, elastic modulus and creep deformation. Vicker’s indentations were the basis for characterising the crack behaviour and fracture toughness of enamel.  Microstructural investigation of the samples using FESEM was conducted. Data were statistically analysed using ANOVA and a Tukey test.

Results: Hardness and elastic modulus of enamel were significantly (P ≥.0001) reduced (13.7% and 6.5% respectively) after treatment with CP bleaching agent.  Enamel treated with HP showed significant reduction of (~10% and 5.5%) in hardness and elastic modulus respectively. Indentation crack lengths of CP and HP bleached enamel were increased (49% and 28%) with a significant (p≤.0001) reduction of fracture toughness (~ 46% and 38.5% respectively). The creep deformation at maximum load has increased and the recovery at unloading was reduced after bleaching treatment.

Conclusions: Tooth bleaching agents can produce detrimental effects on the mechanical properties of dental enamel, possibly as a consequence of damaging or denaturing of its protein components.