Fluoride and Calcium-Phosphate Effects on Fracture Toughness of Bleached Dentin

Objectives: It was the aim of this study to determine whether or not the decrease in fracture toughness (K_1C), observed after carbamide peroxide bleaching, could be either prevented or restored with pre- or post-bleaching treatments using an amorphous calcium phosphate or a fluoride gel in vitro.

Methods: Compact test specimens, approximately 4.6X4.5X1.6 mm, composed of dentin cut from the coronal aspect of recently extracted human molars, were used (n=10/group). Tooth bleach (10% carbamide peroxide, Opalescence, Ultradent Dental Products) was applied directly to the dentin (6hr/day) in a custom-made bleach tray to simulate an overnight conventional (2-weeks) or prolonged (8-weeks) bleaching regimen. The specimen-containing bleach trays were immersed in artificial saliva and stored in a 37^oC humid environment during the bleach treatments. The prevention or recovery groups received, respectively, additional daily topical pre-bleach or post-bleach treatments of either amorphous calcium phosphate (MI paste, GC America) or fluoride gel (Prevident Booster, Colgate Oral Pharmaceuticals) for 30-minutes. A placebo gel acted as a control material for each group. The dentin specimens were stored in 37^oC artificial saliva when not undergoing treatment. 24hrs after the last bleaching session, a tensile load was applied (1cm/min) until specimen fracture. K_1C values were calculated and analyzed using ANOVA and Tukey's test (p<0.05).

Results: The highest mean K_1C results were generally associated with the unbleached (placebo) groups. Preliminary results indicated that the daily amorphous calcium phosphate and fluoride gel treatments did not significantly affect the fracture toughness results.

Conclusions: The in-vitro fracture toughness of dentin was generally reduced by the direct application of a carbamide peroxide bleach. There was insufficient evidence to suggest that a daily amorphous calcium phosphate or fluoride gel treatment could prevent this reduction in K_1C. Materials were provided by Ultradent Dental Products, GC America and Colgate Oral Pharmaceuticals.