Effect of phosphoric acid concentration on wet-bonding to etched dentin

This study was conducted to investigate the influence of phosphoric acid (H3PO44) demineralization of dentin during smear layer removal prior to dentin bonding.

Bovine dentin was pre-treated with either 10 wt% or 35 wt% phosphoric acid for 30 s. Substrates were then rinsed and either kept moist or air-dried before a light-cured bonding system was applied. The adhesive system consisted of a 5 wt% 4-methacryloyloxyethyl trimellitate anhydride (4-META) in acetone primer, and a photocured bonding agent, 4-META/triethylene glycol dimethacrylate-camphorquinone/N-phenylglycine (4-META/TEGDMA-CQ/NPG). Tensile bond strengths of bonded specimens were measured, and specimens were examined with a scanning electron microscope (SEM). Data obtained were subjected to two-way ANOVA and Duncan's New Multiple Range test to examine statistically significant differences (p < 0.05).

Improved tensile bond strengths were measured when the 4-META/acetone primer was applied to wet dentin that had been etched with 10 wt% H3PO4 compared with similarly etched but dry dentin samples. However, when the dentin was etched with 35 wt% H3PO4, there was no significant difference in tensile bond strength between wet and dry specimens. SEM examination of the fractured surfaces revealed a number of cohesive failures in the demineralized dentin. Microscopic examination of cross sections of these surfaces indicated that resin impregnation was inadequate and that the hybrid layers in the cohesively fractured specimens were defective.

Phenomena observed in this investigation suggest that the channels between the H3PO4-denatured collagen fibers were difficult to preserve, so adhesive monomer impregnation of these substrates was impaired in a considerable number of test specimens.