Monomers with low oxygen inhibition as enamel/dentin adhesives

This study was conducted to investigate the depths of polymerization inhibition by oxygen, the shear bond strengths to enamel and dentin, and the marginal adaptation in dentin cavities of experimental adhesives containing BisGMA/HEMA of mixtures of low-inhibition BisGMA-dicarbonate with HEMA or with HEMA-carbonate at ratios of 100/0, 80/20, 60/40, 50/50, and 40/60 by weight.

The inhibition layer thickness was determined microscopically as non-polymerized surface film thickness on three samples each. For bond strength testing, composite cylinders were bonded to enamel and dentin following H3PO4 conditioning with the adhesives dissolved in acetone; marginal adaptation was evaluated in cylindrical dentin cavities. Five specimens each were stored in water for 24 h prior to testing. Shear bond strength results were compared by ANOVA and Duncan's test (p < 0.05), the maximum gap widths by Kruskal-Wallis, ANOVA and Wilcoxon's two-sample test (p < 0.01).

Inhibition layer thickness was significantly smaller for modified than for conventional BisGMA/HEMA monomers and increased significantly with HEMA content. In the BisGMA-dicarbonate/HEMA-carbonate group, the inhibited layer was thinner than 1 micron. Apart from the 40 and 50% BisGMA-dicarbonate/HEMA-carbonate mixtures, enamel bond strengths were not significantly different. The highest bond strengths to dentin were 15 and 12 MPa at 50 BisGMA and 50 BisGMA-dicarbonates/HEMA, respectively. The average bond strength for the BisGMA-dicarbonate/HEMA-carbonate group was 5 MPa. Adhesives with 40 and 60% HEMA in the conventional and 40 to 60% HEMA in the dicarbonate/HEMA group showed significantly better marginal adaptation than all others (p < 0.05).

Carbonate-modified low-inhibition monomers have no advantage as enamel/dentin adhesives compared with conventional BisGMA/HEMA-based resins.