Trans-enamel polymerization: Effect on composite hardness

Objectives: To evaluate the effect of enamel thickness and radiant exposure on composite Knoop Hardness (KH). Methods: Three pairs of proximal enamel discs from lower molars (thickness = 1mm, 2mm or 3mm) were used in this study. The discs were fixed to a metal cylinder with the proximal enamel facing the inner side of the cylinder. Composite specimens were prepared in a 3mm-wide X 3mm-high split-mold with hybrid composite resin (Filtek 250, 3M ESPE) in between two Mylar strips. Composite was applied to the enamel surface facing the external side of the metal cylinder and then cured through the enamel with one of 3 curing units: Curing Light 2500 (3M ESPE), Astralis 10 (Ivoclar Vivadent) and Freelight 2 (3M ESPE) for 10 sec, 20 sec, 40 sec, or 60 sec. Five composite specimens were prepared for each of the 48 combinations of independent variables (a 3X4X4 design). For the control group, 5 composite specimens were cured in contact with the light guide. Nine KH measurements were taken from each specimen on the composite surface facing the enamel disc, using a microhardness tester (Micromet 2004, Buehler, 100g, 15 sec). Data were analyzed with ANOVA with Bonferroni adjustment for multiple comparisons at P<0.05. Results: The 4 enamel thicknesses all resulted in statistical different KH values: 0mm=41.4±1.1; 1mm=33.8±0.8; 2mm= 26.2±0.8; 3mm=8.9±0.8. Curing time also resulted in statistically different KH values: 60sec=32.4±1.2; 40sec=28.1±0.8; 20sec=24.5±1.2; 10sec=18.3±0.8. The Astralis 10 curing unit resulted in statistically higher KH than the other units. The control group resulted in statistically greater KH than any of the trans-enamel polymerization groups. Conclusion: Curing through the enamel may not result in adequate composite polymerization.