Microhardness of different thicknesses of bulk fill composites

To compare the microhardness values and bottom/top hardness ratios of different composites after being cured in 2 or 4 mm increments.

Two bulk fill composites, methacrylate-based and ormocer-based, and one conventional composite were tested. 36 cylindrical discs were prepared (n=12/composite, with six for 2 mm, and six for 4 mm thickness) by pressing each composite into a mold between two glass slides covered by Mylar strips. The top and bottom surfaces of each sample were evaluated using a Buehler hardness tester for Knoop microhardness, with a 50 g static load applied for 10 seconds at three different locations of the central part of each sample. The bottom/top hardness ratio was calculated for each sample. The Knoop microhardness data and bottom/top ratio percentages were analyzed using two-way repeated measures ANOVA and Holm-Sidak post hoc test, with significance at P< 0.05.

The tested methacrylate-based bulk fill had the highest overall microhardness among the three tested composites. All three composite types showed a significant difference in microhardness between the top and bottom of the 4 mm discs. The bottom/top ratio percentages differed significantly for both tested bulk fill composites across different thicknesses. Both tested bulk fill materials had a bottom/top ratio of ≥ 80% at the deepest level of a 4 mm increment.

The type of material significantly affected both the bottom and top microhardness values under the experimental conditions used. While both tested bulk fill composites showed a decline in microhardness at the bottom of the 4 mm depth, they had clinically acceptable microhardness ratios at greater depths.