Beyond High-Speed Curing: Effects of Layer Thickness, Glycerin, and Polishing on the Surface Properties of an...

This in vitro study evaluated the effects of layer thickness, photoactivation protocol, glycerin application, and polishing on top-surface roughness and microhardness. Sixty specimens were prepared from bulk-fill (Tetric PowerFill) and conventional (IPS Empress Direct) composites. Bulk-fill specimens were 2 or 4 mm thick and photoactivated with a conventional (1200 mW/cm², 20 s) or high-speed (3000 mW/cm², 3 s) protocol, giving nominal radiant exposures of 24 and 9 J/cm²; conventional specimens were 2 mm thick and photoactivated using the conventional protocol. Glycerin was applied or omitted; all specimens were polymerized beneath a polyester strip, with no air-exposed group. Roughness and Vickers microhardness were measured exclusively on the top surface, before and after polishing, and analyzed using General Linear Models for repeated measures (α = 0.05). In the bulk-fill composite, polishing reduced roughness and increased microhardness, with significant polishing × glycerin interactions for both outcomes. In the conventional composite, glycerin increased microhardness and roughness before polishing. Polishing increased microhardness, and a significant polishing × glycerin interaction was observed for roughness. The 20 s protocol produced significantly higher top-surface microhardness than the 3 s protocol, although the difference was small (2.58 VHN); because irradiance and exposure time differed simultaneously, their individual contributions cannot be distinguished. Layer thickness did not significantly affect the top-surface properties evaluated; this cannot be extrapolated to deeper regions or taken as evidence of equivalence between the 2 and 4 mm increments. In the condition in which both composites were treated identically, the effects of glycerin and of polishing differed significantly between the materials for both outcomes; for roughness, the difference in glycerin effects further depended on polishing. With no air-exposed controls and no artificial aging, the findings are restricted to the immediate top-surface response under the conditions evaluated.