Influence of high-power light-curing modes on the mechanical and optical properties of no-cap bulk-fill resin...

The clinical performance of no-cap bulk-fill resin composites depends strongly on adequate polymerization, which influences both mechanical durability and optical stability. Although contemporary high-power light-curing units (LCUs) provide ultra-fast curing modes, evidence regarding their effects on modern bulk-fill materials remains limited. This in vitro study aimed to evaluate the influence of different polymerization modes of VALO and Bluephase PowerCure on the microhardness and color stability of no-cap bulk-fill resin composites.

Four no-cap bulk-fill resin composites (Omnichroma Bulk Flow, Charisma Bulk Flow ONE, Simplishade Bulk Fill, and Admira Fusion x-tra) were investigated. Two hundred cylindrical specimens (3 mm thickness) were prepared and polymerized using two LCUs (VALO and Bluephase PowerCure) under five curing modes (VALO Standard, High Power, and Xtra; Bluephase High Power and PowerCure). Vickers microhardness was measured on the top and bottom surfaces after 24 h, and bottom-to-top hardness ratios were calculated as an indicator of depth of cure. Baseline color was recorded and remeasured after 12-day coffee immersion at 37 °C, and color change (ΔE₀₀) was calculated. Data were analyzed using two-way ANOVA and Aligned Rank Transform ANOVA (α = 0.05).

Composite type, polymerization mode, and their interaction significantly affected bottom-to-top hardness ratio, microhardness, and color change values (p < 0.001). VALO Standard and VALO High Power generally produced higher bottom-to-top hardness ratios and bottom-surface microhardness values than the 3-second ultra-short curing modes. Admira Fusion x-tra and Simplishade Bulk Fill generally showed higher microhardness values and/or lower ΔE₀₀ values under the tested conditions, whereas Omnichroma Bulk Flow and Charisma Bulk Flow ONE showed lower bottom-to-top hardness ratios or higher ΔE₀₀ values in some ultra-short curing groups. No consistent correlation was observed between ΔE₀₀ and bottom-to-top hardness ratio across the tested material-curing mode combinations.

Polymerization mode and total radiant exposure influenced the mechanical and optical outcomes of the tested no-cap bulk-fill resin composites. Longer exposure protocols with higher radiant exposure generally resulted in more favorable bottom-to-top hardness ratios and bottom-surface microhardness values than the 3-second ultra-short curing protocols. The findings suggest that ultra-rapid curing modes may not provide equivalent outcomes for all no-cap bulk-fill resin composites. Therefore, curing protocols should be selected with consideration of material-specific behavior, exposure time, and total radiant exposure.