Effect of Incremental Versus Bulk Placement of Composite Resin on Depth of Cure.

Objectives: To test the depth of cure of bulk-fill resin composite compared to nanohybrid resin composite placed in the incremental versus a bulk technique.
Methods: Deep cavity preparation was simulated using a mold (4mm x 6mm). Three types of resin-based composites were used (Filtek One Bulk Restorative, Filtek Supreme Ultra, and Filtek Bulk Fill Flowable). Each material was placed using both incremental and bulk techniques. Samples were light-cured for 20 seconds per 2mm increment for incremental and 20 seconds for bulk technique at 1200mW/cm². Uncured composite was removed and samples were stored in distilled water. Depth of cure was measured immediately and microhardness (top and bottom) were measured using the Knoop Microhardness (KHN) test at 24 hours and 7 days post-cure. Data were statistically analyzed using ANOVA 2-way test (α=0.05).
Results: Statistically significant decrease in depth of cure of all three composites when used in the bulk technique. Highest depth of cure for Filtek Bulk Fill Flowable (5.59mm). At 24 hours, KHN was significantly lower at the bottom of all samples placed in a bulk technique (8.08; 10.55; 10.09) when compared to the bottom of an incrementally placed (58.70; 39.87; 22.93) respectively. KHN significantly higher at the top of nano-composite incremental (48.0) and bulk (54.91) techniques when compared to bulk-fill high viscosity incremental (35.8) and bulk (37.45) and bulk-fill low viscosity incremental (16.16) and bulk (18.14); p<0.05. At 7 days post-cure KHN was slightly higher for most samples.
Conclusions: Nano-composites cannot be placed in increments larger than 4.66mm due to inadequate cure. Bulk-fill composites never resulted in higher hardness values than those achieved when the nano-composite was placed incrementally. Bulk-fill composites placed in a bulk technique may significantly decrease chair time, however, they result in significantly lower hardness at the deepest parts of the restoration when compared to an incrementally placed nano-composite.