Internal Porosity and Surface Roughness of Bulk Fill Resin Composites

Objectives: To examine the effects of the light-curing unit (LCU) and lubricants on the internal porosity and surface roughness of bulk fill resin-based composites (RBCs).
Methods: A clinically relevant mold (Ultradent) that mimics a mesio–occluso–distal (MOD) cavity preparation, 7 mm deep interproximal boxes, 11.5 mm wide, and 5 mm deep at the center, was used to prepare 28 samples of RBC. The flowable bulk fill RBC (3M Filtek Bulkfill Flowable) filled most of the mold, leaving the last 2 mm to be filled incrementally with bulk fill regular RBC (3M Filtek One Bulk Fill). Between each increment, one lubricant was applied: control (no lubricant), uncured Primer & Bond N (Dentsply), 95% ethyl alcohol, and wetting resin (Seamfree) along with either Bluephase Style 20i (BP) used for 20 s, or Monet Laser (ML) used for 1 s for the regular bulk fill and 3 s for the flowable bulk fill. The internal porosity and closed pores were detected by using micro-CT (Bruker). The surface roughness (Ra) was measured sidewise within 2 mm (top), 4 mm (middle), and 6 mm (deep interproximal boxes). A full factorial DOE design was used to analyze the data. Two-way ANOVA and Tukey tests were used to further analyze the Ra values.
Results: The absence of lubricant (P=0.060444) and its interaction with the LCUs (P=0.033401) significantly increased the porosity within the RBC. Bulk fill RBCs cured with any LCU ranged between 6256 and 18681 when no lubricant was applied, whereas the porosity was lower, 580 to 4261, when a lubricant was used. Only the LCUs had a significant effect on the amount of porosity. None of the factors had a significant effect on the Ra values across the evaluated areas. Two-way ANOVA revealed that the Ra values at the top and middle surfaces of the RBCs were notably higher than at the deepest surfaces.
Conclusions: The use of a lubricant and the choice of the LCU significantly influenced the porosity in RBCs, whereas Ra values varied significantly across different surfaces of the RBCs, with the top and middle surfaces exhibiting the highest Ra. A suboptimal handling of the bulk fill RBC used in increments can introduce irregularities into the material, leading to greater surface roughness. Also, the lubricant applied in between each RBC increment could have contributed to a higher surface roughness.