Bulk-Fill Resin-Based Composites
In 2009, the first bulk-fill RBC with flowable consistency was introduced to the market. It was indicated for use as a base in Class I and Class II cavity preparations, requiring an additional layer of 2 mm of conventional RBC on the occlusal surface. The stress decreasing resin (SDR) allows greater molecule flexibility, thus avoiding the stress generated during photocuring (Ilie & Hickel, 2011).
Later, bulk-fill RBCs with similar consistency and application methods were developed for clinical use. For Filtek7'1 BulkFill Flowable (3M ESPE), the manufacturer claims that it is based on four monomers; BisGMA, UDMA, Procrylat, and BisEMA, which have high molecular weight, to reduce the development of polymerization shrinkage. Procrylat monomer is added to allow greater fluidity and thus lower the polymerization stress (Ilie & Hickel, 2011).
For all the above RBCs, the manufacturers have not announced major changes to the polymerization system. Subsequently, a different bulk-fill RBC variation (Tetric N Ceram® Bulk Fill, Ivoclar Vivadent and x-tra fil, VOCO) was introduced, which had conventional consistency and was indicated to be used in increments of up to 4 mm without the need for an extra occlusal layer. In the case of flowable bulk-fill RBCs, the inorganic filler ranges between 64-75 w't% and 38-61 vol%. On the other hand, in most of the high-viscosity bulk-fill RBCs, this component w'as increased up to 79-86 wt% and 61-81 vol%.
Tetric N Ceram® Bulk Fill (Ivoclar Vivadent) has a new polymerization initiation booster called Ivocerin®, which is a germanium-based photoinitiator of greater reactivity than that of CQ, due to its higher absorption of visible light (400-450 nm) (Moszner, Fischer, Ganster. Liska, & Rheinberger, 2008). It was also reported that it has a filter for light pollution that ensures proper clinical w'ork time.
A third variation of bulk-fill was introduced (SonicFill™, Kerr), w'hich depends on a special sonic vibration handpiece and it is used in 5 mm increments. It is activated by means of sound vibration, producing a momentary drop in viscosity during application. This resin is also indicated for Class I and II preparations with no occlusal layer.
Properties of Bulk-Fill Composites
Generally, bulk-fill RBCs have a lower proportion of fillers with an increase in their size, which w'as claimed to decrease problems encountered wfith conventional RBCs. Multiple in vitro studies evaluated different properties for the newly introduced bulk- fill RBCs. One of the main concerns was the depth of cure of 4 mm increment, which was recommended by the manufacturers. In conventional photopolymerized RBCs, limited curing depth was found and the possibility of insufficient monomer conversion in the bottom of the cavity preparation was speculated (Lindberg, Peutzfeldt. & van Dijken, 2005). Some studies found an improved depth of cure of bulk-fill RBCs in comparison to conventional ones, while others found no improvement. Several reasons have been proposed to explain the improved depth of cure. The first suggested reason was the inclusion of more efficient initiation systems in some resins, w'hich was claimed to improve the light penetration in the RBC (Alrahlah, Silikas, & Watts, 2014). A second suggested reason is that bulk-fill RBCs have higher trans- lucency, which w'ould allow deeper light penetration (Bucuta & Ilie, 2014). Another reason is the decreased matrix-filler surface interface, which reduces light refraction and improves light penetration (Ilie, Bucuta, & Draenert, 2013c). Reduction in refractive index differences between resin and filler improved the DC (Fujita, Nishiyama, Nemoto, Okada, & Ikemi, 2005), and increased depth of cure as well as color shade matching (Shortall, Palin, & Burtscher, 2008).
Polymerization Shrinkage and Stress
It w'as found that some bulk-fill flowable RBCs were effectively cured in 4 mm bulk, but shrank more than the conventional non-flowable RBCs (Jang, Park, & Hwang, 2015). However, bulk-fill non-flowable RBC showed comparable shrinkage to conventional non-flowable RBC, but it w'as not sufficiently cured in 4 mm bulk
(Jang, Park. & Hwang, 2015). When evaluating the polymerization stress of bulk-fill RBCs, it was found to be lower than conventional RBCs and conventional flowable RBCs (El-Damanhoury & Platt, 2014; Ilie & Hickel, 2011). However, in another study, it was found that polymerization shrinkage of bulk-fill RBCs might be smaller, similar, or larger than conventional flowable RBCs (Garcia, Yaman, Dennison, & Neiva, 2014). This has shown the relationship between the proportion of filler and polymerization shrinkage, where the resins with the least amount of filler loading, and therefore with the greater proportion of resinous matrix, experienced higher levels of shrinkage by polymerization, and vice versa.
Cuspal flexure was evaluated in premolars with Class II restorations restored with bulk-fill RBCs applied in one increment; it was significantly lower than those observed in conventional RBCs applied in incremental layers (Moorthy et al., 2012). However, it is difficult to determine if the lower cusp flexure observed by using bulk-fill RBC was due to a smaller contraction of the resin or due to changes in the mode of application of the material. A study has determined the existence of a smaller change in the inter-cusp distance when RBC was placed as a single increment in comparison to the incremental method. It was reasoned that each increment would cause deformation of the cavity’s walls with the downward movement of the walls and toward the inside. This movement decreases the total volume of the cavity. In contrast, for RBC placed using the single-increment technique, the cavity volume remains relatively the same (Versluis, Douglas, Cross, & Sakaguchi, 1996).
RBC marginal adaptability is closely related to the development of polymerization shrinkage and stress. Studies found no significant differences in marginal integrity when using bulk-fill RBCs in comparison to conventional RBCs (Roggendorf, Kramer, Appelt, Naumann, & Frankenberger, 2011). Several studies evaluated the viscosity of bulk-fill RBCs in relation to different properties. It was found that the use of flowable RBCs results in better adaptation compared to packable RBCs (Opdam, Roeters, Joosten, & Veeke, 2002).