Effects of curing modes on depth of cure and microtensile bond strength of bulk fill composites to dentin

Abstract Objectives To compare the microtensile bond strength (µTBS) and depth of cure (DOC) of bulk-fill composites cured by monowave (MW) and polywave (PW) LED units using different curing times. Methodology Three composites were tested: Tetric EvoCeram Bulk Fill (TBF), Filtek Bulk Fill (FBF), and Tetric EvoCeram (T; control). Flat dentin surfaces treated with adhesive (AdheSE Universal®, Ivoclar Vivadent) were bonded with 4 mm cylindrical samples of each bulk-fill composite material (n=6) and cured with monowave (Satelec) or polywave (Bluephase Style) curing units for 10 or 20 seconds. After 24 hours, teeth were sectioned into individual 0.9 mm2 beams and tested for µTBS. Failure modes were analysed. Moreover, the DOC scrape test (IOS 4090) was completed (n=5) following the same curing protocols. Two-way ANOVA (a=0.05) was performed, isolating light-curing units. Results For samples cured with the MW light-curing unit, no significant effects were observed in the µTBS results between any of the resin composite brands and the curing times. Conversely, when resins were cured with a PW light unit, a significant effect was observed for TBF resin. In general, bulk-fill composites presented greater DOC and longer curing time resulted in higher DOC for all composites. Conclusion The µTBS of the composites to dentin was not affected by the curing mode of the resins, except for TBF cured with PW light unit. Bulk-fill composites exhibit greater DOC than conventional resin-based composites.


Introduction
With the trend moving away from the use of amalgam restorations, the demand for resin-based direct restorative materials has increased. 1 Their bonding capability, relative stability, and acceptable clinical performance in the oral environment makes these materials well-suited to minimally invasive restorative procedures. Resin composite development has evolved, with changes in fillers, monomers, and/ or curing systems. 2 The recent introduction of bulkfill resin composites aimed to streamline the clinical application of resin composites by accommodating curing in 4-5 mm increments. Bulk-fill composites have been designed to enhance light transmittance and depth of cure (DOC) compared to conventional resin composites. 3,4 Although these modifications vary, most manufacturers aim for a more translucent material with enhanced curing capability through filler modifications, 2,3 incorporation of high molecular weight monomers, and/or addition of new alternative photoinitiators. 2,[5][6][7][8] Several commercially available bulk-fill materials have increased filler size or decreased filler content to minimize the scattering of light, thus encouraging light transmittance. 2,7 Moreover, modifications to monomers and photoinitiator targets improved optical properties, reduced polymerization shrinkage, and increased DOC. [8][9][10][11][12] Bulk-fill composites are commercially available in different viscosities, with different clinical applications; however, the impact of those modifications on performance is not yet fully understood. In addition to camphorquinone in bulk-fill composites, the use of alternative photoinitiator systems has been reported as the main factor contributing to enhanced DOC. 12 Alternative photoinitiators such as Ivocerin, Irgacure 819, and OPPI (onium compound p-octyloxy-phenylphenyl-iodonium hexafluoroantimonate) have absorption peak wavelengths ranging from 290 to 330 nm, which does not match with monowave (MW) or single-peak light-emitting diode (LED) curing units (ranging from 350 to 460 nm). 13 This mismatch identifies a parallel concern related to whether singlepeak LED light-curing units can efficiently cure bulk-fill composites containing alternative photoinitiators. 14,15 However, research on the impact of single-peak LED curing units on the performance of bulk-fill composites is still scarce. 16 Several independent studies have validated the DOC on bulk-fill composites with the use of different light-curing units, [17][18][19] and the degree of conversion of bulk-fill materials. 4,11,14,16,[20][21][22][23][24] Although a few studies have investigated shear bond strength (SBS) 25

Depth of cure (DOC) scrape test
Individual unit-dose capsules of each resin composite were used as molds for the scrape test, as described by ISO 4049 specifications. 28 The capsule back-end and nozzle-end were removed and a 12 mm capsule cylinder remained. The resin composite inside each capsule was condensed against a Mylar strip on a glass slab and then cured using the same protocols described previously for the µTBS test. The uncured resin composite was immediately scraped off from the ejected cylinders with a metallic spatula (765 Premium Instrument, AISI 420, Germany). What remained of the resin composite cylinder after the scraping procedure was measured and divided by two to determine the DOC in millimeters.
Additional analysis of the resin composite cylinders was performed for all groups to assess the presence of poorly polymerized and soft resin composites, as described in previous studies. 10,29,30 Each pre-

Statistical analysis
The microtensile bond strength results were analyzed using two-way ANOVA for each light-curing unit and material independently. A significance level of α=0.05 was set for both analyses. All data were submitted to normality tests (Shapiro-Wilk), followed by the Tukey's post-hoc test for all pairwise multiple comparisons.The DOC scrape test results were analyzed using two separate approaches. First, for the pre-MEK immersion results, two-way ANOVA was independently performed for each light-curing unit, with the significance level set at α=0.05. The data was then subjected to normality tests (Shapiro-Wilk), followed by the Tukey's post-hoc test for all pairwise multiple comparisons. Subsequently, each composite was independently analyzed by paired t-tests (twotailed) with a significance level of α=0.05; this data was also subjected to normality tests (Shapiro-Wilk).
The statistical analyses for both the microtensile bond strength and DOC scrape tests were performed using Sigma Plot 13.5 (Systat Software Inc., Systat Software Inc., CA, USA) software.

Microtensile bond strength (µTBS)
The microtensile bond strength (µTBS) of the composites to dentin obtained using the PW curing Failure mode analysis    *Each light curing unit (LCU) was isolated for statistical analysis. Superscript letters represent statistical differences. Capital letters compare curing times and lower-case letters compare resin composites. *Each resin composite was isolated for statistical analysis. Superscript letters represent statistical differences.

Table 3 -Scrape test results of each resin composite (n=5) analyzed independently (Paired t-test) pre-and post-MEK immersion. Data are represented in mm (SD)
a similar trend with a higher DOC for longer curing times and bulk-fill restorative materials.

Discussion
The curing times selected for our research are consistent with those most commonly recommended for bulk-fill composites available on the market. Thus, the intent was to test both curing intervals (10 and 20 seconds) under similar radiant emittance 31 (≅960 mW/ cm 2 ) for each light-curing unit. The first null hypothesis tested was partially rejected, as significantly lower µTBS values were observed for TBF, cured for 20 seconds with the PW curing unit (Table 1). One would expect higher µTBS values for TBF cured with the PW curing unit at 10 or 20 seconds due to complete compatibility of the adhesive-composite-curing unit system, which was supplied by the same manufacturer.
However, lower µTBS values were observed for TBF cured for 20 seconds when compared with the 10-second curing time. Moreover, no differences were observed for TBF cured with either the PW or MW lightcuring unit for the 20-second curing time (Table 1) (Table 2). Moreover, these results are supported by previous investigations on the polymerization of restorative composites using 2 mm increments to allow adequate curing. 2,6,7,10 Although we did not assess the degree of conversion The scraping method, as indicated by Rueggeberg,et al. 38 (2009)   One Bulk Fill, has recently been introduced. However, most recent studies report data related to the former formulation, compatible with the bulk fill composites used in our study. 5,6,39,40 In addition, our study cannot expand on further discussion related to the potential interference of the two material composition, since they were not directly compared.
In short, our results raise concerns about the quality of polymerization of a resin composite applied in bulk and closer to the pulpal floor, especially in medium to deep cavities. Moreover, it is clear that μTBS test results are poor predictors of a well-cured composite (and vice versa), since other factors, such as material elasticity and its compliance under tensile stress 33,37 may play a significant role, requiring further studies. Furthermore, our study was designed to assess immediate results only and did not propose aging in long-term storage at 37°C or termocycling; however, the information these tests could provide is relevant and should be considered for future studies in the field.