## 4 Discussion
### Adequate Depth of Cure (DoC) Achieved by Most Composites  
As already mentioned before, CR is considered adequately cured at a specific depth (DoC) if the bottom-to-top microhardness ratio exceeds 80% [4,32,48]. Studies have consistently shown that bulk-fill composites, such as SurefiSDR and Venus Bulk Fill, achieve clinically acceptable degrees of conversion and hardness when cured with appropriate LED light-curing units (LCUs), even at greater depths [21; 22; 23; 24]. Specifically, [PERSON] et al. [20], [PERSON] and [PERSON] [19], and [PERSON] et al. [28] confirmed that bulk-fill composites could achieve a bottom-to-top hardness ratio exceeding 80% when cured at depths up to 4 mm using LED LCUs, indicating a sufficient depth of cure. These findings have been corroborated by others as well [29; 30]. However, not all studies support these results. [PERSON] et al. [11] reported that SDR Plus and SonicFill 2 did not exhibit an acceptable depth of cure when polymerized with a monowave LED following the manufacturer's instructions. On the other hand, [PERSON] et al. [31], [PERSON] et al. [32], [PERSON] et al. [33], and [PERSON] et al. [10] found no significant difference in the degree of conversion of bulk-fill composites when comparing monowave and polywave LCUs. Interestingly, [PERSON] et al. [34] reported that a quartz-tungsten-halogen (QTH) LCU produced a higher DoC than monowave or polywave LED LCUs in bleach shade regular composites. In our study, all bulk-fill composites met this criterion at a 4 mm depth when cured with all LCUs, except for SonicFill and Tetric EvoCeram Bulk Fill with the Valo LCU, as well as Tetric EvoCeram Bulk Fill and X-tra Base cured with the Bluephase Style. Composites such as SDR, Filtek Bulk Fill, and SonicFill demonstrated an 80% hardness ratio at depths greater than 4 mm. According to [PERSON] [43] the BlueStyle LCU emits 22% of its irradiation in the violet spectrum, and Valo 26%, which could be about 220 mW/cm\({}^{2}\) and 260 mW/cm\({}^{2}\), respectively, in this study. This energy does not help the polymerization process in materials with only CQ as a photoinitiator, so the spectral irradiance in the blue region (absorbed energy that can excite CQ) was 800 mW/cm\({}^{2}\) for Bluephase Style and 740 mW/cm\({}^{2}\) for Valo according to the measured radiant power in this study. Violet light cannot penetrate the composite beyond 2.5 mm [43], and therefore cannot be useful in properly curing composites, such as Tetric EvoCeram, containing alternative photoinitiators like TPO or Ivocerin at a depth of 4 mm. This could be the reason for the decreased DoC for SonicFill and Tetric EvoCeram Bulk Fill with the Valo LCU, as well as for Tetric EvoCeram Bulk Fill and X-tra Base cured with the Bluephase Style, in this study. [PERSON] et al. [55] reported that the power output of different LED LCUs can vary up to 60%, and sometimes they do not comply with the manufacturers' information, being lower. For SonicFill, which the manufacturer recommends curing up to 5 mm, our results confirm its adequacy, aligning with recent studies by [PERSON] et al. [29], [PERSON] et al. [30], and [PERSON] et al. [29], which found similar DoC results under various curing conditions. However, other research, such as that of [PERSON] et al. [56], [PERSON] et al. [57], and [PERSON] et al. [58], indicates that some composites may fail to achieve an adequate DoC under certain conditions, emphasizing the importance of selecting the appropriate LCU type and exposure parameters. In the present study, microhardness was measured at a time beyond 24 h, and as a result, higher values due to additional post-curing of the specimens were expected [20]. The 24 h post-curing hardness of bulk-fill composites was reported to be between 9% and 100%, and the materials presenting a low initial DoC presented a higher post-curing DC [59].