## 4 Discussion
However, the manufacturer had replaced a majority of the TEGDMA with a blend of UDMA and Bis-EMA in micro-hybrid Filtek Z250 when compared to packable Filtek P60. Both packable Filtek P60 and micro-hybrid Filtek Z250 also contain zirconia/silica inorganic filler with a particle size range of 0.01 to 3.5 um. The inorganic filler percentage is 80% wt and 61% by volume for Filtek P60, and 82% wt and 60% by volume for Filtek Z250, respectively [Table 1]. Although both resins had similar filler types and had the same molecules in their organic matrices, the composition of the matrix and their fillers was different, so their DCs were found to be different. It was stated that the total replacement of BisGMA by BisEMA in composites with TEGDMA resulted in higher conversion. That a higher DC value was obtained for Filtek P60 than Filtek Z 250 might be explained by the presence of UDMA and Bis-EMA in the organic structure of these RBCs [16; 37]. While RBCs polymerize, some part of the light is reflected from the RBC surface, and the other part of the light passing through the RBC is absorbed or scattered depending on the particle size of the fillers as well as the refractive indices of the resin matrix and the fillers [1]. As the particle size of the fillers in the RBC approaches the wavelength of the activating light, the light scattering within the RBC increases. This scattering reduces the amount of light transmitted through the RBC and causes the DC to decrease in the deepest layers [26]. Smaller particles emit and scatter light more than larger particles, making polymerization more difficult for light to penetrate deeper into the material and requiring greater irradiance or longer exposure times to cure small-particle RBCs. It has been shown that larger particles and increased filler content increase mechanical properties as they are least affected by light scattering [26]. Manufacturers claim that bulk-fill RBCs have higher light transmission properties due to a reduced light scattering at the filler-matrix interface by increasing the filler size or reducing the filler amount, which enables restoration to build up in thick layers up to 4-5 mm [40]. In this study, bulk-fill Filtek P60 placed with all placement techniques up to 8 mm showed higher DCs than the other RBCs. The resin matrix of the ormocer Admira comprises inorganic-organic copolymer ormocers, Bis-GMA, urethane (aliphatic and aromatic) DMA, BHT, and TEGDMA. The ormocer Admira contains Ba-Al-B-silicate glass and SiO2 inorganic filler particles, with a particle size range of 0.04 to 1.2 \(\upmu\)m (average particle size is 0.7 \(\upmu\)m) and a filler loading of 78% wt and 56% by volume [Table 1]. The hybrid Tetric Ceram and flowable Tetric Flow also have the same molecules in their organic matrixes, which are composed of Bis-GMA, UDMA, and TEGDMA. They also contain the same types of inorganic fillers (barium glass, ytterbium trifluoride, Ba-Al-fluorosilicate glass, highly dispersed silicon dioxide, and spheroid mixed oxide) with a particle size range of 0.04 to 3 \(\upmu\)m (average particle size is 0.7 \(\upmu\)m). The inorganic filler loading is 79% wt and 60% by volume for Tetric Ceram and 64.6% wt and 39.7% by volume for Tetric Flow, respectively [Table 1]. [PERSON] and [PERSON] (2003) reported that the flowable RBC showed the lowest cure depth compared to micro-hybrid RBCs [34]. This result is in corroboration with the findings of the present study, in which flowable RBCs revealed the lowest DC compared with the packable, micro-hybrid, ormocer, and hybrid RBCs tested. [PERSON] and [PERSON] explained that this finding may be due to the filler content of the flowable RBCs being below a certain percentage [34]. Despite packable Filtek P60 and micro-hybrid Filtek Z250, and hybrid Tetric Ceram and flowable Tetric Flow having the same molecules in their organic matrices and having the same filler type and loading, their DCs were found to be different.