# Surface Gloss, Radiopacity and Shear Bond Strength of Contemporary Universal Composite Resins
## 4 Discussion
It has been reported that the radiopacity of a CR material can exceed the radiopacity of human enamel if the filler loading is raised to 70% or above and the amount of radiopaque oxide in the filler particles is 20% [36; 37]. In a study evaluating the radiolucency of direct aesthetic restorative materials, it was reported that the ratio of the monomer that gives the radiolucent image in the CR material also affects the radiopacity [38]. The thicknesses of the specimens in this study were similar for all CRs, but measurements of mean gray values presented variable radiopacity values. Therefore, the second null hypothesis was also rejected. The detected radiopacity differences may be related to the inorganic phases of the evaluated CRs. The most radiopaque CRs were CD and NS (6.65 mm Al, 6.48 mm Al, respectively). When the tested materials were evaluated in terms of filler content percentages, CD (64%) had the lowest filler load by weight but showed the highest radiopacity and was similar to NS with a filler content of 78-80%. On the other hand, EA (82%) with the highest filler loading by weight showed lower radiopacity compared with other tested CRs (F, 76,5%; GA, 82%). Previous studies have reported that high atomic number fillers increase the radiopacity of CRs by increasing the x-ray adsorption capacity [36; 39; 40]. [PERSON] et al. [41] have stated that the radiopacity of a material tends to increase with the presence of high atomic numbered elements in the filler particles. It was observed that the universal CRs used in this study included ytterbium, barium and zirconium with atomic numbers of 70, 56 and 40, respectively. In line with the findings of the study, the fact that the filler ratio is not directly proportional to the radiopacity values may be related to the atomic number of the filler type used. In the present study, besides the gloss and radiopacity, the bonding capacity of the tested CRs to enamel and dentin with their respective adhesive systems were also evaluated. All universal adhesive systems were applied in self-etch mode in accordance with the manufacturers' instructions and the third null hypothesis--that there would be no differences among the CRs in terms of SBS to enamel and dentin--was also rejected. The differences in the composition of the adhesives may be the reason for their different bond strength performances. Two systematic review and meta-analysis studies [16; 42] have reported that bond strength was advanced by the use of mild universal adhesives with prior acid etching of enamel; however, this effect was not found to be significant for dentin according to the in vitro literature. The findings of this study also support this report, as the tested CRs used with universal adhesives showed higher SBS to dentin substrate compared with enamel in self-etch mode. One of the controversial issues of universal adhesives is that the increase in surface area of the enamel substrate is lower than that caused by the application of phosphoric acid. This effect is related to the pH of the adhesive [43; 44]. The pH of the GPB (pH: 1.5) used with the GA was lower than that of the other adhesives used with the other CRs tested and the bond strength to enamel was the highest, followed by SBU used with F (pH: 2.7) and then TUB used with EA (pH: 2.2) (\(p>0.05\)). Additionally, GPB contains 4-MET and 10-MDTP as functional monomers in addition to 10-methacryloyloxideyl dihydrogen phosphate (10-MDP). [PERSON] et al. [45] have reported that 4-MET creates strong chemical bonds due to the calcium content of the enamel. GPB also contains acetone as a solvent, with its high volatility and hygroscopic nature [46], which can facilitate the permeability of resin monomers. On the other hand, SBU and TUB, which showed similar enamel bond strengths, contain acetone and ethanol, respectively. Therefore, it may not be accurate to interpret the bonding values of CRs by evaluating only the pH value of the adhesive and the solvents in its content.