# Air-Polishing Powders' Effect on the Color of CAD/CAM Restorative Materials
## 4 Discussion
There might be two reasons why the MZ samples do not appear significantly rougher than the other blocks in SEM images (Figures 3-7) but their \(\Delta\)E\({}_{00}\) values are significantly higher: 1--when the glycine powder is sprayed on VE, it might have roughened the surface and splashed, but may have partially penetrated into the MZ sample owing to MZ's lower hardness value compared with the other blocks in this study, and glycine's tiny size, thereby changed the color and 2--the SEM images' magnification is 200\(\times\), and this might not be enough to show the exact surface topography that the small-sized APPs created [49]. In this study, to determine the color difference, the CIEDE2000 system, which is the most recent formula that CIE officially suggests, is used because the CIELAB system is not sufficient for determining especially small color differences [35; 44]. The \(\Delta\)E\({}_{00}\) values of this study were interpreted according to the literature reports on visual thresholds, which provide guidance on the clinical relevance of the results [35]. According to the 50:50% CIEDE2000 acceptability thresholds for \(\Delta\)E\({}_{00}\) = 1.8 [44], \(\Delta L^{\prime}\) = 2.92, \(\Delta C^{\prime}\) = 2.52, and \(\Delta H^{\prime}\) = 1.90 [35], all groups had acceptable color changes. However, according to the 50:50% perceptibility threshold (\(\Delta\)E\({}_{00}\) = 0.8) [44], the VM samples air-polished with aluminum trihydroxide or erythroid; the MZ samples treated with any of the APPs; the LU samples exposed to calcium carbonate; the CS samples air-polished with any of the APPs, except glycine; and the VE samples treated with calcium carbonate or erythroid demonstrated \(\Delta\)E\({}_{00}\) values above the perceptibility threshold. Regarding these results, glycine seems safe to use on restorative materials, except MZ, a resin composite. [PERSON] et al. also reported that glycine is the most efficient in removing biofilm from natural teeth and restorative materials, and it causes the least surface damage compared with the other powders [15]. Another study revealed that glycine and erythird have a similar surface-damaging potential [22]. However, despite its small particle size, as seen in the SEM image (Figure 2), and its low density and hardness, though statistically insignificant, erythroid caused a sufficient color change to exceed the perceptibility threshold of all restorative materials except LU. This effect of erythroid might be because more particles may have struck the sample per unit of time owing to its small and uniform particle size (Figure 2) [37]. In addition, LU seems to be discolored the least among the other CAD/CAM blocks, which is probably due to its small filler size, which sustains better resistance to abrasion due to the quality of the interfacial bonding between the small fillers and the resin matrix [21]. It is reported that large and insufficiently embedded filler particles usually result in a greater loss of volume. In contrast, small filler particles demonstrate higher resistance to air-polishing abrasion and a more regular microstructure, which also leads to better chemical and optical properties [21]. Furthermore, LU is composed of aggregated nanoclusters, and owing to that, polishing causes drifting of the nanofillers instead of turning out of the whole cluster, which provides better resistance to wear [28]. Moreover, [PERSON] et al. revealed that sodium bicarbonate and glycine are safe for resin composites, whereas aluminum trihydroxide and calcium carbonate are not [24]. However, in this study, it was found that in terms of color stability, sodium bicarbonate and glycine are also not safe to use on MZ resin composite blocks. In addition, the \(\Delta\)E\({}_{00}\) values of the MZ samples were above the perceptibility threshold in all the APP groups, probably due to their organic components and relatively big filler particles [21].