# Air-Polishing Powders' Effect on the Color of CAD/CAM Restorative Materials
## 4 Discussion  
In this in vitro study, the effect of five different APPs on the color change in five different CAD/CAM restorative materials was evaluated, and statistically significant differences were found (\(p<0.05\), \(p<0.001\)). According to the results, both the first and second parts of the null hypothesis were rejected since the color of CAD/CAM restorative materials changed in different amounts after applying different APPs. Based on the results, the \(\Delta\)E\({}_{00}\) values of the LU samples that were exposed to erythroid and the CS samples that were exposed to glycine were significantly lower than those exposed to calcium carbonate and aluminum trihydroxide. Considering the APPs, calcium carbonate has the highest density, and aluminum trihydroxide has the highest hardness value (Table 2) and the largest average particle size, as seen in the SEM images in Figure 2. On the other hand, erythritol has the lowest density, the lowest hardness value, and the smallest average particle size, and glycine follows subsequently. Previous studies reported that the APP becomes more abrasive as the mean particle size, hardness, and density increase [21; 23; 26]. In addition, as surface roughness increases, gloss decreases, affecting color perception [13; 19; 34; 45; 47]. Therefore, the color changes in the restorative materials can be attributed to the surface roughness that air-polishing causes, as can be seen in  
Figure 4: SEM micrographs of Paradigm MZ 100 specimens with a half-air-polished, half-non-polished surface at 200\(\times\). Figure 5: SEM micrographs of Lava Ultimate specimens with a half-air-polished, half-non-polished surface at 200\(\times\). Figure 6: SEM micrographs of Cerasmart specimens with a half-air-polished, half-non-polished surface at 200\(\times\). Figure 7: SEM micrographs of Vita Enamic specimens with a half-air-polished, half-non-polished surface at 200\(\times\). the SEM images of the CAD/CAM restorative materials (Figures 3-7). For instance, the LU (Figure 5) and CS (Figure 6) samples treated with calcium carbonate and aluminum trihydroxide show apparent porosity in the SEM images compared with the other APPs, and this can explain the significant color change. Furthermore, [PERSON] et al. and [PERSON] et al. reported that in terms of abrasiveness, glycine! sodium bicarbonate! calcium carbonate! aluminum trihydroxide [24; 29]. This result corresponds to the amount of color change caused by the APPs that are used in the present study. In terms of color change according to the type of CAD/CAM restorative material, the \(\Delta\)E\({}_{00}\) values of the MZ samples were significantly higher than the LU and VE samples treated with sodium bicarbonate or aluminum trihydroxide. Previous research reported that air-polishing with sodium bicarbonate or aluminum trihydroxide increases the roughness of resin composites, and the higher \(\Delta\)E\({}_{00}\) values of the MZ samples could be partially attributed to this [25; 32]. In addition, [PERSON] et al. revealed that an increase in filler size and replacement of the fillers in restorative materials after air-polishing might increase roughness [31]. The filler size of LU is much smaller than MZ, which probably caused less roughness of LU and, thereby, less color change. Among the calcium carbonate-treated groups in this study, the \(\Delta\)E\({}_{00}\) values of the VM samples were found to be significantly lower than those of CS and MZ, which is parallel with the results of a recent study reporting that glass-ceramic materials have significantly better color stability than resin composites [48]. In addition, among the glycine-treated groups, the color change in the MZ samples was significantly higher than the other restorative materials, which is due to the differences in composition and microstructure of the restorative materials [13; 48]. However, when the SEM images of the CAD/CAM blocks were analyzed, the glycine-treated VE samples (Figure 7) seemed more porous than the glycine-treated MZ samples (Figure 4).