## 4 Discussion  
This systematic review aimed to evaluate the influence of different treatments and conditions on the optical properties of monolithic zirconia as compared to other relevant ceramic materials. The optical properties (such as color stability, translucency, and surface gloss) of almost all the monolithic zirconia samples were significantly influenced by different treatments and conditions, including artificial, simulated, chemical, and hydrothermal aging, grinding/oclusal adjustment, glazing/external staining, toothbrushing, bleaching, and artificial gastric acid. For instance, according to [PERSON] et al. [40], a surface glaze could protect against aging impact for up to 1 year of clinical performance. Glazed monolithic zirconia demonstrated the greatest translucency parameters after and before artificial water spray aging as compared to those of only rubber polish with and without utilizing polishing paste [40]. The porosity generated on the surface after aging results in an increase in the incident light scattering that reduces translucency [52; 53]. Moreover, after aging, the presence of the tetragonal and monoclinic stages in the structure decreases the translucency since the individual stage has varying refractive indices [52; 54]. The aging method itself might be a variable influencing the optical properties of zirconia if varying methods are utilized. For example, constant immersion in a highly acidic solution simulating gastric acid with highly acidic pH (i.e., 1.2) at 37 \({}^{\circ}\)C for 4 days had a significant effect on the translucency of pre-sintered zirconia (i.e., 3-mol\(\%\)Y-TZP; [PERSON], Zirkonzhan, and Zenostar, Ivoclar Vivadent, Zirkonzahn, Atlanta, GA, USA) [46]. This protocol was repeated 208 times, representing four regurgitation sessions/week, for 12 months. This period under these specific conditions of time, temperature, and pH is regarded as equal to 10 years of clinical performance. Nevertheless, [PERSON] et al. [55] found that the utilization of acidic solution (i.e., pH = 2.0) with intermittent exposure (i.e., 2 min immersion, 2 min immersion in distilled water, and eventually 2 h storage in water at 37 \({}^{\circ}\)C) did not significantly influence the optical properties (i.e., translucency parameter) of zirconia ceramic. This procedure was repeated for a duration equal to 108 h of exposure [55]. [PERSON] and colleagues [56] found that the colorimetric features of zirconia were maintained after an accelerated aging procedure. Nonetheless, these investigators employed an aging method in an autoclave without exposure to ultraviolet light [47]. According to [PERSON] and colleagues [57], the mean color difference in the zirconia samples was 1.29. This value is much lower than the mean color stability values reported by most of the studies included in this systematic review (i.e., <2.0). This outcome might be due to the utilization of the veneered samples fabricated from the colored pre-sintered block, and hence, a separate coloring method before sintering was not used by the authors of one of the included studies [40]. Some reports mentioned that color change in ceramics owing to aging might be related to metal oxides. Metallic pigments are incorporated for the color shading of ceramic, and these oxides are easily dissolved under ultraviolet light [57; 58; 59]. Consequently, in the present review, the significant color change in the zirconia might also be because of the dissolution of metallic oxides resulting from the ultraviolet radiation used during the aging protocol. The association of aging with surface treatments also influences the color stability of ceramics. According to [PERSON] and colleagues [60], the color change was greater in polished materials followed by the glazed materials after aging. In the present review, the color change score of the polishing paste-treated zirconia samples was revealed to be greater as compared to that of other relevant ceramic materials.