# Surface Morphology Changes of Bleached Dental Ceramics
## 4 Discussions  
Conventional dental ceramics being the most inert among all dental restorative materials can exhibit surface deterioration when interacting with different bleaching materials [24]. Contacting with extremely unstable and reactive free radicals of H\({}^{+}\) or H\({}_{3}\)O\({}^{+}\), released by bleaching agents [18; 21], followed by possible radical diffusion may cause dissolution in ceramic glass networks [19; 25; 26]. Moreover, an increase in surface roughness greater than the threshold (R\({}_{\rm a}\) > 0.2 \(\upmu\)m) may result in an increase of plaque accumulation, thereby increasing the risk of periodontal inflammation or affecting ceramic aesthetics by changing the ceramic texture [21]. The average surface roughness of no one of investigated samples before and after bleaching has exceeded the value of 0.2 \(\upmu\)m which was clinically set as a level for possible bacteria accumulation. The lowest surface roughness between all initially prepared ceramics (polished and glazed) was found for zirconium dioxide. R\({}_{\rm a1}\) of polished zirconium dioxide samples was the lowest and varied between 6.00 and 8.31 nm, R\({}_{\rm q1}\)--between 7.30 and 11.39 nm. The roughness of initial leucite-reinforced glass ceramic samples and lithium disilicate samples was higher, as compared with zirconium dioxide samples, indicating the highest surface roughness of glazed lithium disilicate (R\({}_{\rm a1}\) = 12.36 \(\pm\) 2.76 nm and R\({}_{\rm q1}\) =17.91 \(\pm\) 4.85 nm) and of polished leucite reinforced glass (R\({}_{\rm a1}\) = 13.15 \(\pm\) 5.13 nm and R\({}_{\rm q1}\) = 17.68 \(\pm\) 6.72 nm). There was no significant roughness difference between polished and glazed parts of initial ceramics observed. Bleaching of the experimental samples with carbamide peroxide (16% CP) was responsible for the reduction of surface roughness of lithium disilicate and leucite-reinforced glass ceramic independently from initial preparation of samples (polished or glazed). A small increase of roughness after CP treatment was observed in zirconium dioxide samples, while application of hydrogen peroxide (6% HP) as a bleaching agent was responsible for the significant increase of surface roughness in all of the investigated samples. Application of 6% HP with following LED illumination caused highest increase of surface roughness of polished leucite-reinforced glass ceramic. Performed statistical analysis revealed that the surface roughness of polished lithium disilicate bleached with _Pola Office_ + 6% HP was statistically significant rougher (mean R\({}_{\rm a2}\) = 37.17 nm) (Sig. 0.005) (mean R\({}_{\rm q2}\) = 50.69 nm) (Sig. 0.004) than bleached with _Philips Zoom NiteWhite_ 16% CP or _Philips Zoom_ 6% HP + LED. The reduction of R\({}_{\rm a}\) and R\({}_{\rm q}\) values of the polished lithium disilicate were observed after its treatment with 16% CP (mean R\({}_{\rm a2}\) = 10.22 nm) (F = 7.765; \(p\) < 0.0) (Sig. 0.005) (mean R\({}_{\rm q2}\) = 12.84 nm) (Sig. 0.004). However, significant difference according to paired t-test was not identified. Surface roughness of glazed leucine-reinforced glass-ceramic bleached with _Pola Office_ + 6% HP was statistically significantly rougher (mean R\({}_{\rm a2}\) = 21.76 nm, mean R\({}_{\rm q2}\) = 32.97 nm) than bleached with _Philips Zoom NiteWhite_ 16% CP or _Philips Zoom_ 6% HP + LED. The surface roughness of the glazed leucite-reinforced glass ceramic was reduced after its treatment with 16% CP (mean R\({}_{\rm a2}\) = 9.20 nm) (F = 4.714; \(p\) < 0.05) (Sig. 0.030) and (mean R\({}_{\rm q2}\) = 11.88 nm) (Sig. 0.028), but there was no significant difference between the initial and bleached samples. The surface roughness of the polished leucite-reinforced glass-ceramic was reduced to a statistically significant level after its treatment with 16% CP (R\({}_{\rm a2}\) = 9.12, Sig. 0.035) (R\({}_{\rm q2}\) = 11.72 nm, Sig. 0.025). The surface roughness of polished lithium disilicate (R\({}_{\rm a2}\) = 37.17 nm, Sig. 0.017) (R\({}_{\rm q2}\) = 50.69 nm, Sig.