Influence of Additive Firing on the Surface Characteristics, Streptococcus mutans Viability and Optical Properties of Zirconia

The purpose of this study was to observe whether the repetitive firing of dental zirconia caused changes in surface characteristics, viability, and optical properties of zirconia. Dental zirconia blocks were sintered and randomly distributed into seven experimental groups: F0-F6. Except for F0, which only went through sintering, the additive firing was performed in order for F1-F6. Surface roughness, contact angle, viability by fluorescence, and translucency parameter were measured. They were all highest after sintering (F0) and decreased after additive firings (F1-F6). The additive firing of zirconia after sintering decreased surface roughness, contact angle, viability, and translucency. The number of firings after the first firing was not found to be critical in surface characteristics, viability, and optical property. Changes in surface characteristics might have led to a decrease in viability, while the change of translucency was not clinically significant. This implies that additive firing may prevent secondary caries under zirconia restorations, not compromising esthetic appearance.