Evaluation of Different Polishing Systems and Speeds for Dental Zirconia

To evaluate two polishing systems and three polishing speeds on the gloss, roughness, and heat production of yttria stabilized tetragonal zirconia polycrystal (Y-TZP).

A Y-TZP block (Zenostar Zr Translucent) was sectioned into 4-mm-thick sections. Specimens (n = 30 per polishing system) were first wet ground with a fine diamond bur at 200,000 RPM for 15 seconds. The baseline gloss of all specimens was measured in a glossmeter (60° angle) in a direction perpendicular to grinding. Initial surface roughness (R_a ) was determined on an optical profilometer in a direction perpendicular to grinding. The specimens were then polished with either a Dialite ZR Polishing Kit or a CeraMaster Polishing Kit. All grinding and polishing was performed by the same operator calibrated to apply approximately 2 N pressure. The medium grit polisher was used for 30 seconds, repeated for 30 seconds, and then the fine grit polisher was used. After each step, the gloss and roughness of each specimen was remeasured as described previously. Each polishing system was used at 5000, 15,000, and 40,000 RPM (n = 10 for each polish system/speed combination). A group of glazed specimens (n = 10) was evaluated for gloss and roughness as a control. The heat generated for each polishing step was measured with a thermocouple. The gloss and roughness of the specimens were analyzed using a linear mixed model and Tukey-Kramer post-hoc tests. Each step of polishing was compared to the glazed control group with an ANOVA and Dunnett's test.

Polishing step, system, and speed were significant (p < 0.05) for gloss and roughness. Examination of system and step interaction was significant for gloss only. Post-hoc analysis revealed that 15,000 RPM produced higher gloss and lower roughness than other speeds. Each progressive step produced an improvement in gloss except for initial 30 seconds with the CeraMaster Coarse (medium polisher; p = 0.34). Roughness was also reduced at each progressive step. No polishing speed/system produced an increase in temperature above 41°C. Both polishing systems were capable of achieving a similar or superior roughness and gloss as the glazed control specimens after the final polishing step at 15,000 RPM.

Fifteen thousand RPM is an optimal polishing speed. Progressing through the polishing sequence significantly improves gloss and roughness and can create similar values as glazed zirconia.