The Effect of Grinding and Polishing of Zirconia Surfaces on Surface Roughness, Gloss and Antagonist Wear

Objective. This study was conducted to evaluate the surface roughness and gloss of yttrium-stabilized tetragonal zirconia after grinding and/or polishing of the surfaces and to investigate the effects of grinding and/or polishing on wear rate of zirconia and its composite antagonist. Background. Many clinicians are extremely concern about delivering zirconia restorations after proper glazing procedure, but in some cases, chair-side modification of the occlusal surfaces is required after cementation in order to obtain proper occlusion, which produces rough surfaces. Loss of the original zirconia gloss and wear of opposing dentition or restorations are some of the drawbacks of rough surfaces. Methods. Thirty-two circular discs of zirconia with 16 mm diameter and another twenty-four discs of 5mm in diameter and 2mm in thickness were prepared. Circular 16 mm specimens were divided into four groups: glazed zirconia; ground zirconia; two-step polished zirconia and three-step polished zirconia. All discs were glazed, then ground and/or polished according to their respective groups. Profilometer and glossmeter measured the average surface roughness and gloss for all groups. The 5 mm discs were divided into three groups, glazed, ground and two-step polished groups. The zirconia specimens were prepared, then mounted into the upper chamber of wear testing machine. In the lower chamber, circular-composite resin specimens served as antagonists. Each group was loaded at 100K, 200K, 400K and 1 million cycles in a wear test machine. The average surface roughness and the average step height of composite specimens were quantified using a Profilometer after each cycle. Data were analyzed with 1-way ANOVA for zirconia roughness and gloss, 2-way ANOVA for wear test, and Tukey HSD post hoc tests (α =0.05). Results. Grinding group had the highest average roughness values and significantly differed from Glazed and Two-step polished group, but not three-step polished group. The Two-step and Three-step polishing groups were not significantly different from each other. There was a significant difference in surface gloss among all groups. Surface roughness of ground and polished zirconia surfaces were reduced slightly after 1 million cycles, while the roughness was increased for the glazed group. There was a significant loss of composite material as wear increased from 100K cycles to 1 million cycles. There were no statistically significance differences between the average surface roughness of composite in all groups at different time points set in this study. Conclusion. Polishing techniques could not return the zirconia surface to a similar gloss as glazed surfaces. Wear of the zirconia specimens was not influenced by wear cycles; in contrast to the antagonist composite that showed a cycle dependent wear increase. Glazing and polishing the monolithic zirconia showed similar opposing composite wear rate while grinding the zirconia created more opposing composite wear. Grants. This study was funded by a Nova Southeastern University, Health Professions Division, Research Department