Wear analysis and topographical properties of monolithic zirconia and CoCr against human enamel after polishing and glazing procedures

Abstract

Purpose: Intraoral adjustments of monolithic zirconia (MZ) necessitate intraoral grinding and polishing procedures, causing surface topography changes. The effect of various polishing and glazing procedures on surface roughness, topographical and phase changes of CoCr compared to zirconia, and assessment of the wear of the opposing dentition has been evaluated in this study.

Materials and methods: One group of square Cobaltchromium (CoCr) and four groups of sintered and polished zirconia specimens (12 × 12 × 1mm) were fabricated (N = 5, n = 8) using Cobaltchromium or yttria-stabilized zirconia (ZrO2, Y2O3) blocks (ZENTROSTAR Zr Translucent; Wieland Dental + Technik). Each of the four zirconia groups was treated differently, one group was glazed (G), one was left unglazed (UG), two groups were polished, one using silicon carbide polishers (BG) and one with diamond-impregnated ceramic polisher kit (CG). All specimens were thereafter subjected to chewing simulation using enamel cusps (1'200'000 cycles, 49N force and 1.67 Hz loading frequency). Topographical changes were evaluated considering (a) weight (digital scale), (b) volume loss (digital microscope), (c) vertical height loss (digital microscope), (d) surface roughness (Ra, profilometer), and (e) mean roughness depth (Rz, profilometer). In addition the volume loss (digital microscope) of the used corresponding enamel cusps was measured. The surface roughness, topographical changes and antagonist wear of enamel cusps were measured. Statistical analysis was performed using the statical software R and the kruskal-wallis rank sum test and posthoc pairwise comparisons (a = 0.05).

Results: The mean surface roughness (μm) difference (ΔRa) was lowest for UG (-0.014 ± 0.007) and the highest for CG (-0.806 ± 0.172) (UG^a < CoCr^b < BG^c < G^a,d < CG^e). UG showed the lowest surface peak roughness (μm) difference (ΔRz) -0.316 ± 0.084 and CG the highest (-3.691 ± 0.369) (UG^a < CoCr^b < BG^c < G^a,d < CG^e). Material weight loss (ΔW) ranged between -0.005 ± 0.0001 and -0.000 ± 0.000 g (CoCr^a < UG^a,b < CG^b < BG^b < G^b) compared to baseline, while the volume loss (ΔV) was between -21 ± 52 × 10⁶ and 58 ± 36 × 10⁶ μm3 (CoCr^a < BG^a < UG^a < CG^a < G^a). The vertical height loss (ΔVH) was highest for UG (-12.067 ± 11.624) and lowest for G -0.033 ± 0.034 μm (G^a < CoCr^a,b < CG^c < BG^a,c,d < UG^a,b,d). As for the volume loss (ΔV) of the corresponding enamel cusps, no significant differences could be measured (BG^a < UG^a < G^a < CG^a < CoCr^a), therefore surface treatment did not affect the wear of the enamel antagonist.

Conclusion: CoCr and highly polished monolitic zirconia produced less surface roughness and similar antagonist wear compared to glazed or unglazed zirconia. CoCr and highly polished monolitic zirconia can be considered as a good choice regarding the mechanical wear behaviour of materials and their corresponding enamel antagonist.

Keywords: CoCr; Glazing; Monolithic zirconia; Surface roughness; Tribology; Wear; Y-TZP.