Assessment of Two Surface Treatment Protocols on Monolithic Zirconia at Pre-sintered and Post-sintered Stages

Abstract

The purpose of this study was to evaluate the effect of air abrasion and silica-coating of zirconia in pre-sintered and post-sintered stages on the surface roughness, phase transformation, biaxial flexural strength (BFS), and tensile bond strength (TBS) to resin cement. Zirconia discs (Bruxzir, Glidewell, California, USA) were divided into two groups according to the timing of surface treatment (pre-sintered and post-sintered). Each group was divided into two subgroupsaccording to the type of surface treatment; air abrasion using 50 µm alumina particles (AB) and silica-coating using 30 µm silica-modified alumina particles (SC). A control group with no surface treatment was also tested. Surface roughness was analyzed using a 3D laser microscope and phase transformation was assessed using a diffractometer. BFS was tested using piston-on-three ball technique in a universal testing machine, and TBS to Panavia V5 with Clearfil Ceramic Primer Plus (Kuraray, Osaka, Japan) resin cement was tested both after 3 days of water storage and after 150 days of water storage combined with thermocycling. Data was analyzed using multivariate analysis of variance and Tukey’s post hoc test. Results showed that surface roughness was higher in the group treated in the pre-sintered stage than the one treated in the post sintered stage. The pre-sintered treated group and the control showed no monoclinic phase while the post-sintered group showed significantly higher portions of monoclinic phase. Regarding BFS the post-sintered treated group had statistically significant higher values when compared to the pre-sintered treated group and the control. Highest TBS values were achieved with the post-sintered silica-coated subgroup. No significant differences were found between the post-sintered air-abraded subgroup and the pre-sintered treated group. All subgroups showed higher TBS values when compared to the control group. However, after thermocycling a dramatic decrease in TBS values was noticed in the samples that were silica coated. Conclusions Within the limitation of this study, the following conclusions could be drawn: 1. Surface treatment of monolithic zirconia using air abrasion or silica coating in the pre-sintered stage even with reduced pressure produced a rougher surface than treating it in the post sintered stage. 2. Sintering cycle caused a complete reversal of the monoclinic phase produced due to surface treatment back to tetragonal phase. 3. Surface treatment of zirconia in the post sintered stage increased the initial biaxial flexural strength of the material. 4. Air abrading zirconia using Al2O3 will produced a more stable bond strength to resin cement after long term water storage than silica coating. 5. The bond strength between resin cement and zirconia after air abrading it using Al2O3 in the pre-sintered stage was comparable to that when abradedin the post sintered stage. Clinical recommendations Air abrading zirconia with Al2O3 in the pre-sintered stage using reduced air pressure could be a promising surface treatment method to produce a surface with no monoclinic phase thus retards the process of low temperature degradation of zirconia.