Effect of surface treatment on flexural strength of zirconia bars

Statement of problem

Clinical and laboratory processing techniques induce damage to the surface of zirconia frameworks, which significantly lessens their strength.
Purpose

The purpose of this study was to investigate the influence of 3 surface restoration methods on the flexural strength of zirconia bars.
Material and methods

Bar-shaped specimens were polished and received 1 of 2 surface treatments (n=20): airborne-particle abrasion with 110-μm aluminum oxide particles at a pressure of 0.2 MPa, or grinding with a diamond point under water cooling and a load of 2 N using an air turbine. Polished specimens served as the control (n=20). The induced surface damage was restored using one of the following restoration methods: polishing of the specimens using an aluminum oxide polishing point (2-μm grit) coated with 0.5-μm diamond polishing paste, application of a thin coat of glazing porcelain according to the manufacturer's instructions, or application of a phosphate ester monomer (MDP) containing an adhesive resin. The specimens were subjected to a 4-point flexural strength test and then examined using a scanning electron microscope (SEM). Two-way ANOVA and the Bonferroni post hoc test were used to analyze the data (α=.05).
Results

There was a significant interaction between the type of surface damage and the restoration method (P<.001). For the ground specimens, all restoration methods resulted in a statistically significant regain in strength, with the polishing procedure being the most effective (1027 MPa). For airborne-particle-abraded specimens, application of the bonding agent was the only effective restoration method. SEM analysis of the fractured surfaces of specimens indicated that application of the bonding agent resulted in sealing of the surface damage produced by airborne-particle abrasion, while polishing was successful in removing the grinding lines produced by the diamond point.
Conclusions

Within the limitations of this in vitro study, restoration of surface damage improved the flexural strength of zirconia specimens.