Effect of polishing procedures and hydrothermal aging on wear characteristics and phase transformation of zirconium dioxide

Abstract

Statement of problem. Yttria-stabilized zirconia used for the fabrication of crowns and fixed

prostheses may require intraoral adjustments after placement and cementation. Grinding and

polishing methods may result in changes in the surface characteristics of zirconia.

Purpose. The purpose of this in vitro study was to assess the effect of polishing procedures on

surface roughness, topographical and phase changes of zirconia, and wear of the opposing dentition.

Material and methods. Presintered and precut yttria-stabilized zirconia specimens (10×10×1 mm)

were divided into 4 groups (control, Intensiv, Shofu, 3M ESPE) depending on the polishing method

used to prepare the specimens. All tests were carried out in triplicate. The specimens were polished

depending on the polishing regimen, while the control was left untreated. The specimens were

thermocycled for 3000 cycles, with a temperature range of 5C to 55C. The surface roughness,

elemental, and phase changes caused by polishing before and after thermocycling were assessed

with surface profilometry, energy-dispersive spectroscopy, and X-ray diffraction analysis. The wear on

antagonist steatite balls was also measured after mastication simulation. Statistical analysis was

performed using 1-way ANOVA and the Tukey post hoc test to perform multiple comparison tests (a=.05).

Results. The polishing procedures increased surface roughness (Ra) of yttria-stabilized zirconia from

0.52 for the control specimen to 0.73 for Intensiv, 0.70 for Shofu, and 0.70 for 3M ESPE (P<.05), which

was reduced by thermocycling to 0.44 (control), 0.58 (Intensiv), and 0.58 (Shofu) (P<.001), while

roughness remained unchanged for 3M ESPE specimens (0.75; P=.452). The deposition of aluminum

when using Shofu abrasives and nickel in Intensiv was demonstrated. Phase changes were observed

on the zirconia surface with formation of the monoclinic phase in all polishing methods. Specimen

aging enhanced the surface phase changes and also induced compressive stresses in zirconia

polished with Intensiv. The different polishing protocols did not affect the wear to the antagonist (P>.05).

Conclusions. Polishing zirconia increased surface roughness and led to surface phase changes, but

wear to the antagonist was not affected.