title: Effect of polishing procedures and hydrothermal aging on wear characteristics and phase transformation of zirconium dioxide.
abstract: BACKGROUND
  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.
  
  
  OBJECTIVE
  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.
  
  
  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 5°C to 55°C. 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 (α=.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.
authors.affiliation: Graduate student, Department of Restorative Dentistry, Faculty of Dental Surgery, University of Malta, Malta.
authors.forename: Darrell
authors.initials: D
authors.lastname: Bartolo
authors.affiliation: Lecturer, Head of Department, Department of Metallurgy and Materials Engineering, Faculty of Engineering, University of Malta, Malta.
authors.forename: Glenn
authors.initials: G
authors.lastname: Cassar
authors.affiliation: Graduate student, University of Zurich, Dental Materials Unit, Center for Dental and Oral Medicine, Clinic for Fixed and Removable Prosthodontics and Dental Materials Science, Zurich, Switzerland.
authors.forename: Nadin
authors.initials: N
authors.lastname: Al-Haj Husain
authors.affiliation: Professor, University of Zurich, Dental Materials Unit, Center for Dental and Oral Medicine, Clinic for Fixed and Removable Prosthodontics and Dental Materials Science, Zurich, Switzerland.
authors.forename: Mutlu
authors.initials: M
authors.lastname: Özcan
authors.affiliation: Associate Professor, Department of Restorative Dentistry, Faculty of Dental Surgery, University of Malta, Malta. Electronic address: josette.camilleri@um.edu.mt.
authors.forename: Josette
authors.initials: J
authors.lastname: Camilleri
mesh_terms: D003775:Dental Polishing; D019554:Dental Restoration Wear; D006358:Hot Temperature; D006801:Humans; D066298:In Vitro Techniques; D013499:Surface Properties; D014867:Water; D015040:Zirconium
chemical_list: D014867:Water; D015040:Zirconium; C028541:zirconium oxide