title: The effect of multiple firings on microtensile bond strength of core-veneer zirconia-based all-ceramic restorations.
abstract: OBJECTIVE
  Differences in core and veneer coefficients of thermal expansion, firing shrinkage, and speed of increasing and decreasing the temperature may generate stress in veneered all-ceramic restorations. Given the necessity of performing multiple firing cycles to achieve improved contour, color, and esthetics, the purpose of this study was to determine the effect of multiple firing cycles on the microtensile bond strength (MTBS) of zirconia core to the porcelain veneer in zirconia-based all-ceramic restorations.
  
  
  METHODS
  Thirty blocks (12 × 12 × 4 mm(3)) of semi-sintered zirconia were machined and sintered according to manufacturer's instruction. Specimens were placed in three groups based on the number of firing cycles (4, 6, 8) for the veneering process. After veneering, the specimens were sectioned into microbars with 8 mm length and 1 mm cross-section. Twenty sound microbars in each group were stressed to failure in a microtensile tester machine at 1 mm/min. Fractured specimens were surveyed under a scanning electron microscope and classified as cohesive in core, cohesive in veneer, and mixed. MTBS data were analyzed using one-way ANOVA and Tukey test (p < 0.05).
  
  
  RESULTS
  The mean MTBS (MPa) after 4, 6, and 8 firing cycles were 30.33 ± 2.13, 27.43 ± 1.79, and 25.06 ± 1.76, respectively. There was a statistically significant difference between the bond strengths of each of the three groups (p < 0.001).
  
  
  CONCLUSIONS
  Increase in firing cycles decreased MTBS. Most of the failures (90-95%) in all three groups were cohesive in the veneering porcelain and did not change as the number of firing cycles increased.
authors: Dental Research Center and Department of Prosthodontics, School of Dentistry, Tehran University of Medical Sciences, Tehran, Iran. somayeh.zeighami@gmail.com
authors: Somayeh
authors: S
authors: Zeighami
authors: Hoseinali
authors: H
authors: Mahgoli
authors: Farzaneh
authors: F
authors: Farid
authors: Abbas
authors: A
authors: Azari
mesh_terms: D000538:Aluminum Silicates; D001840:Dental Bonding; D003735:Dental Casting Technique; D003776:Dental Porcelain / Q000737:chemistry; D019232:Dental Restoration Failure; D003799:Dental Stress Analysis; D003801:Dental Veneers; D006358:Hot Temperature; D011176:Post and Core Technique; D013718:Tensile Strength; D015040:Zirconium
chemical_list: D000538:Aluminum Silicates; D003776:Dental Porcelain; C078519:leucite; D015040:Zirconium; C028541:zirconium oxide