Prestresses in bilayered all-ceramic restorations.
BACKGROUND
A general trend in all ceramic systems is to use veneering ceramics of slightly lower thermal expansion coefficients compared with that of the framework resulting in a positive mismatch in thermal expansion coefficient (+DeltaTEC). The concept behind this TEC mismatch is to generate compressive stresses in the weaker veneering ceramic and thus enhance the overall strength of the restoration. This technique had excellent results with porcelain fused to metal restorations (PFM). However, there are concerns to apply this concept to all-ceramic restorations. The aim of this research was to determine the stresses in bilayered all-ceramic restorations due to the mismatch in TEC.


METHODS
Two commercial veneering ceramics with a TEC lower than that of zirconia (+DeltaTEC); NobelRondo zirconiatrade mark and Lava Ceramtrade mark, plus one experimental veneering ceramic with an identical TEC that matches that of zirconia (DeltaTEC = 0) were used to veneer zirconia discs. The specimens were loaded in biaxial flexure test setup with the veneer ceramic in tension. The stresses due to load application and TEC mismatch were calculated using fractography, engineering mathematics, and finite element analysis (FEA).


RESULTS
In this study, the highest load at failure (64 N) was obtained with the experimental veneer where the thermal mismatch between zirconia and veneering ceramic was minimal. For the two commercial veneer ceramics the magnitude of the thermal mismatch localized at the zirconia veneer interface (42 MPa) exceeded the bond strength between the two materials and resulted in delamination failure during testing (ca. 50 MPa).


CONCLUSIONS
For all-ceramic zirconia veneered restorations it is recommended to minimize the thermal mismatch as much as possible.
Aboushelib
Moustafa N
MN
Dental Biomaterials Department, Faculty of Dentistry, Alexandria University, Egypt. mabou@acta.nl
Feilzer
Albert J
AJ
de Jager
Niek
N
Kleverlaan
Cornelis J
CJ
D002516:Ceramics / Q000592:standards*; D019245:Compressive Strength; D019232:Dental Restoration Failure; D003799:Dental Stress Analysis; D003801:Dental Veneers / Q000592:standards*; D020342:Finite Element Analysis; D013718:Tensile Strength; D015040:Zirconium
D015040:Zirconium; C028541:zirconium oxide