Core:Dentine Ratio Effects on a Y-TZP Dental Ceramic

Objectives: The perceived wisdom regarding bilayered ceramic systems is that the laminate should be constructed from the strongest core material used in the greatest thickness possible and the weaker veneering porcelain should be as thin as possible. In the current study the effect of core:dentine thickness ratio on the bi-axial flexure stress and fracture mode and failure origin of bilayered yttria-stabilised tetragonal zirconia polycrystalline (Y-TZP) ceramic specimens was assessed as an in vitro assessment for all-ceramic crowns. Methods: Three sets of 20 bilayered composite discs with a 1.6 (0.1) mm thickness of Lava (3M ESPE, Seefeld, Germany) core and core to Lava™ Ceram dentine overlay porcelain ratios of 1:2, 1:1 and 2:1 were manufactured. Bilayered discs were centrally loaded on the veneering porcelain surface in a ball on ring configuration. Stresses at axial positions throughout the porcelain /zirconia bilayer were calculated according to methods described by Hsueh et al. (2005). The fracture mode and failure origin were assessed by optical microscopy techniques. Analysis of group means were performed using a one-way analysis of variance and Tukey's multiple range tests employed at P<0.05. Results: Significant bi-axial flexure stress variations were evident when the fracture strength data was analysed with the highest flexure stress recorded for the 2:1 group. The fracture mode and failure origin was also dependent on the core:dentine ratio with an increased incidence of Hertzian cone formation with increased dentine thickness. Conclusions: The bi-axial flexure stress at the interface, fracture mode and failure origin of bilayered Y-TZP ceramics is markedly dependent on the core:dentine thickness ratio suggesting that veneering porcelain chipping rather than substructure failures may be a problem with Y-TZP crowns and bridges.