Influence of Core/veneer Thickness Ratios on Failure of Bilayered Ceramics

Objectives: Clinicians have assumed that load-bearing increases with core thickness for bilayered systems (veneer/high-strength core).    However, recent analytical modeling (Hsueh and Miranda, 2003) suggested that failure loads are core-thickness independent for radial cracking of a supported specimen.  This study investigated the influence of thickness ratios on load-bearing of a bilayered ceramic system. Methods: Finite element analysis (FEA) was used to calculate stresses for a range of core/veneer thicknesses (mm): (1) 0.5/1.0; (2) 0.8/0.7; (3) 1.0/0.5; (4) 1.2/0.3; and, (5) 1.5/0.0.  Veneer/core tabs were fabricated for modeled ratios (n=10/group; Vita In-Ceram Alumina/VM7) with thicknesses controlled by a ceramic machinist.  Specimens were non-adhesively cemented to epoxy-glass-fiber bases (NEMA grade G-10).  After 2.5 weeks storage (wet/37°C) specimens were loaded (2mm diam. piston, 5 N/s).  Loads at crack pop-in were calculated using acoustic emission and loading rate. Results: FEA predicted failure loads (core ceramic = 200 MPa) were: (1) 631 N; (2) 663 N; (3) 663 N; (4) 706 N; and, (5) 786 N.  Measured failure loads were used to calculate (FEA) actual core ceramic strengths for each group.  The core ceramics for groups 1, 2 and 3 were weaker (148 ± 4.6 MPa) than for groups 4 and 5 (170.1 ± 4.3 MPa) suggesting a processing difference for the thinner specimens.  Adjusted failure loads (all = 170 MPa) were: (1) 546.0 ± 73.9 N; (2) 573.7 ± 74.1 N; (3) 574.5 ± 51.6 N; (4) 611.0 ± 68.1 N; and, (5) 680.3 ± 71.5 N.  Statistical groupings (ANOVA, 95% Duncan) were: non-adjusted (1,2,3) (4) and (5); adjusted (1,2,3,4) and (5). Conclusions: FEA and measured failure loads support the hypothesis that core thickness is not highly influential in determining load-to-failure.  Crowns may be fabricated to optimize esthetics without decreasing durability.  Support from NIDCR DE007302-15 and Vita Zahnfabrik.