Influence of veneer and cyclic loading on failure behavior of lithium disilicate glass-ceramic molar crowns.

OBJECTIVE: This in vitro study was designed to investigate the influence of the veneer and cyclic loading on the failure behavior of lithium disilicate glass-ceramic (LDG) crowns on maxillary first molar. METHODS: Sixty-four LDG crowns were divided into 4 groups (n=16). Thirty-two monolithic crowns were fabricated from IPS e.max Press (M), and the remaining bilayered crowns using cut-back technique and conventional manual layering technique from IPS e.max Press/Ceram (B). Monolithic or bilayered crowns were subjected to single-load-to-fracture (SLF) testing using a universal testing machine, before (M1 and B1) and after exposure to sliding-contact fatigue (SCF) testing (M2 and B2), consisting of 1,200,000 mechanical cycles (Fmax=98 N). Data were statistically analyzed using two-by-two factorial design ANOVA. Fractographic analysis was performed to determine the fracture modes of the failed specimens. RESULTS: The mean fracture load values (N+/-S.D.) for M1, B1, M2 and B2 were 2686+/-628 N, 1443+/-327 N, 2133+/-578 N and 1464+/-419 N, respectively. Significant differences were found between the failure loads of all groups (P<0.001), except between groups B1 and B2. Bulk fracture initiating from the occlusal surface is the primary failure mode of monolithic and veneered LDG crowns. Cracking that initiated from core-veneer interfacial defects and ultimately resulted in bulk fracture is another major failure origin of veneered all-ceramic crowns. SIGNIFICANCE: Veneer application resulted in significantly lower fracture load values compared to monolithic LDG crowns. Cyclic loading is an accelerating factor contributing to fracture for monolithic LDG crowns but not for bilayered ones.