Shear bond strength of lithium-disilicate and leucite-reinforced veneers

Objectives: This research measured shear bond strength (MPa) and assessed the mode of fracture of lithium disilicate (IPS e.max®) and leucite-reinforced (IPS Empress®) veneers bonded to enamel.
Methods: Twenty-two freshly extracted human molar teeth providing a minimum 6.0mm circumferential enamel for bonding veneer specimens were mounted in epoxy resin and finished flat using wet 600-grit SiC paper. Two groups (n=11) of circular veneer specimens (5.0mm x 0.75mm) were fabricated using IPS e.max® (A) and IPS Empress® (B). The veneer specimens were prepared according to manufacturer instructions and bonded to the etched tooth enamel surfaces using dual-cure resin composite cement. All specimens were thermo-cycled between 5degC and 55degC for 2500 cycles using a 30 second dwell time. Shear bond strengths were determined on an Instron® at a cross-head speed of 1.0mm/minute. Mode of fracture was assessed by visual inspection at 20X magnification of the tooth and veneer specimen.
Results: Mean shear bond strength (MPa) for the veneer groups were: A=10.79±2.86 and B=15.07±3.01. One-way ANOVA revealed that groups A and B were significantly different (p≤0.01). A higher percentage of adhesive fracture was observed for group A (82%) compared to group B (45%). Complete adhesive failure occurred at the veneer/cement interface 89% of the time for Group A and 20% of the time for Group B. Group B had a higher incidence (55%) of combined adhesive/cohesive fracture than Group A (18%).
Conclusions: Shear bond strengths of leucite-reinforced veneers (B) bonded to enamel were significantly (p≤0.01) higher than lithium disilicate veneers (A). While a higher percentage of complete adhesive fracture was observed with Group A, Group B had a higher incidence of combined adhesive/cohesive failure. The lower SBS of Group A and higher incidence of veneer/cement interface failure suggests that surface conditioning with hydrofluoric acid of lithium disilicate may be less than desirable for effective bonding.