Direct Observation of Fracture process in bond system between all-ceramic restorations and adhesive resin cements

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In IPS Empress 2 all-ceramic system, the restorations are fabricated by veneering porcelain on the copings hot-pressed, however, the effects of multiple firing for veneering on the bond strength between IPS Empress 2 core ceramics and a adhesive resin cement are not clarified. In this study, the micro-tensile bond strength between the core ceramics and a resin cement was evaluated. From the statistical analysis, the mean bond strength was not significantly different for unheated (41.5 Mpa) and for heated four times (39.1 Mpa). It was also found that the multiple firing for veneering significantly influenced the Weibull plots of the bond strength. It is thought that the all-ceramic crowns machined under brittle-mode grinding contain various subsurface cracks, and these cracks decrease the strength the crowns. However, the kind, the size and the location of the cracks actually occur in the crowns machined by CAD/CAM systems are not clarified. In this study, evaluation of the subsurface damages in CAD/CAM machined all-ceramic crowns were carried out using the bonded-interface sectioning technique. From the SEM observation, it is concluded that Median and Lateral cracks were generated by grinding at the ridge, the margin, the axial wall and the inner surface of the crowns, and these cracks can be the fracture origin of the crown under cyclic occlusal loadings.