![]() In agreement with these clinical findings, 2-mm-thick full-contour molar crowns of IPS e.max CAD subjected to sliding contact fatigue-testing have demonstrated significantly higher reliability than porcelain-layered Y-TZP crowns ( Guess et al., 2010). Recent 2-year clinical recalls on full-contour lithium disilicate crowns have shown promising results in terms of structural integrity, with no mechanical failures such as fracture or chipping ( Fasbinder et al., 2010 Reich et al., 2010). The lithium disilicate glass-ceramic system, whether CAD/CAM-processed or heat-pressed, is indicated either as a full-contour (monolithic) restoration or as a core for subsequent porcelain veneering. Contributing to the continued use of these crowns have been specific changes in the core design aimed at optimizing porcelain support, as well as improvements observed in reliability and reduced fracture sizes of the porcelain veneer relative to a uniform thickness core for Y-TZP crowns ( Silva et al., 2010). Considering that porcelain veneer fracture in Y-TZP restorations is a complex phenomenon-one likely explained by differences in coefficient of thermal expansion (CTE) between framework and porcelain, residual thermal stresses from cooling, and core/framework design, among other factors-there is still much to be clarified ( Zarone et al., 2011). A method to reproduce these failure modes in the laboratory has been developed using mouth-motion fatigue-testing in water ( Coelho et al., 2009a, b Silva et al., 2010), where mechanical testing has confirmed the susceptibility of zirconia crowns to porcelain veneer failure. Although zirconia fixed partial dentures have been most commonly investigated ( Tinschert et al., 2005 Vult von Steyern et al., 2005 Sailer et al., 2006), the sparse information regarding single crowns has shown similar results concerning the principal failure mode, which involved fractures within the veneering ceramic ( Cehreli et al., 2009 Ortorp et al., 2009 Groten and Huttig, 2010). There was no significant difference between the Weibull characteristic failure loads of MON and BTV (1,535 N and 1,609 N, respectively), which were significantly higher than that of Y-TZP (370 N ) and comparable with that of MCR (1,304 N ), validating the study hypothesis.Ĭompared with other all-ceramic systems, yttrium oxide partially stabilized tetragonal zirconia polycrystal (Y-TZP) exhibits superior mechanical properties, due partly to a transformation toughening mechanism ( Christel et al., 1989 Tinschert et al., 2000). Use level probability Weibull calculation (use load = 1,200 N) showed interval overlaps between MON and BTV. ![]() Crack evolution was followed, and fractography of post mortem specimens was performed and compared with that of clinical specimens. Monolithic 1.0-mm (MON) and 2.0-mm CAD/CAM lithium disilicate crowns, the latter with a buccal thin veneer (BTV) of 0.5 mm, were fabricated and then sliding-contact-fatigued (step-stress method) until failure or suspension ( n = 18/group). CAD/CAM first mandibular molar full-crown preparations were produced with uniform thicknesses of either 1.0-mm or 2.0-mm occlusal and axial reduction, then replicated in composite for standard crown dies. The present investigation hypothesized that the reliability of reduced-thickness monolithic lithium disilicate crowns is high relative to that of veneered zirconia (Y-TZP) and comparable with that of metal ceramic (MCR) systems.
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