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Behr M, Weiser F et al. Acta Odontol Scand 2011; 69: 170–175

Hygroscopic expansion of resin-based core materials and/or luting agents have an impact on the fracture resistance of glass–ceramic restorations. In this in vitro study, zirconia substructures were milled in order to fit simulated cores. The cores were constructed from three resins, two of which were resin composite materials. All resins had different water sorption properties. Zirconia copings that had undergone thermal cycling and mechanical loading had a significantly lower fracture resistance when luted to cores with high water sorption than those cores with moderate water sorption. Although zirconia has superior mechanical properties to most glass-ceramic materials, it does not 'perform like a ductile metal alloy'. The term 'ceramic steel' is disingenuous.