The transverse strength, reliability and microstructural features of four dental ceramics — Part I

Objectives: The flexural strength, reliability and microstructure of a heat pressed ceramic system (Empress) was evaluated in comparison with three current frit materials used in ceramic crown and inlay construction. Methods: Twenty bar specimens per material were constructed, overglazed and transverse tested at a crosshead speed of 0.5 mm/min. Etched and fractured bar specimens were viewed under a scanning electron microscope using secondary electron imaging. Results: Mean strengths (MPa ± SD) were: Empress, 117.3 ± 31.7; Cerinate porcelain, 118.2 ± 8.7; Corum porcelain, 92.9 ± 13.3; and Alpha porcelain, 60.9 ± 10.1. Results were highly significant (P < 0.001) with differences between all groups (P < 0.05) except Cerinate porcelain and Empress (P > 0.05). Weibull analysis of the results provided m-values and the predicted strength at the 1 and 5% probabilities of failure. Microstructural examination using secondary electron imaging showed leucite crystals 1.6–3.5 μm and associated microcracking in Corum porcelain and leucite crystal clusters (1.5 μm) in Empress. Cerinate porcelain exhibited a uniform distribution of fine leucite crystals (1 μm). Spherical porosity combined with crack pore combinations were noted in Vita Alpha porcelain. Conclusion: Empress glass ceramic was not stronger than Cerinate or more reliable than many of the frit materials. Uniform leucite distribution and fine crystal size may be associated with improved strength and reliability.