Comparison of two heat-pressed all-ceramic dental materials

The processing route for two heat-pressed all-ceramic materials (Empress and OPC) is virtually identical. The purpose of this study was to determine the mechanical properties of both materials and determine if significant differences exist between them.

X-ray powder diffraction of the ceramics before and after processing was carried out to identify the crystal phases present. The mechanical properties of both materials were tested. Specimens were tested for hardness, fracture toughness (indentation method) and flexural strength (biaxial method). The results were statistically evaluated and tested for differences using a Mann-Whitney test. Secondary electron imaging of both materials was carried out before and after processing.

X-ray powder diffraction revealed that OPC changes as a result of heat-pressing from being a complex mixture of crystalline oxides to a glass-ceramic. In contrast Empress is a glass-ceramic before and after processing. X-ray diffraction identified leucite as the main crystalline phase in both ceramics. The biaxial flexural strength of OPC was 153.6 (17.8) MPa and for Empress was 134.4 (11.5) MPa. The hardness of OPC was 7.28 (0.62) GPa and for Empress was 6.94 (0.79) GPa. Indentation fracture toughness of OPC was 1.36 (0.29) MPam0.5 and for Empress was 1.33 (0.08) MPam0.5. Secondary electron images show Empress to be the same before and after processing while OPC is clearly very different. Empress also appears to have a higher glass content compared with OPC.

The results of X-ray diffraction show that Empress is pre-cerammed whilst OPC is not. Statistical analysis revealed that no significant difference exists between the two materials for any of the mechanical properties tested at a 95% (p < 0.05) confidence level. It was concluded that no difference exists between the two materials on completion of processing.