Bond strength of ceramics heat-pressed onto three dental alloys

Statement of problem. The press-on-metal (PoM) technique has been used as an alternative fabrication method for metal-ceramic restorations. However, how the PoM technique compares with the conventional porcelain layering (CPL) technique under a variety of conditions is unclear. Purpose. The purpose of this in vitro study was to compare the bond strength of 3 alloy substrates with heat-pressed ceramics or conventionally layered porcelain before and after thermocycling. Material and methods. Specimens (n= 5) of Au, Pd, and Ni-Cr alloys were veneered with heat-pressed ceramics or conventionally layered porcelain. The 3-point bend test was conducted according to the International Organization for Standardization standard 9693-1 as bond strength before and after thermocycling. The metal-ceramic interfaces were characterized by field emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectroscopy (EDS). Two-and 3-way ANOVA followed by the Tukey honestly significant difference (HSD) test were used to analyze the data (alpha=.05). Results. Significantly lower mean bond strength was recorded for the Au and Pd alloys of the PoM group than for those of the CPL group (P<.05). CPL-Au demonstrated the highest bond strength of 50.2 +/- 2.0 MPa, whereas PoM-Pd showed the lowest bond strength of 31.8 +/- 2.7 MPa; significant differences were found among all groups (P<. 05). After 20 000 thermocycles, CPL-Au showed significantly reduced bond strength value (P<. 05). A value of approximately 40 MPa was observed in all groups except for PoM-Pd (26.5 +/- 1.6 MPa, P<. 05). The metal-ceramic interface resulting from the PoM technique revealed 2-to 20-mm pores, with more defects observed in the PoM-Pd group than in any of the other group. Conclusions. Defects and an oxide layer were formed at the metal-ceramic interface during the heat-pressing process, especially for the Pd alloy. After thermocycling, PoM-Pd had the lowest bond strength value, although it exceeded the minimum 25 MPa of the ISO 9693-1 standard. The Au and Ni-Cr alloys exhibited similar levels of porcelain bond strength with both techniques.