Identification of monoclinic phase in CAD/CAM zirconia FPD frameworks

Objective: The aim of this study was to identify the presence of monoclinic zirconia phase (m-ZrO2) in 5 computeraided design/computer-assisted manufacture zirconia systems composed of yttria-stabilized tetragonal zirconia polycrystals (Y-TZP). Materials and methods: Three-unit fixedpartial dentures were prepared from Cercon (CR); Lava (LW); Zenotec Zr Bridge (WD); In-Ceram YZ (YZ); and IPS e-max ZirCAD (ZC), all milled in a pre-sintered stage and then fully sintered according to the manufacturers' instructions. Raman spectroscopy was used to identify and map the distribution of the m-ZrO2 phase at cervical crown margins, pontic and connector regions. Three sets of data were obtained from each material (n = 2) and the percentage volume (%Vm) of the m-ZrO2 phase was calculated per region. Statistical analysis was performed by two-way analysis of variance and Tukey test (a = 0.05). Results: The m-ZrO2 phase was detected in all the specimens, with the highest intensity located at the crown margins. WD showed the lowest %Vm content (0- 3.14%), followed by LW (10.26-12.39%), CR (11.72-13.19%), ZC (11.13-14.10%) and YZ (12.15-14.99%). No statistically significant difference was found among LW, CR, ZC, YZ per region. Within each material group, significant differences were found between margin-pontic/connector (WD, YZ), margin-connector (CR, ZC) and margin-pontic (LW). Significance: The Y-TZP destabilizing m-ZrO2 phase was identified in all the fully sintered frameworks tested, with the highest %Vm located at the margins. The extent to which the presence of this phase may be implicated with zirconia low temperature degradation or porcelain to zirconia bonding is unknown.