Comparison of 3D changes of 3Y, 4Y, and 5Y zirconia single crowns under different sintering protocols

Different sintering protocols had been documented to affect the microstructure, grain size, mechanical, and optical properties, as well as the internal and marginal fit of zirconia prostheses. Yttria-partially stabilized zirconia (3Y-PSZ), 3Y; (4Y-PSZ), 4Y; and (5Y-PSZ), 5Y are indicated for different clinical scenarios, and, how different sintering protocols might affect their respective final dimensions is not well documented. The manufacturers' disk-specific magnification factor does not distinguish between standard or rapid sintering, which may lead to changes in the final dimensions of prostheses.

The primary purpose of this in vitro study was to compare the 3-dimensional (3D) changes of 3Y, 4Y, and 5Y zirconia single crowns under standard and rapid sintering protocols. A secondary purpose was to assess whether the disk-specific magnification factor was applicable to both these sintering protocols.

A standardized complete ceramic crown preparation cast model was fabricated in cobalt chromium (Co-Cr) alloy to receive complete monolithic zirconia molar crowns made of 3Y (Zi; Amann Girrbach AG), 4Y (Zolid HT+; Amann Girrbach AG), and 5Y (Zolid FX; Amann Girrbach AG) zirconia. The crowns were milled from partially sintered zirconia disks and sintered using either standard or rapid sintering protocols. There were 6 experimental groups; 3S, 3R, 4S, 4R, 5S, 5R with a sample size of 15 for each group (n=15). The disk-specific magnification factor was also investigated for interaction with the sintering protocols. Clinical marginal fit was assessed, and the crowns were then scanned using an intraoral scanner (TRIOS 4; 3Shape A/S) to produce standard tessellation language (STL) files, which were imported into a 3D software system (Geomagic Control X; 3D Systems) to compare their 3D changes at the Overall (O), External (E), Intaglio Occlusal (IO), Intaglio Axial (IA), and Margins (M) segments. Color maps and root mean square (RMS) values were generated to measure their deviation from the control model. Two-way ANOVA with the Bonferroni post hoc test (α=.05) was used for both the primary purpose of comparing 3D changes of the 3Y, 4Y, and 5Y zirconia under standard and rapid sintering protocols and for the secondary purpose of assessing whether the disk-specific magnification factor was applicable to both these sintering protocols by assessing their interactions.

All zirconia crowns had clinically acceptable marginal fit. For 3Y zirconia, the RMS was significantly lower under rapid sintering (15 ±2 µm) compared with standard sintering (27 ±2 µm) at the IO segment (P<.001) and significantly lower under rapid sintering (19 ±1 µm) compared with standard sintering (23 ±1 µm) at the IA segment (P=.036). For 4Y zirconia, the RMS was significantly higher under rapid sintering (22 ±1 µm) compared with standard sintering (19 ±1 µm) at the O segment (P=.032),significantly higher under rapid sintering (26 ±1 µm) compared with standard sintering (20 ±1 µm) at the IA segment (P=.010), and significantly higher under rapid sintering (30 ±2 µm) compared with standard sintering (21 ±2 µm) at the M segment (P=.022). For 5Y zirconia, no significant difference in the RMS was found between the 2 sintering protocols for any of the crown segments. The disk-specific magnification factor was more significant under rapid sintering than under standard sintering.

Rapid sintering improved the fit of 3Y zirconia single crowns but worsened the fit of 4Y zirconia single crowns. The sintering protocol did not affect 5Y zirconia single crowns. The disk-specific magnification factor was more significant under rapid sintering than under standard sintering. All zirconia single crowns had clinically acceptable marginal fit, regardless of whether 3Y, 4Y, or 5Y zirconia, the standard or rapid sintering protocol, and the disk-specific magnification factor were used.