Fatigue behavior of multilayered zirconias: Impact of finishing and polishing protocols.

STATEMENT OF PROBLEM: Evidence regarding the effect and standardization of finishing and polishing protocols on the fatigue resistance and surface characteristics of multilayered zirconia remains limited.

PURPOSE: The purpose of this in vitro study was to evaluate the effect of different finishing and polishing protocols on fatigue failure load (FFL), the number of cycles to failure (CFF), and the survival rates of new dental zirconias.

MATERIAL AND

METHODS

Sixty ceramic disks (Ø12×1.5 mm) following the International Organization for Standardization (ISO) 6872 standard were fabricated with 3 zirconia systems: ultratranslucent zirconia (UT), multilayered 3Y-TZP/5Y-PSZ zirconia (GT), and multilayered 4Y-PSZ/5Y-PSZ zirconia (SG) (N=180). After sintering, the specimens from each zirconia were divided into 6 groups (n=15) according to the protocols: DR (diamond rotary instruments + rubber points), R (rubber points), GL (diamond rotary instruments + rubber points + glaze), and C (control). Fatigue resistance was tested using the stepwise stress method (5 Hz, 100 000 cycles). X-ray diffraction (XRD), fractography, and surface roughness analyses were also performed. Two-way ANOVA, the Tukey pairwise comparison test (α=.05), Kaplan-Meier, and Mantel-Cox tests were used for statistical analyses (α=.05).

RESULTS

The finishing and polishing protocol (P=.001), zirconia type (P<.001), and their interaction (P=.028) affected fatigue resistance. For GT, zirconia rubber polishing (GTR- 831.94A MPa) presented higher fatigue resistance than the control group (GTC- 646.53B MPa), while for UT, zirconia diamond rotary instruments + rubber points reduced the resistance (UTDR- 273.44D MPa) compared with the control group (UTC- 383.02C MPa). SG zirconia showed no significant differences across finishing and polishing protocols. For CFF, the finishing and polishing protocol significantly influenced (P<.001) the UT ceramic. Glaze (0.42D µm) reduced the roughness of SG ceramic compared with the control group (0.93AB µm). The highest roughness value was observed in the UT control group (UTC- 0.99A µm).

CONCLUSIONS

Finishing and polishing affected the mechanical strength and surface characteristics of zirconia, depending on zirconia composition and microstructure. For ultratranslucent zirconia, finishing procedures reduced fatigue strength, whereas for multilayered zirconia, finishing protocols resulted in increased fatigue strength.