Effect of different sintering protocols on the fracture strength of 3-unit monolithic gradient zirconia fixed partial dentures: An in vitro study

Statement of problem. Strength-gradient zirconia combining 3 zirconia formulations with different flexural strengths has been reported to have outstanding mechanical properties. However, data concerning the effect of different sintering protocols on the fracture strength of 3-unit monolithic gradient zirconia fixed partial dentures (FPDs) are sparse.Purpose. The purpose of this in vitro study was to test the effect of different sintering protocols on the fracture strength of 3-unit monolithic gradient zirconia FPDs.Material and methods. Two custom-made stainless-steel master dies were designed to replicate a mandibular right second premolar and second molar prepared to receive a 3-unit monolithic zirconia FPD. Thirty monolithic zirconia FPDs were milled from gradient zirconia blanks and allocated to 3 groups (n=10) according to the sintering protocols: high-speed sintering, speed sintering, and conventional sintering. The FPDs were cemented onto the corresponding dies with traditional glass ionomer cement. All FPDs were cyclic loaded (600 000 cycles/49 N/1.7 Hz) in a mastication simulator. Fracture load measurements for each FPD were determined by using a universal testing machine. Scanning electron microscopy (SEM) at x80 magnification was used to examine a fractured FPD from each group. A representative specimen from each group was examined with SEM at x30 000 magnification to determine the grain size. One-way ANOVA, pair-wise Tukey honestly significant difference (HSD), and Pearson correlation tests were used for statistical analysis of the data (alpha=.05).Results. The high-speed sintered FPDs recorded the highest statistically significant fracture load mean +/- standard deviation value (2526 +/- 300 N), followed by the speed sintered FPDs (2136 +/- 127 N), while the lowest statistically significant fracture load mean value was recorded with the conventionally sintered FPDs (1361 +/- 181 N) (P<.001). In addition, the mean +/- standard deviation grain size values were 488 +/- 272 nm for the high-speed sintered specimen, 578 +/- 409 nm for the speed sintered specimen, and 832 +/- 551 nm for the conventionally sintered specimen (P<.001). A significant negative correlation was found between fracture strength and grain size among the 3 groups.Conclusions. The fracture strength of 3-unit monolithic gradient zirconia FPDs sintered by using a high-speed protocol was significantly higher than that of speed and conventionally sintered FPDs (P<.001). The high-speed sintering protocol reduced the mean grain size of gradient zirconia FPDs compared with that of both speed and conventional sintering protocols. (J Prosthet Dent 2023;130:908.e1-e8)