Influence of speed sintering on the fit and fracture strength of 3-unit monolithic zirconia fixed partial dentures

Speed sintering has been introduced to enable single-visit monolithic zirconia prostheses. However, the fit and fracture load of zirconia 3-unit monolithic fixed partial dentures (FPDs) after speed sintering are unknown. The purpose of this in vitro study was to test the properties of zirconia 3-unit monolithic FPDs after speed sintering and to compare the properties with conventional sintering. A calibrated operator digitized an in vitro model with a complete coverage preparation of a maxillary right second premolar and second molar (n=12) by using the CEREC AC Omnicam (Dentsply Sirona) scanner. Twelve zirconia FPDs were designed (CEREC SW 4.1.1), and for each data set (n=12), 1 FPD was designed and milled 4 times (MCXL Premium; CEREC Zirconia; Dentsply Sirona), resulting in 4 identical monolithic FPDs (N=48). The FPDs were divided into 2 groups according to the sintering procedure (n=24): speed sintering (group S) by using the SpeedFire (Dentsply Sirona) and the conventional sintering (group C) by using the inFire HTC speed (Dentsply Sirona). All the FPDs were glazed by using glaze-spray and fired according to the sintering group. The SpeedFire (Dentsply Sirona) was used for group S, and the VACUMAT 6000M (VITA Zahnfabrik) was used for group C. The fit of the FPDs was evaluated with the replica technique by using polyvinyl siloxane and analyzed according to the measurement areas: marginal gap, chamfer area, axial wall, and occlusal area. Subsequently, groups S and C were further subdivided, and 12 specimens per group underwent artificial aging by thermomechanical loading in a mastication machine (50 N for 1.2×106 times at a frequency of 1.7 Hz and a thermal change in distilled water from 5 °C to 50 °C every 120 seconds), resulting in additional subgroups: group SA and group CA. For all the FPDs (groups S, C, SA, and CA), a fracture load measurement was conducted. The Kolmogorov-Smirnov test was used to examine the values of the fit and fracture load for normal distribution. The Mann-Whitney U test for the fit and a 2-way ANOVA for the fracture load were used to detect the differences among the groups (α=.05). Group S showed a better marginal (P=.018) and occlusal (P