RESEARCH AND EDUCATION The effect of extended glaze firing on the flexural fatigue strength of hard-machined ceramics Iana Lamadrid Aurélio, DDS, MSD, PhD, a Catina Prochnow, DDS, MSD, b Luís Felipe Guilardi, DDS, MSD, c Gabriela Freitas Ramos, DDS, MSD, d Marco Antonio Bottino, DDS, MSD, PhD, e and Liliana Gressler May, DDS, MSD, PhD f Critical flaws in ceramic materials can be introduced as a function of the processing method, which includes all steps used in making a ceramic part, from shaping to firing. 1 Com- mon flaws include subsurface and surface damage from computer-aided design and computer-aided manufacturing (CAD-CAM) and rotational diamond grinding during inter- nal adjustments. 2 Recently, Curran et al 3 assessed subsur- face and surface damages on ground feldspar and glass- ceramics for hard machining, 4 in which fully sintered blocks were milled into the expected restoration design, and esti- mated the potential losses in the strength, based on crack size measurements of the generated damage. Based on the re- sults, which suggested a potential strength loss estimated between 33% and 54%, the authors advised examining the margin quality be examined using stereomicroscopy and assess the type of chip damage present in densely sintered milled (hard-machined) ceramic blocks. 3 Hard-machined restorations can be obtained from leucite-based (LEU) and lithium disilicate-based (DIS) ceramics. 4,5 Favorable lifetime predictions indicated that 90% of single- unit milled restorations obtained from these materials would survive at least 10.9 years. 6 However, current in vitro studies suggest that hard a Professor, Faculty of Dentistry, University Center of Serra Gaúcha FSG, Rio Grande do Sul, Brazil. b Doctoral student, Department of Restorative Dentistry, Faculty of Dentistry, Federal University of Santa Maria, Rio Grande do Sul, Brazil. c Doctoral student, Department of Restorative Dentistry, Faculty of Dentistry, Federal University of Santa Maria, Rio Grande do Sul, Brazil. d Doctoral student, Department of Restorative Dentistry, São José dos Campos Dental School, São Paulo State University, São Paulo, Brazil. e Titular Professor, Department of Restorative Dentistry, São José dos Campos Dental School, São Paulo State University, São Paulo, Brazil. f Adjunct Professor, Department of Restorative Dentistry, Faculty of Dentistry, Federal University of Santa Maria, Rio Grande do Sul, Brazil. ABSTRACT Statement of problem. It is unclear whether an extended glaze firing could improve the long-term mechanical performance of densely sintered CAD-CAM ceramics. Purpose. The purpose of this in vitro study was to analyze the effect of an extended glaze firing on the flexural fatigue strength (FFS) of densely sintered milled (hard-machined) leucite-based (LEU) and lithium disilicate-based (DIS) ceramics. Material and methods. Disks were machined from ceramic blocks and divided into 6 groups (n=20) according to the material, LEU or DIS, and to the applied glaze firing: manufacturer- recommended glaze (G group), extended glaze (EG group), and control/no firing (C group). The surface roughness of the disks was measured before and after firing by using a contact profilometer, and data were compared by paired sample tests. Specimens were submitted to fatigue by using the staircase test design in water (piston-on-3 balls; 500 000 cycles, 20 Hz, and sinusoidal loading). Mean (±SD) FFS values were then calculated and analyzed by using 1-way analysis of variance and post hoc Tukey test (a=.05). Results. Surface roughness did not change after the firing (P>.05). The highest FFS value in both ceramics was obtained after EG firing (LEU-EG=80.52 ±6.3 MPa; DIS-EG=147.25 ±10.5 MPa), which was statistically superior to G firing (LEU-G=73 ±6.8 MPa, P=.003; DIS-G=134.34 ±15.6 MPa; P=.023) and C group (LEU-C=61.94 ±6.3 MPa; P<.001; DIS-C=134.13 ±17.3 MPa; P=.023). Conclusions. EG firing optimized the biaxial flexural fatigue strength of hard-machined leucite and lithium disilicate ceramics compared with conventional glaze firing. (J Prosthet Dent 2018;-:---) THE JOURNAL OF PROSTHETIC DENTISTRY 1