Evaluation of the Effect of Chairside Adjustment Extent on the Biaxial Flexural Strength of Aged Monolithic Zirconia

Background and objective Chairside adjustment of monolithic zirconia restorations is a common clinical procedure. This study aimed to investigate how surface treatment procedures and the extent of adjustment affect the biaxial flexural strength (BFS) of monolithic zirconia after artificial aging. Methods Fifty-six disk-shaped specimens (Ø15 × 1.2 ± 0.2 mm) were milled, sintered, and glazed from zirconia blocks (Ceramill, Amann Girrbach, Koblach, Austria) according to ISO 6872:2024. The specimens were then divided into seven groups (n = 8) based on surface treatment (grinding, polishing, or reglazing) and extent of adjustment (one vs. two areas): no adjustment (G1), grinding one area (G2), grinding + polishing one area (G3), grinding + reglazing one area (G4), grinding two areas (G5), grinding + polishing two areas (G6), and grinding + reglazing two areas (G7). All specimens were hydrothermally aged in an autoclave (134 °C, 2 bar, five hours) before BFS testing (piston-on-three-ball method, crosshead speed 1 mm/min). Data were analyzed using two-way ANOVA and Tukey's post hoc test (α = 0.05). Results The highest BFS value was recorded for group G6 (grinding + polishing two areas), whereas group G4 (grinding + reglazing one area) showed the lowest value. Both surface treatment and extent of adjustment significantly affected the BFS of monolithic zirconia specimens (p < 0.05). Grinding and polishing significantly increased BFS values, while reglazing had a significant negative effect. Conclusions Proper polishing after chairside grinding provided the best flexural strength following artificial aging, whereas reglazing may compromise the flexural strength of monolithic zirconia. The flexural strength of monolithic zirconia is significantly influenced by the extent of adjustment.