Introduction: Celtra® Press is a zirconia-reinforced lithium silicate (ZLS) glass-ceramic containing 10 % zirconium oxide (ZrO2). This material is used to fabricate dental restorations by pressing technique. After heat pressing procedures leftover buttons are usually discarded. these leftovers buttons are suggested to be re-pressed with concerns regarding changes in microstructure and mechanical properties.

Objectives: To evaluate the effects of re-pressing on the mechanical properties and microstructure of Celtra® Press.

Materials and methods: A total of 24-disc specimens; 14mm in diameter and 1.5 in thickness were fabricated by lost wax technique. Specimens were divided into two parallel groups, Group I: Pressed specimens (n=12) and Group II: Re-pressed specimens (n=12). Biaxial flexural strength (BFS) was measured using a piston-on-three-ball test. X-ray diffraction (XRD) was used to identify crystalline phases. Scanning electron microscope (SEM) photomicrographs (5000x) were used for microstructure analysis.

Results: The mean and standard deviation values of BFS for the pressed specimens (Group I) and the re-pressed specimens (Group II) were (136.72 ± 29.41) and (167.24 ± 36.46) MPa respectively. BFS of the repressed group was significantly higher than the pressed group (p=0.001). SEM photomicrographs (5000x) showed an increase in the grains size after repressing. X-ray diffraction revealed lithium silicate as the main crystalline phase and the peak intensities of the re-pressed specimens were lower than the pressed specimens.

Conclusion: Re-pressing of Celtra® Press improved the BFS values significantly. Lithium silicate was the main crystalline phase in the pressed and re-pressed specimens. Celtra® Press grains were larger after re-pressing under SEM.