Effect of Surface Modifications and Chewing Cycling on Monolithic Zirconia

Objectives: The objective of this study was to determine the minimum thickness required for monolithic zirconia in order to resist chewing forces by evaluating the flexural strength with or without surface modifications.
Methods: Monolithic zirconia discs (10 mm in diameter) were fabricated at different thicknesses: 1.3 mm, 1.0 mm and 0.7 mm. For each thickness, 21 discs were fabricated. The samples of each thickness group was further divided into 3 subgroups (n=7) according to the surface treatments applied: 1-sandblasting with 50 µm Al₂0₃ particles at a pressure of 4 bars and a distance of 10 mm, 2- grinding with a diamond bur and polished, 3-untreated control group. The samples underwent a chewing cycling at a force of 50 N for 240,000 cycles. The biaxial flexure strength was determined with using a piston-on-three- ball technique in a universal testing machine. Flexural loading was applied by centering on the disc, using a steel cylinder of 1.4 mm diameter at a crosshead speed of 0.5 mm/min until fracture occured. The data were statistically analyzed using 2-way ANOVA tests, followed by Tukey HSD (p<0.05).
Results: Sandblasted and ground samples had higher flexural strength than the samples of control group. However, sandblasting significantly increased the flexural strength of samples compare to grinding with a diamond bur (p<0.05). While sandblasted 0.7 mm thick zirconia samples showed the highest flexural strength (918 ± 259 MPa), the bur ground 1 mm thick samples revealed the lowest (644 ± 105.5 MPa). The change in the thickness of the samples did not affect the flexural strength significantly (p>0.05).
Conclusions: The results indicated that sandblasting strengthens monolithic zirconia. Specimen thicknesses of 0.7, 1.0, and 1.3 mm did not reveal differences in flexural strength.