Flexural Strength and Reliability of Dental CAD-CAM Restorative Materials

Objectives: The aim of this study was to investigate the flexural strength values and Weibull distribution of restorative materials for CAD-CAM; two recent hybrid ceramics, Vita Enamic (EN, Vita Zahnfabrik) and Lava Ultimate (UL, 3M ESPE), and one monolithic zirconia, Lava Plus high translucency zirconia (LZ, 3M ESPE).
Methods: Flexural specimens (4.0 mm x 1.4 mm x 18.0 mm) were prepared by machining from EN (2M2HT), UL (A1-HT), and sintered LZ blanks (Multi L). The rectangular bar specimens were ground lengthwise and polished with diamond paste (6 um and 1 um). The opposing faces were highly flat and parallel within 0.01 mm using a high precision grinding machine and edges were rounded. The specimens were stored in 37°C distilled water for 2 days before testing. Flexural strength of specimens was determined by three-point test with a 14-mm span and a constant loading rate of 1.0 mm/min using Instron 3344 with Bluhill 2 software. Mean flexural strength and modulus values were obtained from 20 specimens for each group. Statistical difference was analyzed by ANOVA and scheffe’s test. Strength data were analyzed by 2-parameter Weibull distribution function to calculate Weibull modulus and characteristic strength according to ISO 6872.
Results: Mean flexural strength values (MPa) and modulus (GPa) were as follows: EN (140.6±6.1, 24.4±0.3), UL (159.3±5.8, 10.8±0.1), and LZ (1106.7±118.6, 86.8±0.5). There was no statistical difference in flexural strength between EN and UL (p>0.05), but these are significantly lower than LZ (p<0.05). In comparison of two hybrids using a unpaired t-test, flexural strength of UL was significantly higher than EN, but flexural modulus of UL was significantly lower (p<0.05). Weibull modulus and characteristic strength were as follows: EN (27.7, 143.4), UL (32.4, 162.0), and LZ (11.0, 1158.1).
Conclusions: Static flexural properties of two hybrid materials (EN, UL) were significantly lower than all-zirconia restorative material (LZ). However, the reliability of flexural strength of hybrid materials was much higher than monolithic zirconia.