Long-term effects of gastric juice alternated with brushing on microhardness, substance loss, flexural strength and reliability of CAD-CAM monolithic materials

The research hypotheses were that the acidic challenge alternated with brushing would promote lower microhardness and biaxial flexural strength and higher substance loss when compared to brushing alone. The CAD-CAM materials evaluated [Lava Ultimate (LU), Vita Enamic (VE), IPS Empress CAD (EMP), IPS e.max CAD (EMAX), and Vita Suprinity (VS)] were submitted to 1) artificial saliva+brushing or 2) gastric juice+brushing, simulating 1, 3, and 5 years of clinical function. Each year of clinical function corresponded to 3 cycles of 4 months. Each cycle of 4 months corresponded to 3 h of immersion in artificial saliva or gastric juice exposure followed by 1,217 cycles of brushing using a toothbrush machine. Microhardness (Vickers diamond indenter under 20-N load and a 20-second dwell time) and substance loss (analytical balance XS105 Dual Range; Mettler Toledo) analyzes were performed at the baseline (T0) and at the end of the 1st (T1), 3rd (T3) and 5th (T5) year. For the substance loss, the difference in mass between each year and the baseline was calculated, resulting in ΔT0-T1, ΔT0-T3, and ΔT0-T5. The biaxial flexural strength test was performed at the end of the 5th year of each treatment in a mechanical testing machine according to the ISO 6872 standard. The Weibull statistics based on the BFS data determined the Weibull modulus (m) of each group. Normality and homoscedasticity were verified using the Shapiro-Wilk and Levene's tests. Although some groups did not meet these assumptions, we decided to proceed with the ANOVA, which is known to be robust for moderate deviations from normality and homocedasticity in cases of block design with balanced groups. Microhardness data were submitted to mixed repeated-measures ANOVA with time as the within-subjects factor and material and treatment as the between-subjects factors. The Bonferroni test was used to assess the differences among groups. The differences in mass were analyzed by 3-way ANOVA with material, treatment and time as independent variables, followed by the Games-Howell test. Finally, BFS data were submitted to 2-way ANOVA with material and treatment as independent variables, followed by the Games-Howell test. The level of significance was set at 0.05. The statistical analyses were performed using the IBM SPSS Statistics v22.0 statistical software. Data analysis revealed that the LU was not influenced by the treatment, while the other materials showed significant lower microhardness in the gastric juice