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+brushing groups in certain years. In both treatments, the materials did not present a significant change in microhardness over the years. Only the LU and the VE showed higher substance loss in the gastric juice+brushing groups in certain times. In general, all materials exhibited an increase in the substance loss over the years. The treatment did not affect the BFS of the materials. The gastric juice+brushing decreased the reliability of the VE. The original data are exposed in “My files”.
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Specimen preparation and treatments CAD-CAM disks (Ø=10.0 mm) were polished with silicon carbide papers (600, 1200, 1500 grits) under water irrigation (final thickness was 1.2 mm ±0.02 mm). The EMAX and VS disks were crystallized (Programat P310; Ivoclar Vivadent AG) according to the manufacturers' instructions. The disks were allocated into two groups: 1) alternation between artificial saliva and brushing (control) and 2) alternation between gastric juice and brushing, simulating 1, 3, and 5 years of clinical function. Figures 1 and 2 show the protocol used to simulate 1 year of each treatment. Considering that bulimic patients generally regurgitate 3x/day and that each episode lasts 30 s, to simulate 4 months of exposure to gastric juice, it would take 3 h of immersion in the solution. The simulated gastric juice was prepared with 0.113% hydrochloric acid (HCl) solution in deionized water, pH 1.2. The specimens were immersed in 1 ml of artificial saliva or simulated gastric juice in 24-well flat-bottom microtiter plates. Regarding brushing, assuming that 1 year corresponds to 3,650 cycles, 1,217 cycles would simulate 4 months. Brushing was carried out in a brushing machine under 2.0 N and 1 Hz with an Oral B 40 soft bristles toothbrush and a solution of distilled water and toothpaste (Colgate, Colgate-Palmolive, São Bernardo do Campo, Brazil) in a 1:1 ratio. The toothbrushes were replaced every 2 h and 40 min, and the toothpaste slurry was replaced every 48 h. Microhardness For the microhardness analysis, 5 indentations were made in each specimen using a Vickers diamond indenter under 20-N load and a 20-second dwell time. Twenty disks were obtained from each material, being 10 for each treatment, since the readings at the baseline (T0) and at the end of the 1st (T1), 3rd (T3) and 5th (T5) year were made in the same specimens. Substance loss For the substance loss (μg), 20 additional disks were obtained and divided as described for the microhardness test. Before the readings, the specimens were ultrasonically cleaned in distilled water for 5 minutes and stored in an oven at 37ºC for 7 days. For the weighing, the specimens were positioned on an analytical balance (XS105 Dual Range; Mettler Toledo), and after 30 seconds, the measurements were recorded. The difference in mass between each year and the baseline was calculated, resulting in ΔT0-T1, ΔT0-T3, and ΔT0-T5. Biaxial flexural strength and reliability For the biaxial flexural strength (BFS) test, 60 disks were obtained from each material, being 30 for each treatment. This analysis was performed after the 5th year of each treatment in a mechanical testing machine (EMIC Equipment and Systems Testing) according to the ISO 6872 standard. The Weibull statistics based on the BFS data determined the Weibull modulus (m) of each group.