Effect of resin-modified glass ionomer containing bioactive glass on the flexural strength and morphology of demineralized dentin.

INTRODUCTION: Recently, bioactive materials have been incorporated into glass ionomer cements to promote the precipitation of calcium phosphates in surrounding tooth structures. This in vitro study was undertaken to evaluate the effect of resin-modified glass ionomer (RMGI) containing bioactive glass (RMGI-BAG) on the flexural strength (FS) of demineralized dentin. MATERIALS AND METHODS: A total of 120 dentin bars (2×2×6 mm) were prepared from sound human third molars. Of these, 60 bars were immersed in a demineralizing solution for 96 hours. This produced dentin in two demineralization conditions (DC): untreated and demineralized. Each dentin bar was immersed for 14 days in simulated body fluid (SBF) at 37°C. Three immersion conditions (IC) were investigated: IC1-SBF only; IC2-SBF + an RMGI bar; IC3-SBF + an RMGI-BAG bar. The combination of the DCs and ICs produced six groups (n=20). FS values of the specimens were measured using a three-point bending test. The microstructural changes and the elemental contents of dentin surfaces were evaluated by scanning electron microscopy. Data were analyzed using a two-way analysis of variance (ANOVA) for the effects of the two independent variables, ie, DC and IC, on mean flexural strength. Tukey multiple comparison tests and simple main effects models were used as needed. The significance level of all tests was set at α=0.05. RESULTS: Both DC (p=0.001) and IC (p=0.049) significantly influenced FS (two-way ANOVA). The interaction between DC and IC did not significantly affect FS (p=0.36). For undemineralized dentin, IC did not affect the mean FS (simple main effects model; p=0.4). However, for demineralized dentin, IC significantly affected FS (small main effects model; p=0.008). The Tukey test showed that for demineralized dentin, the mean FS produced by immersion in SBF + RMGI-BAG was significantly stronger than that produced by either immersion in SBF only (p=0.011) or in SBF + RMGI (p=0.034). Scanning electron microscope/energy-dispersive x-ray spectroscopy analyses revealed more calcium and phosphate ions on the surface of dentin immersed in SBF + RMGI-BAG than on dentin immersed in SBF + RMGI. CONCLUSION: Immersion in SBF + RMGI-BAG increased the FS of demineralized dentin more than immersion in SBF + RMGI.

Effect of prewarming and/or delayed light activation on resin-modified glass ionomer bond strength to tooth structures.

INTRODUCTION: Recent research shows that the acid-base reaction and light-activated polymerization in resin-modified glass ionomers (RMGI) compete with and inhibit one another. In addition, extrinsic energy would improve some properties of RMGI. This in vitro study evaluated the effect of prewarming and/or delayed light activation on bond strength of RMGI to tooth structure. MATERIALS AND METHODS: Ninety-six flat enamel and dentin surfaces of human molars were ground with sequentially finer abrasives to 600-grit silicon carbide paper. Each surface was treated with a cavity conditioner for 10 seconds, rinsed, and gently air-dried (n=12). RMGI was applied to tooth substrates according to the following protocols: group 1) according to manufacturer's instructions; group 2) a delay of two minutes in light activation; group 3) prewarming of the encapsulated material (90 seconds, 40°C); group 4) prewarming plus a delay of two minutes in light activation. After 24 hours of storage at 37°C and 500 rounds of thermocycling, the samples were tested for shear bond strength and analyzed using two-way analysis of variance and Tukey HSD test (α=0.05). RESULTS: Significant differences were observed between study groups (p<0.05). The highest enamel bond strength was recorded in group 2. Regarding dentin groups, the highest bond strength was recorded in group 1. CONCLUSION: Within the limitations of the present study, delaying light activation might improve bond strength of RMGI to enamel; however, the standard procedure recommended by the manufacturer is the best procedure for bonding of RMGI to dentin. More investigations are necessary.

Bond strength of composite resin to bleached dentin: effect of using antioxidant versus buffering agent.

OBJECTIVE: Application of sodium ascorbate as an antioxidant and calcium hydroxide as a buffering agent following intracoronal bleaching has been recommended. The aim of this study was to investigate the effect of using the mentioned materials on shear bond strength of composite resin to the bleached dentin. MATERIALS AND METHODS: In this in vitro investigation, sixty human sound premolars were divided randomly into five groups (n=12). Occlusal dentin surfaces were exposed. The negative control (NC) group was not bleached and the other groups were bleached with 35% hydrogen peroxide gel for 5 days. Afterwards, composite cylinders were built up in the positive control (PC) group immediately after bleaching, in the delay bonding (DB) group after one week, in the sodium ascorbate (SA) and calcium hydroxide (CH) groups after 40 hours of treatment with the materials. Then, the samples were stored in 37°C for 24 hours. The specimens were thermocycled (5-55°C, 500 cycles), subjected to shear bond testing by universal machine. The data were analyzed by One-Way ANOVA and Duncan tests (α =0.05). RESULTS: There was a significant difference between PC and CH groups in comparison with the other groups (p<0.05), but the difference among other groups was not significant (p>0.05). CONCLUSION: Application of sodium ascorbate could significantly increase the bond strength of composite resin to bleached dentin, while the use of calcium hydroxide did not affect bond strength.

Effect of CPP-ACP application on flexural strength of bleached enamel and dentin complex.

INTRODUCTION: Controversy continues over the effects of bleaching treatments on tooth structures in the literature. In addition to tooth sensitivity, a number of studies have reported adverse effects of bleaching procedures on dental hard tissues, including decreased hardness, fracture toughness, flexural strength, and other changes in the mechanical properties. The purpose of this in vitro study was to investigate the effect of casein phosphopeptide-amorphous calcium phosphate (CPP-ACP) application on the flexural strength (FS) of bleached teeth. METHODS AND MATERIALS: One hundred twenty blocks (2 × 3 × 8 mm) were prepared from the middle portion of the facial surfaces of 120 sound bovine teeth. Specimens were randomly divided into six groups (n=20). Group 1 consisted of the control group, stored in distilled water at 37 °C. The experimental groups (2 to 6) were immersed in CPP-ACP (0.5 h/d for 14 days, twice daily), 9.5% hydrogen peroxide (HP) (0.5 h/d, twice daily for 14 days), 9.5% HP+CPP-ACP, 38% HP (1 h/d, twice weekly for 2 weeks), and 38% HP+CPP-ACP, respectively. Flexural strength test was performed 24 hours after the last treatment session using a universal testing machine with a crosshead speed of 0.5 mm/min. Results were analyzed by two-way analysis of variance (ANOVA) and a post hoc Tukey's test (á=0.05). RESULTS: Mean ± SD values for FS were 179.50 ± 24.16, 194.00 ± 21.31, 155.25 ± 32.7, 177.50 ± 30.15, 158.50 ± 27.49, and 177.50 ± 28.09 MPa, respectively. Statistically significant differences were observed in FS values between groups (p<0.05). Tukey analysis showed that using CPP-ACP subsequent to both bleaching techniques has a significant effect on tooth flexural strength. CONCLUSION: Simulated in-office or at-home bleaching regimens performed in the absence of saliva decrease the FS of tooth structure. Application of CPP-ACP subsequent to both bleaching regimens could compensate for the decreased FS of the bovine enamel-dentin complex in this environment.