| Object: Characteristic of dental hard tissue is key for microtensile bond strength. Nowadays, there are many reports about the effect of normal dental tissue , carious dental tissue and non-carious sclerotic dental tissue on microtensile bond strength, but few about irradiated dental tissue. One purpose of this study was to evaluate the effect of irradiation on microhardness and microstructure of enamel and dentin, and the other was about the effect of irradiation on microtensile bond strength of enamel and dentin by total-etch adhesive system and all in one self-etching adhesive system, so as to provide experimental evidence for selecting suitable adhesive and method in clinical practice.Material and methods:16 extracted intact third molars were used to study the effect of irradiation on microhardness and microstructure of enamel and dentin. They were randomized into enamel group and dentin group (n=8), and in each group 6 teeth were used for determining microhardness and the remaining 2 for observing microstructure.After sample preparation, either lingual or buccal parts of the enamel group and occlusal flat dentin of the dentin group were irradiated fractionally up to 60Gy, 2Gy/day, 1time/day, 5times/week. After microhardness measured, microstructures of the irradiated enamel and dentin were observed by scanning electron microscope (SEM).Another 60 extracted intact third molars were used to study the effect of irradiation on microtensile bond strength of enamel and dentin. They were randomized into enamel group and dentin group (n=30). Each group was further divided into 4 trial subgroups (P8, P15, P30, G1 n=5 ) and 2 control subgroups(PR, GR, n=5) . After sample preparation, either lingual or buccal parts of all the enamel trial subgroups and occlusal flat dentin of all the dentin trial subgroups were irradiated fractionally up to 60Gy, 2Gy/day, 1 time/day, 5 times/week. Then the irradiated enamel and dentin was bonded with total-etching adhesive Prime& Bond NT(PB) in different etching time(8s, 10s, 30s)and self-etching adhesive G-Bond(GB) according to the manufacturer's instructions, and a block of 4-5mm thick light-cured resin TPH3 was built up with each bond. After that, the specimens were stored in distilled water for 24h at 37±1℃, and then cut into rectangular-shaped bonded specimens with cross-sectional area of about 0.81mm 2 by a hard tissue cutting machine. The specimens were then tested to failure in tension mode at a crosshead speed of 1 mm/min. The data was submitted to one-way ANOVA, then followed by LSD post hoc test (alpha=.05), and every fractured specimen was examined by stereomicroscope. Finally, the microstructure characteristics of the enamel/dentin-adhesive resin interface of each group were observed by SEM.Results:1 The microhardness of enamel and dentin in trial groups were obviously lower than that in control groups (p<0.05). SEM displayed that the enamel surface appeared cracks and crater-like pores with irregular edge,loose structures and melted.The diameters of tubules were not equal and some were even obstructed.2 The microtensile bond strength for PB and GB adhesives bonding irradiated enamel was lower than that of the control enamel, but the difference for GB was not significant (P15R P=0.00, G1R P>0.05). With etching time extending (8s, 15s, 30s), microtensile bond strength for PB bonding radiated enamel increased accordingly (P81530). The differences for P8 and P30, P15 and P30 were both significant (P<0.05), but P8 and P15 is not significant (P>0.05). Extending etching time to 30s microtensile bond strength of PB bonding irradiated enamel could be improved, but still lower than that of the control dentin (P<0.05). The microtensile bond strength of PB bonding irradiated enamel was higher than that of GB, but the difference was not significant(P15>G1 , p>0.05. Most of fracture modes were adhesive and mixed failure, and differences were not significant among the groups. SEM displayed that the hybrid layer thickness of PB adhesive bonding the irradiated enamel was about 5μm, which was similar with the control group, but the thickness was uneven, and the resin tags were disorderly and in cluster distribution. The hybrid layer of PB adhesive bonding the irradiated enamel became thicker with etching time extending(8s: 4μm, 15s: 5μm, 30s: 6μm), and the resin tags became stronger. Thickness of the hybrid layer formed by GB bonding irradiated enamel was about 3μm, which was slightly thinner than that of the control group, and only a few thin resin tags formed .3 The microtensile bond strength of PB and GB adhesives bonding the irradiated dentin was significantly lower than that of the control dentin(PB P<0.05 ; GB P<0.05 ). With the etching time extending(8s,15s,30s),microtensile bond strength of PB adhesive bonding irradiated dentin increased accordingly(P81530),which was all significantly lower than that of PR ( P<0.05 ) ,but the differences among the 3 groups were not significant(P>0.05). For irradiated enamel, microtensile bond strength of PB was obviously higher than that of GB(p<0.05).SEM displayed that the hybrid layer thickness of PB adhesive bonding the irradiated dentin was about 4μm, less than the control group (7μm),and only a few resin tags were observed in lateral branches of dentinal tubules. The hybrid layer thickness of GB adhesive bonding the irradiated dentin was about 4μm, less than that of control group (4μm). In addition, there was no resin tags observed in lateral branches of dentinal tubules in irradiated dentin bonded by GB adhesive. The hybrid layer of PB adhesive bonding the irradiated dentin turned thicker with etching time increasing (8s, 15s, 30s), and resin tags were observed in lateral branches of dentinal tubules in 8s, 15s and 30s etching time .Conclusion:1 Radiation obviously reduced the microhardness of enamel and dentin and changed the microstructures of them.2 Microtensile bond strength of PB and GB adhesives bonding irradiated enamel was both lower than that of the control dentin. The microtensile bond strength of PB bonding irradiated dentin was slightly higher than that of GB. Extending the etching time to 30s could improve microtensile bond strength of PB adhesive bonding irradiated enamel.3 Microtensile bond strength of PB and GB adhesives bonding irradiated dentin was both significantly lower than that of the control dentin. The microtensile bond strength of PB bonding irradiated dental was much higher than that of GB. With etching time extending, microtensile bond strength of PB adhesive bonding irradiated enamel tended to increase accordingly. 4 The results of this study suggested that using total-etching adhesive system and appropriately extending etching time could clinically improve the bonding effect of teeth damaged after radiotherapy. |
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