| Objective The purpose of this study was to evaluate the microtensile bond strengths of 3 dentin adhesive systems (Prime & Bond NT, Xeno III, Xeno V) at different application times (10, 20, 30, 40 seconds), and to observe the microstructure characteristic of the dentin-adhesive resin fracture interface under SEM.Methods 60 intact and fresh premolars extracted from orthodontic patients aged 14-21 years old were employed. Put every tooth into two parts along the long axis in the maxillofacial median, completely wore away the enamel, and polished the dentin surface. And then the prepared teeth were randomly divided into three classes (NT, X III and XV). Every class in accordance with adhesive coating time were randomly assigned to four groups with 10 specimens each, that is, 10sec group, 20sec group (standard group), 30sec and 40sec group. Composite build-ups were constructed incrementally according to the use of instructions. After physiological saline storage at 37℃for 24 h, teeth were sectioned to obtain sticks with cross-sectional areas of 1.0 mm~2 to be tested in tension (0.5 mm/min) immediately, then to observe the fracture interface of the dentin-adhesive resin under SEM. The microtensile bond strength (uTBS) values were analyzed by two way repeated measures ANOVA and SNK test (a = 0.05).Results uTBS was significantly lesser with the 10-second application time than with the 20-second, 30-second and 40-second time for Prime&Bond NT. There was no significant difference among the later three. The ruselts were similar for Xeno III and Xeno V, There was no significant difference between 20-second and 30-second application time ,but they had significantly higher mean uTBS value than 10-second. The 40sec group's bonding strength decreased compared with 20sec or 30sec group (P <0.05).Observing the fracture interface of the dentin-adhesive resin under SEM, we found that in Prime & Bond NT, Xeno III and Xeno V classes, the occluding situation of the dentinal tubules of 20sec and 30sec groups are better than 10s group. The occluding situation of the dentinal tubules of 20sec and 30sec groups in Xeno III and Xeno V classes are better than that in Prime & Bond NT class respectively. In Xeno III and Xeno V classes the pit-like structure after the dissolving of smear layer caused by etching agent appeared on the dentin fracture of the 40sec group. But the same phenomenon can't be found in 40sec group of Prime & Bond NT class.Conclusions1. In Total-etching adhesives Prime & Bond NT case, the desired dentin bonding strength can be achieved when the application time spent reaches to 20 sec. The extension of time has no significance on bonding strength.2. In Self-etching adhesives Xeno III and Xeno V cases, the desired dentin bonding strength can be achieved when the application time spent is no less than 20 sec. The extension of time may takes in negative impact instead of positive effect.3. In order to obtain a wonderful occluding situation of the dentinal tubules, 20sec is the limit application time of Prime&Bond NT while 20～30sec is the proper application time of Xeno III and Xeno V. In addition, it's presented that occluding situation of the dentinal tubules is greater in Self-etching adhesives Xeno III and Xeno V cases, compared with that in Total-etching adhesives Prime & Bond NT case.
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