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The longevity of resin restorations is currently an area of great interest in adhesive dentistry. However, no work has been conducted to investigate the durability of resin-dentin bond structures using human substrate in vivo. The purpose of this study was to investigate the degradation of the resin-dentin bond structures aged in an oral environment for 1, 2, or 3 years. Cavities were prepared in primary molars, and an adhesive resin system (Scotchbond Multi-Purpose) was applied to the cavity. After 1 to 3 years, following the eruption of the succedaneous permanent teeth, the resin-restored teeth were extracted. Immediately after extraction, those teeth were sectioned perpendicular to the adhesive interface and trimmed to produce an hourglass-shaped specimen. Then, a micro-tensile test was performed at a crosshead speed of 1.0 mm/min. The mean bond strengths were statistically compared with one-way ANOVA and Fisher's PLSD test (p < 0.05). Further, all fractured surfaces were observed by SEM, and the area fraction of failure mode was calculated by means of a digital analyzer on SEM photomicrographs. There were significant differences in tensile-bond strength among all 3 groups (p < 0.05), with mean values ranging from 28.3 +/- 11.3 MPa (control), to 15.2 +/- 4.4 MPa (1 to 2 years), to 9.1 +/- 5.1 MPa (2 to 3 years). Moreover, under fractographic analysis, the proportion of demineralized dentin at the fractured surface in specimens aged in an oral environment was greater than that in control specimens. Furthermore, degradation of resin composite and the depletion of collagen fibrils was observed among the specimens aged in an oral environment. Analysis of the results of this study indicated that the degradation of resin-dentin bond structures occurs after aging in the oral cavity.

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The long-term durability of bonds between adhesive resins and dentin is of significant importance for the longevity of bonded restorations. We carried out an in vivo one-year study to evaluate the durability of resin-dentin bonds in the oral cavity, as well as to test the hypothesis that the adhesive interface would show morphological changes in vivo over time. Very shallow saucer-shaped dentin cavities were prepared in 12 intact teeth of one Japanese monkey (Macaca fuscata) under general anesthesia. The cavities were restored with Clearfil Liner Bond II and Clearfil Photo Posterior resin composite. The teeth were extracted at three different times: immediately, and 180 and 360 days after placement of the restorations. One day after the monkey was killed, specimens of the three time periods were subjected to the micro-tensile bond test at a crosshead speed of 1 mm/min. The surfaces of the failed bonds were observed under a field emission scanning electron microscope (FE-SEM). Bond strength measurements in this study were successfully performed and were stable at approximately 19 MPa during the one-year testing. Scanning electron microscopic observations of the failed surfaces revealed, at the top of the hybrid layer and within the adhesive resin, porosity which increased over time. Long-term bonds can be assessed in vivo by the combined evaluation of the microtensile bond strength and SEM morphological examination of the adhesive interface.

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