Interfacial chemistry of the dentin/adhesive bond

To date, the dentin/adhesive (d/a) bond has primarily been studied by morphologic analysis in conjunction with bond strength measurement. Although these analyses have enhanced our understanding, numerous questions about the chemistry have not been answered. The purpose of this study was to determine, at the molecular level, quantitative differences in the composition of the d/a interface formed under "wet" bonding conditions. The occlusal one-third of the crown was removed from 10 extracted, unerupted human third molars. The prepared dentin surfaces were treated, per manufacturers' instructions, with either Single Bond (3M) or One-Step adhesive (Bisco). Three-micron-thick sections of the d/a interface were cut and stained with Goldner trichrome for light microscopy. Companion slabs were analyzed with micro-Raman spectroscopy; the sample was placed at the focus of a 100x microscope objective, and spectra were acquired at 1-mu m intervals across the d/a interface. Reference spectra were collected on model compounds of type I collagen and adhesive; the relative ratios of the integrated intensities of spectral features from adhesive and collagen were determined and plotted as a function of wt% adhesive. The same ratios were determined for the interface samples; by comparing these ratios with the calibration curve generated from the model compounds, we determined the percent of adhesive as a function of spatial position across the d/a interface. The relative percent of Single Bond adhesive was < 50% throughout more than half of the hybrid layer; One Step adhesive was greater than or equal to 50% throughout most of the hybrid. The results from this study provide the first direct chemical evidence of phase separation in a dentin adhesive and its detrimental effect on the dentin/adhesive bond.