Interfacial chemistry of the dentin/adhesive bond

To date, the dentin/adhesive (d/a) bond has primarily been studied by morph ologic analysis in conjunction with bond strength measurement. Although the se analyses have enhanced our understanding, numerous questions about the c hemistry have not been answered. The purpose of this study was to determine , at the molecular level, quantitative differences in the composition of th e d/a interface formed under "wet" bonding conditions. The occlusal one-thi rd of the crown was removed from 10 extracted, unerupted human third molars . The prepared dentin surfaces were treated, per manufacturers' instruction s, 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 trich rome for light microscopy. Companion slabs were analyzed with micro-Raman s pectroscopy; the sample was placed at the focus of a 100x microscope object ive, 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 an d adhesive; the relative ratios of the integrated intensities of spectral f eatures from adhesive and collagen were determined and plotted as a functio n of wt% adhesive. The same ratios were determined for the interface sample s; by comparing these ratios with the calibration curve generated from the model compounds, we determined the percent of adhesive as a function of spa tial 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. T he results from this study provide the first direct chemical evidence of ph ase separation in a dentin adhesive and its detrimental effect on the denti n/adhesive bond.