Micro-Raman spectroscopic studies on the adhesive-dentine interface and the degree of conversion of dental adhesives
A series of studies on monomer to polymer conversion in adhesive systems was undertaken using micro-Raman spectroscopy. A database of micro-Raman spectra was compiled for identification of tooth tissues and materials. The degree of conversion was assessed as a function of time and light source. Linear and two-dimensional micro- Raman characterisations of the adhesive-dentine and resin-based composite-adhesivedentine interfaces were performed. The degree of monomer to polymer conversion of adhesive systems was correlated with the amount of eluted monomers obtained by highperformance liquid chromatography. The degree of conversion varied significantly depending on adhesive chemical composition, curing time and light source. It was impossible to specify one curing time applicable to all adhesive systems, due to differences in conversion kinetics. In general, conventional halogen light-curing units at twenty seconds curing time produced similar or higher degree of conversion in adhesive systems compared to high-power LED units at ten seconds. Significantly higher monomer conversion was found in the adhesive layer compared to the hybrid layer in both etch-and-rinse and self-etch systems. Etch-and-rinse adhesive systems formed thicker hybrid layers compared to self-etch systems. Micro-Raman spectroscopy gave a more precise indication of dentine demineralisation and adhesive penetration than scanning electron microscopy and indicated that the hybrid layer is a gradual transitional zone between the adhesive layer and un-affected dentine. The absolute amount and weight percent of eluted monomers varied in all tested adhesive systems. In most adhesive systems, more than 90% of eluted monomers were detected within the first one hour of immersion. Overall, no correlation was found between the degree of conversion and the amount of eluted monomers.