Dentin bonding--questions for the new millennium.

PURPOSE: The objective of this review article is to summarize the most recent concepts in dentin bonding and, simultaneously, challenge some convictions that have not been backed up by sound, convincing research. After the introduction of the enamel acid-etch technique by Dr. Buonocore in 1955, the increasing demand for restorative and non-restorative esthetic treatments, as well as the refinement in the utilization of fluoride-containing products, have totally transformed the practice of operative dentistry. While bonding to enamel is a reliable technique, bonding to dentin represents a greater challenge: dentin is an intrinsically wet organic tissue penetrated by a tubular labyrinth containing the odontoblastic process, which communicates with the pulp. The manufacturers of new adhesive systems recommend the application of their adhesive materials on moist dentin. The main reason is that the spatial alteration that occurs upon drying demineralized dentin may prevent the monomers from penetrating the labyrinth of nano-channels formed by dissolution of hydroxyapatite crystals between collagen fibrils. Several of the current concepts in dentin bonding have been built from the observation of clinical studies, which measure retention rates, and from laboratory bond strength studies, which measure the force needed to debond a composite post from a flat dentin area. Although their utility is doubtful as far as clinical performance is concerned, bond strength studies are still necessary in order to compare similar parameters among materials. However, both clinical and laboratory studies lack information on the dynamic behavior of the substrate in a vital environment. For example, the changes induced by adhesive systems in the resin-infiltrated demineralized dentin have not been fully characterized. Also, the role of collagenous and non-collagenous proteins in dentin bonding has been generally overlooked.