Current aspects on bonding effectiveness and stability in adhesive dentistry

Improved dental adhesive technology has extensively influenced modern concepts in restorative dentistry. In light of minimal-invasive dentistry, this new approach promotes a more conservative cavity design, which basically relies on the effectiveness of current enamel-dentine adhesives. Nowadays, the interaction of adhesives with the dental substrate is based on two different strategies, commonly described as an etch-and-rinse and a self-etch approach. In an attempt to simplify the bonding technique, manufacturers have decreased the number of steps necessary for the accomplishment of the bonding procedure. As a consequence, two-step etch-and-rinse and one-step (self-etch) adhesives were introduced and gained rapid popularity in the dental market due to their claimed user-friendliness and lower technique sensitivity. However, many concerns have been raised on the bonding effectiveness of these simplified adhesives, especially in terms of durability, although this tends to be very material dependent. In order to blend all the adhesive components into one single solution, one-step adhesives were made more acidic and hydrophilic. Unfortunately, these properties induce a wide variety of seemingly unrelated problems that may jeopardize the effectiveness and stability of adhesion to the dental substrate. Being more susceptible to water sorption and thus nanoleakage, these adhesives are more prone to bond degradation and tend to fail prematurely as compared to their multi-step counterparts. Incidentally, another factor that may interfere with the bonding effectiveness of adhesives is the technique used for caries removal and cavity preparation. Several tools are on the market today to effectively remove carious tissue, thereby respecting the current trend of minimum intervention. Despite their promising performance, such techniques modify the tooth substrate in different aspects, possibly affecting bonding effectiveness. Altogether, we may conclude that not only the adhesive formulation, but also substrate nature must be taken into account to achieve a stable bonding interface, rendering the restorative treatment more predictable in terms of clinical performance. In this review, we analyse the current theoretical and clinical aspects of adhesion to enamel and dentine, and discuss the diverse possibilities to overcome problems which nowadays still challenge clinicians in their achievement of a more stable and effective bond to tooth enamel and dentine.