Review on the Durability of Adhesive Systems

Nowadays, restoring teeth with minimal sacrifice of sound tooth structure forms the basis of restorative practice. To achieve this concept, adhesives should essentially provide strong and durable bonding. Bonding to enamel is known to be more stable over time, but stability of contemporary dentin bondings are still questionable. Although their short term bonding effectiveness are sufficient, nowadays, there is an obvious trend in the development of adhesives with a simplified and easier application procedure but durabilitity of simplified adhesives in comparison to conventional adhesives is questionable. Impregnation of water through nanometer-sized prosities within hybrid and adhesive layer as nanoleakage is considered very detrimental to bond integrity and durability. Impregnation of water lead to hyrolysis (break up covalent bonds) and plasticizing of resin components due to reduction of the frictional forces between the polymer chains. Penetration and extension of water into adhesive interface is related to the permeability of simplified adhesives that behave as permeable or semipermeable membranes after polymerization and do not provide any hermetic seal on the prepared dentin. So, two-step etch & rinse adhesives with combination of primer and adhesive resin, are more susceptible to water sorption due to less-optimal hybridization and residual solvent and these are susceptible to water degradation due to the presence of hydrophilic monomer in final cured adhesive layer compared to three step etch & rinse. The surrounding resin-enamel bond plays a protective role against degradation. One-step self-etch adhesives are susceptible to water sorption and osmotically- induced water movement from dentin to bonding surface, water-tree formation and hydrolytic degradation due to the presence of acidic, hydrophilic monomer and the residual water (as essential solvent). During solvent evaporation, the monomer/water ratio may change and subsequently result in phase separations and blistering. Therefore the hydrolytic stability of cured adhesives is of crucial importance. The best way to achieve this goal is to apply a solvent-free, neutral–pH, hydrophobic adhesive resin layer in a separate step. Three-step etch & rinse adhesives remain the “gold standard” in terms of adhesion durability. Any kind of simplification in the clinical application procedure results in a loss of bonding effectiveness. Only two-step, self-etch most adhesives closely approach this standard, and have additional clinical benefits.