Current modifications of dental adhesive systems for composite resin restorations: a review in literature

Currently, dental caries is prevalent all around the world and composite resins are widely used as restorative materials for its treatment. Adhesion of composite resins to tooth tissues is achieved using a variety of dental adhesive systems and is crucial for the survival of composite resin restorations. Adhesive dentistry began in 1955 by Buonocore and up to date developing technology led to evolution of adhesive strategies from no-etch to total etch (4th and 5th generation) and to self-etch (6th, 7th and 8th generation) adhesive systems. Although adhesive agents allow a more conservative restorative approach, achieving durable bond strength remains a matter of concern mainly due to degradation of the resin-dentin interface in the challenging oral environment. Endogenous collagenolytic enzymes, matrix metalloproteinases and cystein cathepsins are responsible for the time-dependent degradation of hybrid layer collagen. Additionally, bacterial enzymes, metabolites and oral fluids can penetrate into the resin-dentin interface and may be involved in the degradation of the hybrid layer. Various modifications have been suggested to develop adhesive systems with particular functions to tackle these problems such as incorporation of matrix metalloproteinase inhibitors, antibacterial and remineralizing agents into adhesive systems, as well as improvement of their mechanical and chemical properties. This literature review focuses on the principles, current status and future of the different techniques and materials designed to prevent the degradation of hybrid layer and bond strength of composite resin restorations.