The bond stability between dentin and filling material is the major challenge in modern adhesive techniques. A resin-dentin bond created by the etch-and rinse technique considerably loses strength within 0.5 to 5 years. This may lead to secondary caries, hypersensitivity and finally loss of restorations. The decrease in bond strength is mainly due to two factors which are related to the collagen network of dentin and its constituent matrix-metalloproteinases (MMPs). Firstly, the collagen fibres exposed by the etching process using 37% phosphoric acid may not be completely infiltrated by subsequently applied adhesive bonding agents. In consequence, a thin layer of exposed, but non-infiltrated collagen remains at the bottom of the hybrid layer, resulting in nanoleakage. Secondly, this non-infiltrated collagen contains active MMPs that may degrade collagen by hydrolysis. Within dentin, these enzymes physiologically are inactive, but they become activated by phosphoric acid and acidic components of bonding agents. As a result, the hybrid layer disintegrates and the bond strength gradually diminishes. However, when chlorhexidine is used as a therapeutic primer following the etching process using 37% phosphoric acid, MMPs are inhibited in a non-specific manner, such that both the hybrid layer and the dentin bond strength are supposed to be preserved for a longertime period. Based on the studies included in this literature review, the use of a pure aqueous solution of 0.2% chlorhexidine as an additional therapeutic primer can be recommended for etch-and-rinse systems.

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