The Basics of Adhesion to Enamel and Dentin According to the AD Concept

The mechanism of bonding to tooth tissue is principally based upon micro-mechanical interaction, potentially supplemented with primary chemical bonding. The ‘adhesion-decalcification concept' or ‘AD-concept' describes how acidic molecules interact with hydroxyapatite-based substrates. In a first step, the molecule electro-statically interacts with hydroxyapatite. Depending on the stability of this bond, the second phase involves ‘decalcification' or ‘adhesion'. Following an ‘etch-and-rinse' approach, acids like phosphoric acid (but also maleic and lactic acid, the latter in case of caries) do not form a stable bond to hydroxyapatite and, besides being readily detached, withdraw calcium from the surface (decalcification). When etching is followed by water rinsing, the dissolved calcium-phosphates are removed from the surface. At enamel, the resulting micro-retentive surface has been shown to most effectively interlock the dental adhesive. At dentin, an organically rich substrate up to several micrometers deep is exposed. Bonding to ‘etched' dentin should then be realized through diffusion of resin in the exposed collagen-fibril network, which can hardly be achieved completely. Also following a ‘self-etch' approach, the acidic monomers get initially bonded to hydroxyapatite. This bond is unstable in case of ‘strong' self-etch adhesives (decalcification); as such self-etch primers/adhesives are not rinsed off, the dissolved calcium-phosphates are embedded in the exposed collagen network. This bond to dentin is unstable, while ‘strong' self-etch adhesives bond reasonably effectively to enamel. Most effective bonding to dentin is however thought to be achieved through ‘mild' self-etching using functional monomers that ionically bond to hydroxyapatite within a submicron hybrid layer (adhesion); they keep collagen protected by hydroxyapatite with direct benefit to bond durability. In this process, specific monomers self-assemble in ‘nano-layering', as was demonstrated morphologically and chemically.