Evolution of Adhesive Technology in Generations

Modern Classification of Adhesives

Classifying dental adhesives into different categories is not straightforward, because of the great supply and vast turnover of manufactured adhesives. Although the classification in generations was referred to above, this chronological classification is not logical, lacks scientific background with regard to the bonding strategy followed, is often commercially misused, and most importantly does not provide any clear information to practitioners with regard to their correct use.14 Classifying adhesives into etch-and-rinse adhesives, self-etch adhesives, and glass-ionomers is simple, reliable and consistent, because of which this classification has been internationally accepted ( Fig. 14.1 ). Further subdivisions are/can be made based on the number of application steps, the number of components (that need to be mixed), and especially with regard to the self-etch systems on the basis of their interaction intensity as self-etch adhesives ( Fig. 14.2 ).15

• strong (pH < 1)

• intermediately strong (pH ~1.5)

• mild (pH ~2)

• ultra-mild (pH ≥ 2.5)

Interaction with Hydroxyapatite-Based Tissues

The fundamental mechanism of bonding to enamel and dentin is essentially based on an exchange process, in which minerals removed from the dental hard tissues are replaced by resin monomers that upon polymerization become micromechanically interlocked in the created porosities.4,14 This so-called hybridization is thus a process primarily based upon diffusion. While the resultant micromechanical interlocking is needed to achieve good bonding in clinical circumstances, the potential benefit of additional chemical interaction between functional monomers and tooth substrate components is thought to particularly improve bond durability.14,16,17

The way molecules interact with hydroxyapatite-based tissues has been described in the so-called AD-concept or Adhesion–Decalcification concept ( Fig. 14.3 ).18 This model dictates that initially all acids bond chemically (ionically) to the calcium of hydroxyapatite (Phase I). Whether the molecule will remain bonded (Phase II, Option 1), or will de-bond (Phase II, Option 2), depends on the stability of the formed bond, or in other words on the stability of the respectively formed calcium salt.

More specifically, molecules like 10-MDP (as functional monomer in a mild self-etch adhesive), but also polyalkenoic acids (as functional polymers in conventional and resin-modified glass-ionomers), will readily bond chemically to the calcium of hydroxyapatite (Phase II, Option 1). They form stable calcium-phosphate and calcium-carboxylate salts, respectively, along with only a limited surface decalcification effect. Mild self-etch adhesives and glass-ionomers only superficially interact with enamel and dentin, and hardly dissolve hydroxyapatite crystals, but rather keep them in place (within a thin, submicron hybrid layer; see below). In this way, the hydroxyapatite remains to protect the more vulnerable collagen fibrils.

On the contrary, molecules like phosphoric and maleic acid, but also strongly etching monomers like HEMA phosphate (as in specific strong self-etch adhesives), will initially bond to calcium of hydroxyapatite (Phase I), but will readily de-bond (Phase II, Option 2) with the negatively loaded phosphate or carboxylate ions, taking the positively loaded calcium ions with them. This results in a severe decalcification or etching effect, as is best known for phosphoric acid which is used as the etchant as part of the etch-and-rinse approach (see below). Because the calcium phosphate/carboxylate bond originally formed (Phase I) at the enamel/dentin surface is not stable, the bond will dissociate, leading to a typical etch pattern at the enamel and a relatively deep (thick) hybrid layer at the dentin that no longer contains any hydroxyapatite crystals.

Current Strategies for Bonding to Enamel

On enamel, phosphoric-acid etching selectively dissolves the enamel rods, thereby providing micro-etch pits that increase the surface energy so much that an ordinary hydrophobic bonding agent is readily sucked in by capillary attraction ( Fig. 14.4 ).19 Without doubt, this micro-mechanical interlocking of multiple, tiny resin tags within the acid-etched enamel surface is still today the best achievable bond to enamel ( Figs. 14.5, 14.6 ).14 It not only effectively seals the restoration margins in the long term, but also protects the more vulnerable bond to dentin against degradation.20

While strong self-etch adhesives generally perform not that unfavorably at enamel, bonding of mild self-etch adhesives to enamel (and certainly to aprismatic enamel) remains so far unsatisfactory ( Fig. 14.6 ).21 Clinical research has clearly revealed that restoration defects develop rather quickly at the enamel margins, whereas the dentin margins appear to maintain their marginal integrity for much longer ( Fig. 14.7 ).22,23 This is somewhat odd, since one might expect that the chemical bonding potential to hydroxyapatite (that can be achieved with specific mild self-etch adhesives that contain functional monomers like 10-MDP) should also show its benefit at the enamel, which even contains much more hydroxyapatite than dentin does. Hence, the specific enamel hydroxyapatite crystal must be less receptive to primary chemical interaction than the dentin hydroxyapatite crystal. This enigma quite obviously demands further in-depth investigation.

Obstacles in Bonding to Dentin

The more reliable that adhesion to enamel is, the less predictable and durable the bonding to dentin is—still today! The main hindrance is due to the mixed inorganic/organic nature of dentin, with dentinal hydroxyapatite deposited on a mesh of collagen.4 In addition, dentin is intimately connected with pulpal tissue by means of numerous fluid-filled tubules, which i.e. transverse through dentin from the enamel–dentin junction to the pulp. This constant outward fluid flow renders the exposed dentin surface naturally moist and thus intrinsically hydrophilic. This hydrophilic character definitely represents one of the major challenges for modern adhesives to durably interact with dentin, and has in essence led to the different bond strategies today available.

The presence of cutting debris on instrumented dental surfaces in the form of a smear layer and smear plugs that obstruct the dentin tubules is another clinical factor that may seriously interfere with bonding.24,25 As regards the evolution in adhesive technology, surface smear formation precluded the intimate interaction of early dentin bonding agents with the underlying tooth tissue, which led to the low bond strengths and totally unsatisfactory clinical performances in the 1980s.4 The first bonding protocol that revealed a clinically acceptable outcome involved complete removal of the smear layer by the etch-and-rinse approach ( Fig. 14.4b ).14 The self-etch (or etch-and-dry) approach only dissolves the smear layer (but does not remove it, as there is no rinse phase) and embeds the dissolved calcium phosphates within the interfacial transition zone ( Fig. 14.8b ).4 Especially for the relatively high-pH self-etch adhesives, rather thick and compact smear layers may negatively influence their bonding effectiveness.25,26

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