Adhesion: 14 Basics in Adhesion Technology

10.1055/b-0034-84417

Adhesion: 14 Basics in Adhesion Technology

Bart Van Meerbeek, Yasuhiro Yoshida

Although noninvasive interventions might be successful in inhibiting the development or further progression of a caries lesion, their success is dependent on the adherence of the patient (Chapter 13). Thus, self-management of the caries disease is of limited sustainability, which is reflected by the number of dentinal caries lesions that can still be observed worldwide (Chapter 8). In former days, amalgam and, if affordable for the patient, gold cast restorations were placed in the teeth mainly for the purpose of providing the patient with a functioning dentition. In recent years, more esthetic solutions have been demanded, which has been accompanied by a discussion on the biocompatibility of restorative materials, with the consequence that amalgams, at least, have lost some of their status in modern dentistry. Tooth-colored filling materials can be mainly divided into cements (glass-ionomers) and resinbased materials (composites) (Chapter 19), the latter being in need of a pretreatment procedure before they can be applied. This procedure comprises the use of several chemical agents to allow for a durable interface between tooth hard substances and the resin-based material. This intriguing step in invasive dentistry has been referred to as adhesion technology.

Adhesive technology has evolved rapidly since it was introduced nearly 60 years ago. The main challenge for dental adhesives is to provide an equally effective bond to two substrates of different nature. Bonding to enamel has proven to be durable. Reliable bonding to dentin is more intricate and can apparently only be achieved when rather complicated and time-consuming application procedures are followed. Consequently, today′s adhesives are often regarded as technique-sensitive with the smallest error in the clinical application procedure being penalized, either by rapid de-bonding or by early marginal degradation. As a consequence, the demand for simpler, more user-friendly and less technique-sensitive adhesives has been high, urging manufacturers to develop new adhesives at a rapid pace.

In detail, this chapter will cover:

  • A critical review on the laboratory and clinical performance of dental adhesive technology that dentists can use today in their practice

  • The basic mechanisms of bonding to enamel and dentin

  • The strengths and weaknesses of the two major bonding approaches, being the etch-and-rinse and the selfetch (or etch-and-dry) bonding strategies

  • The more challenging bonding to caries-affected dentin

  • Clear guidelines for predictable, reliable, and durable bonding to enamel and dentin, both separately and jointly

Evolution of Adhesive Technology in Generations

Early Days

After observing the industrial use of phosphoric acid to improve adhesion of paints and resin coatings to metal surfaces, Buonocore, in 1955,1 applied acid to teeth to “render the tooth surface more receptive to adhesion”. Imitating his enamel acidetch technique, Brudevold et al. reported in 19562 that glycerophosphoric acid dime-thacrylate (GPDM) could bond to hydrochloric acid-etched dentinal surfaces. The bond strength attained with this primitive adhesive technique was only 2–3 MPa, in contrast to the common 15–20 MPa bond strength obtained to acid-etched enamel. Predating the experiments of Buonocore, other investigators used the same monomer, GPDM, in the early 1950s with the introduction of Sevriton Cavity Seal (Amalgamated Dental Company).3

After the failures of this early dentin acid-etch technique, numerous dentin adhesives with complex chemical formulas were designed and developed with the objective of promoting chemical adhesion. So-called dentin bonding agents were no longer unfilled resins intended purely to better wet the dentinal surface prior to the application of a stiff resin composite. They became bifunctional monomers with specific reactive groups that were claimed to react chemically with the inorganic calcium-hydroxyapatite and/or organic collagen component of dentin.4 The development of N-phenylglycine glycidyl methacrylate (NPG-GMA) was the basis of the first commercially available dentin bonding agent, Cervident (SS White).5 This first generation dentin bonding agent theoretically bonded to enamel and dentin through chelation with calcium. Clearfil Bond System F (Kuraray), introduced in 1978, was the first product of a large second generation of dentin adhesives, such as Bondlite (Kerr/Sybron), J&J VLC Dentin Bonding Agent (Johnson & Johnson Dental), Dentin Adhesit (Vivadent), and Scotchbond (3 M), among others. These products were based on phosphorous esters of methacrylate derivatives for enhanced surface wetting as well as ionic interaction with calcium.6 Clinically, they failed rather early, as these adhesives primarily bonded to the weak smear layer. The basis for the third generation of dentin adhesives was laid when the Japanese philosophy of etching dentin to remove the smear layer gained acceptance.7 This had earlier been discouraged in America and Europe until the end of the 1980s because of pulpal inflammation concerns.8 Based on this total-etch concept, Clearfil New Bond (Kuraray) was introduced in 1984, which contained 2-hydroxyethyl methacrylate (HEMA) and 10-methacryloyloxy decyl dihydrogenphosphate (10-MDP). Another approach, at that time unique, to deal with the cavity smear layer was the use of Scotchprep (3 M), which was an aqueous HEMA solution containing also maleic acid at low concentration; it was followed by the application of an unfilled adhesive resin, Scotchbond 2 (3 M), which contained bisphenol A diglycidyl dimethacrylate (Bis-GMA) and HEMA. In this way, this acidic primer simultaneously etched and impregnated the dentinal surface and was in fact the precursor of the current self-etch adhesives. At that time, Scotchprep (3M) was, however, advocated to be used solely on dentin. Other systems, such as Superlux Universalbond 2 (DMG), and Syntac (Ivoclar-Vivadent), followed this smear layer-dissolving approach.

Breakthrough in Adhesion Technology

Significant advances in adhesive dentistry were made with the development of the multistep fourth generation adhesives in the early-to-mid 1990s. They are today referred to as three-step etch-and-rinse adhesives, and are considered as the “gold standard” and consequently still widely used. Essential is the pretreatment of dentin with conditioners and primers that make the heterogeneous and hydrophilic dentin substrate more receptive to bonding. Although etchants formerly contained phosphoric acid at a concentration well below 30%, or contained alternative milder acids (such as maleic, nitric or oxalic acid), today the fourth generation adhesives generally come with 30%–40% phosphoric-acid gels. An important final step in this multistep approach, regarding interface sealing and thus bond stability, involves the separate application and light curing of a hydrophobic adhesive resin. Representative adhesives in this group are All-Bond 3 (Bisco), OptiBond Fl (Kerr), Permaquik (Ultradent) and Scotchbond Multi-Purpose (3 M).

Because of the alleged complexity of the fourth generation systems, the fifth generation adhesives involve a separate etch-and-rinse phase followed by a combined application of a primer-adhesive resin or so-called one-bottle. Although most of these two-step etch-and-rinse adhesives have somewhat fallen short of achieving at least similar or even improved bonding with fewer bottles, this generation has been adopted very well in routine clinical practice. Representative commercial products of this generation are Excite (Ivoclar-Vivadent), iBond Total Etch (Hereaus-Kulzer), One-step (diverse versions, Bisco), OptiBond Solo (diverse versions, Kerr), Prime&Bond (diverse versions, Dentsply), Scotchbond 1 (diverse versions, 3 M ESPE), and TECO (DMG).

No Separate Etching?

The sixth generation is one of self-etch adhesives that can be further subdivided into two-step self-etch and one-step, two-component self-etch adhesives, depending respectively on whether a separate self-etching primer is provided or not. To ensure etching, the presence of water as an ionizing medium is necessary. The bonding performance attained by the sixth generation adhesives varies a great deal, depending on the actual composition and more specifically on the actual functional monomer included in the adhesive formulation. Representative of the two-step self-etch adhesives are AdheSE (Ivoclar-Vivadent), Clearfil SE Bond and Protect Bond (Kuraray), Contax (DMG), and OptiBond Solo Plus Self-Etch (Kerr). Representative of the one-step, two-component self-etch adhesives are Adper Prompt L-Pop (3 M ESPE), One-up Bond F (Tokuyama), and Xeno III (Dentsply).

The latest and seventh generation comprises single-component, one-step self-etch adhesives, or the only true one-bottle or all-in-one adhesives; they combine conditioning, priming, and application of adhesive resin, but do not require mixing. These most simple-to-use adhesives are intricate mixes of hydrophilic and hydrophobic components, and have been abundantly documented as having several shortcomings (see box). Representative (single-component) one-step self-etch adhesives are Adper Easy Bond and Scotchbond Universal (3 M ESPE), Bond Force (Tokuyama), Clearfil S 3 Bond Plus (Kuraray), G-Bond and G-aenial Bond (GC), iBond Self Etch (HereausKulzer), and Xeno V (Dentsply).

NOTE

Shortcomings of simple-to-use adhesives (all-inone)913:

  • Reduced immediate and long-term bonding effectiveness

  • Increased interfacial nanoleakage

  • Enhanced water sorption for adhesives that are rich in HEMA

  • Phase-separation for HEMA-free/poor adhesives

  • Reduced shelf-life

  • Especially, less favorable clinical performance

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

Fig. 14.1 Classification of adhesives and number of application steps for each method.
Schematic illustration of the differential interaction of self-etch adhesives with dentin depending on the pH of the self-etching solution.15 HAP: hydroxyapatite.
  • strong (pH < 1)

  • intermediately strong (pH ~1.5)

  • mild (pH ~2)

  • ultra-mild (pH ≥ 2.5)

NOTE

Internationally accepted classification of adhesives:

  • Etch-and-rinse

  • Self-etch

  • Glass-ionomers

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.

Schematic illustration of the AD-concept, dictating whether molecules after initial attachment (Phase I) either remain bonded (Phase II, Option 1) or decalcify hydroxyapatite-based hard tissues (Phase II, Option 2).17,18

NOTE

The AD-concept or Adhesion–Decalcification concept describes the way molecules interact with hydroxyapatite-based tissues.

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

NOTE

The micromechanical interlocking of multiple, tiny resin tags within the acid-etched enamel surface is still today the best achievable bond to enamel.

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.

Fig. 14.4a, b Collage of TEM photomicrographs illustrating the interfacial ultrastructure of an etch-and-rinse adhesive at enamel (a) and dentin (b).
Typical 5-year-old Class-V restoration illustrating a well-maintained marginal integrity at enamel versus severe marginal discoloration at the dentin.
General trends in microtensile bond strength of the different classes of adhesives to enamel and dentin (taken from pooled data for different adhesives per class, as tested at KU Leuven following a standard protocol). Note that the higher bond strength to dentin than to enamel should not be literally interpreted as “higher,” but rather should be ascribed to the brittleness of enamel, which leads to the measurement of lower values in microtensile bond-strength experiments.
Clinical examples of Class-V composite restorations (3-year-old) either bonded with an etch-and-rinse (green boxes, cervical) or a self-etch adhesive (white arrows). While a smooth enamel margin was obtained with the etch-and-rinse adhesive, a less optimal marginal adaptation with slight discoloration can be observed at the enamel margin when a mild self-etch adhesive was employed.
Fig. 14.8a, b Collage of TEM photomicro-graphs illustrating the interfacial ultrastructure of a mild self-etch adhesive at enamel (a) and dentin (b).

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

NOTE

Two main factors are responsible for lower durability of dentin bonding compared with enamel bonding:

  • Mixed inorganic/organic nature of dentin

  • Presence of cutting debris on instrumented dental surfaces in the form of a smear layer and smear plugs that obstruct the dentin tubules

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May 23, 2020 | Posted by in General Dentistry | Comments Off on Adhesion: 14 Basics in Adhesion Technology
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