Universal adhesives – a new direction in the development of adhesive systems

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Authors: A. Tichý ^1,2; K. Hosaka ²; J. Tagami ²
Authors place of work: Stomatologická klinika, 1. lékařská fakulta Univerzity Karlovy a Všeobecná fakultní nemocnice, Praha ¹; Department of Cariology and Operative Dentistry, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, Tokyo, Japan ²
Published in the journal: Česká stomatologie / Praktické zubní lékařství, ročník 120, 2020, 1, s. 4-12
Category: Přehledový článek

Summary

Objectives: In the past decade, many new adhesive systems labeled as universal were introduced. Their common characteristic is that they can be applied to hard dental tissues either in self-etch or etch-and-rinse mode, and they are also able to bond to various restorative materials. Moreover, universal adhesives are mostly one-step and user-friendly. Because numerous papers about their properties were published, the purpose of this review article is to critically discuss the available information about the properties of universal adhesives and their bonding to various materials.

Conclusions: The simplification of the application procedure is accompanied by several drawbacks associated with the necessity to mix all components into a single bottle. Due to the content of hydrophilic monomers, water and volatile solvents, adhesive layers of universal adhesives are more susceptible to water sorption. Consequently, they are more prone to hydrolytic and enzymatic degradation, thus exhibiting a lower durability. Universal adhesives also have limitations in bonding to some materials and require preceding surface treatments. To enhance the bond strength to enamel, selective enamel etching with phosphoric acid is recommended because the self-etching effect of universal adhesives may be insufficient. In contrast, more durable bonding to dentin was reported in self-etch mode compared to etch-and-rinse. For glass ceramics, a silane coupling agent should be applied in a separate step prior to the application of universal adhesives despite that some of them are silane-containing. The silanes are unstable in acidic conditions and their premature hydrolysis precludes the chemical interaction with glass. Lastly, zirconia ceramics require mechanical pre-treatment using air-abrasion because the chemical bond alone is not sufficient. In conclusion, universal adhesives can be used in various indications, however, it is necessary to be aware of their drawbacks and limitations.

Keywords:

Dentin – adhesion – universal adhesives – enamel – ceramics – resin composite – review

INTRODUCTION

Adhesive dentistry has developed substantially in recent decades and it has allowed highly aesthetic and minimally invasive treatment options in many indications [1, 2]. However, a laborious and technique-sensitive procedure is required to overcome the different properties of the hydrophilic dentin and hydrophobic restorative materials. Therefore, the effort to simplify and shorten the adhesives’ application procedure has become one of the trends in adhesive dentistry and lead to the development of adhesives labeled as universal. Although no formal definition exists, most universal adhesives are one-bottle systems which can be applied to hard dental tissues either in self-etch or etch-and-rinse mode. Universal adhesives are also characterized by the ability to bond to almost all restorative materials. As numerous scientific texts and commercial claims have been published about their properties over the past decade, it is difficult to follow the current state-of-the-art. Therefore, the aim of this review article is to critically discuss and summarize the available information about universal adhesives and their bonding to various materials.

CLASSIFICATION OF ADHESIVE SYSTEMS

The classification of adhesives has been extensively described in the literature [3]; therefore, it will be just briefly reviewed in this article. It seems to be most suitable to classify the adhesives as etch-and-rinse, which utilize phosphoric-acid etching, or self-etch which contain acidic methacrylate monomers. These two groups are further subdivided according to the number of application steps.

Etch-and-rinse adhesives

Conventionally, a 32–40% phosphoric acid gel is used as the etchant to remove the smear layer from enamel and dentin surfaces and to form irregular surface patterns suitable for micromechanical bonding (Fig. 1, 2). In the case of enamel, porosities created by the selective dissolution of minerals are infiltrated by the adhesive and the bond is created by its polymerization. In the case of dentin, the monomers of the adhesive penetrate exposed collagen fibers and after polymerization, the so-called hybrid layer [4] is formed and provides the bond to dentin. In addition, the adhesive flows into dentinal tubules exposed by the smear layer removal and forms resin tags which also contribute to the bond strength.

Enamel surface with exposed prisms after phosphoric acid etching.
Scanning electron microscope, magnification 2000×. — **Fig. 1. Enamel surface with exposed prisms after phosphoric acid etching. Scanning electron microscope, magnification 2000×.**

Dentin surface with open dentinal tubules after phosphoric acid etching. Remnants of silica particles from the etching gel were observed.
Scanning electron microscope, magnification 2000×. — **Fig. 2. Dentin surface with open dentinal tubules after phosphoric acid etching. Remnants of silica particles from the etching gel were observed. Scanning electron microscope, magnification 2000×.**

Etch-and-rinse adhesives are available as three-step or two-step. Three-step systems consist of 1) phosphoric acid etching, 2) priming with solvated hydrophilic methacrylate monomers, and 3) bonding with hydrophobic methacrylate monomers. Long-term results showed an excellent clinical reliability of these systems [5], they are therefore considered as the standard reference for new products. In two-step etch-and-rinse adhesives, primer and bond are combined into one bottle in order to simplify the application procedure. Nowadays, they are rarely used due to the increased hydrophilicity of the primer-bond mixture [6], which resulted in their inferior clinical performance [5].

Self-etch adhesives

The development of self-etching primers was a breakthrough in adhesive dentistry because the acidic methacrylate monomers [7, 8] enabled simultaneous etching and priming. Consequently, the risk of insufficient impregnation of collagen fibers and nanoleakage [9] within the hybrid layer decreased. The use of self-etching primers is also easier, it considerably shortens the adhesive application procedure, and it eliminates the critical step of dentin air-drying after phosphoric-acid etching and rinsing. Furthermore, lower post-operative sensitivity was observed with self-etch adhesives compared to etch-and-rinse, because phosphoric acid etching completely removes the smear layer and exposes the orifices of dentinal tubules [10].

The pH of self-etch systems has a crucial influence on their bonding performance [10, 11]. Strongly acidic (pH<1) and intermediately acidic (pH»1–2) adhesives exhibited an etching effect similar to phosphoric acid and achieved promising results on enamel. However, their bonding to dentin was not stable in the long term due to the increased diffusion of water into the hybrid layer. As a consequence of remaining deposits of hydrolytically unstable salts, an osmotic gradient between dentin and the hybrid layer was formed and resulted in the accelerated decomposition of the adhesive joint [12]. Therefore, mildly or very mildly acidic adhesives (pH»2–3) which hybridize the smear layer and interact with dentin to a depth of about 1 µm are mostly used at present. While this is much less compared with phosphoric acid etching, which demineralizes dentin to a depth of approximately 5–10 mm, the laboratory and clinical results of mild self-etch adhesives are comparable to the three-step etch-and-rinse systems [5]. However, for very mildly acidic adhesives whose demineralizing effect is insufficient on enamel, selective enamel etching with phosphoric acid is recommended, especially when bonding to unprepared enamel [13].

Self-etch adhesives are available as two-step and one-step. While the two-step systems consist of 1) a self-etching primer and 2) a hydrophobic bond, the two components are applied simultaneously in the one-step approach. Because one-step adhesives contain large amounts of solvents which must be thoroughly air-dried, these systems form very thin adhesive layers (Fig. 3). Although most of the current one-step adhesives are one-bottle, some are separated into two bottles and the solutions are mixed just before the application. This can solve issues associated with the long-term instability of all components mixed into one bottle but it cannot prevent the principal drawback of one-step adhesives – their higher hydrophilicity and water sorption, which may lead to a lower durability of the adhesive joint [10].

Fig. 3. A cross-sectional view of the adhesive joint between dentin and a hybrid resin composite formed by a thin (~5 μm) adhesive layer of the universal adhesive G-Premio Bond (GC, Tokyo, Japan). Scanning electron microscope, magnification 2000×.

UNIVERSAL ADHESIVES

By their composition, universal adhesives are similar to one-step self-etch systems and their use is very fast and simple as well. The distinctive property of universal adhesives is that they can be applied either in self-etch or etch-and-rinse mode, which is the reason why they have also been labeled as multi-mode [14, 15]. Besides hard dental tissues, universal adhesives can bond to most restorative materials. The major advantage of their versatility is the option to adapt to many clinical situations without the need for more adhesive systems.

The versatility of universal adhesives is mainly provided by 10-MDP (10-methacryloyloxydecyl dihydrogenphosphate) which bonds chemically with calcium ions of hydroxyapatite. The calcium salts of 10-MDP are almost insoluble in water, thus the bond with hard dental tissues is stable over the long term [16]. Transmission electron microscopy also revealed nanolayering of 10-MDP applied to hard dental tissues, a regular arrangement in 4nm layers which apparently also contributes to the bond stability [17]. Another advantage of 10-MDP is its ability to adhere to restorative materials, such as zirconia ceramics [18, 19]. In order to promote the adhesion in other clinical situations, some universal adhesives also contain silanes for bonding to silica-based materials, and chemical polymerization initiators which enable polymerization even under insufficient light irradiation.

On the other hand, the need to mix all components into one bottle poses several disadvantages. Higher hydrophilicity of the adhesive layer resulting from the presence of hydrophilic monomers, water and solvent residues appears to be the most serious drawback. The most commonly used hydrophilic monomer is 2-hydroxyethyl methacrylate (HEMA), which is essential for the infiltration of wet dentin, but it also increases water sorption, impairs the mechanical properties and affects the polymerization of adhesives [20]. However, HEMA cannot be simply eliminated for its amphiphilic nature that keeps hydrophobic methacrylates in solution with water, which is necessary to ionize the acidic monomers. It was reported that HEMA-free adhesives have a lower water sorption [21] but phase separation was observed (Fig. 4), i.e. the formation of water droplets separated from hydrophobic methacrylates [22] which could negatively affect the adhesive’s properties. Therefore, some modern materials replace HEMA partially or completely by monomers with amide groups which are also able to prevent phase separation [23, 24], while being less hydrophilic [25] and exhibiting a higher degree of conversion [24] and durability [23, 24].

Phase separation observed in the adhesive layer of the HEMA-free universal adhesive G-Premio Bond (GC, Tokyo, Japan). The round-shaped voids were formed by water droplets separated from hydrophobic methacrylates.
Scanning electron microscope, magnification 2000×. — Fig. 4. Phase separation observed in the adhesive layer of the HEMA-free universal adhesive G-Premio Bond (GC, Tokyo, Japan). The round-shaped voids were formed by water droplets separated from hydrophobic methacrylates. Scanning electron microscope, magnification 2000×.

The presence of water and volatile organic solvents also plays an important role in the bonding performance of adhesives. The hydrophilicity and low viscosity of solvents improve the infiltration of hard dental tissues by monomers and solvents also facilitate the evaporation of water [26]. Alcohols (ethanol, less tert-butanol or isopropanol) and acetone are used as solvents in most adhesives and may account for up to 80% of their weight. However, the solvents cannot be completely evaporated under clinical conditions, and their residues may adversely affect the polymerization of the adhesive layer and increase its permeability [27–29]. This is one of the reasons why universal adhesives underperform multi-step systems, whose adhesive layers mainly constitute of hydrophobic monomers that prevent water sorption.

As a consequence of the hydrophilicity and permeability of the adhesive layer of universal adhesives, increased nanoleakage was observed [10]. Moreover, microscopic studies revealed water-treeing [30] described as a network of channels allowing fluid movement within the adhesive layer. All the aforementioned factors might accelerate the hydrolytic degradation of the adhesive joint and consequently impair its long-term resistance [12, 31, 32]. Besides that, combining all the adhesives’ components into one bottle might also reduce their shelf life, as the ester bond of the conventional methacrylates is susceptible to hydrolysis in acidic conditions [33]. In this aspect, hydrolytically more stable methacrylates with an ether or an amide bond appear to be more suitable [33]. The shelf life may also be affected by improper storage as a consequence of volatile solvent evaporation (acetone in particular), so it is necessary to close the bottles immediately after use, to avoid their exposure to high temperatures, and to store them according to the manufacturer’s recommendation.

BONDING OF UNIVERSAL ADHESIVES TO VARIOUS MATERIALS

Adhesive systems in dentistry serve to bond a wide range of materials. These include primarily hard dental tissues, but also composite resins, various types of ceramics, metal alloys and others. Because of the different characteristics of these materials, a truly universal adhesive procedure is very difficult to be achieved. Moreover, it is required that the adhesive joint is sufficiently strong and stable in the long term even in the aggressive environment of the oral cavity.

Adhesion to enamel

Compared to phosphoric acid, the acidity and hence the etching effect of universal adhesives is lower, which together with their shorter application time negatively affects the quality of adhesion to enamel. A meta-analysis of laboratory studies showed that the initial bond strength of universal adhesives to enamel and its durability were significantly improved if enamel was etched with phosphoric acid prior to the application of universal adhesives [14, 34–36]. Alternative strategies for the enhancement of the adhesion to enamel include a prolonged [37] or repeated [38] application of the universal adhesive.

Adhesion to dentin

One of the characteristics defining universal adhesives is the possibility of application in either self-etch or etch-and-rinse mode. The bond strength to dentin in both these modes was investigated in many studies and meta-analyses of short-term data, gathered mostly after 24 h, revealed no significant difference between the two modes [14, 39]. Only for the very mildly acidic All-Bond Universal (pH=3.1; Bisco, Schaumburg, IL, USA), phosphoric acid etching increased its bond strength to dentin significantly [14]. On the other hand, a negative effect of phosphoric acid etching on the durability of the adhesive joint was observed in some studies [32, 40–42]. This is probably due to insufficient sealing of the demineralized zone in the etch-and-rinse mode [43], which may result in an accelerated hydrolytic degradation of the resin polymer network and an enzymatic decomposition of unprotected collagen by matrix metalloproteinases and cysteine proteases [31]. In addition, the removal of smear layer unplugs dentinal tubules, which might increase the postoperative sensitivity and deteriorate the adhesive’s properties due to higher outflow of the tubular fluid [10, 44]. It is therefore appropriate to limit the phosphoric acid etching to enamel.

Several studies have also examined how to improve the adhesion of universal adhesives. It has been shown that active [45, 46] or prolonged [25] application can improve their bond strength, probably due to better adhesive infiltration of the treated dentin. This is in contrast to the “no-waiting” concept proposed by manufacturers of some recent universal adhesives, which recommends immediate solvent evaporation and polymerization of the adhesive. However, laboratory studies suggested that this strategy might have some limitations [47, 48]. Other studies aimed to reduce the negative effect of universal adhesives’ increased hydrophilicity by extended air-drying [27, 49] or the use of warm air [50] which improved the solvent evaporation. More demanding procedures include, for example, the deproteinization of the smear layer with agents such as NaOCl. The objective is to eliminate the organic component of the hybrid layer, which is susceptible to enzymatic degradation, and to improve chemical bonding to dentin [51]. The major drawback of this method are residual free radicals in the structure of dentin produced by the decomposition of NaOCl, which adversely affect the polymerization of the adhesives. In order to improve the bond strength, a subsequent application of a reducing agent is necessary to decompose the radicals [52]. It was also proposed to use hydrophobic bonding agents to coat universal adhesives applied in the first step. In most of the studies, this two-step approach led to an improved durability of the adhesive joint [23, 53–56] but its efficiency was material-dependent and influenced by whether the universal adhesive was light-cured prior to the application of the bonding agent [53, 54]. However, it must be noted that universal adhesives and bonding agents may be incompatible, especially their photoinitiating systems [53], so they cannot be combined arbitrarily.

Adhesion to glass ceramics

To improve the adhesion to glass ceramics, it is essential to create a suitable surface relief by hydrofluoric acid which etches the silicate structure of glass [57]. The concentration of hydrofluoric acid and its application time is dependent on the type of glass ceramics and manufacturer’s instructions for use should be followed. The application of trialkoxysilanes, usually 3-methacryloxypropyltrimethoxysilane (MPTS) [58], in the next step is recommended to achieve chemical bonding. By the hydrolysis of alkoxy groups and their reaction with silanol groups of the ceramic’s glass phase, MPTS is chemically bonded to the ceramic surface and by polymerization, its methacrylate group connects with the monomers of the adhesive [59]. Silanes are also contained in some universal adhesives, but their efficacy was doubted because they are unstable in acidic environment containing water. As a result, they prematurely hydrolyze and condensate into oligomers, by which they lose the ability to bond to the silicate structures [60, 61]. Several studies demonstrated that the bond strength of a silane-containing universal adhesive to glass ceramics was significantly improved if a silane coupling agent was applied in a separate preceding step [60–63]. Alternatively, MPTS can be added to the adhesive just before its application, but just a day of storage has deactivated the silane again [60]. This implies that it cannot be relied upon the silane content in the adhesive and that a silane coupling agent should be applied to glass ceramics in a separate step.

Adhesion to zirconia ceramics

Given its excellent mechanical properties, zirconia ceramics have been increasingly used in clinical practice. Unlike ceramic materials containing a glass phase, zirconia ceramics cannot be effectively etched by hydrofluoric acid [57]. Therefore, many alternative pretreatments have been tested to achieve optimal adhesion [64, 65]. Air-abrasion with Al₂O₃ particles followed by the application of 10-MDP-based primers is most used because 10-MDP can chemically bond with zirconia [18, 19]. Alternatively, tribochemical silica coating and a subsequent application of a silane coupling agent can be used as well. According to a meta-analysis of laboratory studies, these methods combining mechanical and chemical surface treatment provide the strongest and most durable adhesion to zirconia ceramics [65] . Since most universal adhesives contain 10-MDP, they can provide a chemical bond with zirconia and they have been successfully tested as an alternative to ceramic primers [66–70], provided that the surface was previously air-abraded. However, the stability of the adhesion of universal adhesives to zirconia ceramics has not yet been sufficiently investigated.

Adhesion to resin-based materials

Universal adhesives can be used for the repair of composite restorations as well. To date, there is no consensus on how to achieve a stable bond between repaired and freshly applied composites. Usually, air-abrasion of the restoration surface is recommended prior to adhesive application [71–74] . Although the effect of silane application is questionable, promising results were obtained when silanization was combined with tribochemical silica coating [75–77]. Using universal adhesives, a few studies showed that a good composite-to-composite bond strength can be achieved by their application to the air-abraded surface of the repaired composite [76–79] and that some systems may benefit from silanization [77, 78].

In combination with resin cements, universal adhesives are also used for cementing posts and indirect reconstructions. However, it is necessary to use the materials in accordance with the manufacturer's recommendations because the incompatibility of one-step adhesives with self-cured and dual-cured resin cements has been reported [80, 81]. It is caused by the reaction of unpolymerized acidic monomers with tertiary amines, which are used as co-initiators in the cements. The decreased activity of the initiating system leads to an insufficient polymerization of the cement, lower mechanical properties and a decreased durability of the adhesive joint [80, 81]. In contrast, so-called "touch and cure" systems can improve the quality of polymerization by the reaction of an initiator in the cement and co-initiators contained in the adhesive which promote chemical polymerization. It was reported that these systems can significantly improve bond strengths especially under insufficient light irradiation [82–84].

CONCLUSION

Universal adhesives offer dentists a simple and quick bonding procedure to various materials in many clinical situations. However, the results of laboratory studies suggest that the simplification of the workflow has several drawbacks and that it might lead to lower long-term durability of the adhesive joints. Nevertheless, the development and optimization of universal adhesives are still ongoing, and some new products show promising results in laboratory studies. However, it is necessary to wait for more clinical data which are scarce to date.

MDDr. Antonin Tichy

Institute of Dental Medicine, First Faculty of Medicine of the Charles University and General University Hospital in Prague, Karlovo namesti 32

121 11 Prague

Czech Republic

E-mail: antonin.tichy@lf1.cuni.cz

Zdroje

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