Etch and Rinse

Etch-and-rinse adhesive systems are the oldest of the multi-generation evolution of resin bonding systems. The three-steps total-etch adhesive systems were introduced in early 1990s ( ⁴⁷ ), that involve acid-etching, priming and application of a separate adhesive. Each of the three-steps can accomplish multiple tasks ending with sealing the bonded interface with a relatively hydrophobic adhesive layer. Consequentially, an inter-diffusion layer is formed that called hybrid layer. Etch-and-rinse adhesives are characterized by an initial etching step, followed by a compulsory rinsing procedure which is responsible for the complete removal of smear layer and smear plugs. On enamel, acid-etching selectively dissolves the enamel rods, creating macro-and micro porosities which are readily penetrated, even by ordinary hydrophobic bonding agents, by capillary attraction ( ⁴⁸ ). Upon polymerization, this micromechanical interlocking of tiny resin tags within the acid-etched enamel surface still provides the best achievable bond to the dental substrate ( ⁴⁹ ). Dentin adhesion is more challenging than enamel adhesion due to dentin composition, rendering the etch-and-rinse strategy a highly sensitive technique ( ⁵⁰ ). Concurrently, acid-etching promotes dentine demineralization over a depth of 3–5 lm, thereby exposing a scaffold of collagen fibrils that is nearly totally depleted of hydroxyapatite ( ²³ ). The following step consists of the application of a primer containing specific monomers with hydrophilic properties, such as 2-Hydroxy ethyl meth-acrylate (HEMA), dissolved in organic solvents like acetone, ethanol or water. While HEMA is responsible for improving the wettability and promoting the re-expansion of the collagen network, the solvents are able to displace water from the dentine surface, thus preparing the collagen network for the subsequent adhesive resin infiltration ( ⁵¹ ). In the bonding step, a solvent-free adhesive resin is applied on the prepared surface, leading to the penetration of hydrophobic monomers not only into the inter-fibrilar spaces of the collagen network but also into dentine tubules. After infiltration, these monomers are polymerized in situ , resulting in the formation of a hybrid layer, which in combination with the presence of resin tags inside dentine tubules provides micromechanical retention to the composite restoration ( ⁵² ). From the traditional three-step etch-and-rinse adhesives, simplified two-step adhesives have been developed that combine the primer and the adhesive resin into one single solution. These simplified adhesives present a reduced ability to infiltrate the demineralized dentine substrate, thereby producing suboptimal hybridization when compared to their three-step counterparts ( ⁵³ ). Moreover, the hydrophilic nature of such adhesives render them more prone to water sorption and consequently more susceptible to the effects of hydrolytic degradation. The solvent present in such adhesives is also more difficult to evaporate, frequently remaining entrapped within the adhesive layer after polymerization ( ⁵⁴ ). The etch-and-rinse technique is considered to be critical and highly sensitive, because the over-dried dentin causes both demineralized collagen fibers to collapse and low monomer diffusion among the fibers, hampering the formation of a functionally suitable hybrid layer (HL), however the sensitivity is mostly related to the etching step itself and to the ostensibly antagonistic role of water in the bonding protocol. In ‘over-wet’ conditions, seems to cause phase separation between the hydrophobic and hydrophilic components of the adhesive, resulting in the formation of blister- and globule-like voids at the resindentine interface ( ⁵⁵ ). In addition, the excessive presence of humidity may result in incomplete monomer polymerization and water adsorption in the HL. These effects can decrease the mechanical quality of the HL formed, causing its early degradation ( ⁵⁶ ). However the conditions of over-dry and over-wet remains a major concern and difficult to standardize; must be considered not only extrinsic, but also intrinsic sources of humidity when an adhesive procedure is clinically performed. Therefore, the surface should be gently dried until the etched enamel presents its white-frosted appearance and dentine loses its shine and turns dull ( ⁵⁷ ). Although etch-and-rinse adhesives are still the gold standard for dental adhesion and the oldest of the marketed adhesives, it seem to be incapable of preventing nanoleakage, ( ⁵⁸ ) despite their satisfactory long-term clinical performance ( ⁵⁰ ). Although occurring even in the absence of interfacial gaps, nanoleakage seems to play a negative role in bonding, especially in terms of durability ( ⁵⁹ ). Thereby, the current trend is to develop simplified self-etching materials ( ⁶⁰ ).

Self-etch

Self-etching systems were introduced to control the sensitivity to humidity of the etch-and-rinse technique as well as to simplify the clinical procedures of adhesive application, reducing clinical time ( ⁶¹ ). The self-etch adhesive systems are classified based on the number of clinical application steps: two-steps or one-step adhesives. The basic composition of self-etch primers and self-etch adhesive systems an aqueous solution of acidic functional monomers, with a pH relatively higher than that of phosphoric acid etchants. Therefore, self-etching adhesives have been classified according to their acidity: as strong (pH≤1), intermediate (pH=1.5), and mild (pH≥2) ( ⁶² ) Mild self-etch adhesives demineralize dentin only superficially leaving hydroxyapatite crystals around the collagen fibrils available for possible chemical interaction. Usually, the smear plug is not completely removed from the dentine tubule. As a result, a shallow hybrid layer is formed with submicron measures ( ⁶³ ), as do the ultra-mild self-etch adhesives ( ⁶⁴ ); on the contrary, strong self-etch adhesives demineralize dentin comparably to etch-and-rinse adhesives. The mild self-etch adhesives are assumed to cause less post-operative pain, as they use the smear layer as bonding substrate, leaving residual smear plugs that cause less dentinal fluid flow than etch-and-rinse adhesives. The role of water is to provide the medium for ionization and action of these acidic resin monomers. Self-etch adhesive systems also contain HEMA (2-hydroxyethyl-methacrylate) hydrophilic monomer, because of its low molecular weight HEMA acts as a co-solvent, minimizing phase separation and increasing the miscibility of hydrophobic and hydrophilic components into the solution and to increase the wettability of dentin surface ( ⁶⁵ ). Bi or multi-functional monomers are added to provide strength to the cross-linking formed from monomeric matrix ( ³ ). Because self-etch adhesive systems do not require a separate acid conditioning step as they contain acidic monomers that simultaneously ‘condition’ and ‘prime’ the dental substrate ( ⁶⁶ ), they are considered as simplified adhesive materials. Possibly, self-etching systems alter the “smear layer” that covers the dentin after tooth bur preparation, creating a thin HL of 0.5–1.2 mm thickness ( ⁶⁷ ). For this system, the created tags are short (16 mm) and narrow. However, due to low acidity, the presence of a “smear layer” or “smear plugs” obliterates the tubule orifices is common after adhesive procedures, limiting hybridization of the peritubular dentin and resin tag formation. In spite of forming a thin HL, this system exhibits a chemical bond to the dentin substrate. Furthermore, self-etch dentin adhesive claimed to minimize post-operative hypersensitivity, because residual smear plugs are left which expose less dentinal tubules and causes less dentinal fluid flow than etch-and-rinse bonds, but the disadvantage is an insufficient enamel etching ability resultant from their less acidity and less injurious to the dental substrate than etch-and-rinse adhesives ( ⁶⁸ ). Thus, it is very important to use these dentin adhesives properly in various clinical situations. On the basis of the steps of application, they can be categorized as: a two-steps “self-etch primers” (SEP) that is mostly solvent-free and a one-step “self-etch adhesives” (SEA) depending on whether a self-etching primer and adhesive resin are separately provided or are combined into one single solution. Two-step self-etching adhesive systems (SEA) require the use of two separate components: the first bottle containing primer and acid and the second bottle containing hydrophobic bond resin. The self-etching primer (SEP) used to condition the dental substrate, followed by the application of a hydrophobic bonding resin ( ⁶⁹ ). The self-etching primer are aqueous acidic solutions containing various vinyl monomers (acidic, hydrophilic and hydrophobic monomers) which can simultaneously etch and infiltrate dental tissues, then photopolymerize with the bonding resin, thus forming a bond between the dental substrate and the restorative material applied after wards. Single-step self-etch adhesives that combine the functions of a self-etching primer and a bonding agent have been developed. One-step adhesives can be further subdivided into ‘two-component’ and ‘single-component’ one-step self-etch adhesives. By separating ‘active’ ingredients (like the functional monomer from water), two-component self-etch adhesives theoretically possess a longer shelf life, but additional and adequate mixing of both components is needed. The single-component one step adhesives can be considered as the only true ‘one-bottle’ or ‘all-in-one’ adhesives, as they combine ‘conditioning’, ‘priming’ and ‘application of the adhesive resin’, and do not require mixing ( ⁶⁹ ). This kind of adhesive system combines acidic functional monomers, hydrophilic monomers, hydrophobic monomers, fillers, water and various solvent (acetone, ethanol, buthanol) and resin component, photo-inhibitors for bonding in a single solution. They are so-called as 7 ^th generation dentin adhesive and undoubtedly the most convenient. The use of water as a solvent is indispensable for single-step self-etch adhesives to ensure the ionization of the acidic functional monomers, and the organic solvents are added to facilitate mixing of the hydrophilic and hydrophobic components ( ⁶⁹ ). The presence of water and acidic functional monomers may compromise the bonding durability of single-step self-etch adhesives. However, the main disadvantages of one-step self-etch adhesives is related to their excessive hydrophilicity that makes the adhesive layer more prone to attract water from the intrinsically moist substrate ( ³⁹ ). Due to such increased water affinity, these adhesives have been reported to act as semi-permeable membranes, even after polymerization, allowing water movement from the substrate throughout the adhesive layer ( ⁴⁶ ). As a consequence, small droplets can be found at the transition between the adhesive layer and the lining composite, especially when polymerization of the latter is delayed. Besides promoting a decrease in bond strength between composite and substrate ( ⁷⁰ ), such permeability of the adhesive layer seems to contribute to the hydrolysis of resin polymers and the consequent degradation of tooth-resin bond over time ( ⁷¹ ). In addition, acetone has a so-called “water-chasing” effect ( ⁷² ), thus it can infiltrate rapidly into the exposed dentinal tubules. However, its vapor pressure is much higher than that of other solvents like ethanol or water, and the adhesive may not infiltrate sufficiently in some situations. It was observed that the poor performance of self-etch adhesives could depend upon shallow resin tag penetration produced by the self-etching process, an inefficient curing caused by their acidic nature, or solvent retention and phase separation phenomena due to the coexistence of both hydrophilic and hydrophobic moieties in the same product ( ⁷³ ). Most single-step dentin adhesives are very hydrophilic so that they can interact with underlying dentin. However, it may form water permeable adhesive layer, thus compromising bonding performance ( ⁷⁴ ). To overcome this problem, All-Bond Universal contains minimum amount of ethanol and water as their solvent.

Universal adhesive systems

One of the most recent novelties, in adhesive dentistry, was the introduction of universal adhesives, that have been used since 2011 in clinical practice. These new products are known as “multi-mode” or “multi-purpose” adhesives because they may be used as self-etch (SE) adhesives, etch-and-rinse (ER) adhesives, or as SE adhesives on dentin and ER adhesives on enamel (a technique commonly referred to as “selective enamel etching”) ( ^{75 , 76} ). This versatile new adhesion philosophy advocates the use of the simplest option of each strategy, that is, one-step self-etch (SE) or two-step etch-and-rinse (ER) ( ⁷⁷ ), using the same single bottle of adhesive solution which is definitely much more challenging to dental substrates of different natures (i.e., sound, carious, sclerotic dentin, as well as enamel) ( ⁷⁸ ). Beforehand etching enamel with phosphoric acid is often recommended, in particular when bonding to unground enamel. Indeed, the priming and bonding components can be separated or combined, resulting in three steps or two steps for etch-and-rinse systems, and two steps or one step for self-etch adhesives. Contemplating these two bonding strategies, adequate bonding to dentin can be completely achieved with either etch-and-rinse or self-etch adhesives; however, at enamel, the etch-and-rinse approach using phosphoric acid remains the preferred choice ( ^{79 , 80} ). In relation to the application mode, self-etch adhesive systems reduce the possibility of iatrogenic induced clinical mis-manipulation during acid conditioning, rinsing and drying, which may occur when etch-and-rinse systems are used ( ⁸¹ ). On the other hand, some drawbacks may be listed for these SE materials. Unfortunately, one of the main drawbacks from applying SE adhesives to dentin and enamel is their inability to etch enamel to the same depth that phosphoric acid does, which is likely responsible for the higher rates of marginal discoloration in the enamel margins of cervical restoration due to their lower acidity. Thereby the degradation of SE was attributed to its acidic content, which increases the hydrophilicity of the adhesive layer and leads to water uptake and plasticization ( ⁸² ). So the long-term performance of simplified one-step adhesives is inferior in terms of bond durability ( ^{60 , 83} ), in particular when compared to the gold-standard three-step etch-and-rinse approach. To overcome the weakness of previous generations of single-step self-etch adhesives, universal adhesives have been developed that allow for application of the adhesive with phosphoric acid pre-etching in the total etch or selective-etch approaches in order to achieve a durable bond to enamel and has been accepted by showing good results in vitro ( ⁸⁴ ) and in vivo studies ( ^{85 – 87} ). Despite the similarities between adhesives, the composition of universal adhesive differs from the current SE systems by the incorporation of monomers that are capable of producing chemical and micromechanical bond adhesion to the dental substrates ( ^{75 , 76} ). Its composition is an important factor to be taken account, since most of these adhesive contain specific carboxylate and/or phosphate monomers that bond ionically to calcium found in hydroxyapatite (Ca10[PO4]6[OH]2) ( ^{88 , 89} ), that could be influence the bonding effectiveness ( ⁷⁷ ). For example, Methacryloyloxydecyl Dihydrogen phosphate (MDP) is a functional monomer found in certain new adhesives, but not for older-generation bonding agents. This is a hydrophilic monomer with mild-etching properties. MDP is one of the monomers that enable a universal adhesive to be used with any etching techniques. Stable MDP-calcium salts are formed during this reaction and deposited in self-assembled nano-layers of varying degrees and quality depending on the adhesive system ( ^{90 , 91} ). It also helps promote strong adhesion to the tooth surface via formation of non-soluble Ca ² salts. Furthermore, it contains biphenyl dimethacrylate (BPDM), dipentaerythritol pentaacrylate phosphoric acid ester (PEN-TA) ( ⁹² ) and polyalkenoic acid copolymer may enhance adhesion to tooth structures and have been part of the composition of different materials for decades. This may be important in terms of durability, as water sorption and hydrolytic breakdown of the adhesive interface over time has been implicated as one of the primary causes of bond failure ( ^{93 , 68} ). Additionally, the matrix of universal is based on a combination of monomers of hydrophilic (hydroxyethul methacrylate /HEMA) hydrophobic (decandiol dimethacrylite /D3MA) and intermediate (bis-GMA) nature. This combination of properties allows universal adhesives to create a bridge over the gap between the hydrophilic tooth substrate and hydrophobic resin restorative, under a variety of surface conditions. Moreover, some universal adhesives may contain silane in their formulation, potentially eliminating the silanization step when bonding to glass ceramics or resin composites, for instance. Nevertheless, it is known that simplified materials are associated with lower in vitro bond strength results and poorer in vivo longevity of restorations, ( ^{94 , 95} ). These findings are probably a result of the complex formulation of simplified adhesives and their high content of solvents, which may impair complete solvent volatilization and consequently lead to poorer adhesive polymerization ( ⁹⁶ ). This multi-approach capability enables the clinician to apply the adhesive with the so-called selective enamel etching technique that combines the advantages of the etch-and-rinse technique on enamel, with the simplified self-etch approach on dentine with additional chemical bonding on remnant carbonated apatite crystallites in those bonding substrates. Therefore, the universal adhesives have much broader applications than 7 ^th generation systems. Additionally, manufacturers typically state that universal adhesives can be used for the placement of both direct and indirect restorations and are compatible with self-cure, light-cure and dual-cure resin-based cements and bonds to metals, zirconia, porcelain and composite. While, the manufacturers of some universal adhesives still recommend the use of separate “activator” and dedicated primers to optimize bond strength to substrates such as porcelain and zirconia. Thus, it appears, at least in certain situations and with some products, that universal adhesives actually consist of two bottles, or require the use of an additional activator, or have chemistries that must be mixed prior to use, or bond most optimally to porcelain and zirconia with separately applied and dedicated primers, or are not compatible with a total-etch protocol. Further, there is an advantage in having an adhesive that can operate on these two procedures since it allows the dentist to choose his procedure according to the clinical case in order to optimize the final result. For instance, when the restoration requires strong bonding to enamel or in case of sclerotic dentin, it may be advisable to apply prior etching. The etching step can be modulated according to the length of time the phosphoric acid gel is applied prior to rinsing. On the other hand, it may be preferable to benefit completely from the self-etch path way, when dealing with cases confronting difficult access, limited time or poor patient compliance in very young patients.

Conclusions

Increasing demands for aesthetic restorative treatments have led to recent advances in dentistry, developing adhesive integrated materials (such as adhesive systems and composites) and techniques aimed at restoring the natural tooth appearance, especially in the anterior segment ( ⁹⁷ ). The major requirement of adhesive aesthetic materials is the ability to achieve an excellent color matching with the natural teeth and the maintenance of the optical properties over time. The goals for esthetic dental restorations are to obtain morphologic, optical and biologic result miming natural enamel and dentine. This color matching is performed in order to obtain harmony with the surrounding anatomical structures ( ⁹⁸ ).

Further the evolution of these materials and techniques has recently took steps forward and succeed in preserving teeth instead of extracting them. Most of these improvements were evident in conservative dentistry and in particular, adhesive dentistry ( ⁹⁹ ).

This review about adhesive dentistry describes all the “generations” and types of adhesive product designs that have been introduced during the last 30 years. Since the introduction of the acid etched into clinical practice, various dentin bonding agents were developed to improved the quality of adhesives and composites restoration. The manufactures have been ongoing progress in the development of new dentin adhesive aiming to simplify the process, attempted to improve clinical results correlates to their stability over time and to their bond strength performance consequently lead to improve their effect on the durability of the resin bond. The new adhesive systems also can be attributed to their ability to decrease or eliminate postoperative sensitivity, improve marginal seal, reduce microleakage and enhance the flow of resin into fissure. The development of functional monomers with strong and stabile chemical affinity to hydroxyapatite is without doubt a valuable direction to continue for improvement of dental adhesion. Furthermore, long-term ageing also requires evaluation of its effect in establishing a long-term success of composite restoration.

References