REVIEW ARTICLE

Self-etching adhesive systems, bond strength and nanofiltration: a review

Maritza Parra Lozada¹; Herney Garzón Rayo²

¹ Undergraduate intern. Specialization in Oral Rehabilitation, Universidad del Valle, Cali
² Oral Rehabilitation Specialist, Universidad Militar Nueva Granada, Fundación CIEO, Professor, Graduate Program, Specialization in Oral Rehabilitation, Universidad del Valle, Cali

SUMBITTED: JANUARY 17/2012-ACCEPTED: JULY 31/2012

Parra M, Garzón H. Self-etching adhesive systems, bond strength and nanofiltration: a review. Rev Fac Odontol Univ Antioq 2012; 24(1): 133-150.

ABSTRACT

Self-etching adhesive systems have greatly enhanced clinical procedures as they have allowed not only developing components and their functions but also reducing application times. During the latest decade they have been subjected to permanent clinical and experimental tests in order to assess their performance. Their multiple advantages contrast to their low effectiveness in terms of bond strength and permeability, as water drops develop on the adhesive interface, thus favoring nanofiltration. As self-etching adhesive systems offer lower reliability in relation to bond strength and nanofiltration in comparison to adhesives of fourth and fifth generation, the latter stand as the golden standard for comparative studies, as they keep using the acid etching protocol, by means of 35 or 37% ortophosphoric acid. The goal of this review article is to present the evolution of self-etching adhesive systems and to discuss bond strength and nanofiltration as variables that intervene in their poor performance, providing dentists with the fundamentals for a critical accurate selection of etching systems.

key words: dental adhesives, self-etching systems, dental filtration, degradation.

INTRODUCTION

The advances of contemporary dentistry increasingly focus on the evolution of materials and clinical techniques due to the frequent demand for esthetical procedures and long-lasting results. A great variety of restoring materials is currently available, and selecting them has become a critical process for dentists, as each procedure must guarantee not only a correct technique but also the peace of mind of using the right materials in order to provide patients with reliable and highly esthetical treatments while fulfilling their expectations.

The evolution of dental biomaterials currently focus on improving components and materials performance, while simplifying clinical procedures in order to achieve better results in a shorter time.

Adhesive systems are a group of biomaterials with an essential role in most treatments dealing with adhesive restorations for esthetic purposes, and are therefore one of the critical aspects of clinical protocols. A great deal of research in the field of dentistry focus on adhesion to the diverse dental substrates, with special emphasis on microfiltration and bond strength.^{1, 2} Advances on these systems, as it commonly happens in most studies on dental materials, focus on improving their components and simplifying their techniques.^{3, 4} A great variety of adhesive systems is currently available in the market; self-etching adhesives are among the latest generation, often addressed by diverse studies that support commercial houses in their strong promotion and marketing processes.² However, their popularity contrast to their poor performance in both in vitro and in vivo studies

El objetivo de esta revisión es conocer la evolución de los sistemas adhesivos autograbadores y discutir las variables de resistencia de unión y nanofiltración que intervienen en su pobre desempeño, permitiendo al clínico tener fundamentos para la crítica y adecuada selección del sistema adhesivo utilizado.

CLASSIFICATION OF ADHESIVE SYSTEMS

the origins of adhesive dentistry date back to 1955 when buonocore, by using industrial adhesion techniques, pointed out that treating the dentine surface with acid etching could make adhesion properties last longer. ^5-8 By 1960 he suggested that the formation of resin tags was the main cause of resin adhesion to the etched enamel. ⁷ Acid etching on the dentin allowed removing the smear layer and preparing its external layer, thus removing part of the inorganic content, exposing collagen mesh, and increasing permeability of the dentinal tubules, which would be infiltrated by the adhesive system to form the so-called hybrid layer, a critical mechanism in the process of resin adhesion to dentine. ^{7, 9} In 1998, van Meerbeek et al suggested a classification of adhesive systems based on mode of interaction with the substrate; they also considered the number of clinical steps required to apply the adhesives: 1. One-step adhesives that modify the smear layer, 2. two-step adhesives that: a) modify the smear layer, b) dissolve the smear layer, c) eliminate the smear layer. 3. three-step adhesives that eliminate the smear layer.¹⁰

Similarly, a classification based on the adhesive strategy was proposed; three adhesion mechanisms are currently used by modern adhesive systems: 1) etch-and-rinse adhesives, 2) Self-etching adhesives, and 3) Glass ionomer adhesives and resin-modified glass ionomers.⁹

However, the most commonly used current classification of adhesive systems is the one based on dentine treatment following the chronological availability of these materials in the market, sorted out in generations. this classification was proposed by Kugel et al.⁷

the first generation of adhesive systems included the use of acid glucosaminyl muramyl dipeptide (GNDP) to improve adhesion of resin to dentine; they were developed by buonocore et al in 1956. ¹¹ they would later evolve to the bifunctional molecule N-Phenylglycine Glycidyl Methacrylate (NPGGMA), but its bond strength was very low, of only 1 to 3 MPa.⁷

The second generation focused on improving the adhesives' bonding agents, so during the early 70's bisphenol halophosphate esters were added to either glycidyl methacrylate (bis-GMA) or hydroxyethyl methacrylate (HeMA), with careful attention on the calcium-chloride-phosphate groups ionic bond; however, bond strength was still very low, 5 to 7 MPa, producing hydrolysis due to saliva exposure, and therefore microfiltration.⁷

During the third generation by the late 70's, partial acid etching of the dentine was introduced to partially modify the smear layer, increasing dentinal permeability. two other components were introduced: the primer, with molecules of bifunctional monomers containing a hydrophilic end and a hydrophobic end (carboxylic end), which have the capacity of transporting hydrophobic molecules, such adhesive monomers, to a tissue with relative humidity such as the dentine, with the capacity of adhering to hydrophobic monomers by its hydrophilic end and to the hydrophobic monomers by its carboxylic end, significantly increasing the capacity of adhering the dentine, between 8 and 15 MPa. This eliminated the need of retentive cavity preparations for adhesive restorations, while also reducing post-operatory sensitivity.⁷

by 1980, with the fourth generation of adhesive systems, the total etch technique was introduced to entirely remove the smear layer, simultaneously etching both enamel and dentine by using phosphoric acid; nevertheless, the main purpose was to avoid collapsing of the collagen fiber mesh exposed on the demineralized dentine layer and to favor the formation of resin tags and lateral branches on the dentinal tubules —which form the so-called hybrid layer, that was described by nakabayashi in 1982¹² as the dentine-resin inter-diffusion area formed by infiltration monomers of both primer and adhesive in the collagen fibers mesh exposed by action of the acid conditioner on the periand inter-tubular dentine. These components may be used separately or combined at the time of application, which could increase technique sensitivity. ^{7, 13-15} Some of the advantages of phosphoric acid total etch include: increasing surface contact angle, increasing surface energy in order to improve humidity on the adherent surface, facilitating resin tags formation, and increasing micromechanical retention, achieving bond strength values of about 31 MPa. ^{16, 17}

The fifth generation allowed simplifying the clinical procedure of adhesive system application, relatively reducing operation times; nevertheless, just as for the fourth generation, collagen fibers mash collapsing must be avoided during the process of total etching. ⁷ During the 90's, this generation initiated the ''one-bottle system'' by combining primer and adhesive in a solution applied after etching both enamel and dentine with 35-37% phosphoric acid during 15 to 20 seconds, thus allowing the formation of resin tags, lateral branches, and hybrid layer, and creating the resin's micromechanical retention into the demineralized substrate. ^16-19 This demonstrated bond strength values both to enamel and dentine of approximately 29 MPa. ^{7, 17}

The steady evolution of adhesive systems aiming at simplifying clinical procedures and operation times, as well as sensitivity towards operative technique, favored the development of adhesive systems of the sixth generation by the mid 90's —the so-called self-etching systems—. these allowed eliminating the step of acid etch application, by simultaneously etching the dental substrate and preparing it to receive the adhesive, by means of self-etching primers and mixtures of adhesives and primers, producing micromechanical retention in hard tissues and enabling direct bonding on the smear layer that covers the dentine. ^20-22 this new system differs from etchand-rinse adhesives in several aspects, such as initial pH, type of acidic monomers, number of bottles and steps, water and solvents concentration, and bond layer hydrophilicity. ^{23, 24} Bond strength values of approximately 26 MPa are reported for two-step self-etching adhesives. ¹⁷ These are composed of aqueous mixtures of functional acidic hydrophilic monomers, usually esters of phosphoric acid, with a pH of 1.5 to 2.5 ?a bit higher than acid phosphoric acid gels. ^{6, 13, 25}

They may also be classified according to their application technique as: sixth generation type I, in which initially the first self-etching adhesive is applied and ventilated, and later the adhesive is applied, ventilated again, and then photopolymerized. Another characteristic of this system is that it is usually compatible with self-curing resin cements, by initially applying the primer on the tooth, ventilating it and then the adhesive is applied and light cured; finally, the self-cured resin luting is applied in order to cement the restoration. The sixth generation type II mixes the primer and the adhesive before applying it on the tooth; the first layer is ventilated for ten seconds and the second one is light cured. This system is not usually compatible with dual resin cements or with self-etching or post core-reconstructing resins such as self-etching conventional composite resins. The acidity of this adhesive agent may interfere with the shaping of composite resin.^31-33 This lack of compatibility occurs because the chemical polymerization of resins take place by a binary system of dual etching or Redox consisting on a peroxide and an aromatic tertiary amine.

Activation of light-curing systems occurs after production of free radicals in the activation of a photoinitiator (diketones such as camphoroquinone) followed by reduction of a photoinitiator activated by an alymphatic amine, allowing liberation of free radicals. Therefore, the resin cements' bond strength to dentine is influenced by incompatibility between the polymerization modes of the cement and the adhesive system, and some authors believe that the incompatibility between self-etching simplified adhesive systems and dual curing resins is related to the acidity of this system. The thorough removal of water or adhesive solvents may also delay the polymerization of self-curing composites.

The oxygen inhibition layer of self-etching adhesives contains dimethacrylates, acidic monomers with phosphate or carboxylic ester groups, and when these adhesives are used along with self-curing resins, an interaction occurs among the adhesive's residual acidic monomers, inhibiting the catalytic components of the peroxide-amine system, typical of the self-curing mechanism. These resin acidic monomers poorly polymerize in presence of the Redox peroxide-amine system, the tertiary amines are therefore neutralized by the acidic monomers, and lose their agent-reducing capacities.^34-36

To reduce the harmful effects of self-etching adhesives' permeability, the general recommendation is to first apply a layer of adhesive as primer, followed by a layer of resin without a solvent agent. The use of anion exchangers in the self-curing resin cement has also been reported.

Due to the water-permeable nature of bonding areas, water penetration is allowed from adjacent dentine, leading to the formation of a great number of micropores at the interface that favor degradation of the adhesive bond layer in the long run—a mechanism that has been attributed to the acidic nature of the interface.

Sulfonic acid has been used as an initiator composite to favor compatibility because it is not sensitive to an acid environment in the dentinal surface treated as a simplified adhesive system.

Sulfonic acid salt is present in several adhesive systems as a promoter of chemical curing composite resins bonding. Another initiator that may be used to transmit a bonding potential of the adhesive system in an acid environment is ascorbic acid.

The use of desensitizing oxalates after dentinal etching and before application of the adhesive has also been reported as a way to decreasing the dentine's hydraulic conductivity due to occlusion of dentinal tubules by liberation of calcium ions in the form of calcium oxalate crystals in both intertubular and peritubular dentine matrices. This reduces water retention at the adhesive interface, but is still under research.^34-38

Adhesive systems of the seventh generation are one-bottle, all-in-one self-etching adhesives whose technique has been fully simplified, offering in one single solution all the hydrophilic acidic monomers, the organic solvents and the water required to activate both the dentine demineralization process and proper system operation. ^{26, 39, 40} LSolvents such as acetone or alcohol are kept inside the solution and their evaporation initiates once they are applied, launching the separation phase reaction, as well as formation of a number of water drops and inhibition by oxygen; also, its conversion degree decreases, favoring hydrolytic degradation and affecting bond strength at the adhesive interface.^{21, 40, 41} Bond strength values of about 20 MPa are usually reported.¹⁷ This system is not compatible with self-curing resin cements either.

In contrast to their advantages, such as procedure simplification, technique sensitivity reduction, simultaneous resin demineralization/infiltration, operation time reduction, and post-operatory sensitivity, the results in terms of bond strength and nanofiltration leave serious doubts on the clinical effectiveness of adhesive systems of the seventh generation due to their instability over time.²⁷

BOND STRENGTH OF SELF-ETCHING ADHESIVE SYSTEMS

In spite of the many generations of adhesives, new materials are launched every year, but the availability of in vivo studies on the clinical performance of self-etching adhesive systems is still scarce^{4, 6} The introduction of self-etching adhesives has simplified adhesion procedures but it is important to take into account that bond to the dentin may be negatively affected if manufacturers' instructions are not closely followed.^{20, 25, 42} Similarly, basic requirements to produce good adhesion must be considered, not only for the adhesion surface but also for the adhesive itself. These requirements include the surfaces being clean and soft and surface energy being high. Concerning the adhesive, the contact angle must tend to zero; this is achieved by means of good humidity, low surface tension, low viscosity, and adequate fluency, so that it presents penetration capacity for capillarity in narrow gaps, with as minimum polymerization concentration as possible in order to avoid the separation phase due to solidification of the adhesive.⁴³

In spite of the strong commercial campaigns, ''allinone'' self-etching adhesives have not performed as expected. Experimental studies have reported low rates of adhesion to the dentine, as well as pre-test failures, thermo-cycling vulnerability, poor adhesion to the enamel, filtrations on the enamel (expressed in low bond strength and nanofiltration), incompatibilities with self-curing resins, semipermeable membrane-like behaviors, nanofiltrations, and monomers-solvent separation phase.^{3, 6, 20, 44, 45}

Most experimental studies compare the performance of adhesives considered to be gold standard (the so-called conventional etching adhesives and threesteps rinsing adhesives of fourth generation) due to their excellent characteristics and functionality during clinical and experimental tests.^{26, 27} These adhesives have shown high bond strength levels in comparison to self-etching adhesives of sixth and seventh generation due to the formation of water vesicles at the adhesive interface, creating nanofiltration gaps and failure of restorations performed with self-etching systems.

Despite the poor results of self-etching adhesives, the two-step Clearfil SE^® adhesive of type I sixth generation has stood out for having the best results in this group due to its good performance in both in vivo and in vitro studies for its chemical composition and its good polymerization.^{14, 27, 39, 46, 47} This adhesive contains 10-methacryloxydecil dihydrogen phosphate (10-MDP), a functional monomer dissolved in water with a pH close to 2, that interacts with surface dentine producing a reaction-integration layer, with a depth of about 1 µm and 300 nm according to its pH—equivalent to the hybrid layer of conventional etch-and-rinse adhesives. Its clinical effectiveness responds to simultaneous and smooth demineralization and infiltration of dentinal surface, which creates a stable layer of resininfiltrated dentine.^{28, 48} The adhesion-decalcification concept (AD) describes the way molecules chemically interact with hydroxyapatite-based tissues by means of chemical reactions such as calcium chelation. Soft self-etching adhesives and glass ionomers interact with the enamel and the dentine only in their external surfaces, hardly dissolving hydroxyapatite crystals and keeping them in their place instead, creating ionic bonds from them. On the contrary, strong selfetching adhesives have demineralizing effects on both enamel and dentine, just as etch-and-rinse adhesive systems do, but the difference is that the dissolved calcium phosphates are not rinsed; these embedded phosphates are very instable in aqueous environments so they considerably debilitate the integrity of the adhesive interface with the exposed dentine and collagen. Better effects are therefore obtained with soft self-etching adhesives.²⁹

In general, self-etching adhesives present better bond strength to dentine than to enamel due to their low acidity in comparison to conventional etch-andrinse adhesives.^{1, 49} This low bond effectiveness can be especially observed by the action of ultrasoft selfetching adhesives on the enamel due to their low micromechanical retention potential and to low chemical reactivity with the enamel's hydroxyapatite. This is why some authors have proposed a selective etching of the enamel margins with phosphoric acid, thus converting a two-step etchant into a three-step adhesive and a one-step etchant into a two-step adhesive.

Obviously, this procedure disagrees with the advantages usually attributed to self-etching adhesives, such as reduction of treatment time and technique simplification; however, some authors recommend combining an etch-and-rinse treatment on the enamel with soft self-etching adhesives on the dentine, since the etchant with phosphoric acid may be considered aggressive on the dentine as it may produce collagen denaturalization in relation to etching time and acid concentration, discrepancy between penetration capacity (10-30 nm) and etchant depth (5-8 µm),⁵⁰ nanopercolation, activation of metalloproteinases that may degrade the hybrid layer, and possible collapse of the collagen mesh exposed by overdrying avoiding resin infiltration and the adequate formation of hybrid layer. The procedure of mixing total etching on enamel and soft self-etching adhesives on dentine may improve results in the long run, and due to its amount of clinical success is therefore recommended by authors such as Peumans and Van Meerbeek B in 2010.⁵¹ Self-etching adhesive systems demonstrate in vitro and in vivo degradation over time of the interface of resin-dentine adhesion with bi-functional highly hydrophilic monomers, which allow a fast absorption of water, debilitating the polymer mesh; also, full evaporation of solvents is hard to achieve even with ventilation, and it decreases rigidity of the adhesive layer due to reduction of the monomers' conversion degree. This condition is much more evident in onestep self-etching adhesives, since this last generation of adhesive systems presents a complex mixture of hydrophilic and hydrophobic components that enable more water absorption and more solubility in comparison to two-step self-etching adhesive systems, significantly reducing bond strength.^{29, 52}

Several mechanisms to measure effectiveness of the adhesives' bonding to enamel and dentine are currently available; they may be macroor microtests that basically depend on the size of the bonding area. The bond strength macrotests include areas greater than 3 mm2; they might be tensile tests, shear tests, and the like. Microtests generally use the micro tensile bond strength test (µTB S) that was developed by Sano et al in 1994;⁵³ their testing area are usually much smaller, close to 1 mm² or less.¹⁷

The micro tensile bond strength test (µTB S) is the one most commonly used and the most relevant due to its reliability; although the bond strength value cannot be considered as a material property, it allows evaluating the material's performance in presence of external forces.^{9, 17, 20, 54, 55}

Although some studies have concluded that pH does not have a predictive value for bond strength, several authors suggest that composition and result of the mechanical strength is more relevant than the adhesive's acidity.²⁷

NANOFILTRATION

Filtration allows water and other products circulating along the interface through gaps produced during the restoration process. According to the size of these gaps, there are two types of filtration: ''microfiltration'' occurs when the gaps are long allowing clinically undetectable circulation of bacteria, fluids, molecules or ions between the cavity's wall and the restoration material. This separation occurs when bond strength to the cavity's wall is lower than the restoration's contraction stress. If this process occurs between the hybrid layer and the intact dentine, where small holes of nanometric dimensions are created allowing penetration of small molecules, the filtration receives the name of ''nanofiltration''.^{9, 56}

The adhesive systems' bonding reliability is essential to the long-lasting success of resin restorations; sealing the margins protects against microfiltration and subsequent complications such as postoperatory sensitivity, marginal discoloration, and recurring caries⁵⁶ Nanofiltration may be avoided by more complex processes such as: Entire solvent evaporation, an adequate sealing of dentinal tubules by means of resin interdigitations that block the outcome by pressuring on dentinal fluid, a complete infiltration of the adhesive system, and inactivation of the metalloproteinases. ^{9, 57, 58} Numerous in vitro studies have demonstrated that the response of the pulpodentinal complex depends on both the material used as a liner or dentinal-pulpal protection and its capacity to avoid microfiltration.⁵⁹

The adhesive systems' nanofiltration has been observed by means of silver nitrate infiltration, which allows observing gaps in the interface that may be the cause of nanofiltration of fluids.^{28, 60, 61} All the adhesive systems reviewed, from conventional adhesives of the fifth generation up to the selfetching adhesives of the seventh generation, present nanofiltration. Tay et al⁶² described two types of nanofiltration that may occur at both the hybrid layer and the adhesive layer: in the form of a cluster or as a stain. ²⁷ There are several explanations to the different types of nanofiltration at self-etching adhesive systems; these explanations include: zones of suboptimum polymerization, and discrepancies between demineralization and infiltration of the resin due to the infiltration depth of demineralizing monomers that do not polymerize, thus increasing nanofiltration susceptibility of self-etching adhesives, especially those that are classified as all-in-one of the seventh generation. ^{27, 32} EThese systems present highly hydrophilic monomers which allow extracting water from the subjacent dentine through the adhesive, forming water drops in the adhesive interfaces and favoring the formation of water clusters and therefore nanofiltration, particularly when the resin system is not immediately cured to block this osmosis effect, mainly in the case of one-step self-etching adhesives, which are rich in HEMA.^{29, 32, 47, 57, 61} This water retention mechanism due to the processes of osmosis and aqueous absorption may be increased by the presence of pulpal pressure.63 Due to water movements inside the adhesive-dentine interface, resinous adhesives and collagen fibers in the hybrid layer may suffer hydrolytic and bacterial degradation by products such as acids and enzymes resulting in deterioration of bond strength and failure of the restoration when using different adhesive systems. ^{9, 39, 61, 64} Instability of all-in-one solutions of self-etching adhesives of the seventh generation and of some type II of the sixth generation is usually exceeded by separation of the components and steps of fourth generation adhesives in which the surface is initially etched with acid as a necessary procedure to eliminate or modify the smear layer and to demineralize the dentinal surface to later proceed to preparation of the demineralized surface by means of a primer with hydrophilic monomers and finally to application of hydrophobic adhesive resins.^{20, 65} This adhesion protocol reduces the risk of forming semi-permeable membranes and water drops due to separation of hydrophilic and hydrophobic monomers, improving their performance.²¹

Hydrogen ion potential or pH measure of the adhesive systems by action of the conditioner produces metalloproteinase activation, favoring hydrolytic degradation of the collagen fibers. In order to improve stability of the adhesive interface, generating a compact and homogeneous hybrid layer, several mechanisms to control dentinal metalloproteinases have been developed. Metalloproteinases are zinc/ calcium-dependent endopeptidases that are capable of degrading components of the extracellular matrix including collagen in its natural or denaturalized forms. Metalloproteinases are expressed by odontoblasts and other pulpal cells during synthesis of the extracellular matrix; they are gelatinases (MMP2-9), collagenases (MMP1-8), and estromelisines (MMP 10-11). ^{66, 67} These latent forms are released from the dentine by dissolution of inorganic components as a result of pH reduction during the acid etching processes with the use of both phosphoric acid and application of selfetching adhesives. When the demineralized dentine is not fully infiltrated by the adhesive system, zones of exposed dentine are formed in both conventional etch-and-rinse adhesives and two-step self-etching adhesives. Exposed dentine and collagen fibers are vulnerable to hydrolytic degradation favored by dentinal metalloproteinases exposed during acid etching, increasing the formation of porosities at the hybrid layer, which turn into a permeable membrane for water absorption and the subsequent nanofiltration, hydrolysis, and degradation of the collagen exposed at the base of the hybrid layer. Some studies have demonstrated that chlorhexidine digluconate solutions at concentrations ranging from 0.02 to 2% can inhibit metalloproteinases and reduce solubility of collagen fibers in an aqueous environment, and that its action on the etched dentine does not negatively influence microtensile bond strength of adhesive systems; on the contrary, it has been proven that besides their antimicrobial effect they prevent or slow down the degradation of collagen fibers exposed at the base of the hybrid layer by action of the metalloproteinases when applied after acid etching during etch-and-rinse adhesive systems; this favors dentine adhesion in the long run without cytotoxic effects.

Chlorhexidine is an amphipathic molecule (that is, it presents hydrophilic and hydrophobic groups) that connects to several proteins by a chelation mechanism, preventing that zinc or calcium ions connect to metalloproteinases and inhibiting their catalytic activity. Several in vitro studies have therefore suggested that incorporating chlorhexidine digluconate in the primer of some two-step self-etching adhesives may produce an inhibitory effect of the metalloproteinases and does not affect immediate bond strength, while using it as disinfecting agent, that is, before application of one-bottle self-etching adhesives on cavities restored with light-curing resins, increases microfiltration. ^68-74 Other substances have also been added to the diverse components of adhesive systems in order to inactivate metalloproteinases, such as 0.51% benzalkonium chloride, which has been added to phosphoric acid without affecting bond strength of the adhesive process.⁷⁵ Polyvynil phosphonic acid, EDTAC, tetracyclines, and alcohol have also been used as MMP inhibitors in in vitro studies^{76, 77}

Galardin (GM6001), used in ophthalmology, is a synthetic and selective inhibitor of metalloproteinases MMP-2, -3, -8, and -9, improving the hybrid layer's long term stability and reducing the amount of nanofiltration.⁷⁸

CONCLUSIONS

Self-etching adhesive systems offer lower reliability in terms of bond strength and nanofiltration in comparison to three-step etch-and-rinse adhesives of fourth generation, which remain as the golden standard for comparative studies.

Out of all the self-etching adhesives, type I sixth generation has offered the best results; therefore, two-step self-etching adhesives present a better clinical and experimental performance than the one-step one-bottle systems. Self-etching adhesive systems present a better performance in dentine adhesion than in enamel adhesion.

Self-curing dual resin cements are incompatible with self-etching adhesives due to their different modes of polymerization and acidity of the adhesive system.

Adding chlorhexidine to the primer of some twostep self-etching adhesives during the protocol of adhesion allows somehow inhibiting metalloproteinases and does not have harmful effects on immediate bond strength to the dentinal substrate.

CORRESPOONDING AUTHOR

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