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Adhesive dentistry has undergone great progress in the last decades. In light of minimal-invasive dentistry, this new approach promotes a more conservative cavity design, which relies on the effectiveness of current enamel-dentine adhesives. Adhesive dentistry began in 1955 by Buonocore on the benefits of acid etching. With changing technologies, dental adhesives have evolved from noetch to total-etch (4th and 5th generation) to selfetch (6th, 7th and 8th generation) systems. Currently, bonding to dental substrates is based on three different strategies: 1) etch-and-rinse, 2) self-etch and 3) resin-modified glass-ionomer approach as possessing the unique properties of self-adherence to the tooth tissue. More recently, a new family of dentin adhesives has been introduced (universal or multi-mode adhesives), which may be used either as etch-and-rinse or as self-etch adhesives. The purpose of this article is to review the literature on the current knowledge for each adhesive system according to their classification that have been advocated by many authorities in most operative/restorative procedures. As noted by several valuable studies that have contributed to understanding of bonding to various substrates helps clinicians to choose the appropriate dentin bonding agents for optimal clinical outcomes.
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Classification review of dental adhesive systems: from the IV generation to the universal type
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Classification review of dental adhesive systems: from the IV generation to the universal type
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                                                                                                     Reviews article


    Classification review of dental adhesive systems:
    from the IV generation to the universal type




                                                                                                                  i
                                                                                                                al
    Eshrak Sofan, PhD                                          Introduction




                                                                                                   n
                                                               The development and regular use of adhesive materi-
    Afrah Sofan, PhD

                                                               als has begun to revolutionize many aspects of
    Gaspare Palaia, PhD




                                                                                                io
                                                               restorative and preventive dentistry. Attitudes to-
    Gianluca Tenore, MD, DDS

                                                               wards cavity preparation are altering since, with ad-
    Umberto Romeo, MD, DDS

                                                               hesive materials, it is no longer necessary to prepare
    Guido Migliau, MD, DDS




                                                                   az
                                                               the cavity to provide mechanical retention through
    Department of Oral and Maxillofacial Sciences,             such features as dovetails, grooves, undercuts, sharp
    University of Rome “Sapienza”, Rome, Italy                 internal angles in order to retain the ﬁlling (1). These
                                                               techniques are, therefore, responsible for the conser-




                                                                 rn
                                                               vation of large quantities of sound tooth substance,
                                                               which would otherwise be victim to the dental bur. Mi-
    Guido Migliau
    Corresponding author:
                                                               croleakage a major dental problem, which is probably
    Department of Oral and Maxillofacial Sciences,             responsible for many cases of secondary caries, may




                                                               te
    “Sapienza” University of Rome                              be reduced or eliminated. These adhesive are there-
    Via Caserta 6                                              fore critical for the success of aesthetic materials
    00162 Rome, Italy
                                                               restorative in modern dentistry.
    E-mail: guido.migliau@uniroma1.it
                                                       In      Dental adhesives are solutions of resin monomers
                                                               that make the resin dental substrate interaction
                                                               achievable (2). Adhesive systems are composed of
                                                               monomers with both hydrophilic groups and hy-
    Summary
                                                ni
                                                               drophobic groups. The former enhance wettability to
                                                               the dental hard tissues, while the latter allow the in-
    Adhesive dentistry has undergone great progress in
                                                               teraction and co-polymerization with the restorative
    the last decades. In light of minimal-invasive
                                        io


                                                               material (3). The chemical composition of adhesives
    dentistry, this new approach promotes a more con-
                                                               also includes curing initiators, inhibitors or stabilizers,
    servative cavity design, which relies on the effec-
                                                               solvents and, in some cases, inorganic ﬁllers (3).
    tiveness of current enamel-dentine adhesives. Ad-
                               iz



                                                               However, it is necessary to consider the anatomy of
    hesive dentistry began in 1955 by Buonocore on the

                                                               tooth. In particular, composition and structure of two
    benefits of acid etching. With changing tech-

                                                               main tissues, enamel and dentine, need to be exam-
    nologies, dental adhesives have evolved from no-
               Ed




                                                               ined in order to understand how they influence adhe-
    etch to total-etch (4th and 5th generation) to self-etch

                                                               sive bonds. Details of the composition of these tis-
    (6th, 7th and 8th generation) systems. Currently,

                                                               sues are shown in Table 1. The mineralized part of
    bonding to dental substrates is based on three dif-

                                                               the tooth is a complex structure made of different
    ferent strategies: 1) etch-and-rinse, 2) self-etch and

                                                               hard tissues, which have a quite distinct ultra-mor-
    3) resin-modified glass-ionomer approach as pos-

                                                               phology and composition. Enamel is composed of a
    sessing the unique properties of self-adherence to
 IC




                                                               hard solid crystalline structure-hydroxyapatite (HAp)
    the tooth tissue. More recently, a new family of

                                                               with strong intermolecular forces, high-energy sur-
    dentin adhesives has been introduced (universal or

                                                               face, besides water and organic material. Dentin is a
    multi-mode adhesives), which may be used either

                                                               biological composite of HAp that envelops collagen.
    as etch-and-rinse or as self-etch adhesives.
C




                                                               Dentin is intrinsically humid, and less hard than
    The purpose of this article is to review the litera-

                                                               enamel, with low intermolecular forces and low-ener-
    ture on the current knowledge for each adhesive

                                                               gy surfaces. The dentin is different from enamel, as it
    system according to their classification that have

                                                               has smear layer, organic contents and presence of
    been advocated by many authorities in most op-
@




                                                               fluid inside the dentinal tubules. In addition, the den-
    erative/restorative procedures. As noted by sever-

                                                               sity of dentinal tubules varies with dentinal depth and,
    al valuable studies that have contributed to un-

                                                               as well as the water content of dentin, is lowest in su-
    derstanding of bonding to various substrates

                                                               perficial dentin and highest in deep dentin. In superfi-
    helps clinicians to choose the appropriate dentin

                                                               cial dentin, which contains fewer tubules, the perme-
    bonding agents for optimal clinical outcomes.

                                                               ation of resin into intertubular dentin will be responsi-
                                                               ble for most of the bond strength. In deep dentin,
    Key words: dental bonding agents, smear layer,
    adhesive systems, self-etch, etch-and-rinse.

    Annali di Stomatologia 2017;VIII (1):1-17                                                                           1
    E. Sofan et al.


    Table 1. Composition of tooth tissues.                       could provide a surface suitable for bonding with risen
                                                                 and also to improve the retention of acrylic resin to pit-
                                                                 and-ﬁssures (8). The mechanism of acid-etch en-
                                                                 hanced adhesion was not published until 1968 (9),
    Components                        Enamel         Dentine
    Inorganic phase (mainly           94-96          50-70
    hydroxyapatite) (%)                                          when Buonocore, Matsui and Gwinnett discussed the
                                                                 effect of phosphoric acid conditioning, which produced
    Calcium phosphate ratio           1.64           1.56        “prism-like” tags of resin materials that penetrated




                                                                                                                      i
                                                                                                                    al
    Organic phase (mainly             4-5            20-30       enamel surfaces. These resin tags were not seen in
    collagen ) (%)                                               unconditioned enamel. The effect of phosphoric acid
                                                                 on enamel resulting in increased adhesion was now
    Water (%)                         1-4            10-20
                                                                 part of the dental literature, but it would be many years




                                                                                                     n
                                                                 later that this principle would be widely accepted. This
                                                                 was the pioneering research of Minimally Invasive




                                                                                                  io
    dentinal tubules are more in number: the intratubular        Dentistry (10). Enamel conditioning with phosphoric
    permeability of resins will be responsible for higher        acid results in the formation of microporosities where
    bond strength (4). Dentin is also a substrate that un-       resin penetrates to form “prism-like” resin tags. This




                                                                     az
    dergoes change with age in an asymmetrical physio-           yields an enamel bonding predominantly microme-
    logical aging process, leading to an increase of             chanical (11). While the same concept applied to
    dentin thickness and decrease in dentin permeability         dentin in 1958 remained problematic, due to the use of
    (5). Furthermore, sclerotic and carious dentin suffers       strictly hydrophobic resins. As well, the high polymer-




                                                                   rn
    structural changes that result in a higher mineraliza-       ization shrinkage of acrylic filling materials gave
    tion and a consequently reduced permeability (5). Un-        Buonocore’s invention only little impact on Restorative
    like dentin, enamel can be dried easily: so bonding          Dentistry at this time. The advent of composite materi-
    process to enamel is different from that of dentin.




                                                                 te
                                                                 als with reduced polymerization shrinkage gave the
                                                                 necessary input to finally enter the era of “Adhesive
                                                                 Dentistry”. By the mid 1960S, the first commercially
                                                                 available pit-and-fissure sealants and composite resin
                                                            In
                                                                 materials utilizing this new adhesive technology were
    History and Evolution

    The history of dental adhesives started as early as          used clinically. Buonocore theorized that risen tags fill-
    1949, when Dr. Hagger, a Swiss chemist who worked            ing the defects created by the etchant were responsi-
    for DeTrey/Amalgamated Dental Company, applied the           ble for enamel adhesion, and by the late 1960s, he al-
                                               ni
    patent for the first dental adhesive: only dentin was ini-   so proposed that bonding to dentin was possible (11).
    tial substrate for bonding not the enamel. Hagger            Since then, dental adhesive has been developed that
    patented a “Cavity Seal” material to be used in combi-       provide numerically higher bond strength and more
                                     io


    nation with the chemically curing resin “Sevriton”, in       substantive bonded interfaces to both enamel and
    1951. This product contained an adhesive called glyc-        dentin. In the 1970s, for the first time, the concept of
    erolphosphoric acid dimethacrylate, which was poly-          smear layer that blocked adhesion to dentin, as identi-
                              iz



    merized using a sulfinic acid initiator, later known as      fication by Eick, using the scanning electron micro-
    “Sevriton Cavity Seal”. This adhesive rely on acidic         scope (SEM) (12), and simultaneously, total-etch con-
    monomers capable of etching and interacting on a             cept were being used. By the 1980s, etch-and-rinse
                Ed




    molecular level with tooth surfaces in order to form         adhesive had gained widespread acceptability.
    physical/chemical bonds between the restoration and          Nakabayashi, in 1982 (13), was the first to demon-
    the tooth. Hagger’s concept was soon adopted by oth-         strate true hybrid layer formation, and also who named
    er investigators and different generations of dental ad-     this new biocomposite by name of hybrid layer. More-
    hesives evolved thereafter, despite the fact it was the      over, he demonstrated that resin could infiltrate into
    first time that bonding to tooth structure became com-       acid-etched dentin to form a new structure composed
 IC




    mercially available through the formation of an inter-       of a resin-matrix reinforced by collagen ﬁbrils. At the
    face very similar to what is called today the hybrid lay-    same time, hybrid layer was considered as the main
    er (6). In 1952, it was postulated by Mclean and             bonding mechanism of bonding agents. This was best
    Kramer, that this material, “Sevriton Cavity Seal”,          observed by transmission electron microscopy, but
C




    chemically bonded to tooth structure (7). This was the       was later demonstrated by scanning electron mi-
    first report of changes in dentin promoted by an acidic      croscopy following argon ion beam etching (14). In the
    monomer and may be considered to be the precursor            early 1990s, the introduction of the three-step total-
    of the hybrid layer concept (7). That concept is obvious     etch adhesive system represented a revolution in ad-
@




    in the development of newer generation of dentin ad-         hesive dentistry. Once dentin is etched with phosphor-
    hesive. In 1954, Buonocore conducted successfully his        ic acid and the etchant is rinsed off, hydrophilic primers
    first experiments on adhesion to enamel trough acid          are used before applying a uniform layer of hydropho-
    etching and he focused on altering the enamel surface        bic resin to complete hybridization. However, two-step
    to obtain a bond with filling material. Besides his          total-etch adhesive systems and two-step self-etch ad-
    groundbreaking research, in 1955 he described using          hesives were introduced into the market in the late
    85% phosphoric acid to alter the enamel surface that         1990s (Fig. 1). Whereas original simple bonding agents

    2                                                                                 Annali di Stomatologia 2017;VIII (1):1-17
    Classification review of dental adhesive systems: from the IV generation to the universal type


                                                                                            Figure 1. The Evolution of Bonding
                                                                                            Adhesives.




                                                                                                                      i
                                                                                                        n           al
                                                                                                     io
                                                                         az
                                                                       rn
                                                                                        Figure 2. Adhesives by generations.




                                                                     te
                                                           In
                                                ni
                                        io
                               iz
               Ed
 IC




    evolved to multi-step systems, recent development fo-           tubule to a depth of 1-10 mm is known as smear
    cuses on simplification of the application procedure in         plugs. These smear plugs are contiguous with smear
C




    order to abate technique sensitivity and reduce manip-          layer consisting of shattered and crushed hydroxyap-
    ulation time (Fig. 2).                                          atite, as well as fragmented and denatured collage
                                                                    that should not be underestimated. The thickness and
                                                                    morphology of the smear layer to the underlying den-
@




                                                                    tine is related to the cavity preparations, while its
                                                                    composition has the characteristics of the tissue that
    Smear layer

    Cavity preparation alters the uppermost layer of tooth          was cut (these may also be contaminated by bacteria
    tissue, covering the tooth surface with a 1.0 µm layer          and saliva). In clinical conditions, a smear layer be-
    of cutting debris, called smear layer (15) (Fig. 3).            haves as a true physical barrier, reducing dentinal
    However, the orifices of the dentin tubules are ob-             permeability by 86% (16). In order to overcome this
    structed by debris tags which may extend into the               smear layer obstacle, a certain degree of etching is

    Annali di Stomatologia 2017;VIII (1):1-17                                                                                 3
    E. Sofan et al.


                                                                               Figure 3. SEM: micrograph of smear
                                                                               layer.




                                                                                                                  i
                                                                                                 n              al
                                                                                              io
                                                                  az
                                                                rn
    required before chemical bonding to the dentin sur-       erties by considering the hydrophilic groups enhance
    face regarding to the bond strength and durability of     the wettability to the dental hard tissues; however,




                                                              te
    adhesion to dental hard tissues. Early non acidic ad-     the hydrophobic groups interact and copolymerize
    hesives failed enough to establish a bond with the        with the restorative material and are thus called am-
    underlying intact dentin. There are basically two op-     phiphilic (18). In other words, adhesives are com-
    tions to overcome low bond strengths due to smear         pounds containing both hydrophilic and hydrophobic
                                                       In
    layer: the removal of the smear layer prior to bonding    monomers. The major difference between hydrophilic
    following an etch-and rinse procedure, or the use of      and hydrophobic adhesives is the chemistry of their
    bonding agents that can penetrate beyond the smear        monomers and solvents. The monomers most used in
    layer while incorporating it following a self-etch ap-    adhesive system are the hydroxylethyl methacrylate
                                           ni
    proach. In case of total-etch adhesive systems, the       (HEMA) and the Bisphenol glycidyl methacrylate (bis-
    smear layer is essentially dissolved with phosphoric      GMA). The first one, HEMA, is totally miscible in wa-
    acid (H3PO4) and subsequently washed away during          ter and serves as an excellent polymerizable wetting
                                   io


    the rinsing step. With self-etching systems, various      agent for dental adhesives. Bis-GMA, instead, is the
    acidic primers are used to modify, disrupt, and/or sol-   main monomer used in most dental composites and
    ubilize the smear layer and, although the remnants        many adhesives, is much more hydrophobic and will
                           iz



    are not washed away as with total-etch systems, still     only absorb about 3% water by weight into its struc-
    permit direct adhesive interaction with the dentin sub-   ture when polymerized (19). A mixture of the two has
    strate. For both approaches, micromechanical inter-       intermediate characteristics and serves as a useful
              Ed




    locking is the basic mechanism of adhesion to enam-       adhesive for the tooth. In order to enhance the wet-
    el and dentin.                                            ting, spreading and penetration of the polymerizable
                                                              monomers into the dentin substrate, solvents are al-
                                                              ways added to the mixture as “thinning” agents.
                                                              These solvents are typically water, ethyl alcohol,
                                                              butyl alcohol or acetone. The first three are very hy-
    Variables in adhesive dental bonding system
 IC




    Many resin adhesive systems and types have been           drophilic and thus enhance the interaction of the
    developed to achieve a durable bond to dental tis-        monomers with surface water, while acetone is good
    sues. Further complication are associated with the        at displacing water from within the dentin. However,
    heterogeneity of tooth structure and composition, the     any solvent not displaced during the placement pro-
C




    hydrophilicity of the exposed dentine surface, the fea-   cedure, such as by drying appropriately, will be incor-
    tures of the dental substrate after cavity preparation    porated into the bonding layer and may serve as a
    and the characteristics of the adhesive itself, such as   weakening contaminant. The monomers present in
    its physicochemical properties and its strategy of in-    dental adhesives are similar to those used in dental
@




    teraction with enamel and dentine (3, 17). Despite the    composite restoratives, thus ensuring that there will
    major difference in the manner of etching between         be strong interaction between the adhesive and the
    etch-and-rinse and self-etch adhesives, the other fun-    overlying composite.
    damental steps for adhesion, namely the ‘priming’         Although adhesion is established and predictable
    and actual ‘bonding’ phase, can be either separate or     clinical procedure, acid etching of dentin has always
    combined. Dental bonding systems are resin blends         concerned both clinical and researchers, as critical
    that possess both hydrophilic and hydrophobic prop-       and definite factor for the quality of adhesion. More-

    4                                                                             Annali di Stomatologia 2017;VIII (1):1-17
    Classification review of dental adhesive systems: from the IV generation to the universal type


    over, inadvertent over-drying of etched dentin after               one are more likely to survive polymerization contrac-
    acid rinsing substantially increases the risk of col-              tion stresses and remain bonded to the tooth. This
    lapse of collagen mesh, which restricts the diffusivity            may be a problem, because Class I preparations
    of resin monomers throughout the intertubular                      have a mean C-factor of 4.03 and Class II prepara-
    dentin. De Goes et al. (19) recommended to brush                   tions have a mean C-factor of 1.85 (21). The negative
    out the excess water with a cotton pellet, a dispos-               effect of C-factor is supported by He et al. (22), who
    able brush, or a tissue paper. In the same fashion,                reported that bulk filling a cavity with a C-factor of five




                                                                                                                          i
                                                                                                                        al
    over-wet conditions also results in lower bond                     produced the lowest bond strength: more microleak-
    strengths due to dilution of the adhesive. In addition,            age has been reported as the C-factor increases. An
    excessive etching of dentin may produce weak                       in vivo study has also reported that the resin-dentin




                                                                                                            n
    bonding due to the possibility that the resin                      interdiffusion zone was detached from the overlying
    monomers may not be able to penetrate into the                     resin in restorations with a C-factor of five (23). Con-
    open dentinal tubules and diffuse across the hydrat-               sequently, the higher the value of C-factor, the




                                                                                                         io
    ed demineralized collagen network as deep as the                   greater is the polymerization shrinkage. Therefore,
    etchant agent and allows fluid movement in the                     three-dimensional tooth preparations (Class I) have
    dentinal tubules. This movement of fluid pulls on the              the highest (most unfavorable) C-factor and thus are




                                                                           az
    odontoblastic process and the patient experiences it               at more risk to the effects of polymerization shrink-
    as pain or postoperative sensitive. Thus, this lack of             age. C-factor plays a significant role when tooth
    penetration leaves behind non-impregnated or poor-                 preparation extends up to the root surface causing a
    ly infiltrated, unsupported areas at the base of hy-               V-shaped gap formation between the composite and




                                                                         rn
    brid layer, which are more prone to micro-and nano-                root surface due to polymerization shrinkage.
    leakage, collagen hydrolysis and degradation of the
    interface over time. The literature and manufacture




                                                                       te
    established the time for etching enamel and dentin
    that should be 15-30 s respectively, in order to ob-
                                                                       Actuality and Classification of Contemporary

    tain adequate bond performance. All of these molds
                                                                       Adhesives

    can affect bond strength, physical properties of the               Dentin bonding agent can be defined as “a thin layer
                                                              In
    cured resin composite and stress generated during                  of resin applied between the conditioned dentin and
    resin polymerization. Also, the large, flat surfaces               resin matrix of composite”. Over the years, there
    used in most laboratory bonding studies may over-                  have been numerous classifications of dentin bonding
    estimate the actual clinical bond strengths achieved.              agents that have been advocated by many authori-
                                                 ni

                                                                       ties. Some of them are based on generation, the
                                                                       number of clinical steps and on the modern adhesive
                                                                       strategy.
                                        io

    Configuration or “C-factor”

    The cavity configuration, or C-factor, was introduced
    by Prof. Carol Davidson and his colleagues in 1980s.               The concept of generation was used because of the
                                                                       Classification by generation
                               iz



    The configuration factor (C-factor) is the ratio of                complexity of bonding agents, the variety of classifica-
    bonded surface of the restoration to the unbonded                  tions refers to when and in what order this type of ad-
    surfaces (20). The C-factor can be used to predict                 hesive was developed by the dental industry. Adhesive
               Ed




    which restorations are most likely to exhibit bond fail-           dentistry began in 1955 by Buonocore on the benefits
    ures between the resin and the tooth. According to                 of acid-etching. With changing technologies, dental ad-
    Feilzer et al., restorations with a C-factor less than             hesives have evolved from no-etch to total-etch (4th and


    Table 2. Classification of dental bonding systems by generations.
 IC




    Generation          Number of steps         Surface pre-treatment        Components          Shear bond strength (MPa)
    1st                 2                       Enamel etch                  2                   2
C




    2nd                 2                       Enamel etch                  2                   5
    3nd                 3                       Dentine conditioning         2-3                 12-15
    4th                 3                       Total etch                   3                   25
@




    5th                 2                       Total etch                   2                   25
    6th                 1                       Self-etch adhesive           2                   20
    7th                 1                       Self-etch adhesive           1                   25
    8th                 1                       Self-etch adhesive           1                   Over 30


    Annali di Stomatologia 2017;VIII (1):1-17                                                                                   5
    E. Sofan et al.


    5th generation) to self-etch (6th, 7th and 8th generation)   existed that dentin ought not to be etched. After the
    systems (24) and the details of these are shown in           primer was added, an unfilled resin was placed on
    Table 2. Each generation has attempted to reduce the         both dentin and enamel. The weak link with this gen-
    number of bottles involved in the process, to minimize       eration was the unfilled resins that simply did not
    the number of procedural steps, to provide faster appli-     penetrate the smear layer effectively according to
    cation techniques and to offer improved chemistry to fa-     Tao et al. in 1988 (30).
    cilitate stronger bonding (Tab. 2).




                                                                                                                     i
                                                                                                                   al
                                                                 In 1980s and 1990s, fourth generation dentin bonding
                                                                 Fourth Generation

    The first generation bonding systems were published          agents were introduced. The fourth generation mate-
    First Generation

    by Buonocore in 1956, who demonstrated that use of           rials was the first to achieved complete removal of




                                                                                                    n
    glycerophosphoric acid dimethacrylate (NPG-GMA)              smear layer (27) and still considered as the golden
    containing resin would bond to acid etched dentin            standard in dentin bonding. In this generation, the




                                                                                                 io
    (25). These bonding agents were designed for ionic           three primary components (etchant, primer and bond-
    bonding to hydroxyapatite or for covalent bonding            ing) are typically packaged in separate containers
    (hydrogen bonding) to collagen. However, immersion           and applied sequentially. The concept of total-etch




                                                                     az
    in water would greatly reduce this bond. After nine          technique and moist dentinal hallmarks of the 4th gen-
    years, Bowen used a coupling agent to overcome this          eration systems (27, 31), where dentin and enamel
    problem (26). He addressed this issue using that act-        are etched at the same time with phosphoric acid
    ed as NPG-GMA a primer or adhesion promoter be-              (H3PO3) for a period of 15-20 s (32). However, the




                                                                   rn
    tween enamel/dentin and resin materials by chelating         surface must be left moist “wet bonding”, in order to
    with surface calcium, where one end would bond to            avoid collagen collapse. The application of a hy-
    dentin, and other would polymerize with composite            drophilic primer solution can infiltrate the exposed




                                                                 te
    resin (26). Overall, this generation leads to very poor      collagen network forming the hybrid layer (27, 33).
    clinical results as well as low bond strength in the 1-3     The hybrid layer is formed by the resin infiltrated sur-
    MPa range (27).                                              face layer on dentin and enamel. The goal of ideal
                                                                 hybridization is to give high bond strengths and a
                                                          In
                                                                 dentin seal (13). Bond strengths for these adhesives
    The second generation of dentin bonding agents               were in the low- to mid-20 MPa range and significant-
    Second Generation

    were introduced in the late 1970s, and sought to im-         ly reduced margin leakage compared to earlier sys-
    prove the coupling agents that were utilized in the          tems (8). This system was very technique sensitive
                                             ni
    first generation of adhesives. The 2nd generation of         and required an exacting technique of controlled
    dentin adhesives primarily used polymerizable phos-          etching with acid on enamel and dentin, followed by
    phates added to bis-GMA resins to promote bonding            two or more components on both enamel and dentin.
                                    io


    to the calcium in mineralized tooth structure (27, 28).      These systems are very effective when used correct-
    Bonding mechanism involves formation of ionic bond           ly, have good long-term clinical track record, and are
    between calcium and chlorophosphate groups. This             the most versatile of all the adhesive categories, be-
                            iz



    ionic bond would rapidly degrade in water submer-            cause they can be used for virtually any bonding pro-
    sion (again analogous to saliva) and even the water          tocol (direct, indirect, self-cure, dual-cure or light-
    within the dentin itself, and cause debonding and/or         cure). These systems are still the standards by which
              Ed




    micoleakage (27). The smear layer was still not re-          the newer systems are judged. However, these sys-
    moved, and this contributed to the relatively weak           tems can be very confusing and time consuming with
    and unreliable bond strengths of this second genera-         so many bottles and application steps. Because of
    tion (27). The smear layer is really a smooth layer of       the complexity of multiple bottles and steps, dentists
    inorganic debris that remains on the prepared dentin         began requesting a simplified adhesive system.
    surface as a result of tooth preparation with rotary in-
 IC




    struments (the drill). This generation of bonding
    agents is no longer used, due mainly to failed at-           In the 1990s and in the ongoing decade, the fifth gen-
                                                                 Fifth Generation

    tempts to bond with a loosely bond smear layer. Bond         eration bonding systems sought to simplified the
    strength: 4-6 Mpa (29).                                      process of fourth generation adhesion by reducing
C




                                                                 the clinical steps which results in reduced working
                                                                 time. These are distinguished by being “one step” or
    In the late 1970s and early 1980s, third generation          “one bottle” system. In addition, an improved way
    Third Generation

    dentin bonding agents were presented. The third gen-         was needed to prevent collagen collapse of deminer-
@




    eration bonding systems introduced a very important          alized dentin and to minimize if not totally eliminate,
    change: the acid etching of the dentin in an effort to       postoperative sensitivity (17, 27, 34). So the most
    modify or partially remove the smear layer (27). This        common method of simplification is “one bottle sys-
    opened the dentin tubules and allowed a primer to be         tem” combined the primer and adhesive into one so-
    placed after the acid was completely rinsed away.            lution to be applied on enamel and dentin simultane-
    While this method achieved a greater bond, it was            ously with 35 to 37% phosphoric acid for 15-20 s.
    considered controversial in dentistry as the feeling         This single bottle, etch-and-rinse adhesive type

    6                                                                                Annali di Stomatologia 2017;VIII (1):1-17
    Classification review of dental adhesive systems: from the IV generation to the universal type


    shows the same mechanical interlocking with etched              “over-etch” situation where the demineralization zone
    dentin occurs by means of resin tags, adhesive later-           is too deep for subsequently placed primers to com-
    al branches and hybrid layer formation and shows                pletely penetrate (33). While data indicates that 6th
    high bond strength values to dentin with marginal               generation adhesives will adhere well to dentin (41 MPa
    seal in enamel (33). These kinds of adhesives sys-              at 24 hours), the bond to enamel is at least 25% weak
    tems may be more susceptible to water degradation               to enamel then both the 4th and 5th generation adhe-
    over time than the fourth generation. This is because           sives in pooled data studies. Several respected clini-




                                                                                                                     i
                                                                                                                   al
    the polymerized primer of the “one bottle system”               cians have utilized 6th generation adhesives for bonding
    tends to be hydrophilic in nature. However, when us-            to dentin after selectively etching the enamel.
    ing the fourth generation, the hydrophilic primer is
    covered by a more hydrophobic resin, making it less




                                                                                                        n
    susceptible to water sorption. Not all 5th generation           The seventh generation bonding systems was intro-
                                                                    Seventh Generation

    adhesives are compatible with dual and self-cured or            duced in late 1999 and early 2005. The seventh gen-




                                                                                                     io
    core materials. The lower PH of the Oxygen-inhibited            eration or one-bottle self-etching system represents
    layer, or the monomers in some simplified products,             the latest simplification of adhesive systems. With
    are too acidic and thereby de-activate the tertiary             these systems, all the ingredients required for bond-




                                                                         az
    amine in chemical-cured composites. As well as the              ing are placed in and delivered from a single bottle
    same in regards to the number of applications (un-              (33, 37). This greatly simplifies the bonding protocol
    filled need more applications), so it is critical to follow     as the claim was that could be achieved consistent
    the manufacturer’s directions.                                  bond strengths while completely eliminating the er-




                                                                       rn
    Several long term studies indicate that 5th generation          rors that could normally be introduced by the dentist
    dental adhesive achieve high clinical bond strengths. In        or dental assistant who had to mix the separate com-
    addition, the resin-dentin bond is prone to water degra-        ponents with other more complicated systems. How-




                                                                     te
    dation, 5th generation adhesives are more prone to wa-          ever, incorporating and placing all of the chemistry
    ter degradation than 4th generation dental adhesive.            required for a viable adhesive system into a single
    Representative dentin bond strength is 3 to 25 MPa.             bottle, and having it remain stable over a reasonable
                                                                    period of time, poses a significant challenge (33).
                                                           In
                                                                    These inherently acidic systems tend to have a signif-
    The sixth generation bonding systems introduced in              icant amount of water in their formulations and may
    Sixth Generation

    the latter part of the 1990s and the early 2000s also           be prone to hydrolysis and chemical breakdown (37,
    known as the “self-etching primers”, were a dramatic            38). Furthermore, once placed and polymerized, they
                                                ni
    leap forward in technology. The sixth generation                are generally more hydrophilic than two-step self-
    bonding systems sought to eliminate the etching step,           etching systems; this condition makes them more
    or to include it chemically in one of the other steps:          prone to water sorption, limits the depth of resin infil-
                                        io


    (self-etching primer + adhesive) acidic primer applied          tration into the tooth and creates some voids (39).
    to tooth first, followed by adhesive or (self-etching ad-       The advantage of this generation was not any mixing
    hesive) two bottles or unit dose containing acidic              required and the bond strengths were consistent.
                               iz



    primer and adhesive; a drop of each liquid is mixed             However, the seventh generation adhesives have
    and applied to the tooth. It is recommended that the            proven to have the lowest initial and long term bond
    components are mixed together immediately before                strengths of any adhesive on the market today that
               Ed




    use. The mixture of hydrophilic and hydrophobic resin           may be considers as disadvantage. Seventh genera-
    components is then applied to the tooth substrate               tion adhesives involve the application of etch, primer,
    (35). Evidently, these bonding systems are character-           and adhesive which have already been mixed, fol-
    ized by the possibility of achieving a proper bond to           lowed by light curing the tooth. Seventh generation
    enamel and dentin using only one solution (27). The             adhesives are “all-in-one” (40) if there has ever been
    biggest advantage of the sixth generation is that their         such a thing. The clinical and scientific data on these
 IC




    efficacy appears to be less dependent on the hydra-             adhesives proves that they are hydrophilic and de-
    tion state of the dentin than the total-etch systems            grade more rapidly. In addition, the chemistry mast
    (33). Unfortunately, the first evaluations of these new         be acidic, as etch is involved in this liquid, and this
    systems showed a sufficient bond to conditioned                 has been shown to adversely react with the compos-
C




    dentin while the bond with enamel was less effective.           ite initiator systems.
    This may be due to the fact that the sixth generation
    systems are composed of an acidic solution that can-
    not be kept in place, must be refreshed continuously            In 2010, voco America introduced voco futurabond
                                                                    Eighth Generation
@




    and have a pH that is not enough to properly etch               DC as 8th generation bonding agent, which contains
    enamel (36). In order to overcome this problem, it is           nanosized fillers (41). In the new agents, the addition
    recommended to etch enamel first with the traditional           of nano-fillers with an average particle size of 12 nm
    phosphoric acid prior to using it. However, those uti-          increases the penetration of resin monomers and the
    lizing this technique should take care to confine the           hybrid layer thickness, which in turn improves the me-
    phosphoric acid solely to the enamel. Additional etch-          chanical properties of the bonding systems (42, 43).
    ing of the dentin with phosphoric acid could create an          Nano-bonding agents are solutions of nano-fillers,

    Annali di Stomatologia 2017;VIII (1):1-17                                                                              7
    E. Sofan et al.


    which produce better enamel and dentin bond                     etch bonding agents is typically an acidic
    strength, stress absorption, and longer shelf life (24).        monomer that also serves as the primer.
    It has been observed that filled bonding agents pro-        2. Primer: the primer is composed of hydrophilic
    duced higher in vitro bond strength. These new agent            monomers usually carried in a water-soluble sol-
    from self-etch generations have an acidic hydrophilic           vent (acetone, ethanol, water) to promote good
    monomers and can be easily used on the etched                   flow and penetration into hydrophilic dentin, which
    enamel after contamination with saliva or moisture              can influence the resulting bond strength. Self-




                                                                                                                    i
                                                                                                                  al
    (44). Based on the manufacturer, nano-particles act-            etch bonding agents utilize primers that are acidic
    ing as crosslinks, will reduced the dimensional                 monomers.
    changes (42, 43). The type of nano-fillers and the          3. Dentin bonding agent (or Dentin Adhesive): can
    method that these particles are incorporated affect             be defined as a thin layer of (usually unfilled)




                                                                                                   n
    the adhesive viscosity and penetration ability of the           resin applied between the conditioned dentin and
    resin monomers into collagen fibers spaces (43).                resin matrix of a composite. The adhesive pro-




                                                                                                io
    Nano-fillers, with dimensions larger than 15-20 nm or           motes bonding between enamel or dentin and
    a content of more than 1.0 percent by weight, both              resin composite restorative material or resin ce-
    can increase the viscosity of the adhesives, and may            ment. Adhesives act as a link between the hy-




                                                                    az
    cause accumulation of the fillers over the top of the           drophilic resin primer and the hydrophobic resin
    moistured surface. These clusters can act as flaws              composite. Proper curing is required to provide
    which may induce cracks and cause a decrease in                 good retention and sealing. Seventh generation
    the bond strength (43).                                         bonding agents utilize primer-adhesives that are




                                                                  rn
                                                                    acidic monomers.
                                                                4. Fillers: recently nanofillers have been added
                                                                    ranging from 0.5% to 40% by weight in the 8th
    Classification by mechanism of adhesion/clinical




                                                                te
    At this stage it was proposed a classification of bond-         generation adhesive systems. Fillers control han-
    step

    ing systems, which reflects their essential mode of             dling and may improve strength. Fillers may in-
    use, rather than historical development:                        crease film thickness of the adhesive layer.
    1. Three-steps: involving etch, prime and bond.             5. Solvent: solvents include acetone, ethanol and
                                                         In
        These bonding systems are supplied as three bot-            water. The solvent affects the evaporation rate on
        tles, one each from etchant, primer and bonding             the tray and in the mouth. Acetone evaporates
        agent. These are the most complicated to use in             quickly and requires the shortest drying time in
        the clinic, but result in highest bond strengths (17)       the mouth. Ethanol evaporates more slowly and
                                             ni
        and greatest durability.                                    requires moderate drying time. Water evaporates
    2. Two-steps 1: here the steps are etch, then finally           very slowly and requires longest drying time.
        prime and bond in a single coating. Bonding sys-            Bonding agents should be dispensed immediately
                                    io


        tems of this type employ substances in two bot-             before use to prevent premature evaporation of
        tles, one consisting of etchant, and the other of           the solvent.
        the combined prime and bond formulation.                In current times, development of new products is oc-
                           iz



    3. Two-steps 2: for these systems, the two steps are        curring at an unprecedented rate. Dentin adhesives
        etching and priming combined followed by bonding.       are currently available as three-step, two-step and
        It uses two bottles of components, the first contain-   single-step systems, depending on how the three car-
              Ed




        ing a self-etching primer and the second the bond-      dinal steps of etching, priming and bonding to tooth
        ing agent. The self-etching primer modifies the         substrate are accomplished or simplified (46). More-
        smear layer on the surface of the dentine, and in-      over, they also considered the number of clinical
        corporates the products in the coating layer.           steps required to apply the adhesives: 1. one-step
    4. One-step: this uses a single bottle containing a         adhesives that modify the smear layer; 2. two-step
        formulation that blends a self-etching primer and       adhesives that: a) modify the smear layer; b) dissolve
 IC




        bonding agent. Clinically, this is the easiest to       the smear layer; c) eliminate the smear layer; 3.
        use, and bond strengths are generally reported to       three-step adhesives that eliminate the smear layer.
        be acceptable, despite the simplicity of bonding        However, the classification based on the adhesive
        operation (45).                                         strategy was proposed; three adhesion mechanisms
C




    In order to understand the hybrid layer formation us-       are currently used by modern adhesive systems:
    ing total etch technique and the self etch technique, it    1. etch-and-rinse adhesives; 2. self-etching adhe-
    is necessary to understand the components of bond-          sives; 3. glass ionomer adhesives and resin-modified
    ing systems that consist of three main components:          glass ionomers (19), which differ signiﬁcantly in the
@




    1) etchant, 2) primer and 3) bonding resin:                 manner they deal with tooth tissue (17). Considering
    1. Etchant: in total-etch technique the etchant used        the differences in professional judgment and manu-
        is 35-37% phosphoric acid. It prepares enamel           facturers’ instructions regarding the selection of the
        and dentin to receive the primer. It creates micro-     adhesive strategy and the number of steps that give
        porosities, up to 7.5 microns which helps to cre-       the dentist the opportunity to decide which bonding
        ate the resin tag formation and thereby results in      agents and techniques to utilize for different clinical
        micro mechanical bonding. The etchant in self-          treatment (Fig. 4; Tabs. 3-7).

    8                                                                               Annali di Stomatologia 2017;VIII (1):1-17
    Classification review of dental adhesive systems: from the IV generation to the universal type




                                                                                                                           i
                                                                                                            n            al
                                                                                                         io
                                                                            az
                                                                          rn
    Figure 4. Modern adhesive strategies.




                                                                        te
    Table 3. List of bonding agents available of 4th generation.
                                                             In
     Generation          Brand name                                Manufacturer                                Polymerisation
     4th generation      All-Bond 2                                Bisco Schaumburg, IL, USA                   Dual cured
     Three-steps         All-Bond 3                                Bisco Schaumburg, IL, USA                   Light cured, Dual
     Etch-Rinse          Clearfil Liner Bond                       Kuraray (Kurashiki, Japan)                  Light or self cured
                                                ni
                         Scotchbond Multi-Purpose                  (3M ESPE, St. Paul, Minn. USA)              Light cured
                         Adper Scotchbond Multi Purpose            (3M ESPE, St. Paul, Minn. USA)              Light cured, Dual
                         Plus
                                        io

                         Optibond Dual Cure                        (Kerr, Orange, CA, USA)                     Light cured
                         Optibond FL                               (Kerr, Orange, CA, USA)                     Light cured
                         Permagen                                                                              Light cured
                               iz


                         Syntac Classic                            (Ultradent Prod Inc, Utah, USA)             Light cured
                         Denthesive                                (Ivoclar-Vivadent, Schann, Liechtenstein)   Light cured
                         Gluma Solid Bond                          (Heraeus Kulzer, Wehrheim Germany)          Light cured
               Ed




                         EBS                                       (Heraeus Kulzer Hanau, Germany)             Light cured
                         Gluma CPS                                 ESPE (now 3M ESPE; Seefeld, Germany)        Light cured
                                                                   Bayer (Heraeus-Kulzer; Leverkusen,
                         Permaquik                                 Germany)                                    Self cured
                         Amalgabond                                Kerr (Ultradent)                            Light cured
                         Cmf                                       Parkell, Farmingdale, NY                    Light cured
                         FL Bond                                   Saremco, Rebstein, Switzerland)             Light cured
 IC




                         ProBond                                   (Shofu Inc. Kyoto, Japan)                   Light cured, Dual
                         Bond-it                                   (Dentsply Caulk)                            Light cured, Dual
                                                                   Pentron Corporation, Wallingford, CT,
                         Ecusit-Primer/Mono                        USA                                         Light cured
C




                         Solobond Plus                             DMG, Hamburg, Germany                       Light cured
                         Luxa bond total etch                      VOCO, Cuxhaven, Germany                     Light cured, Dual
                                                                   DMG America
@




                                                                        sive. Each of the three-steps can accomplish multiple
                                                                        tasks ending with sealing the bonded interface with a
    Etch and Rinse

    Etch-and-rinse adhesive systems are the oldest of                   relatively hydrophobic adhesive layer. Consequential-
    the multi-generation evolution of resin bonding sys-                ly, an inter-diffusion layer is formed that called hybrid
    tems. The three-steps total-etch adhesive systems                   layer. Etch-and-rinse adhesives are characterized by
    were introduced in early 1990s (47), that involve acid-             an initial etching step, followed by a compulsory rins-
    etching, priming and application of a separate adhe-                ing procedure which is responsible for the complete

    Annali di Stomatologia 2017;VIII (1):1-17                                                                                        9
    ineare a cosa? il mio originale è diverso comunque ora forse è corretta E.

    Sofan et al.


    Table 4. List of bonding agents available of 5th generation.

     Generation        Brand name                                  Manufacturer                                  Polymerisation
         th
     5 Generation Admira Bond                                      Voco, (Cuxhaven, Germany)                     Light cured
     Two-steps    Solobond M                                       Voco, (Cuxhaven, Germany)                     Light cured




                                                                                                                             i
     Etch-Rinse   Polibond                                         Voco, (Cuxhaven, Germany)                     Dual cured




                                                                                                                           al
                  Excite                                           Ivoclar Vivadent (Schaan, Lichtenstein)       Light cured
                  Excite DSC                                       Ivoclar Vivadent (Schaan, Lichtenstein)       Dual cured
                  ExciTE F                                         Ivoclar Vivadent (Schaan, Lichtenstein)       Light cured
                  Gluma 2000                                       Bayer, (now Heraeus-Kulzer; Leverkusen,       Light cured




                                                                                                            n
                                                                   Germany)
                       Gluma Comfort Bond                          Heraeus Kulzer, Hanau, Germany                Light cured




                                                                                                         io
                       Gluma One Bond                              Heraeus Kulzer, Hanau, Germany                Light cured
                       One-Coat Bond                               Coltène Whaledent (Altstätten,                Light cured
                                                                   Switzerland)




                                                                             az
                       Optibond Solo Plus                          Kerr (Orange, Calif. USA)                     Light cured
                       Optibond SoloPlus Dual cure                 Kerr (Orange, Calif. USA)                     Dual cured
                       Prime&Bond 2.0                              Dentsply-Detrey (Konstanz, Germany)           Light cured
                       Prime&Bond 2.1                              Dentsply-Detrey (Konstanz, Germany)           Light cured




                                                                           rn
                       Prime&Bond NT                               Dentsply-Detrey (Konstanz, Germany)           Dual cured
                       XP Bond                                     Dentsply-Detrey (Konstanz, Germany)           Self cured
                       Stae                                        Southern Dental Industries (Victoria,         Light cured
                                                                   Australia)




                                                                         te
                       Syntac Single-Component                     Ivoclar Vivadent (Schaan, Liechtenstein)      Light cured
                       One Step                                    Bisco Inc., Schaumburg, IL, USA               Light cured
                       One-Step Plus                               Bisco Inc., Schaumburg, IL, USA               Light cured
                       Adper Single Bond Plus, (Adper              3M ESPE, St. Paul, MN,USA                     Light cured
                       Single Bond 2)
                       Scotchbond 1 (Single Bond)
                                                             In    3M ESPE (Seefeld,Germany)
                                                                   Kuraray (Osaka, Japan)                        Light cured
                       Clearfil Liner Bond 2                       Kuraray (Osaka, Japan)                        Light cured
                       Clearfil SE                                 Kuraray Medical Inc, Tokyo, Japan             Dual cured
                                                ni
                       Clearfil Photobond                          Kuraray Medical Inc, Tokyo, Japan             Self cured
                       Clearfil New Bond                           Pentron Corporation, Wallingford, CT, USA     Light cured
                       Bond-1                                      Sun Medical Co, Shiga, Japan                  Dual cured
                                      io

                       Superbond C&B                               Bisco Schaumburg, IL, USA                     Self cured
                       All bond plus                               Bisco Schaumburg, IL, USA                     Light cured
                             iz



    removal of smear layer and smear plugs. On enamel,                  (51). In the bonding step, a solvent-free adhesive
    acid-etching selectively dissolves the enamel rods,                 resin is applied on the prepared surface, leading to
    creating macro-and micro porosities which are readily               the penetration of hydrophobic monomers not only in-
              Ed




    penetrated, even by ordinary hydrophobic bonding                    to the inter-ﬁbrilar spaces of the collagen network but
    agents, by capillary attraction (48). Upon polymeriza-              also into dentine tubules. After inﬁltration, these
    tion, this micromechanical interlocking of tiny resin               monomers are polymerized in situ, resulting in the
    tags within the acid-etched enamel surface still pro-               formation of a hybrid layer, which in combination with
    vides the best achievable bond to the dental sub-                   the presence of resin tags inside dentine tubules pro-
    strate (49). Dentin adhesion is more challenging than               vides micromechanical retention to the composite
 IC




    enamel adhesion due to dentin composition, render-                  restoration (52). From the traditional three-step etch-
    ing the etch-and-rinse strategy a highly sensitive                  and-rinse adhesives, simpliﬁed two-step adhesives
    technique (50). Concurrently, acid-etching promotes                 have been developed that combine the primer and
    dentine demineralization over a depth of 3-5 lm,                    the adhesive resin into one single solution. These
C




    thereby exposing a scaffold of collagen ﬁbrils that is              simpliﬁed adhesives present a reduced ability to inﬁl-
    nearly totally depleted of hydroxyapatite (23). The fol-            trate the demineralized dentine substrate, thereby
    lowing step consists of the application of a primer                 producing suboptimal hybridization when compared
    containing speciﬁc monomers with hydrophilic proper-                to their three-step counterparts (53). Moreover, the
@




    ties, such as 2-Hydroxy ethyl meth-acrylate (HEMA),                 hydrophilic nature of such adhesives render them
    dissolved in organic solvents like acetone, ethanol or              more prone to water sorption and consequently more
    water. While HEMA is responsible for improving the                  susceptible to the effects of hydrolytic degradation.
    wettability and promoting the re-expansion of the col-              The solvent present in such adhesives is also more
    lagen network, the solvents are able to displace water              difficult to evaporate, frequently remaining entrapped
    from the dentine surface, thus preparing the collagen               within the adhesive layer after polymerization (54).
    network for the subsequent adhesive resin inﬁltration               The etch-and-rinse technique is considered to be crit-

    10                                                                                       Annali di Stomatologia 2017;VIII (1):1-17
    Classification review of dental adhesive systems: from the IV generation to the universal type


    Table 5. List of bonding agents available of 6th generation.

     Generation                  Brand name                         Manufacturer                            Polymerisation
     6th Generation              ART Bond                           Coltene (Alstatten, Switzerland)        Light cured
     Two-steps                   PUB 3                              Denstply (Konstanz, Germany)            Light cured
     Self-Etch                   Clearfil SE                        Kuraray (Tokyo, Japan)                  Light cured




                                                                                                                            i
                                 Clearfil Protect Bond              Kuraray (Osaka, Japan)                  Light cured




                                                                                                                          al
                                 Denthesive 2                       Heraeus Kulzer (Wehrheim, Germany)      Light cured
                                 Tyrian SPE                         Bisco (Schaumburg, IL, USA)             Light cured
                                 Adhe SE                            Ivoclar Vivadent (Schaan,               Dual cured




                                                                                                           n
                                                                    Liechtenstein)
                                 Adper Scotchbond SE                3M ESPE (St. Paul, MN, USA)             Light cured




                                                                                                        io
                                 self-etch
                                 FL bond II                         Shofu Dental                            Light cured
                                 Clearfill Liner bond 2V            Kuraray (Tokyo, Japan)                  Dual cured




                                                                             az
                                 Contax                             DMG America                             Dual cured
                                 Nanobond                           Pentron Clinical                        Dual cured
                                 Clearfil S3 Bond                   Kuraray (Osaka, Japan)                  Light cured
                                 G Bond                             GC Corp (Tokyo, Japan                   Light cured
                                 AQ Bond plus                       Sun Medicals                            Light cured




                                                                           rn
                                 Hybrid Bond                        Vivadent (Schann, Liechtenstein)        Light cured
                                 All Bond SE                        Bisco (Inc., Schaumburg, IL, USA)       Light cured
                                 iBond Gluma inside                 Heraeus Kulzer (Hanau, Germany)         Light cured




                                                                         te
                                 Fluoro bond Shake One              Shofu, (Tokyo, Japan)                   Light cured
                                 One up Bond F+                     Tokuyama Corp, (Tokyo, Japan)           Light cured
                                 PSA Dyract                         Dentsply, (Konstanz, Germany)           Light cured
                                 Xeno III                           Dentsply, (Sankin)                      Light cured
                                 Prompt Adper Prompt L-Pop
                                 L-Pop
                                                               In   3M ESPE (St. Paul, Minn. USA)
                                                                    3M ESPE (St. Paul, Minn. USA)
                                                                                                            Light cured
                                                                                                            Light cured
                                 Brush and bond                     Parkell                                 Light cured



    Table 6. List of bonding agents available of 7th and 8th generation.
                                                 ni

     Generation              Brand name                   Manufacturer                                       Polymerisation
                                        io

     7/8th Generation        One Coat 7.0                 Coltène/Whaledent (AG, Altstätten, Switzerland)    Light cured
     One-step                Xeno IV                      Dentsply Caulk (Milford, DE, USA)                  Light cured
                             AdheSE One F (no mix)        Ivoclar Vivadent, (Schaan, Principality of         Light cured
                               iz


     Self-Etch
                                                          Liechtenstein)
                             G-BOND                       GC America (Alsip, IL, USA)                        Light cured
                             OptiBond All-In-One          Kerr (Orange, CA, USA)                             Light cured
               Ed




                             Clearfil S3 Bond Plus        Kuraray (Tokyo, Japan)                             Light cured
                             Adper Easy one               3M ESPE (St. Paul, Minn. USA)                      Light cured
                             Bond force (no mix)          Tokuyama Dental                                    Light cured
                             Clearfill DC bond            Kuraray (Tokyo, Japan)                             Dual cured
                             Xeno IV DC                   Dentsply Caulk (Milford, DE, USA)                  Dual cured
                             Futura bond DC               Voco (Germany)                                     Dual cured
 IC




    Table 7. List of bonding agents available of Universal generation.
C




     Generation       Brand name                         Manufacturer                                   Polymerisation
     Multi-           All-Bond Universal                 Bisco (Inc., Schaumburg, IL, USA)              Light cured, Dual
     mode or          Prime&Bond Elect                   Dentsply Caulk (Milford, DE, USA)              Light cured
@




     Universal        Xeno Select                        Dentsply Caulk (Milford, DE, USA)              Light cured
                      AdheSE Universal                   Ivoclar Vivadent (Schaan, Principality of      Light cured
                                                         Liechtenstein)
                      G-aenial Bond                      GC America (Alsip, IL, USA)                    Light cured
                      Clearfil Universal Bond            Kuraray (Tokyo, Japan)                         Light cured, Self cured
                      Scotchbond Universal               3M ESPE (St. Paul, MN, USA)                    Light cured
                      Adhesive
                      Futurabond U                       Voco (Cuxhaven, Germany)                       Light cured


    Annali di Stomatologia 2017;VIII (1):1-17                                                                                     11
    E. Sofan et al.


    ical and highly sensitive, because the over-dried          cause less dentinal fluid flow than etch-and-rinse ad-
    dentin causes both demineralized collagen ﬁbers to         hesives. The role of water is to provide the medium
    collapse and low monomer diffusion among the               for ionization and action of these acidic resin
    ﬁbers, hampering the formation of a functionally suit-     monomers. Self-etch adhesive systems also contain
    able hybrid layer (HL), however the sensitivity is         HEMA (2-hydroxyethyl-methacrylate) hydrophilic
    mostly related to the etching step itself and to the os-   monomer, because of its low molecular weight HEMA
    tensibly antagonistic role of water in the bonding pro-    acts as a co-solvent, minimizing phase separation




                                                                                                                   i
    tocol. In ‘over-wet’ conditions, seems to cause phase      and increasing the miscibility of hydrophobic and hy-




                                                                                                                 al
    separation between the hydrophobic and hydrophilic         drophilic components into the solution and to in-
    components of the adhesive, resulting in the forma-        crease the wettability of dentin surface (65). Bi or
    tion of blister- and globule-like voids at the resin-      multi-functional monomers are added to provide




                                                                                                  n
    dentine interface (55). In addition, the excessive         strength to the cross-linking formed from monomeric
    presence of humidity may result in incomplete              matrix (3). Because self-etch adhesive systems do




                                                                                               io
    monomer polymerization and water adsorption in the         not require a separate acid conditioning step as they
    HL. These effects can decrease the mechanical qual-        contain acidic monomers that simultaneously ‘condi-
    ity of the HL formed, causing its early degradation        tion’ and ‘prime’ the dental substrate (66), they are




                                                                   az
    (56). However the conditions of over-dry and over-         considered as simplified adhesive materials. Possi-
    wet remains a major concern and difficult to standard-     bly, self-etching systems alter the ‘‘smear layer’’ that
    ize; must be considered not only extrinsic, but also in-   covers the dentin after tooth bur preparation, creating
    trinsic sources of humidity when an adhesive proce-        a thin HL of 0.5-1.2 mm thickness (67). For this sys-




                                                                 rn
    dure is clinically performed. Therefore, the surface       tem, the created tags are short (16 mm) and narrow.
    should be gently dried until the etched enamel pre-        However, due to low acidity, the presence of a
    sents its white-frosted appearance and dentine loses       ‘‘smear layer’’ or ‘‘smear plugs’’ obliterates the tubule
    its shine and turns dull (57). Although etch-and-rinse     oriﬁces is common after adhesive procedures, limiting




                                                               te
    adhesives are still the gold standard for dental adhe-     hybridization of the peritubular dentin and resin tag
    sion and the oldest of the marketed adhesives, it          formation. In spite of forming a thin HL, this system
    seem to be incapable of preventing nanoleakage,            exhibits a chemical bond to the dentin substrate. Fur-
                                                        In
    (58) despite their satisfactory long-term clinical per-    thermore, self-etch dentin adhesive claimed to mini-
    formance (50). Although occurring even in the ab-          mize post-operative hypersensitivity, because resid-
    sence of interfacial gaps, nanoleakage seems to play       ual smear plugs are left which expose less dentinal
    a negative role in bonding, especially in terms of         tubules and causes less dentinal fluid flow than etch-
    durability (59). Thereby, the current trend is to devel-   and-rinse bonds, but the disadvantage is an insuffi-
                                            ni

    op simplified self-etching materials (60).                 cient 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 impor-
                                   io


                                                               tant to use these dentin adhesives properly in various
                                                               clinical situations. On the basis of the steps of appli-
    Self-etch

    Self-etching systems were introduced to control the        cation, they can be categorized as: a two-steps “self-
                           iz



    sensitivity to humidity of the etch-and-rinse technique    etch primers” (SEP) that is mostly solvent-free and a
    as well as to simplify the clinical procedures of adhe-    one-step “self-etch adhesives” (SEA) depending on
    sive application, reducing clinical time (61). The self-   whether a self-etching primer and adhesive resin are
              Ed




    etch adhesive systems are classified based on the          separately provided or are combined into one single
    number of clinical application steps: two-steps or         solution. Two-step self-etching adhesive systems
    one-step adhesives. The basic composition of self-         (SEA) require the use of two separate components:
    etch primers and self-etch adhesive systems an             the ﬁrst bottle containing primer and acid and the
    aqueous solution of acidic functional monomers, with       second bottle containing hydrophobic bond resin. The
    a pH relatively higher than that of phosphoric acid        self-etching primer (SEP) used to condition the dental
 IC




    etchants. Therefore, self-etching adhesives have           substrate, followed by the application of a hydropho-
    been classified according to their acidity: as strong      bic bonding resin (69). The self-etching primer are
    (pH≤1), intermediate (pH=1.5), and mild (pH≥2) (62)        aqueous acidic solutions containing various vinyl
    Mild self-etch adhesives demineralize dentin only su-      monomers (acidic, hydrophilic and hydrophobic
C




    perficially leaving hydroxyapatite crystals around the     monomers) which can simultaneously etch and infil-
    collagen fibrils available for possible chemical inter-    trate dental tissues, then photopolymerize with the
    action. Usually, the smear plug is not completely re-      bonding resin, thus forming a bond between the den-
    moved from the dentine tubule. As a result, a shallow      tal substrate and the restorative material applied after
@




    hybrid layer is formed with submicron measures (63),       wards. Single-step self-etch adhesives that combine
    as do the ultra-mild self-etch adhesives (64); on the      the functions of a self-etching primer and a bonding
    contrary, strong self-etch adhesives demineralize          agent have been developed. One-step adhesives can
    dentin comparably to etch-and-rinse adhesives. The         be further subdivided into ‘two-component’ and ‘sin-
    mild self-etch adhesives are assumed to cause less         gle-component’ one-step self-etch adhesives. By sep-
    post-operative pain, as they use the smear layer as        arating ‘active’ ingredients (like the functional
    bonding substrate, leaving residual smear plugs that       monomer from water), two-component self-etch adhe-

    12                                                                             Annali di Stomatologia 2017;VIII (1):1-17
    Classification review of dental adhesive systems: from the IV generation to the universal type


    sives theoretically possess a longer shelf life, but ad-        that have been used since 2011 in clinical practice.
    ditional and adequate mixing of both components is              These new products are known as “multi-mode″ or
    needed. The single-component one step adhesives                 ″multi-purpose″ adhesives because they may be
    can be considered as the only true ‘one-bottle’ or ‘all-        used as self-etch (SE) adhesives, etch-and-rinse
    in-one’ adhesives, as they combine ‘conditioning’,              (ER) adhesives, or as SE adhesives on dentin and
    ‘priming’ and ‘application of the adhesive resin’, and          ER adhesives on enamel (a technique commonly re-
    do not require mixing (69). This kind of adhesive sys-          ferred to as “selective enamel etching”) (75, 76). This




                                                                                                                      i
                                                                                                                    al
    tem combines acidic functional monomers, hydrophilic            versatile new adhesion philosophy advocates the use
    monomers, hydrophobic monomers, fillers, water and              of the simplest option of each strategy, that is, one-
    various solvent (acetone, ethanol, buthanol) and resin          step self-etch (SE) or two-step etch-and-rinse (ER)
    component, photo-inhibitors for bonding in a single             (77), using the same single bottle of adhesive solu-




                                                                                                        n
    solution. They are so-called as 7th generation dentin           tion which is deﬁnitely much more challenging to den-
    adhesive and undoubtedly the most convenient. The               tal substrates of different natures (i.e., sound, cari-




                                                                                                     io
    use of water as a solvent is indispensable for single-          ous, sclerotic dentin, as well as enamel) (78). Before-
    step self-etch adhesives to ensure the ionization of            hand etching enamel with phosphoric acid is often
    the acidic functional monomers, and the organic sol-            recommended, in particular when bonding to un-




                                                                         az
    vents are added to facilitate mixing of the hydrophilic         ground enamel. Indeed, the priming and bonding
    and hydrophobic components (69). The presence of                components can be separated or combined, resulting
    water and acidic functional monomers may compro-                in three steps or two steps for etch-and-rinse sys-
    mise the bonding durability of single-step self-etch            tems, and two steps or one step for self-etch adhe-




                                                                       rn
    adhesives. However, the main disadvantages of one-              sives. Contemplating these two bonding strategies,
    step self-etch adhesives is related to their excessive          adequate bonding to dentin can be completely
    hydrophilicity that makes the adhesive layer more               achieved with either etch-and-rinse or self-etch adhe-




                                                                     te
    prone to attract water from the intrinsically moist sub-        sives; however, at enamel, the etch-and-rinse ap-
    strate (39). Due to such increased water afﬁnity,               proach using phosphoric acid remains the preferred
    these adhesives have been reported to act as semi-              choice (79, 80). In relation to the application mode,
    permeable membranes, even after polymerization, al-             self-etch adhesive systems reduce the possibility of
                                                           In
    lowing water movement from the substrate through-               iatrogenic induced clinical mis-manipulation during
    out the adhesive layer (46). As a consequence, small            acid conditioning, rinsing and drying, which may oc-
    droplets can be found at the transition between the             cur when etch-and-rinse systems are used (81). On
    adhesive layer and the lining composite, especially             the other hand, some drawbacks may be listed for
                                                ni
    when polymerization of the latter is delayed. Besides           these SE materials. Unfortunately, one of the main
    promoting a decrease in bond strength between com-              drawbacks from applying SE adhesives to dentin and
    posite and substrate (70), such permeability of the             enamel is their inability to etch enamel to the same
                                        io


    adhesive layer seems to contribute to the hydrolysis            depth that phosphoric acid does, which is likely re-
    of resin polymers and the consequent degradation of             sponsible for the higher rates of marginal discol-
    tooth-resin bond over time (71). In addition, acetone           oration in the enamel margins of cervical restoration
                               iz



    has a so-called “water-chasing” effect (72), thus it            due to their lower acidity. Thereby the degradation of
    can infiltrate rapidly into the exposed dentinal                SE was attributed to its acidic content, which increas-
    tubules. However, its vapor pressure is much higher             es the hydrophilicity of the adhesive layer and leads
               Ed




    than that of other solvents like ethanol or water, and          to water uptake and plasticization (82). So the long-
    the adhesive may not infiltrate sufficiently in some sit-       term performance of simpliﬁed one-step adhesives is
    uations. It was observed that the poor performance of           inferior in terms of bond durability (60, 83), in particu-
    self-etch adhesives could depend upon shallow resin             lar when compared to the gold-standard three-step
    tag penetration produced by the self-etching process,           etch-and-rinse approach. To overcome the weakness
    an inefficient curing caused by their acidic nature, or         of previous generations of single-step self-etch adhe-
 IC




    solvent retention and phase separation phenomena                sives, universal adhesives have been developed that
    due to the coexistence of both hydrophilic and hy-              allow for application of the adhesive with phosphoric
    drophobic moieties in the same product (73). Most               acid pre-etching in the total etch or selective-etch ap-
    single-step dentin adhesives are very hydrophilic so            proaches in order to achieve a durable bond to enam-
C




    that they can interact with underlying dentin. Howev-           el and has been accepted by showing good results in
    er, it may form water permeable adhesive layer, thus            vitro (84) and in vivo studies (85-87). Despite the sim-
    compromising bonding performance (74). To over-                 ilarities between adhesives, the composition of uni-
    come this problem, All-Bond Universal contains mini-            versal adhesive differs from the current SE systems
@




    mum amount of ethanol and water as their solvent.               by the incorporation of monomers that are capable of
                                                                    producing chemical and micromechanical bond adhe-
                                                                    sion 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
    Universal adhesive systems

    One of the most recent novelties, in adhesive den-              phosphate monomers that bond ionically to calcium
    tistry, was the introduction of universal adhesives,            found in hydroxyapatite (Ca10[PO4]6[OH]2) (88, 89),

    Annali di Stomatologia 2017;VIII (1):1-17                                                                              13
    E. Sofan et al.


    that could be influence the bonding effectiveness           two bottles, or require the use of an additional activa-
    (77). For example, Methacryloyloxydecyl Dihydrogen          tor, or have chemistries that must be mixed prior to
    phosphate (MDP) is a functional monomer found in            use, or bond most optimally to porcelain and zirconia
    certain new adhesives, but not for older-generation         with separately applied and dedicated primers, or are
    bonding agents. This is a hydrophilic monomer with          not compatible with a total-etch protocol. Further,
    mild-etching properties. MDP is one of the monomers         there is an advantage in having an adhesive that can
    that enable a universal adhesive to be used with any        operate on these two procedures since it allows the




                                                                                                                    i
                                                                                                                  al
    etching techniques. Stable MDP-calcium salts are            dentist to choose his procedure according to the clini-
    formed during this reaction and deposited in self-as-       cal case in order to optimize the ﬁnal result. For in-
    sembled nano-layers of varying degrees and quality          stance, when the restoration requires strong bonding
    depending on the adhesive system (90, 91). It also          to enamel or in case of sclerotic dentin, it may be ad-




                                                                                                   n
    helps promote strong adhesion to the tooth surface          visable to apply prior etching. The etching step can
    via formation of non-soluble Ca2 salts. Furthermore, it     be modulated according to the length of time the




                                                                                                io
    contains biphenyl dimethacrylate (BPDM), dipen-             phosphoric acid gel is applied prior to rinsing. On the
    taerythritol pentaacrylate phosphoric acid ester (PEN-      other hand, it may be preferable to beneﬁt completely
    TA) (92) and polyalkenoic acid copolymer may en-            from the self-etch path way, when dealing with cases




                                                                    az
    hance adhesion to tooth structures and have been            confronting difﬁcult access, limited time or poor pa-
    part of the composition of different materials for          tient compliance in very young patients.
    decades. This may be important in terms of durability,
    as water sorption and hydrolytic breakdown of the ad-




                                                                  rn
    hesive interface over time has been implicated as
    one of the primary causes of bond failure (93, 68).
                                                                Conclusions

    Additionally, the matrix of universal is based on a         Increasing demands for aesthetic restorative treat-




                                                                te
    combination of monomers of hydrophilic (hydrox-             ments have led to recent advances in dentistry, de-
    yethul methacrylate /HEMA) hydrophobic (decandiol           veloping adhesive integrated materials (such as ad-
    dimethacrylite /D3MA) and intermediate (bis-GMA)            hesive systems and composites) and techniques
    nature. This combination of properties allows univer-       aimed at restoring the natural tooth appearance, es-
                                                         In
    sal adhesives to create a bridge over the gap be-           pecially in the anterior segment (97). The major re-
    tween the hydrophilic tooth substrate and hydropho-         quirement of adhesive aesthetic materials is the abili-
    bic resin restorative, under a variety of surface condi-    ty to achieve an excellent color matching with the nat-
    tions. Moreover, some universal adhesives may con-          ural teeth and the maintenance of the optical proper-
                                             ni
    tain silane in their formulation, potentially eliminating   ties over time. The goals for esthetic dental restora-
    the silanization step when bonding to glass ceramics        tions are to obtain morphologic, optical and biologic
    or resin composites, for instance. Nevertheless, it is      result miming natural enamel and dentine. This color
                                    io


    known that simplified materials are associated with         matching is performed in order to obtain harmony
    lower in vitro bond strength results and poorer in vivo     with the surrounding anatomical structures (98).
    longevity of restorations, (94, 95). These findings are     Further the evolution of these materials and tech-
                           iz



    probably a result of the complex formulation of simpli-     niques has recently took steps forward and succeed
    fied adhesives and their high content of solvents,          in preserving teeth instead of extracting them. Most
    which may impair complete solvent volatilization and        of these improvements were evident in conservative
              Ed




    consequently lead to poorer adhesive polymerization         dentistry and in particular, adhesive dentistry (99).
    (96). This multi-approach capability enables the clini-     This review about adhesive dentistry describes all the
    cian to apply the adhesive with the so-called selective     “generations” and types of adhesive product designs
    enamel etching technique that combines the advan-           that have been introduced during the last 30 years.
    tages of the etch-and-rinse technique on enamel, with       Since the introduction of the acid etched into clinical
    the simpliﬁed self-etch approach on dentine with ad-        practice, various dentin bonding agents were devel-
 IC




    ditional chemical bonding on remnant carbonated ap-         oped to improved the quality of adhesives and com-
    atite crystallites in those bonding substrates. There-      posites restoration. The manufactures have been on-
    fore, the universal adhesives have much broader ap-         going progress in the development of new dentin ad-
    plications than 7 th generation systems. Additionally,      hesive aiming to simplify the process, attempted to
C




    manufacturers typically state that universal adhesives      improve clinical results correlates to their stability
    can be used for the placement of both direct and indi-      over time and to their bond strength performance
    rect restorations and are compatible with self-cure,        consequently lead to improve their effect on the dura-
    light-cure and dual-cure resin-based cements and            bility of the resin bond. The new adhesive systems al-
@




    bonds to metals, zirconia, porcelain and composite.         so can be attributed to their ability to decrease or
    While, the manufacturers of some universal adhe-            eliminate postoperative sensitivity, improve marginal
    sives still recommend the use of separate “activator”       seal, reduce microleakage and enhance the flow of
    and dedicated primers to optimize bond strength to          resin into fissure. The development of functional
    substrates such as porcelain and zirconia. Thus, it         monomers with strong and stabile chemical affinity to
    appears, at least in certain situations and with some       hydroxyapatite is without doubt a valuable direction to
    products, that universal adhesives actually consist of      continue for improvement of dental adhesion. Fur-

    14                                                                              Annali di Stomatologia 2017;VIII (1):1-17
    Classification review of dental adhesive systems: from the IV generation to the universal type


    thermore, long-term ageing also requires evaluation                            dimethacrylate-based dental resins. Biomaterials. 2003;24:
    of its effect in establishing a long-term success of                           655-665.
    composite restoration.                                                   20.   Feilzer AJ, de Gee AJ, Davidson CL. Setting stress in com-
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                                                                                   Dentl Researc. 1987;66(11):1636-1639.
                                                                             21.   De la Macorra JC, Gomez-Fernandez S. Quantification of the
                                                                                   configuration factor in Class I and II cavities and simulated




                                                                                                                                        i
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                                                                             rn
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    Annali di Stomatologia 2017;VIII (1):1-17                                                                                            17
