The Science of Bonding From First to Sixth Generation

20S JADA, Vol. 131, June 2000

As we enter the new millennium, it is importantfor us to examine the past. The principles of adhe-sive dentistry date back to 1955 when Buonocore,using techniques of industrial bonding, postulatedthat acids could be used as a surface treatmentbefore application of the resins.1 He subsequentlyfound that etching enamel with phosphoric acidincreased the duration of adhesion under water.In 1963, Buonocore demonstrated his insight intoadhesion dentistry when he discussed the differ-ence in bonding to enamel and to dentin,2 particu-larly when he referred to Dr. Bowen’s attempts toinvestigate substances that will displace waterfrom tooth surfaces3 with the idea that they couldbe used as pretreatment for enamel or dentin.Buonocore then stated that they could even beincorporated into the adhesives.2

In the late 1960s, Buonocore suggested that itwas the formation of resin tags that caused theprincipal adhesion of the resins to acid-etchedenamel.4 The idea that resin penetrates themicroporosities of etched enamel and results in amicromechanical bond is well-accepted today.

As time went on, variations in duration of the

acid-etching procedure and concentration of thephosphoric acid, along with alternative acids,were tested for the etching of enamel.5-7 The cur-rent thinking is that a 30 to 40 percent phos-phoric acid etch of 15 seconds is acceptable.

The ability to bond reliably to enamel is nowwell-accepted, but as Buonocore suggested in1963, adhesion of our restorative materials todentin has proved to be more elusive.2

Early attempts to bond to dentin resulted inpoor bond strengths.8 This is not surprising giventhe fact that while enamel contains little protein,dentin is 17 percent collagen by volume. This col-lagen is inaccessible due to surrounding hydroxy-apatite crystals.9 The dentinal tubules are theonly pores available for micromechanical reten-tion. These tubules contain fluid, which would bean impediment to bonding. The number oftubules available for bond also varies dependingon location, with deep dentin having moretubules than superficial dentin.9 Other factorssuch as age of teeth, direction of tubules and ofenamel prisms, presence of cementum and typeof dentin can affect dentin bonding.10,11

A B S T R A C T

Background. Adhesive dentistry has revo-lutionized restorative dental practice during the past30 years. Improved adhesive materials have maderesin-based composite restorations more reliable andlong-standing.

Clinical Implications. This articlereviews the evolution of bonding from the first gen-eration to current bonding materials. It discussesthe composition and effectiveness of the various iter-ations. Current products are highlighted to improveclinical use and performance of the materials.

THE SCIENCE OF BONDING: FROM FIRST TO SIXTH GENERATIONGERARD KUGEL, D.M.D., M.S.; MARCO FERRARI, M.D., D.D.S., PH.D.

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ARTICLE 2

Copyright ©1998-2001 American Dental Association. All rights reserved.

Early dentin bonding wasfurther complicated by the pres-ence of the smear layer. (Thesmear layer is the organicdebris that remains on thedentin surface after the prepa-ration of the dentin duringrestoration of a tooth.) Thesmear layer blocks the dentinaltubules and acts as a “diffusionbarrier.” This was originallythought of as an advantage inthat it protected the pulp bydecreasing the permeability ofdentin.12 As dentin bondingimproved, the removal of thesmear layer became necessary,but not without controversy.

Several factors characterizethe change in bonding systemsfrom the Buonocore era to today:detching enamel;detching-conditioning dentin;dsmear layer treatment;dhandling properties.

This article reviews severalaspects of the different genera-tions of bonding systems.

FIRST GENERATION

In 1956, Buonocore and col-leagues demonstrated that useof a glycerophosphoric aciddimethacrylate-containing resinwould bond to acid-etcheddentin.13 This bond wasbelieved to be due to the inter-action of this bifunctional resinmolecule with the calcium ionsof hydroxyapatite. Of course,immersion in water wouldgreatly reduce this bond. Nineyears later Bowen14 tried toaddress this issue usingN-phenylglycine and glycidylmethacrylate, or NPG-GMA.NPG-GMA is a bifunctionalmolecule or coupling agent.This means that one end of thismolecule bonds to dentin whilethe other bonds (polymerizes) tocomposite resin. The bondstrengths of these early systems

were only 1 to 3 megapascals.The clinical results with thesesystems were poor.

SECOND GENERATION

As improvements were made inthe adhesive coupling agentsfor composites, the adhesion todentin increased. In the late1970s, the second-generationsystems were introduced. Themajority of these incorporatedhalophosphorous esters ofunfilled resins such as bisphe-nol-A glycidyl methacrylate, orbis-GMA, or hydroxyethylmethacrylate, or HEMA.15

The mechanism by whichthese second-generation sys-tems bonded to dentin werepostulated to be through anionic bond to calcium bychlorophosphate groups. Thesewere weak bonds (in compari-

son to fifth- and sixth-genera-tion systems) but they were asignificant improvement overfirst-generation systems.

One major concern withthese systems was that thephosphate bond to calcium inthe dentin was not strongenough to resist the hydrolysisresulting from water immer-sion. This hydrolysis, resultingfrom either saliva exposure ormoisture from the dentin itself,could result in composite resindebonding from the dentin andcausing microleakage.

Since dentin was not etchedin these early bonding systems,

much of the adhesion was dueto bonding to the smear layer.Some of the second-generationsystems were thought to softenthe smear layer and thusimprove resin penetration.However, these systems result-ed in bond strengths to dentinthat were weak and unreliable.

THIRD GENERATION

With the third-generation sys-tems, the acid etching of thedentin partially removes and/ormodifies the smear layer.16 Thiseffect is due to the pK of theprimer solution. The acid opensdentinal tubules partially andincreases their permeability.The acid must be rinsed com-pletely before the primer isapplied. The primer containshydrophilic resin monomerswhich include hydroxyethyltrimellitate anhydride, or4-META, and biphenyldimethacrylate, or BPDM. Theprimers contain a hydrophilicgroup that infiltrates the smearlayer, modifying it and promot-ing adhesion to dentin, and thehydrophilic group of the primercreates adhesion to the resin.

Following primer application,an unfilled resin is placed ondentin and enamel. Thesethird-generation adhesion sys-tems usually use a hydrophilicdentin-resin primer. Dentinprimers may be 6 percent phos-phate penta-acrylate, orPENTA; 30 percent HEMA; and64 percent ethanol. Followingetching and primer application,the unfilled resin adhesive isapplied to dentin and enamel.

In most of these systems, thephosphate primer modifies thesmear layer by softening it;after penetration, it cures,forming a hard surface. Theadhesive is then applied,attaching the cured primer to

JADA, Vol. 131, June 2000 21S

As dentin bondingimproved, removal of the smear layerbecame necessary,but not without controversy.


the composite resin. Bonding tosmear-layer–covered dentin wasnot very successful before 1990,however, because the resins didnot penetrate through thesmear layer and the smearlayer was very weak.17

FOURTH GENERATION

The complete removal of thesmear layer is achieved withfourth-generation bonding sys-tems. Fusayama and colleaguestried to simplify bonding toenamel and dentin by etchingthe preparation with 40 percentphosphoric acid.18 Unfortu-nately, it was not understoodthat this procedure overetcheddentin and resulted in the col-lapse of exposed collagen fibers.

In 1982, Nakabayashi andcolleagues reported the forma-tion of a hybrid layer resultingfrom the polymerized methacry-late and dentin.9 The hybridlayer is defined as “the struc-ture formed in dental hard tis-sues (enamel, dentin, cemen-tum) by demineralization of thesurface and subsurface, fol-lowed by infiltration ofmonomers and subsequent poly-merization.”9

The use of the total-etchtechnique is one of the main

characteristicsof fourth-gener-ation bondingsystems.19,20 Thetotal-etch tech-nique permitsthe etching ofenamel anddentin simulta-neously usingphosphoric acidfor 15 to 20 sec-onds. The sur-face must beleft moist (“wetbonding”), how-ever, in order toavoid collagen collapse (Figure1); the application of ahydrophilic primer solution caninfiltrate the exposed collagennetwork forming the hybridlayer (Figure 2).21,22 Unfor-tunately, “moist dentin” is noteasily defined clinically andmay lead to less-than-idealbonds if the dentin is excessive-ly wet23 or dried.16

The formation of resin tagsand adhesive lateral branchescomplete the bonding mecha-nism between the adhesivematerial and etched dentin sub-strate (Figure 3).22,24,25 The min-eralized tissues of the peritubu-lar and intertubular dentin are

dissolved by the acidic action;the initial surface penetrationexposes the collagen fibers. Inthis area, for a depth of 2 to 4micrometers, hybridizationtakes place, and resin tags canseal the tubule orificesfirmly.16,26

FIFTH GENERATION

To simplify the clinical proce-dure by reducing the bondingsteps and thus, the workingtime, a better system was need-ed. Also, clinicians needed abetter way to prevent collagencollapse of demineralizeddentin. The fifth generation ofbonding systems was developed


Figure 1. Collagen fibers exposed after etchingdentin (scanning electron microscopy X5000;reprinted with permission of P.N. Mason).

Figure 2. Hybrid layer formed between etcheddentin and a one-bottle system (scanning electronmicroscopy × 1,550).

Figure 3. Resin tags and adhesive lateral branchescontribute to micromechanical bonding formation(scanning electron microscopy × 2,000).


to make the use of adhesivematerials more reliable forpractitioners.

The fifth generation consistsof two different types of adhe-sive materials: the so-called“one-bottle systems” and theself-etching primer bondingsystems.

One-bottle systems. Tofacilitate clinical use, “one-bot-tle” systems combined theprimer and adhesives into onesolution to be applied afteretching enamel and dentinsimultaneously (the total-etchwet-bonding technique) with 35to 37 percent phosphoric acidfor 15 to 20 seconds.27 Thesebonding systems create amechanical interlocking withetched dentin by means of resintags, adhesive lateral branchesand hybrid layer formation andshow high bond-strength valuesboth to the etched enamel anddentin.28,29

Self-etching primer.Watanabe and Nakabayashideveloped a self-etching primerthat was an aqueous solution of 20 percent phenyl-P in 30percent HEMA for bonding toenamel and dentin simultane-ously.30

The combination of etchingand priming steps reduce theworking time, eliminate thewashing out of the acidic geland also eliminate the risk ofcollagen collapse. However, theself-etching primer solution alsohas some disadvantages. Forexample, the solution must berefreshed continuously becauseits liquid formulation cannot becontrolled where it is placed,31

and often a residual smearlayer remained in betweenadhesive material and dentin.16

Also the effectiveness of self-etching primer systems onproperly etching the enamelwas less predictable than theresult obtained with phosphoricacid gel (Figure 4).31 Toida32

advised that removal of thesmear layer by a separate etch-ing step before bonding wouldproduce a more reliable anddurable bond to dentin.

Bond strength tests madeunder laboratory conditionsoften did not demonstrate sta-tistically significant differencesbetween one-bottle systems andself-etching primer bonding sys-tems.33 Leakage tests conductedunder laboratory and clinicalconditions showed that the seal

achieved at the enamel marginswith one-bottle systems is supe-rior to that resulting from self-etching primer.31

SIXTH GENERATION

Recently, several bonding sys-tems were developed and pro-posed as the sixth generation ofadhesive materials. These bond-ing systems are characterizedby the possibility to achieve aproper bond to enamel anddentin using only one solution.These materials should reallybe a one-step bonding system.Unfortunately, the first evalua-tions of these new systemsshowed a sufficient bond to con-ditioned dentin while the bondwith enamel was less effective.This may be due to the fact thatthe sixth-generation systemsare composed of an acidic solu-tion that cannot be kept inplace, must be refreshed contin-uously and have a pK that isnot enough to properly etchenamel.34

However, any improvementin the direction of clinical sim-plification of bonding proce-dures can bring us closer toachieving an ideal bondingsystem.

JADA, Vol. 131, June 2000 23S

Figure 4. Enamel surface after etching with a self-etching primer solution; the enamel surface is lessretentive than that obtained with phosphoric acid(scanning electron microscopy × 1,500).

Figure 5. Laminate veneers of maxillary central inci-sors; the porcelain restorations are luted thanks tothe adhesive technique.


DISCUSSION

Bonding to etched enamel wasconsidered a safe and reliableprocedure for many years.Because of the inorganic compo-sition of the enamel, the acidattack produces interprismaticand prismatic dissolution, cre-ating irregularities into whichthe resin can flow and, afterpolymerization, create amechanical interlocking. Thedemineralization of the enameldepends on the low pH of theacid and on the etching time.The pH and the etching timemust be sufficient to provideadequate enamel retentionwithout the need for additionalsteps. The morphological stud-ies made on the first five gener-ations of bonding systems, inwhich phosphoric acid was usedto etch enamel, showed a uni-form etch pattern. When phos-phoric acid was not used orwhen self-etching primers (fifthand sixth generations) wereapplied, the bonding mecha-nism of adhesives to enamelwas less effective.

The bonding mechanism todentin was effective and pre-dictable when the smear layerwas completely dissolved, inter-tubular and peritubular dentinwere dissolved, collagen fibersexposed and, after infiltrationof resin monomers, a hybridlayer formed. This bondingmechanism was evident fromfourth to sixth generations ofenamel dentin bonding systems.

A wide range of clinical indi-cations for bonding systems arecommonly followed by practi-tioners. Bonding systems can beused as coupling resin sealantsto enamel in young patients.35

Also, the sealing effect of bond-ing systems can be used on pro-tecting dentinal surfaces after

preparation36 or beneath amal-gam restorations.37,38

Desensitization of root expo-sure is another clinical indica-tion for bonding systems.39-41

Also, bonding systems are indi-cated in any direct estheticrestorations. In fact, themechanical properties of thebonding mechanism achievedwith hybrid layer and resin tagformation can be greater thanthe forces of polymerizationcontraction.16 Finally, bondingsystems are essential for aproper bonding-luting proce-dure of any indirect restora-tions (Figure 5).42

CONCLUSIONS

To satisfy the growing estheticdemands of today’s dentalpatients, improvements inmaterials and procedures havebeen made to make it possible

to reproduce the naturalappearance of natural teethwith direct and/or indirectesthetic restorations. Esthetictechniques involve a bondingstep to ensure durability andreliability. Thus, the ideal bond-ing system should be biocompat-ible, bond indifferently to enam-el and dentin, have sufficientstrength to resist failure as aresult of masticatory forces,

have mechanical propertiesclose to those of tooth struc-tures, be resistant to degrada-tion in the oral environmentand easy to use for the clinician.

Although important improve-ments in bonding have beenmade in the last 30 years, notethat the requirements of anideal bonding system are quitesimilar to those indicated byBuonocore.2 Apparently, thefuture has a sound backgroundin the past. ■

Dr. Kugel is professor and assistant deanfor research, Tufts University School ofDental Medicine, Boston, Mass. 02111Address reprint requests to Dr. Kugel.

Dr. Ferrari is a research professor at TuftsUniversity School of Dental Medicine, Boston,and professor, University of Siena, Italy.

The authors dedicate this paper to thememory of Gaia Gotti. They also expresstheir appreciation to Jennifer Towers for herhelp in the preparation of this manuscript.

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The Science of Bonding From First to Sixth Generation

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