Operative Dentistry

Clinical considerations for optimal dentinal bondingPaula R. Walshaw*/Dorothy McComb*

Abstract A review of current clinical research has demonstrated that successful attachmentwith resin-based bonding systems is achieved through brief acidic conditioning ofdentin, followed by thorough coverage with resin priming and bonding agents. Thisarticle discusses faclors of chnical relevance in achieving optimal results. Effectivepriming, using multiple coats to enhance resin penetration to the full depth ofdentinal demineralization, is crucial. A thin, uniform layer of bonding resin is acritical, elastic intermediar}'for absorbing stresses of polymerization shrinkage. Anair stream should be used only for evaporation of solvent and not for spreadingbonding resin, because use of an air stream causes uneven thinning of this valuableintermediate layer. Contamination ofthe dentinal surface with excessive moistureor solvent or the presence of air voids will make bonding unpredictable underclinical conditions. Adequate etching of peripheral enamel continues to be animportant factor in the long-term retention of adhesive restorations.(Quintessence int ¡996:27:619-625.)

Clinicai reievance

Attention to technical detail at each stage of adentinal bonding procedure is necessary for clinicalsuccess. Practical guidelines, based on a review ofcurrent research, are presented.

IntrodnctiOD

Current bonding techniques, which can produce awell-defined attachment to dentin. have increased theacceptance and reliability of resin-based restorativematerials in clinical use. A large body of in vitro work,recently reviewed by Burke and McCaughey, ' and asmaller number of in vivo studies-"^ have confirmedthe superior performance of bonding systems thatcondition the dentinal surface, allowing diffusion of

" Assistant Professor, Restorative Depanment. Faculty of Dentistry,LJniversUy of Toronto, Toronto. Ontario, Canada,

" Professor and Head, Restorative Departmenl, Faculty of Deniistry.University of Toronto, Toronto. Ontario, Canada,

Reprini requests: Dr Paula R. Walshaw. Assistant Professor, RestorativeDepartmenl, University of Toronto, Faculty of Dentistry, 124 EdwardStreet, Toronto. Ontario. M5G 1G6 Canada,

resin and formation of a hybrid resin-dentin attach-ment. Such resin impregnation of surface dentin hasimplications for reduced microleakage, reduced sen-sitivity, increased retention, and even increased cariesresistance. The improved strength of attachment al-lows greater opportunity for tooth reinforcement andmore conservative restorative options.

Research in human subjects under clinicai con-ditions has demonstrated that successful attachment isachieved with proprietary materials that use acidicconditioning, as a separate step, prior to priming,^'^These scanning electron microscopic (SEM) studiesprovided insight into some factors relevant to optimalefficacy of clinical technique which, together withpertinent data from a review of current research, will bediscussed in this article. All SEM micrographs wereprepared by methods described in those humanstudies,̂ ••'

Characteristics of a successfni dentinal ix»nd

The most effective adhesion of resin composite todentin has proved to be via a micromechanicalattachment between resin and demineraiized. primed,surface layers of intertubular dentin. This complex,known as the hybrid layeP, is best achieved by brief useof acidic conditioning agents to remove the instrti-

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Fig 1 Freeze-fractured interface showing a secure attach-ment produced intraorally with Ail-Bond 2 between resincomposite (c). adhesive resin (a), and dentin (d). Afterfracture, this surface was treated with 1% hydrochloric acidfor 2 minutes to demonstrate resin impregnation of theinfertubuiar matrix and widened tubular orifices within theacid-resistant hybrid layer (h). Deeper iayers of denfincontain resin tags loosely wifhin the tubuies, which havebeen exposed by the acid treatment. ¡Scale bar = 6 |im )

Fig 2 Fractured intertace showing a iocaiized area oiinadequate primer coverage to the extreme right of anotherwise secure attachment, produced intraoraiiy witfiScofchbond Muiti-Purpose, similar to that in Fig 1, (a)Adhesive resin; (c) resin composite; (d) dentin; {h| hybridiayer (Scale bar - 6 ^m.)

mented smear layer and to effect surface demineral-ization and exposure of the dentinal collagen frame-work. Application of hydrophilic resin primers facili-tates subsequent penetration of low-viscosi^' adhesiveresin into the microspaces between the collagen fíbrilsand into the dentinal tuhules. Photopolymerization ofthese cohesive resin components stabilizes the surfacehybridization against the shrinkage stresses occurringwhen the bulk of restorative resin composite ispolymerized, Careilil application of these steps caneffect a gap-free bond in vivo (Figs 1 and 2).

Effective priming

The function of the primer is to promote resin diffiasioninto the moist, demineraiized dentin with the intent ofachieving complete resin infiltration. Use of volatilesolvents and hydrophilic resins, such as hydroxyethylmethacrylate, promotes surface wetting and dentinalpenetration, but complete saturation of the micro-spaces does not aiways occur. Microscopic examina-

tion of attachments produced in vivo, by methodspreviously described,^-' has shown deficiencies thatappear to comprise three categories: (I) incompletesurface coverage (Figs 2 and 3); (2) incompleteinterflbrillar saturation within the hybrid zone (Fig 4);and (3) incomplete penetration to the Hill depth of thedemineralized dentin (Fig 5). Development of bettersurface-prittiing agents, capable of more efficientwetting and complete saturation, is required. Untilthen, the potential for incomplete priming and theneed for meticulous application of currently availablematerials should be emphasized.

One method of improving surface coverage anddiffusion of the primer is by the application of tnultiplecoats. Even for materiais utilizing a single-coat tech-nique, the use of further applications may provebeneficial. Titley et al'° found that a second coat ofScotchbond Multi-Purpose primer (3M Dental) im-proves the in vitro dentina! shear bond strengthsignificantly; however, there is no additional increasewith up to five further applications of primer. An

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Fig 3 Microspaces between exposed coiiagen ijbrjis (i) ona dentinai surface thai appears lo be inadequateiy primed.thus preventing surface wetting with tiydrophobic adhesiveresin (a). Interfaciai separation has occurred, and coiiagenfibriis (arrpw) appear lo have been puiied from the dentin bypoiymerization shrinkage. (Scaie bar - 1 |im )

Fig 4 Incomplete diifusion of resin throughout the demin-eralized intertubular dentin (d) Microspaces between thecollagen fibrils are seen beiow the bonded interface ¡i).(Scale bar= 1 |im.)

increase in bond strerrgth was also reported by Vargaset al," who utilized additional coats of both Scotch-bond Multi-Purpose and All-Bond 2 (Bisco Dentai).

A second consideration is the degree of surfacedrying that promotes optimal cohesive hybridization.It is important that the surface dentin not be dehyd-rated following conditioning, because the unsupporteddemineralized collagen fibers may collapse and pre-vent efficient diffusion of primer. On the other hand,excessive surface moisture may result in voids at theresin-dentin interface. This has been described as theoverwet phenomenon, seen with apphcation of All-Bond 2 primers'^ and thought to be caused byseparation of the hydrophobic primer B from themixed primers.'^ Until optimal dentinal surface mois-ture is defined, the clinician should avoid extremeconditions of surface wetness or dehydration.

Microspaces are visible among the exposed colla-gen fibers in the deeper layers of the hybrid zoneshown in Fig 5, which demonstrates an area ofincomplete penetration of resin to the full depth ofthedemineralized dentin. Although infrequently observedin vivo,^' possibly because of the brief conditioningtimes, concertis have been raised about the implications

Fig 5 Resin diffusion at the periphery of the widenedtubuie (t>, in addifion to impregnation of the intertubulardentin (d) adjacent to adhesive resin (a). A demarcated zone(z) with no evidence of resin impregnation is visible beneaththe hybrid layer (Scaie bar = 1 (im.) (From Walshavvi andMcComb.^ Reprinted with permission from Butterworth-Heinemann, Kidiington, United Kingdom.)

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of extensive areas of such unreinforced collagenousframeworks beneath restorations.- A new type ofmicroleakage, termed nanoleakage. has been sugge-sted via this pathway by Sano et al. ''' They demonstra-ted microscopic percolation among collagen fiberswithin the hybrid layer, but its elfect on the longevity ofthe dentinal bond is not known.

It is therefore critical that resin penetrate to the flilldepth of dentinal demineralization. Complete penetra-tion can best be achieved by brief acidic conditioning,ideally for no longer than 15 seconds, followed bythorotigh coverage with resin priming and bondingagents. If a longer etching time is desired for enamel,conditioning acid may be placed first on the majorenamel surfaces for 15 to 30 seconds and thenextended to dentin for a further 15 seconds, to ensureadequate enamel etching and prevent overdemineral-ization of dentin.

Bonding systems that use a single-step conditionerand primer combination have not always shown thesame chnical reliability.*'''^ Although Duke et al'^reported excellent clinical results with one such agent,their SEM results confirm those of other studies,which have shown iittle evidence of dentina! hybridiza-tion. The efficacy of some of the newer bondingformnlations that utilize self etching primers, particu-larly those designed for enamel bonding, requiresfurther evaluation.

Role of resin tags

Although resin tags can penetrate vital dentinal tubulesto a limited depth (approximately 10 to 20 |im),' theyare attached only at the top 2 to 3 pm within thefunneled tubular orifices, widened by removal ofthehighly mineralized intrambular {peritubuiar) dentinduring acidic conditioning. Hybridization occurs later-ally at this point into the exposed intertubular dentin,but any deeper extensions ofthe resin tags lie passivelywithin the dentinal tubules (see Fig 1). They do notappear to contribute to the attachment mechanism;hence, there is no benefit to be gained by prolongeddentinal conditioning to effect maximum resin taglength. In a recent in vitro study of the structure ofresin tags within dentinal tubules, Titley et al" haveobserved three different types. Many of the longer,hoilow extensions visible with the scanning electronmicroscope are now thought to be the internal lining ofthe tubuie, or lamina limitans.

Primer constituents can, however, penetrate deeplyinto dentin. Hydroxyethyl methacrylate and trieth-

ylene glycol methacrylate have been shown to leachfrom bonding agents and resin composites and todiffuse through the tubules to the pulp chamber invitro,'^ but the significance of this is not known.

Importance of the adhesive resin layer

The presence of a distinct layer of adhesive resinbetween the hybrid zone and the overlying resincomposite appears to provide an important flexible,intermediate layer that can absorb stresses and preventseparation during polymerization shrinkage of theresin composite. The effect of such shrinkage when theintermediate adhesive resin layer is absent is shown inFig 6.

When low-viscosity bonding resin is applied to abox-shaped cavity, some difficulty is experienced inachieving an even coverage. Usually, the resin isapplied, and a gentle air stream is used to coat thecavity and remove excess. The resin frequently pools inthe line angles, leaving bare areas, devoid of adhesiveresin, on the cavity ñoor (Fig 7). In studies inhumans,^'^ a secure attachment was usually foundcoincident with such pooling and at other sites where adistinct layer of adhesive resin was clearly evident.This observation supports the stress-absorbing theoryelaborated by Kemp-Scholte and Davidson'^ and VanMeerbeek et al.-° A recent report has shown that suchstresses can also be reduced by a thick layer ofglass-ionomer lining material.^'

The areas of detachment often associated with lackof an adhesive resin layer in the human studies (seeFigs 6 and 7) are thought to be caused by use of an airsyringe on the cavity floor, even if the air is lightlyapplied, Eliades-- has drawn attention to the im-portance of a uniform adhesive film and the tendencyof air blasts to promote surface defects and variationsin film thickness that may significantly reduce effectivebonding at the interface (Fig 8). He concluded that anair stream should be used only for the evaporation ofsolvent and not for spreading the resin film. Excessiveair thinning has recently been shown to reducesignificantly the shear bond strength of ScotchbondMulti-Purpose to dentin.-^ Optimal clinical techniqueshould, therefore, aim to produce a thin, uniform layerof adhesive resin; careful brush application should beused to coat the cavity floor and walls, sparingly,avoiding the need for ah" thirming.

Additional factors influencing the presence ofinterfacial gaps are the cavity configuration andmethod of insertion of the resin composite. In a

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Fig 6 Separation between resin composite (c) and con-ditioned, primed dentin (d| in a localized area that lacks theoptimal thickness of intermediate adhesive resin. (Scale bar- 15 um.)

Fig 7 Gap-free intraorai bonding coincident with a distinctlayer of adhesive resin (a). An adjaoent area lackingintermediate adhesive resin coverage shows detachmentbetween dentin (d] and resin composite (cl (Scale bar -15 um.)

box-shaped cavity, the restoration has only one freeouter surface, which may restrict free flow ofthe resincomposite during polymerization and produce highcontraction stresses at the tooth hiterface.-'*•-' This canbe reduced by use of composite insertion techniquesthat place increments diagonally against lateral walls,maximizing the available free surface area duringphotopolymerization.

Clinical studies

The clinical difficulties of optimizing all factors aredemonstrated in Fig 9, in which experimental fracturethrough a clinically bonded interface revealed failure ofthe bond at different levels withiti the adhesiveinterface. In vitro studies have confirmed that unevenbonding is found in adjacent parts of the samespecimen,-^-^^ and a recent three-dimensional micro-leakage study has concluded that the quality ofbonding is not uniform over the entire cavity wall.̂ ^These findings suggest that attachments to dentiti,particularly when formed under real clinical con-ditions, tnay not be totally predictable. Bacterialmicroleakage is a distinct possibility.

Fig 8 Undersurface of detached Ail-Bond 2 resin com-posite (c) showing an area of nonattachment (n), causedeither by air pockets or fluid contamination, at the interfacebetween dentin (d) and composite. (Scale bar = 15 \im.)

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Fig 9 Higher magnification ot an area adiacent to Ihat ofFig 8, showing interfacial fracture through different levels ofthe hybrid layer. Collagen fibriis (f] are seen above the thickadhesive resin layer (a), which shoves evidence of anemerging resin tag (t). The smcoth area Is) indicates iackof bending, probabiy respiting from air or moisture contam-ination. (Scaie bar = 1.5 [im.)

Bacteria can become established more quicklyunder composite restorations than under amalgam-'-'''and significantly higher proportions of Streptococcusmiitans have been found at cavity margins of resincomposite restorations than at cavity margins ofamaigam or glass-ionomer restorations.-'' It wouldseem, therefore, that secondary caries is a real risk ifbonding procedures are unsuccessñil, particularly ifthe patient exhibits poor oral hygiene. One methodthat may prove useful in counteracting surface degra-dation and bacteriai microleakage at dentinal marginsis the addition of bacterial inhibitors to the resincomposite. Imazato et al̂ ^ have shown that there isless accumulation of Streptococcus mutans on com-posite surfaces when an experimental antibacterialformulation is used, and some manufacturers have nowdeveloped antimicrobial dentin primers. Torii et al̂ ^showed that effective resin impregnation of cavity wallsrenders them less soluble in acid and actually providesincreased caries prevention in vitro. It is, therefore,evident that, when performed effectively, dentinalbonding techniques can reduce the potential forrecurrent caries. However, beeause of inherent techni-

que sensitivity, dentinal botiding may also allowiticreased bacterial microleakage with associated pro-blems.

Recent clinical studies of the retention of Class Vabrasion lesions have shown improved performance ofseveral newer dentinal bonding agents tbat conditiondentin.'^-''' Van Meerbeek et al'= drew attention toseverai factors, other than the dentinal adhesive itself,on which retention of cervical restorations depends.These include flexibility of the resin composite,occlusal stress, and presence of sclerotic dentin. Theyfound that additionai retention from enamel bevelingmarkedly improves the 2- and 3-year chnical per-formance of many ofthe materials tested.

Enamel bonding

Attention has been drawn to the important adjunctiverole of enamel bonding in the long-term retention ofadhesive restorations.'^-'^ Wherever possible, cavo-surface margins should be placed in enamel to takeadvantage ofthe proven reliability of enamel bonding.Use of a bevel or chamfer is advisable. Watts et al̂ ^found that removal of surface enamel along a 45-degree bevel at cavity margins produces a uniformlygood etch with 35% phosphoric acid, in contrast withunprepared enamel.

Use of total-etch techniques, in which enamel anddentin are simultaneously and briefly conditioned withdilute acidic solutions, are time saving and popular butmay not give a consistently good enamei etch. Inci-dental observations made during previously reportedwork^' revealed occasional gaps at enamel marginsthat were conditioned intraorally with 10% maleic or10% phosphoric acid for 15 seconds, indicating apossible benefit of a longer enamel etching time. VanMeerbeek et al" have stated similar concerns.

Conciusions

For optitnal results with currently available resinbonding materials, the following technique is recom-mended:

1. Enamel margins, beveled or chamfered as clinicallyappropriate, should be utilized wherever possible.Optimal acidic conditioning should be ensured.

2. Brief conditioning of dentin. for 15 seconds or less,should be used to remove the instrumented smearlayer atid efTcct surface demineralization only.

3. Desiccation ofthe conditioned dentin should beavoided. Thorough dentinal priming should beensured by use of additional coats of primer.

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M. Identification of adentin created in vivo:ceint 1992,23:135-141,

4, The importance ofa low-viscosity resin layer mustbe recognized. A thin, utiiform layer of adhesivebonding resin shouid be apphed. The use of airthinning should be avoided.

5, Resin composite should be inserted in diagonallyplaced increments to minimize contraction stressesat the bonded interface.

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