Frankenberger 2002

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The use of owable composites as lled adhesivesRoland Frankenberger a, *, Manuela Lopes b, Jorge Perdigao c, Wallace W. Ambrose d,

Bruno T. Rosa e

aPoliclinic for Operative Dentistry and Periodontology, University of Erlangen-Nuremberg, Glueckstrasse 11, D-91054 Erlangen, Germanyb Minnesota Dental Research Center for Biomaterials and Biomechanics, University of Minnesota, 16-212 Moos Tower B 515 Delaware St SE, Minneapolis,

MN 55455, USAc Division of Operative Dentistry, Department of Restorative Sciences, University of Minnesota, 8-450 Moos Tower B 515 Delaware St SE, Minneapolis,

MN 55455, USAdSchool of Dentistry, Dental Research Center, University of North Carolina at Chapel Hill CB no. 7455, Chapel Hill, NC 27599-74550, USA

ePrivate practice, Londrina, PR, Brazil

Received 17 October 2000; received in revised form 23 March 2001; accepted 9 April 2001

Abstract

Objective: The effect of lled adhesives on bonding resin composites to dentin has not been fully understood. Due to their ller content,lled adhesives may act as stress breakers. The aim of this in vitro study was to evaluate the useof owable composites of different viscositieson bonding to enamel and dentin without the use of an intermediate bonding resin.

Materials and methods: Enamel and dentin bond strengths of OptiBond FL, Syntac Classic, and EBS Multi combined either with theirproprietary bonding agent or a owable resin composite (Ultraseal XT Plus or Revolution) were measured. The tests were carried out with amicrotensile device at a crosshead speed of 1 mm/min after 24 h of storage at 37 8C in water. Mean bond strengths were analyzed using theWilcoxon test and multiple comparisons according to the Mann-Whitney U -test. The micro-morphology of corresponding resindentininterfaces of the same teeth were analyzed using SEM and TEM.

Results: The control groups with adhesive systems used as per manufacturers' protocol showed bond strengths of 38.941.1 MPa to

enamel and 28.833.4 MPa to dentin. With respect to bond strength to etched enamel, only Ultraseal XT Plus as bonding resin reached thelevel of the control groups. When used as bonding agents on dentin, both owable composites produced lower microtensile bond strengths toetched and primed dentin than did the control groups. Micro-morphological analysis using SEM and TEM resulted in hybrid layer formationfor both control and experimental groups. However, many areas of the resindentin interface showed insufcient penetration of the owablecomposites at the top of the hybrid layer as well as numerous tubules obstructed by ller particles.

Conclusions: The owable composites tested in this study should not be used to replace bonding agents. Flowable composites of thinnerviscosity, such as Ultraseal XT Plus, may bond to enamel adequately without the requirement of an intermediate bonding resin. q 2002Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved.

Keywords : Enamel bonding; Dentin bonding; Hybrid layer; Resin composites; Flowable composites; Filled adhesives

1. Introduction

The increasing attractiveness of tooth-colored restora-tions has promoted research in this particular area of operative dentistry during the last few years [13].For Class II restorations, resin-based restorative materialsare now being used instead of amalgam [4,5]. Due to thecharacteristic polymerization shrinkage of resin-basedcomposites, clinical success with composite restorative

materials is fundamentally dependent on effective anddurable adhesion to enamel and dentin [68].

Bonding to enamel is now accepted as clinically reliable,because acidic etchants, such as 3040% phosphoric acid, arecapable of creating microporosities on the enamel surfacewhich enable the penetration of polymerizable monomersto provide micromechanical retention [9,10]. However,dentin still remains an unpredictable substrate for adhesion.The wet tubular microstructure, combined with the highorganic content, is responsible for bonding to dentin beingfar more difcult to perform [1114]. Although differentapproaches of surface pre-treatment are available, acid-etching and subsequent penetration of reactive primermolecules into the decalcied dentin surface is the most

Dental Materials 18 (2002) 227238

dentalmaterials

0109-5641/02/$22.00 + 0.00 q 2002 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved.PII: S0109-5641(01)0 0040-9

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* Corresponding author. Tel.: 1 49-9131-853669; fax: 1 49-9131-853603.

E-mail address: [email protected] (R. Frankenberger).


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recommended [2,14,15]. While modern one-bottle adhesivescombine primer and bonding resin in one solution, conven-tional two-bottle systems rely on a low-viscosity primeracting exclusively for penetration and impregnation of thecollagen network exposed by acid etching [13,16,17]. Highvapor-pressure solvents, like acetone or ethanol, arecommonly used to facilitate the penetration and carry themonomers into a direct contact with collagen bers [12,1719]. The primer is followed by a hydrophobic bonding resindesigned for lling the interbrillar spaces as a backbone andmechanically stabilizing the established interface generatinga mixed zone of resin-entangled collagen brils, the hybridlayer [20]. The use of these hybridizing adhesive systems hasbeen reported to produce high bond strengths, primarily whenthe dentin was left moist instead of being dried after etchingand rinsing [6,12,18].

A modication was introduced when lled bondingresins, such as OptiBond FL (Kerr, Orange, CA, USA)and Permaquik (Ultradent Products, Inc., Salt Lake City,

UT, USA) were marketed [21,22]. After air thinning, thehigh viscosity of these adhesives results in thicker layers atboth the cavity oor and margin [6]. It has been repeatedlyreported to be promising regarding marginal and internalseal of resin composite restorations [2326].

This phenomenon characterized by a thicker adhesivelayer between restorative material and tooth substrateswas described as elastic cavity wall concept [26]. Usingunlled adhesives, extensive air thinning due to differentcavity geometries might result in thin layers which couldnot be polymerized due to the polymerization inhibition byoxygen [2,12]. Thicker layers of unlled resins may there-

fore be as advantageous for marginal adaptation as theapplication of lled adhesives, but it cannot be recom-mended clinically, because these materials usually provideno radiopacity [27]. Therefore, this modied use couldmislead clinicians to interprete the adhesive radiotran-sparency as gap formation or recurrent caries at the marginof the restoration [27].

Flowable resin composites have been reported to adaptwell to the cavity wall [5]. This optimal adaptation mayresult in an improvement of the adhesive performance of resin composites [5,28,29]. Unterbrink and Liebenbergsuggested using a owable resin composite as bondingagent to solve this problem by forming a strong and radio-paque material at the resindentin interface [30,31].

Due to the fact that lled adhesives and owablecomposites appear to have similar viscosities, this tech-nique may have a strong impact for the restoration of carious lesions with direct resin composites. Nevertheless,the effect of owable composites on bond strengths andinterfacial micro-morphology has not been fully studied.Therefore, the null hypothesis to be tested in this projectwas that owable composites would be as effective asunlled bonding resins when used as part of a bondingsystem, by using a microtensile bond strength test ( m -TBS) combined with scanning electron microscopy

(SEM) and transmission electron microscopy (TEM)evaluation.

2. Materials and methods

2.1. Specimen preparation and microtensile testing

A total of thirty six caries-free human third molars wereused in this investigation. The teeth were stored in 0.1%thymol solution at ambient temperature for less than 4weeks after extraction and were consecutively debridedand examined to ensure that they were free of defects. Forbond strength testing, the occlusal enamel was removed and700900 m m thick enameldentin disks were cut from themid-coronal level of the tooth, perpendicular to the toothaxis by slow-speed diamond-saw sectioning (Isomet,Buehler, Lake Bluff, IL, USA) under continuous water cool-ing. A standardized smear layer was created in both corre-

sponding surfaces by wet-sanding with 600-grit sandpaperfor 60 s [17]. Eighteen teeth were randomly assigned tothree dentin adhesives. Further, the teeth were subdividedinto three groups with a uid resin and two owable resincomposites acting as bonding agent. Both surfaces weretreated with the adhesive systems (Table 1) OptiBond FL(Kerr), Syntac Classic (Vivadent, Schaan, Principality of Liechtenstein), and EBS Multi (ESPE, Seefeld, Germany)according to the manufacturer's instructions except forSyntac classic which was applied upon etching dentin for15 s like alternatively recommended in the manufacturer'sprotocol. The experimental groups were treated with the

primers like in the control groups but afterwards bondedwith the owable resin composites Ultraseal XT Plus (Ultra-dent Products, Inc., Salt Lake City, UT, USA) and Revolu-tion (Kerr, Orange, CA, USA) replacing the manufacturers'bonding resins and scrubbed continuously for 15 s onto theprimed surface. The owable composites acting as bondingagent were thinned with a brush but not air-thinned andconsecutively light-cured for 40 s. The crowns of the at-tened teeth were then reconstructed with four 1 mm-layersof a minilled hybrid resin composite (Tetric Ceram,Vivadent) each layer being light-cured for 40 s with anElipar Highlight curing unit (without softstart). The intens-ity of the light was checked periodically with a radiometer(Demetron/Kerr, Danbury, CT, USA) to ensure that anintensity 400 mW/cm 2 was exceeded. The disks made formicromorphological investigations were pretreated simul-taneously and covered with resin composite in one layerof 1 mm thickness, then cut in half and stored in distilledwater for making the SEM and TEM specimens (Fig. 1).

The m -TBS specimens were stored in distilled water for24 h at 378C and then sectioned. The peripheral areas of thetooth were removed resulting in a 5 5 mm square centralpart. The remaining specimen was sectioned into ve slices,which were sectioned again to receive 25 resindentinbeams. The saw was adjusted to steps of 1 mm, due to the

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Fig. 1. (a) Set up of the study divided in SEM/TEM investigation and m -TBS measurement. (b) Set up for m -TBS measurement on attened enamel.

Table 1Investigated adhesive systems, owable resin composites replacing the bonding resins, and manufacturers

Adhesivesystem (code)

Etchant Primer Bonding resin Manufacturer

OptiBond FL Kerr Etchant OptiBond Primer OptiBond Adhesive Kerr, Orange,CA, USA

37.5% phosphoric acid HEMA, GPDM, MMEP, ethanol,water, initiators Bis-GMA, HEMA, GPDM, barium-aluminum borosilicate glass, disodiumhexauorosilicate, fumed silica(total 48% ller)Ultraseal XT Plus (Ultradent)Bis-GMA, TEGDMA, 53% llerRevolution (Kerr)Bis-GMA, TEGDMA, 53% ller

Syntac classic Total Etch Syntac classic primer Heliobond Vivadent, Schaan,Liechtenstein

36% phosphoric acid Maleic acid, TEGDMA, water, acetone Bis-GMA, TEGDMA, UDMASyntac classic adhesive Ultraseal XT Plus(2nd primer) RevolutionPEGDMA, glutaraldehyde, water

EBS Multi MiniTip Etch Gel EBS Multi Primer EBS Multi Bond ESPE, Seefeld,

Germany32% phosphoric acid HEMA, MMC, water Bis-GMA, HEMA, TEGDMA, MAM

Ultraseal XT PlusRevolution


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thickness of the blade (300 m m) resulting in sticks with across-sectional area of 700 700 m m (0.5 mm 2). From theresulting 25 central sticks of each tooth, ten were selectedrevealing a distance to the pulp of 2.0 ^ 0.5 mm resulting in20 microtensile specimens for each group ( n 20; Fig. 1a).

For evaluating enamel bond strength, 18 third molarswere cut longitudinally and the buccal and lingual aspectsof the teeth were attened on an area of 4 4 mm withoutdentin exposure using 600 grit sandpaper under continuouswater cooling. Then the same specimen preparation proce-dure was carried out like in the dentin specimens withoutrubbing of adhesive components. From the resulting 64sticks of each tooth, ten beams with an enamel thicknessof 0.5 ^ 0.1 mm were selected resulting in 20 microtensilebeams in each group ( n 20; Fig. 1b). For the simulation of mixed cavities, also during enamel bonding procedures theprimers were applied according to the manufacturer'srecommendations for use.

The sticks were mounted in a Bencor Multi-T device

(Danville Engineering, Danville, CA, USA) with a specialcyanoacrylate glue (Zapit, Dental Ventures of America,Corona, CA, USA) and debonded using an universal testingmachine model 4411 (Instron Co., Canton, MA, USA) witha 50-N load cell traveling at a crosshead speed of 1 mm/min.m -TBS was determined by computing the quotient of maxi-mum load (N) and adhesion area. After the m -TBS test, thedentin sides of the fractured interfaces were analyzed undera stereo light microscope at 40 magnication to deter-mine the fracture mode.

2.2. Statistical analysis

The statistical analysis was performed using SPSS 8.0 forWindows (SPSS Inc., Chicago, IL, USA). The mean bondstrength and margin analysis were non-normally distributed(as shown by the Kolmogorov-Smirnov test). Therefore,non-parametric tests were used to compute differencesbetween dependent groups (Wilcoxon matched-pairs signed-ranks test) and among independent groups (Mann-WhitneyU -test, correction method according to Bonferroni-Holm)for pairwise comparisons at the 0.05 level of signicance(a ). To assess the inuence of the different bonding resins,the levels of signicance were adjusted to a * 1-(1 2 a )1/ k

(k number of performed pairwise tests) [3].

2.3. SEM analysis

The disk halves for the resindentin interfacial SEMinvestigation ( n 2) were xed in 2% paraformaldehyde,dehydrated in ascending concentrations of ethanol (507095100%) for 1 h, immersed in hexamethyldisilazane(HMDS, Ted Pella, Redding, CA, USA) and nallyembedded in epoxy resin (Epo-Thin, Buehler) exposingthe adhesive interfaces of the middle of the tooth. Theexposed interfaces were polished with wet silicon carbidepaper of decreasing abrasiveness (up to 1200 grit) and softtissue with increasingly ne diamond suspension (Buehler

Inc.) to a particle size of 1 m m. Finally the blocks weretreated with 6N hydrochloride acid for 30 s followed by a10 min immersion in 2.5% sodium hypochlorite to enablethe visualization of a three-dimensional interface relief.

One further specimen for each enamel group was madeby etching attened buccal aspects of third molars, applyingthe same protocol as in the m -TBS study and dissolving thethinned enamel by use of HCl for 3 days resulting in resinreplicas of the etch pattern. The nished SEM specimenswere mounted on aluminum stubs (Ted Pella) using adhe-sive carbon sticks and a quick drying silver paint (TedPella).

The specimens were sputter-coated (E 5100, PolaronInstruments Inc., VG Microtech, Watford, England),mounted on aluminum specimen holders to be observedunder a SEM (4000 magnication at 15 kV, JSM6300 V scanning electron microscope, JEOL, Peabody,MA, USA). Furthermore the thickness of the adhesive/ow-able composite layer above the hybrid layer was measured

on ten points in the dentin specimens (TiffMes 1.9, Univer-sity of Erlangen).

2.4. TEM analysis

For TEM evaluation of the resindentin interfaces, theresin composite layers of the corresponding disk halveswere thinned and six small disks with a cross-section areaof 1.01.5 mm 2 were cut from the central area using a slow-speed diamond saw under continuous water cooling, toexpose the opposite sides of the interfaces viewed by SEM.

Three of the resulting six sticks in each group were then

demineralized in 10% buffered EDTA (pH 7.4) for 72 h andconsecutively xed using 2.5% glutaraldehyde and 2%paraformaldehyde in 0.1 M sodium cacodylate buffer atpH 7.4 for 12 h at 48C. After the xation process, the speci-mens were rinsed with 0.1 M sodium cacodylate buffer atpH 7.4 for 2 h at 48C. Post-xation was performed with 2%osmium tetroxide for 1 h followed by washing in 0.1 Msodium cacodylate buffer at pH 7.4 for 1 h. After rinsingin deionized water for 20 min, both the demineralized andthe non-demineralized sticks were dehydrated in ascendinggrades of ethanol in analogy to the SEM specimens. Thespecimens were then immersed in propylene oxide for20 min and embedded in a 50/50 mix of propylene oxideand epoxy resin (Eponate 12, Ted Pella) in a rotator (PelcoInltrator, Ted Pella) for 6 h at 6 rpm. Finally, the speci-mens were immersed in 100% epoxy resin and set undervacuum for 12 h for resin inltration into the sticks. Afterpositioning of the specimens in rubber molds, the stickswere embedded in fresh resin exposing the central aspectsof the interfaces and dried in an oven for 12 h at 65 8C. Theblocks were sectioned in an ultra-microtome (MT 2-B Ultra-microtome, Ivan Sorvall Inc., Norwalk, CT, USA) equippedwith a material science diamond knife (Micro Star Tech-nologies Inc., Huntsville, TX, USA). The demineralizedsections were additionally stained with 2% uranyl acetate



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for 10 min and 3% lead citrate for 5 min. After drying atroom temperature, the sections were analyzed using a CM-12 LaB 6 Transmission Electron Microscope (Philips Elec-tronic Instruments, Mahwah, NJ, USA) at an acceleratingvoltage of 100 kV.

3. Results

An overview of the results is shown in Tables 2 and 3.Both owable resin composites resulted in lower meandentin m -TBS when used as bonding resin (11.615.9 MPa for Ultraseal XT Plus vs 7.113.3 MPa forRevolution; Table 2) compared with the control groups(28.833.4 MPa; P , 0.05). The different adhesive systemstested exhibited no signicant differences in mean m -TBS(P . 0.05). The failure modes were exclusively adhesivewithin the resindentin interface.

For enamel m -TBS, the control groups exhibited no statis-tical differences and were between 38.9 and 41.1 MPa(Table 3). The use of Ultraseal XT Plus as a bonding resinresulted in similar mean m -TBS (35.440.6 MPa) to the

control groups ( P . 0.05). The application of Revolutionwithout an intermediate bonding resin resulted in statisti-cally lower mean m -TBS to etched enamel (20.824.6 MPa;P , 0.05) than did the original bonding agents and UltrasealXT Plus. The failure modes detected in the enamel groupswere adhesive between resin and enamel for the experi-mental groups. Within the control groups, cohesive failuresin resin were detected (OptiBond FL: 20%, Syntac Classic:10%, EBS Multi: 30%) (Table 3).

SEM analysis of the dentin specimens showed a well-dened interface of the control groups (Figs. 2a, 3a, 4a),but also penetration of the owable composites into super-cially acid-demineralized and primed dentin, because ahybrid layer of similar thickness as in the control groupswas clearly evident (Figs. 2b,c, 3b,c, 4b,c). However, closerinvestigation exhibited distinct zones of ller accumulation

(Fig. 2c) as well as insufcient hybridization of the peritub-ular and intertubular dentin at the top of the hybrid layer(Figs. 2b, 3b,c, 4b,c). The resin replicas of etched enamelexhibited a less pronounced formation of resin tags extend-ing into acid-conditioned enamel when Revolution was usedfor bonding (asterisks in Fig. 5c) compared with UltrasealXT Plus and the control specimens (Fig. 5a,b).

The mean layer thicknesses of the adhesives in the controlgroup have been 1.7 ^ 1.6 m m for EBS Multi, 3.2 ^ 1.9 m mfor Syntac Classic, and 13.3 ^ 5.7 m m for OptiBond FL.

The layer thicknesses of the owable composites exhibitedno statistically signicant difference and have therefore beenpooled revealing a mean thickness of 101.8 ^ 44.9 m m forUltraseal XT Plus and 173.3 ^ 57.1 m m for Revolution. Allgroups have been signicantly different from each other(layer thickness: EBS , Syntac , OptiBond , Ultraseal ,Revolution; P , 0.05).

TEM investigation revealed total-etch interfaces for thecontrol groups (Fig. 6a) as provided by the literatureand also penetration of the owable composites into acid-demineralized dentin (Fig. 6b,c). In analogy to the SEMobservations, ller accumulation near the top of the hybrid

layer was clearly evident in the TEM (Fig. 6b,c). Fillerparticles of the diameter of the dentinal tubule entranceoften tended to form plugs and consequently insufcientresin tag formation (Fig. 6b,c) like displayed in the SEMgures (Figs. 2b, 4b,c).

4. Discussion

The objective of this in vitro study was to clarify whetherowable resin composites could be used as lled adhesivesfor bonding of resin composites. The method utilized forbond strength testing was the microtensile bond testreported to be well-suited for the evaluation of bondstrengths to enamel and dentin [3236]. This methodologyprovides the possibility to investigate interfacial bond


Table 2Results of dentin microtensile bond strength [MPa] (SD) for different restorative combinations. Groups with same superscript letters are not statisticallydifferent (Wilcoxon and Mann-Whitney U -test, P . 0.05)

Adhesive system Manufacturer's adhesive Ultraseal XT Plus Revolution

OptiBond FL 33.4 (7.2) A 15.9 (3.4) B 13.3 (4.0) B

Syntac classic 28.8 (8.9) A 11.6 (3.5) B 7.1 (4.1) C

EBS Multi 29.5 (7.7)A

12.5 (5.0)B

8.0 (3.0)C

Table 3Results of enamel microtensile bond strength [MPa] (SD) and percentages of cohesive failures in enamel for different restorative combinations. Groups withsame superscript letters are not statistically different (Wilcoxon and Mann-Whitney U -test, P . 0.05)

Adhesive system Manufacturer's adhesive Ultraseal XT Plus Revolution

OptiBond FL 40.4 (8.6) A 20% 37.4 (7.1) A 0% 24.6 (5.9) B 0%Syntac classic 38.9 (9.2) A 10% 35.3 (6.0) A 0% 25.1 (4.5) B 0%EBS multi 41.1 (10.1) A 30% 40.6 (9.2) A 0% 20.8 (5.8) B 0%


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strengths on small areas below 1 mm 2. Conventional shearand tensile bonding procedures reveal much larger bondingareas with diameters between 3 and 5 mm [12,33]. Whendentin is used as a bonding substrate in combination withrecently developed adhesives reaching bond strengths over15 MPa, these tests often produce non-uniform stressesresulting in pulling out dentin [3739]. The observationsof the present study indicate that this problem can beprevented with microtensile testing because only adhesivefailures were observed for the dentin groups. This conrmsthe ndings of other studies dealing with microtensile test-ing [3235]. As shown by Shono et al. in recent investiga-

tions using the same methodology used in the present study,bonding to attened dentin surfaces at variable distancesfrom the pulp, results in regional differences in dentinbond strength [40,41]. Therefore, only the very centralareas of the specimens were used to obtain a reliable ran-domization of test specimens. Compared with the results of Shono et al. having involved 50 specimens of differentocclusal regions per group, the present standard deviationsare in a comparable range [41]. This consistency demon-strates the reliability of the modied methodology using 20central specimens in each group providing equal distancesfrom the bonded surface to the pulp chamber. According to


Fig. 2. Scanning electron micrographs of resindentin interfaces bonded with OptiBond FL. (nal magnication: 4000 ). (a) (as per manufacturer'sinstructions. The lled adhesive (A) penetrates well into the dentinal tubules forming resin tags (T) and various lateral branches (asterisks). The hybridlayer (H) is approximately 4 m m thick (regionbetween arrowheads). (b) Specimen etched with phosphoric acid, primed with OptiBondPrimer and bonded withUltraseal XT Plus replacing the OptiBond adhesive (A). Note the porous upper half (hand) of the hybrid layer (H, region between arrowheads). The resin tags(T) are approximately 10 m m long and show thinendings (asterisks). (c) Specimen etched with phosphoric acid,primed with OptiBondPrimer and bonded withRevolution instead of the OptiBond adhesive (A). The resin tags (T) show lateral branches (asterisk), a hybrid layer (H, between arrowheads) is clearly evident.Note the accumulation of llers and polishing particles at the top of the hybrid layer (right hand) and the gap (left hand) resulting from insufcient peritubularpenetration at the tubule entrance.


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the suggestion of Pashley et al., statistics have been carriedout with n 20 as number of specimens under investigationper experimental group [35,35].

Variation of the bonding substrate is less evident in enamelbonding procedures. Nevertheless, the lingual and buccalaspects of the teeth were attened in order to achieve similarbonding areas. Due to the method involving sticks with across-sectional area of 0.5 mm 2, the recorded values forboth enamel and dentin bonding are higher compared withconventional shear or tensile studies [17,32,33,41]. For theevaluation of enamel bond strengths, the primers were alsoapplied to simulate the situation in cavities, when dentin andenamel are treated simultaneously, but without rubbing,becausedetrimentaleffects of scrubbing motions during appli-cation on enamel have been reported in the literature [42].

The bond strength investigation was accompanied withmicro-morphological analyses of the resindentin interface.SEM analysis of the interfaces displays the resin-side of theinterface. TEM evaluation of both demineralized and non-demineralized specimens represents a complement of theinvestigation at an ultra-morphological level of thepreviously tested interfacial characteristics [2,43].

The bonding systems used different priming philosophiesaccording to different solvents. OptiBond FL primercontains water and ethanol, Syntac Classic contains acetoneand water, and EBS Multi contains water as solvent. Syntacclassic was used with phosphoric acid etching despite beinga self-etching primer when used on dentin [44,45]. As repre-sentative owable resin composites two materials wereselected because of their different viscosities [46] and


Fig. 3. Scanning electron micrographs of interfaces bonded with Syntac Classic (nal magnication: 4000 ). (a) as per manufacturer's instructions. Theinterface shows the adhesive (A), hybrid layer (H, between arrowheads), resin tags (T) and various lateral branches (asterisks). (b) Specimen etched withphosphoric acid, primed with Syntac Classic Primer/Adhesive and bonded with Ultraseal XT Plus as adhesive instead of the Heliobond adhesive. Note the gap(asterisks) between hybrid layer (H) and owable resin composite acting as adhesive resin (A). T: resin tags. (c) Specimen etched with phosphoric acid, primedwith Syntac Classic Primer/Adhesive and bonded with Revolution instead of the Heliobond adhesive. Between the top of the hybrid layer (H) and the owableresin acting as adhesive (A) a distinct gap is detectable (asterisks). Multiple voids (small asterisks) are visible within the applied owable resin compositeforming the tags (T).


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similar radiopacity [47]. Therefore, Ultraseal XT Plus (ow:534 mm 2 /30 s at 0.5 MPa [46]) was used as thinner materialand Revolution (ow: 243 mm 2) was used providing ahigher viscosity although it is equally lled (both materials53% by weight [46]) like Ultraseal XT Plus. This way, theeffect of the consistency was also investigated as there arecharacteristic differences among the numerous owableresin composites on the market.

Another important point when discussing bond strengthsto dentin is the thickness of the bonding agent[13,17,21,23,26]. The evaluation of the SEM specimens,however, clearly demonstrated that thicker layers did notautomatically lead to higher microtensile bond strengthresults. In contrast, the owable composites revealed thethickest layers (up to 173 m m due to not possible air-thinning) and lowest dentin bond strengths. However, the

thicker adhesive layers in the OptiBond FL control groupsmay have contributed to slightly higher dentin bondstrengths, but this particular difference was not statisticallysignicant (P . 0.05; Table 2).

Unterbrink and Liebenberg describe the use of SyntacSingle Component with the owable resin composite TetricFlow [31]. However, the adhesive system Syntac SingleComponent is a one-bottle adhesive combining features of primer and bonding resin (which is light-cured), leading tohybridization itself. To act as a real lled adhesive likeOptiBond FL, the dentin should have been acid-etchedand primed with a primer of a two-bottle system (which isnot light-cured). The owable resin applied after primingwould play the role of a bonding resin. However, due to itshigher viscosity compared with unlled resins, OptiBondFL adhesive may behave as a owable resin composite.


Fig. 4. Scanning electron micrographs of interfaces bonded with EBS Multi (nal magnication: 4000 ). (a) as per manufacturer's instructions. The hybridlayer (H) is approximately 4 m m thick (arrowheads), the resin tags (T) exhibit various lateral branches (asterisks). Due to the thin layer of adhesive, the resincomposite (RC) has contact to the hybrid layer (H). (b) Specimen etched with phosphoric acid, primed with EBS Multi Primer and bonded with Ultraseal XTPlus replacing the EBS Bond. Note the short resin tags (T) and the gap formations at the top of the hybrid layer (H, asterisk) and peritubular at the tubuleentrances (asterisks). (c) Specimen etched with phosphoric acid, primed with EBS Multi Primer and bonded with Revolution as Bonding Agent (A). Note thepartially short resin tags (T, lower asterisk) and the gaps (upper asterisk) at the top of the hybrid layer (H, between arrowheads).


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Therefore, a owable resin composite may have the capabil-ity to act in the same way as a lled adhesive replacing thebonding agent of two-bottle systems. The results of thepresent study clearly demonstrate that the owable compo-sites tested are not capable of hybridizing etched and primeddentin as efciently as the commercially available bondingagents recommended by the manufacturers.

In contrast, bonding to enamel shows far less complexity-only the penetration into the porosities of acid-etched enamelis an prerequisite for successful adhesion [9,10]. The micro-tensile bond strength results reveal that Ultraseal XT Plusmay act as an enamel bonding agent due to its lower viscos-ity. The viscosity represented by the material Revolutionseems to be too high for complete penetration into etchedenamel, as demonstrated by the signicantly lower bond

strengths. Although there is not much noticeable differencebetween the viscosity of the lled adhesive OptiBond FL andthe owable composites during application, there seems toexist a borderline of viscosity avoiding penetration intoetched and primed dentin. This point of view is supportedby the fact that the lled one-bottle adhesive OptiBond Solo(Kerr, Orange, CA, USA) is recently marketed in a lowerlled formula by the manufacturer. A further explanationfor the differences in bond strength could be the differentchemical composition of the lled adhesive vs the owablesmarketed primarily as restorative materials.

Penetration of liquids into narrow capillaries, such as themicroporosities of etched enamel, is inuenced by proper-ties of the liquid, such as viscosity and the surface freeenergy of the capillary wall [48]. Additionally, viscosity


Fig. 5. Resin replicas of composite bonded with EBS Multi (nal magnication 5000 ). (a) as per manufacturer's instructions. The microporosities andchannels provided by acid etching are poured out. (b) Specimen etched with phosphoric acid, primed with EBS Multi Primer and bonded with Ultraseal XTPlus replacing the EBS Bond. Also all structures of etched enamel are indirectly visible. (c) Specimen etched with phosphoric acid, primed with EBS MultiPrimer and bonded with Revolution as Bonding Agent. Distinct ller accumulation prevents the complete penetration of the owable composite into the acid-conditioned enamel on the left resulting in short resin tags providing less retetion (asterisks).


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of the restorative resin has been described as a parameterthat inuences the penetration of restorative resins into acid-etched enamel [49].

Depth of resin penetration into enamel decreases onlyslightly with increasing viscosity [50]. Several studieshave reported that, when applied to acid-etched enamel,there are no signicant differences between low-viscosityresins and high-viscosity resins in terms of adaptation anddepth of penetration [5052]. These studies were performedby comparing early-generation high-viscosity compositeresins with low-viscosity unlled resins. In fact, compositeresins seem to adapt themselves to etched enamel surfacesequally well as do low-viscosity resins [51].

SEM and TEM evaluations corroborate the bond strengthresults on a micro-morphological level. The SEM speci-mens show an acid resistant hybrid layer formation in thegroups with owable resin composites acting as lled adhe-sive resins. The detection of accumulated llers as well aspolishing particles under the SEM alone would not primar-ily be suspicious for inferior bonding to dentin, and incom-plete resin penetration may not be visible due to the acidtreatment during the specimen production using hydro-chloric acid and sodium hypochlorite. The distinct gapformations at the top of the hybrid layer, however, indicatethe inferior dentin bonding perfomance of the experimentalgroups (Figs. 2b, 3b,c, 4b,c). The additional TEM analysis


Fig. 6. Transmission electron micrographs of interfaces bonded with OptiBond FL. (a) As per manufacturer's instructions (non-decalcied specimen, nalmagnication: 6300 ). H: hybrid layer; Note the complete penetration of the hybrid layer (H) and the `shag carpet'-extension of collagen bers into theadhesive layer (black hand). The small llers of the lled adhesive penetrate to the entrances of lateral branches (white hand). (b) With Ultraseal XT Plusreplacing the EBS Bond (non-decalcied specimen, magnication 6,300 ). Note the ller particle obstructing the tubule entrance (white hand) and resulting inshort tag formation (black hand). This corresponds to the observations made in Fig. 4b. H: Hybrid layer. (c) With Revolution as bonding agent (decalciedspecimen, magnication 5000 ). Note the ller particle clogging the dentinal tubule (arrow) and preventing complete resin tag formation (asterisk) andhybridization (H) of the demineralized peritubular dentin. The arrow shows a slight `shag carpet' appearance of the top of the hybrid layer resulting fromscrubbing the owable resin composite.


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proved to be an important method to look closer at themechanisms of dentin hybridization. These more detailedviews received by TEM clearly indicate an insufcient tubu-lar penetration of the owables applied after etching andpriming (Fig. 6b,c).

Due to the similar results for the differently viscous ow-able resin composites when used as lled adhesives, themicroscopic results conrm the hypothesis that viscosityalone may not be the only crucial factor for the inferiorbonding behavior of the owables. The chemical composi-tion such as the addition of HEMA to unlled resins (as anhydrophilic agent) as well as the ller size and ller distri-bution in lled resins could be another factor for the stillsuperior bonding performance of the adhesive resins testedas controls.

The resin replicas of enamel produced for SEM investi-gation show less pronounced penetration of the owableRevolution into acid-conditioned enamel when comparedwith Ultraseal XT Plus or the bonding resins of the control

groups (Fig. 5). This explains the less predictable resultsobtained with the more viscous material Revolution actingas lled adhesive. However, only in the control groupscohesive failures in resin occurred, while in the groupsbonded with owables adhesive failures were observedexclusively (Table 3). Nevertheless, bond strength resultsclearly indicate that Ultraseal XT Plus bonded equallycompared to the control groups.

5. Conclusions

The owable composites tested in this study do not fulllthe requirements to act as lled dentin adhesives.

Both owable composites were unable to fully hybridizeetched and primed dentin.

The owable composite Ultraseal XT Plus may be usedas enamel bonding resin without an intermediary bondingagent.

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