Adhesion of Endodontic Sealers: Scanning ElectronMicroscopy and Energy Dispersive Spectroscopy

Iman M. Saleh, BDS, MSc, I. Eystein Ruyter, Dr. Rer. Nat., Dr. Philos., Markus P. Haapasalo, DDS, PhD,and Dag Ørstavik, DDS, PhD

The microscopic details of the debonded inter-faces between endodontic sealers and dentin orgutta-percha were assessed in this study. Dentin,conditioned with 37% H3PO4 for 30 s, 25% citricacid for 30 s, 17% EDTA for 5 min, or a rinse with 10ml of distilled H2O (control), and gutta-percha sur-faces were coated with freshly mixed sealer:Grossman’s sealer, Apexit, Ketac-Endo, AH Plus,RoekoSeal Automix, or RoekoSeal Automix with anexperimental primer. The surfaces were pressedtogether and the sealers were allowed to set. Aftertensile bond strength testing, the morphologicalaspects of the fractured surfaces were assessedby scanning electron microscopy and energy dis-persive spectroscopy. The energy dispersive spec-troscopy successfully traced sealer componentsto the debonded surfaces. Some of the sealerspenetrated into the dentinal tubules when the den-tin surface had been pretreated with acids. How-ever, these sealer tags remained occluding the tu-bules after bond failure in some instances only(Grossman’s sealer, RoekoSeal Automix with anexperimental primer, AH Plus/EDTA). Penetrationof the endodontic sealers into the dentinal tubuleswhen the smear layer was removed was not asso-ciated with higher bond strength.

The standard method of obturation of the root canal system is byusing a core material in combination with a root canal sealer. Adesirable property of the root canal sealer is to have adhesivestrength both to the dentin and to the core material, which isusually gutta-percha. The sealer also must have cohesive strengthto hold the obturation together (1). The ability of root canal sealersto adhere to dentin and gutta-percha may be expected to result insuperior sealing ability, which should reduce leakage in clinicalsituations. Adhesion also should improve the stability of the rootfilling, e.g. during preparation for post space.

The role of the smear layer in endodontics has been the subjectof considerable debate since it was first described in 1975 (2). The

smear layer was found to prevent the penetration of root canalsealers inside the dentinal tubules (3, 4). Therefore, its removal hasbeen recommended to improve the obturation seal (5, 6) and reducecoronal leakage (7). On the other hand, presence or absence of thesmear layer was found to have no significant effect on the apicalseal (8). In a previous study (9) we also did not find improved bondstrength of the various sealers after removal of the smear layer.

Documenting the mode of failure of sealers to different dentinsurfaces and gutta-percha might improve our understanding of therole of the smear layer. The purpose of this investigation was2-fold: (a) to assess the microscopic details of the morphologicalaspects of the debonded surfaces by scanning electron microscopy(SEM); and (b) to identify elements of the sealers on the dentin andgutta-percha surfaces by energy dispersive spectroscopy (EDS)after debonding.

MATERIALS AND METHODS

Human single-rooted teeth, stored in 0.01% NaOCl at 4°C afterextraction, were used in this study. Root dentin cylinders, 4 mm indiameter, were cut in a buccolingual direction at a right angle to thetooth’s long axis. The cylinders were mounted in brass holders andground flat against 500-grit silicon carbide abrasive paper (Struers,Copenhagen, Denmark). Gutta-percha cylinders of 4-mm diameterwere prepared from heat-softened gutta-percha (Roeko, Langenau,Germany). The cylinders were mounted in brass holders and theirend surfaces ground flat in a similar manner to the dentin cylinders.

The dentin specimens were randomly divided into four equalgroups and their surfaces conditioned with 37% H3PO4 for 30 s,25% citric acid for 30 s, 17% EDTA for 5 min, or 10 ml of distilledH2O (control). The conditioned dentin surfaces were then rinsedwith 10 ml of distilled H2O and dried with an air stream for 5 s.Each group of conditioned specimens were further divided into sixequal subgroups (n � 4), according to the type of sealer used.

For the Grossman’s sealer (GS)/phosphoric acid subgroup, thepH of the acid-treated dentin surface was measured using pH-indicator strips (Merck KGaA, Darmstadt, Germany) after rinsingthe surface with 10 ml of distilled H2O.

Five sealers of different chemical composition were tested (Ta-ble 1). An experimental primer supplied with RoekoSeal Automix(RS) also was investigated. The procedures for testing were pre-viously described (9). Briefly, the dentin and gutta-percha surfaceswere coated with a thin layer of the freshly mixed sealer and the

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cylinders immediately pressed together. The test specimens werekept in an incubator at 37°C and a relative humidity of 90 � 5%.The sealers were allowed to set for 1.5 times the manufacturer’sstated setting time. Tensile bond strength was measured in auniversal testing machine (Instron, Instron Limited, Bucks, UK).

The debonded dentin and gutta-percha specimens were sepa-rated from their brass holders and air-dried. Two specimens fromeach subgroup were selected for SEM and EDS examination. Eachspecimen was mounted on an aluminium stub and sputter-coatedwith carbon in a Balzers SCD 050 coating unit (Balzers AG,Balzers, Liechtenstein). The specimens were first observed undera scanning electron microscope (Philips XL 30, Eindhoven, TheNetherlands) at an operating voltage of 10 kV and photographed ata magnification of �200, �800, and �2000. The debonded sur-faces were examined to indicate the mode of failure: adhesive (atthe dentin/sealer or gutta-percha/sealer interface), cohesive withinthe sealer, or a mixture of the two. The specimens were thenanalyzed by an energy-dispersive spectroscope (EDAX, DX-4-i,Mahwah, NJ) to identify elements of the sealers on the debondedsurfaces. X-ray intensities in counts per second were set at 100 andthe accelerating voltage at 20 kV.

Control root dentin discs were prepared in the same manner asthe experimental ones. Each was treated with one of the fourpretreatment solutions used in the study and processed in a similarmanner for SEM examination. Control discs were prepared foreach sealer, as well as for gutta-percha and dentin, and processedfor EDS analysis. Basic elements in each material were detectedand used as a reference for their presence on the fractured surfaces.

RESULTS

Control SEM Specimens

The control dentin specimen pretreated with distilled H2Oshowed the retained smear layer (with tracks from the carbidepaper polishing). The smear layer was completely removed withexposure of the underlying dentinal tubules in the control speci-mens pretreated with phosphoric or citric acids. No packed mate-rial was found in the tubule openings, whereas the specimenpretreated with EDTA showed evidence of smear plugs in thetubules.

Control EDS Specimens

The elements detected in each of the control specimens—den-tin, gutta-percha, and the different sealers—are shown in Table 2.

Experimental Specimens

For AH Plus (AH) sealer, debonded dentin surfaces showedareas of cohesive failure within the sealer as well as areas ofadhesive failure to dentin (Fig. 1a). In the adhesive failure area,EDS confirmed that particles remaining on the phosphoric acid-and citric acid-treated dentin were not from the sealer (Fig. 1b). Allgutta-percha surfaces showed remnants of the sealer, althoughmacroscopically no sealer residues could be detected (Fig. 1c). Zrand Si were detected by EDS analysis on gutta-percha surfaces thatappeared clean. Tags protruding from the sealer remaining ongutta-percha were observed where adhesive failure to dentin,which had been pretreated with phosphoric and citric acids, wasobvious (Fig. 1d). This was not the case when dentin was pre-treated with EDTA, where sealer tags occluded the dentinal tubulesas evident by EDS (Fig. 1, e and f).

For RS, SEM examination of the debonded surfaces revealed anadhesive failure to dentin treated with distilled H2O only (Fig. 2,a and b). The gutta-percha surface was completely covered by thesealer. The same mode of failure was observed when dentin waspretreated with phosphoric acid or EDTA. In this case the dentinaltubules were exposed with no sealer inside (Fig. 2c). EDS con-firmed that particles detected on the dentin surface were not fromthe sealer (Fig. 2d). On the citric acid-treated dentin the sealer waspartly detached from the surface (Fig. 2e). The undersurface of thesealer layer revealed Ca and P peaks (Fig. 2f). No sealer could beidentified in the dentinal tubules.

Examination of the distilled H2O-treated dentin surface, towhich a primer was applied before RS, showed complete coverageby the sealer indicating an adhesive failure to gutta-percha (Fig. 3,a and b). The failure was adhesive to both dentin and gutta-perchawhen the dentin was pretreated with phosphoric acid, citric acid,and EDTA. The dentin surface was partly covered by sealer rem-nants and partly showing exposed dentinal tubules (Fig. 3c). Zr andSi were detected in the material occluding some of the tubules (Fig.

TABLE 2. Elements detected by EDS for each of the materialsused

Material Elements

Dentin Ca, P, MgGutta-percha Zn, Ba, SAH Plus Ca, W, ZrApexit Ca, Bi, SiKetac-Endo Ca, Si, Al, FRoekoSeal Automix Si, ZrGrossman’s sealer Zn, Ba, S

TABLE 1. Root canal sealers tested

Sealer Code Chemical Composition Manufacturer Batch Number

Grossman’s sealer GS Zinc oxide-eugenol–based Prepared according toGrossman, NIOMlaboratory, Haslum, Norway

2

Apexit AP Calcium hydroxide-based Vivadent, Schaan,Liechtenstein

912697

Ketac-Endo KE Glass-ionomer–based Espe, Seefeld, Germany FW0055094AH Plus AH Resin-based Dentsply DeTrey GmbH,

Konstanz, Germany9810000713

RoekoSeal Automix RS Silicone-based Roeko, Langenau, Germany B239b/B240bRoekoSeal Automix � Primer RP Silicone-based Roeko, Langenau, Germany M496

596 Saleh et al. Journal of Endodontics

FIG 1. Debonded AH specimen showing both adhesive failure to dentin (AD) and cohesive failure within the sealer (C). (a) Scanning electron micrograph(�800) of dentin surface partly covered with sealer; citric acid pretreatment. (b) EDS analysis of area (white square in a). The area appears to be clear dentinwith complete absence of sealer elements. (c) Scanning electron micrograph (�200) of gutta-percha surface covered with a continuous layer of sealer; citricacid pretreatment. (d) Tags protruding from the sealer remaining on gutta-percha; phosphoric acid pretreatment (�800). (e) Sealer tags occluding the dentinaltubules; EDTA pretreatment (�800). (f) EDS of area (white cross in e) showing the presence of W and Zr in addition to sealer elements. The Zr peak isoverlapped by P.

FIG 2. Debonded RS specimen. (a) Scanning electron micrograph (�800) of dentin surface covered with smear layer; distilled H2O pretreatment. (b) EDSanalysis shows clear dentin with complete absence of sealer elements. (c) Scanning electron micrograph (�800) of dentin surface with exposed dentinaltubules and particles on the surface; phosphoric acid pretreatment. (d) EDS analysis of the particles reveals no sealer elements. (e) Scanning electronmicrograph (�800) of dentin surface with partly detached sealer; citric acid pretreatment. (f) EDS analysis of area (white cross) reveals Ca and P.

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FIG 3. Debonded RP specimen. (a) Scanning electron micrograph (�800) of dentin surface completely covered with sealer; distilled H2Opretreatment. (b) Scanning electron micrograph (�800) of gutta-percha surface; distilled H2O pretreatment. (c) Scanning electron micrograph(�800) of dentin surface partly covered with sealer. Open dentinal tubules with sealer tags; citric acid pretreatment. (d) EDS analysis of thecontent of the dentinal tubules revealed Si and Zr. (e) EDS analysis of area (white square in c) revealed Ca and P. (f) Scanning electronmicrograph (�2000) of gutta-percha surface covered with sealer. Tags were protruding from the sealer surface; citric acid pretreatment.

FIG 4. Debonded KE specimen. (a) Scanning electron micrograph (�800) of dentin surface completely covered with sealer; citric acidpretreatment. (b) Scanning electron micrograph (�200) of gutta-percha surface; citric acid pretreatment. (c) EDS analysis of gutta-perchasurface revealed traces of sealer. (d) Scanning electron micrograph (�800) of dentin surface partly covered with sealer. Open dentinal tubuleswith no sealer tags; EDTA pretreatment. (e) EDS analysis of area indicated (white square in d). The area appears to be clear dentin. (f) Scanningelectron micrograph (�800) of gutta-percha surface partly covered with sealer; EDTA pretreatment.

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3d). Some crystals, not sealer particles as confirmed by EDS, wereprecipitated on the phosphoric acid- and citric acid-treated dentinsurfaces (Fig. 3e). Sealer tags protruding from the detached sealersurface were occasionally seen. The gutta-percha surface was onlypartly covered by sealer remnants showing protruding tags (Fig.3f).

Except for EDTA-treatment, SEM examination of the debondeddentin surfaces revealed a continuous layer of Ketac-Endo (KE)sealer (Fig. 4a). The gutta-percha surface occasionally revealedsmall particles identified as KE by EDS (Fig. 4, b and c). Thesurface of dentin that had been pretreated with EDTA was onlypartly covered by KE (Fig. 4d). Open dentinal tubules were oth-erwise seen with no evidence of sealer in them. No peaks otherthan dentin elements were identified (Fig. 4e). The debondedgutta-percha surface was similarly partly covered by KE (Fig. 4f).

SEM examination of the debonded surfaces of GS revealed anadhesive failure to dentin that had been treated with distilled H2O.The gutta-percha surface was completely covered with the sealer,whereas a smear layer was covering the dentin surface. In case ofacid treatment, the failure was cohesive within the sealer andadhesive to dentin. The gutta-percha surface was completely cov-ered with the sealer showing protruding tags. Ca and P wereidentified in addition to the elements of GS (Fig. 5, a and b).Remnants of the sealer were partly covering the dentin surface andoccasionally occluding the exposed dentinal tubules (Fig. 5c). Znpeaks were always detected inside the tubules (Fig. 5d). In phos-phoric acid-treated dentin, Ca and P were detected in the thread-like particles remaining on the dentin surface (Fig. 5, e and f). ThepH of the phosphoric acid-treated dentin was measured to approx-imately 5.5 after rinsing with distilled H2O.

For Apexit (AP), a mixed type of failure, cohesive within thesealer and adhesive to dentin, was observed for all pretreatmentconditions. A continuous layer of the sealer covered the gutta-percha surfaces (Fig. 6a). The debonded dentin surfaces were onlypartly covered by the sealer (Fig. 6b). No sealer was found toocclude the exposed dentinal tubules. This was evident by the tagsthat projected from the surface of the sealer covering thegutta-percha.

DISCUSSION

This study is the sequel to an earlier investigation of the effectsof dentin pretreatment on the tensile bond strength of variousendodontic sealers (9). We found that the highest bond strengthsfor all sealers with the exception of GS were recorded when thesmear layer was not removed. Pretreatment of dentin with phos-phoric and citric acids resulted in the highest bond strength for GS.The SEM observations of the debonded surfaces were confirmedby EDS analysis, which provides qualitative identification of theelements present in a given SEM field.

The endodontic smear layer is known to form on the surface ofdentinal walls when the root canals are instrumented (2). Oksan etal. (3) and Kouvas et al. (4) observed that the smear layer ob-structed the penetration of sealer tags into the dentinal tubules. Thepresent study confirms these findings. SEM pictures revealed pen-etration of sealer tags only when the smear layer had been re-moved. Such mechanical interlocking, however, did not result inhigher bond strengths as demonstrated in the first part of this study(9). The highest bond strengths were in fact recorded when dentin

FIG 5. Debonded GS specimen. (a) Scanning electron micrograph (�800) of gutta-percha surface completely covered with sealer. Tags wereseen on the sealer surface; phosphoric acid pretreatment. (b) EDS analysis of sealer revealed Ca and P in addition to sealer elements. (c)Scanning electron micrograph (�800) of dentin surface partly covered with sealer. Sealer tags are seen inside the dentinal tubules; phosphoricacid pretreatment. (d) EDS analysis revealed Zn inside the dentinal tubules. (e) Scanning electron micrograph (�800) of dentin surface showingthread-like precipitations. (f) EDS analysis of the precipitated material revealed Ca and P.

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had been treated with distilled H2O in most instances. This wasinterpreted as an enhancement of adhesion of sealers to dentin inthe presence of the smear layer. The findings of Lalh et al. (10) alsosuggest an important role played by the smear layer in adhesion;they found that for glass-ionomer–based sealers higher bondstrengths were recorded when the smear layer was not removed byEDTA.

The smear layer, at low magnification, has a typical amorphousstructure and, at higher magnification, a globular pattern (11). Ithas been suggested that the bond that develops essentially occursbetween the material and the smear layer and not the underlying,mineralized dentin matrix. The strength of the bond may be de-termined and limited by the strength of the forces holding theglobules to each other and to the underlying dentin (11). The bondstrengths recorded in the case of distilled H2O-treated dentin werenot the same for all sealers, which suggests a different mechanismof bonding to the smear layer for the different sealers.

AH and AP had the same pattern of failure (adhesive to dentinand cohesive within the sealer), yet their bond strengths differedsignificantly. This difference can be attributed to the differentcohesive strengths of the two sealers. The low bond strength andthe adhesive failure to distilled H2O-treated dentin for GS reflectits weak bond to smear layer. However, the bond strength in-creased significantly after treatment of dentin with phosphoric andcitric acids, and the mode of failure was predominantly cohesivewithin the sealer. This may be related to the effects from the acidscausing precipitation of zinc phosphate aggregates at the interface,which is confirmed by the detection of Zn inside the dentinaltubules. It has been shown earlier that precipitation of aggregates

or crystals at the interface may increase bond strength (12). Ad-hesion between KE and the smear layer was strong enough so thatfailure occurred at the weakest adhesive bonds, which were formedat the sealer–gutta-percha interface. For RS, the mode of failurewas adhesive to dentin with all pretreatment conditions, yet thebond strength was higher in case of distilled H2O, which means astronger bond to smear layer than to smear-free dentin. This bondwas even stronger with the application of the primer, as demon-strated by the higher bond strength recorded and by the failuremode becoming adhesive to gutta-percha. This suggests a possibleinteraction of the primer with the smear layer resulting in improvedadhesion of RS.

In the present study, phosphoric and citric acids and EDTA wereused as dentin pretreatments. Pretreatment of dentin with phos-phoric acid or citric acid was found to be more effective thanEDTA in removing the smear layer (13).

The particles that were occasionally observed on the dentinsurface in the case of citric acid pretreatment and confirmed byEDS not to originate from the sealers are presumably a result of thecrystallization of the acid that had remained after final irrigation.Copious irrigation is necessary to avoid residual crystals, presum-ably of calcium citrate.

AH, AP, and GS were able to penetrate into the dentinal tubuleswhen the smear layer was removed. However, these sealer tagsremained occluding the tubules after bond failure in some instancesonly (GS, AH/EDTA). Otherwise the sealer together with the tagswas detached from the surface. There was no indication of pene-tration of KE into the dentinal tubules after EDTA pretreatment. Incases of pretreatment with phosphoric or citric acids, however, itwas not possible to assess the penetration inside the dentinaltubules, because the failure was adhesive to gutta-percha. Simi-larly, RS did not penetrate inside the tubules. For RoekoSealAutomix with an experimental primer (RP), the use of the primergreatly enhanced the penetration of the sealer into the tubules butwith no significant increase in bond strength. The opening of thetubules and the loss of the smear layer are apparently not favorableprocesses for improving adhesion. There may be at least twopossible explanations for this. The opened dentinal tubules may actas stress raisers, which would promote failure in the adhesive joint,and/or the smear layer and plugs are rich in calcium and phosphateand are potential sites for strong adhesive bonding (14).

The present study showed that not all sealers penetrated insidethe exposed dentinal tubules and that the bond strength was nothigher for all sealers that were able to penetrate inside the tubules.This confirms the suggestion that micromechanical retention bypenetration of sealer tags inside the tubules is not the only, andmaybe not an important, factor affecting adhesion of root canalsealers (3). Moreover, tubular penetration is clearly dependent onthe chemical and physical properties of the sealer (3). Adhesion ofsealers to dentin and gutta-percha is a complex process withseparate mechanisms operating for the various sealers and differentdentin surfaces.

An overall variability was observed among sealers for adhesiveproperties to dentin and gutta-percha. The mode of failure for AHand AP did not vary with the different dentin pretreatments,whereas for GS, KE, RS, and RP more than one type of failure wasobserved. Some of the sealers penetrated into the dentinal tubuleswhen the dentin surface had been pretreated with acids. However,these sealer tags remained occluding the tubules after bond failurein some instances only. The penetration of the sealer tags into thedentinal tubules was not associated with higher bond strengthvalues (9).

FIG 6. Debonded AP specimen/EDTA pretreatment showing bothadhesive failure to dentin (AD) and cohesive failure within the sealer(C). (a) Scanning electron micrograph (�800) of gutta-percha sur-face covered with a continuous layer of sealer. (b) Scanning electronmicrograph (�800) of dentin surface partly covered with sealer.

600 Saleh et al. Journal of Endodontics

Drs. Saleh, Ruyter, and Ørstavik are affiliated with the NIOM–ScandinavianInstitute of Dental Materials, Haslum, Norway. Drs. Saleh and Haapasalo areaffiliated with the Department of Endodontics, Faculty of Dentistry, Universityof Oslo, Oslo, Norway.

Address requests for reprints to Iman M. Saleh, NIOM–Scandinavian In-stitute of Dental Materials, P.O. Box 70, N-1305 Haslum, Norway.

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