Basic Research—Technology

Bonding of Self-adhesive (Self-etching) Root Canal Sealersto Radicular DentinBrian R. Babb, DMD,* Robert J. Loushine, DDS,* Thomas E. Bryan, BS,* Jason M. Ames, DMD,*

Mark S. Causey, BS,* Jongryul Kim, DMD, MS, PhD,†

Young Kyung Kim, DDS, PhD,‡

R.NormanWeller,DMD,MS,*DavidH.Pashley, DMD, PhD,§ andFranklinR.Tay, BDSc (Hons),PhD*,§

AbstractThe latest generation of methacrylate resin–based sealershas eliminated the use of separate self-etching primers byincorporating acidic resin monomers in the sealers torender them self-adhesive to dentin. This study examinedthe adhesive strengths, interfacial ultrastructure, andtracer penetration of a nonetching (EndoREZ; Ultradent,South Jordan, UT) and two self-adhesive methacrylateresin–based sealers (MetaSEAL; Parkell, Farmington,NY, and RealSeal SE; SybronEndo, Orange, CA) whenthey were applied to radicular dentin following the manu-facturers’ recommended use of EDTA as the active finalrinse. A modified push-out testing design was used toevaluate the dislodgement of core-free sealers. The mixedsealers were placed in dimensionally identical, artificiallycreated canal spaces prepared in the coronal, middle,and apical thirds of radicular dentin. After setting, eachsealer-filled cavity was subjected to compressive loadinguntil failure. Additional specimens were prepared fortransmission electron microscopy to examine the ultra-structure and nanoleakage within the sealer-radiculardentin interface. The two self-adhesive sealers MetaSEALand RealSeal SE exhibited higher push-out strengths thanthe nonetching sealer EndoREZ when EDTA was used asthe active final rinse. All three sealers showed a 1- to1.5-mm thick zone of partially demineralized dentin,with the EDTA dentin demineralization effect maskingthe true self-etching potential of MetaSEAL and RealSealSE. The true self-etching potential of self-adhesive sealersis a clinically important attribute that should be furtherinvestigated. Incomplete smear layer removal from theapical third of instrumented canal walls may jeopardizethe performance of self-adhesive sealers should they failto self-etch without the adjunctive use of calciumchelating irrigants. (J Endod 2009;35:578–582)

Key WordsAdhesion, dislocation resistance, EDTA, hybrid layer,nanoleakage, push-out test, root canal sealers

Interests in adhesive endodontics (1) have led to the introduction of three generations ofmethacrylate resin–based root canal sealers. EndoREZ (Ultradent, South Jordan, UT),

the first generation (2–4), uses nonacidic, hydrophilic resin monomers to enhancesealer penetration into dentinal tubules after the removal of canal wall smear layers(5, 6). The second generation (7–9) (eg, RealSeal; SybronEndo, Orange, CA) is techno-logically analogous to those resin-based luting cements that use separate self-etchingprimers (10, 11) before the application of flowable composites to the primed dentin.The use of self-etching primers reintroduces the concept of incorporating smear layerscreated by hand/rotary instruments in the sealer-dentin interface (12). Provided that theyare aggressive enough to etch through thick smear layers (13), the technique sensitivity ofbonding to root canals may be reduced when smear layers are inadvertently retained inthe apical third of instrumented canal walls.

The third generation of methacrylate resin–based sealers (eg, MetaSEAL; Parkell,Farmington, NY, and RealSeal SE) (14, 15) is comparable to self-adhesive resin lutingcements (16) in that both were designed with the intention of combining a self-etchingprimer and a moderately filled flowable composite into a single product. They representa milestone in bonding step reduction, in that acidic resin monomers that are originallyfound in dentin adhesive primers are now incorporated into the resin-based sealer/cement to render them self-adhesive to dentin substrates.

There have been concerns regarding the limited aggressiveness of self-adhesiveresin cements in creating micromechanical retention via dentin hybridization (17).This probably accounted for their weaker adhesive strengths and poorer marginalintegrity when compared with conventional resin cements that use etch-and-rinse orself-etch adhesives for bonding (18–20). In theory, the bonding mechanism of self-adhesive sealers is similar to self-adhesive resin cements. However, the latter areused on smear layer–covered dentin, whereas the former are presumably applied tosmear layer–depleted dentin after irrigation with EDTA. As EDTA demineralizes radic-ular dentin apart from removing smear layers (5, 6, 21), the adhesive mechanism ofself-adhesive sealers may be different from the limited dentin hybridization observedfor the self-adhesive resin cements. Thus, the objective of this study was to examinethe adhesive strengths, interfacial ultrastructure, and tracer penetration of two self-adhesive methacrylate resin–based sealers when they are applied to radicular dentinfollowing the manufacturers’ recommended use of EDTA as the active final rinse.

From the *Department of Endodontics, School of Dentistry, Medical College of Georgia, Augusta, GA; †Department of Conservative Dentistry, School of Dentistry,KyungHee University, Seoul, Korea; ‡Department of Conservative Dentistry, School of Dentistry, Kyungpook National University, Daegu, Korea; and §Department of OralBiology, School of Dentistry, Medical College of Georgia, Augusta, GA.

Supported by the Dental Research Center, School of Dentistry, Medical College of Georgia, Augusta, GA.Address requests for reprints to Dr Franklin R. Tay, Department of Endodontics, School of Dentistry, Medical College of Georgia, Augusta, GA 30912-1129. E-mail

address: ftay@mcg.edu.0099-2399/$0 - see front matter

Copyright ª 2009 American Association of Endodontists.doi:10.1016/j.joen.2009.01.005

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Materials and MethodsSimulated Canal Spaces

Forty-two human canine teeth were used in this study. A 0.90 �0.05-mm thick longitudinal tooth slice was prepared from each toothusing an Isomet saw (Buehler, Lake Bluff, IL) under water cooling.Simulated canal spaces were prepared in the coronal, middle, andapical thirds of the radicular dentin using the protocol introduced byHuffman et al (22). Briefly, a minidrill press was used to generate verti-cally oriented truncated cavities (Fig. 1A) that were enlarged with size40, 0.04 taper ProFile rotary instruments (Dentsply Tulsa, Tulsa, OK)to their D16 diameter. Each cavity had standardized diameters of 0.94mm and 1.04 mm along its top and base. The experimental designalso ensured that the artificial canal spaces were devoid of calcospher-ites (23) that could have augmented a sealer’s retention from the non-instrumented parts of a true root canal. Tooth slices were randomlydivided into three groups. For each sealer, 20 simulated canal spacesfrom 10 slabs were used for evaluating the push-out strengths at threeradicular dentin locations (N = 20).

Specimen PreparationTwo self-etching, dual-curable, methacrylate resin-based sealers

were investigated. MetaSEAL uses 4-methacryloyloxyethyltrimellitateanhydride, and RealSEAL SE uses a polymerizable methacrylate carbox-ylic acid/anhydride as the respective acidic resin monomer. EndoREZ,a dual-curable sealer that contains nonacidic diurethane dimethacrylateand triethyleneglycol dimethacrylate, was used as the ‘‘nonetch’’ sealerfor comparison.

Tooth slices were ultrasonicated in 6.15% sodium hypochlorite(NaOCl), 17% EDTA, and sterile distilled water for 2 minutes each toremove organic debris and smear layers. En masse cleaning ensuredthat potential differences in push-out strength at the three dentin loca-tions were not caused by inadequate cleaning of the apical dentin. Thecavities were bulk filled with sealer without a main thermoplastic corematerial, according to the method invented by Jainaen et al (24). Thesealer-filled, glass slide–covered cavities were stored in light-protectedhumidors for 3 days until the sealers had completely set in the self-curedmode, simulating the curing condition encountered in the middle andapical thirds of true root canals.

Push-out StrengthBonding of the set sealers to radicular dentin was evaluated with

a thin-slice push-out test design (25, 26) using a custom-built, light-illuminated, Plexiform push-out device (Fig. 1B). The use of high-inten-sity fiberoptic illumination greatly enhanced the alignment of a 0.7-mmdiameter plunger with the inverted truncated cavities, with a 0.1-mmclearance from either side of the dentinal wall (Fig. 1C). The fiberopticillumination ensures that the sealer may be dislodged into the under-lying cylindrical well without the plunger touching the cavity walls.Each sealer-filled cavity was subjected to compressive loading via a Vi-trodyne universal testing machine (Liveco Inc, Burlington, VT) ata cross-head speed of 10 mm/sec until failure.

The circumferences of the coronal (C) and apical aspects (A) ofeach cavity were measured using image analysis software (Scion Corp,Frederick, MA). The sealer-dentin interfacial area was approximated by0.5(C + A)h, where h represents the tooth slice thickness. Push-outstrength was computed by dividing the maximum load at failure bythe interfacial area. Failure modes were classified as adhesive failurealong the sealer-dentin interface, cohesive failure within the sealer,or mixed failure.

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Statistical AnalysisEach sealer-filled hole was treated as a statistical unit. Because the

normality and homoscedasticity assumptions of the push-out strengthdata appeared to be valid, they were analyzed using two-way analysisof variance, with sealer type and dentin location as independent vari-ables. Post hoc comparisons were performed by using a Tukey test.Statistical significance was set at a = 0.05.

Transmission Electron MicroscopyThe remaining four slabs from each sealer group were filled with

sealer as previously described. After setting, excess sealers were polishedoff to expose the sealer-dentin interfaces. For each sealer, two slabs wereprocessed for examination of the ultrastructure of the sealer-dentininterface. They were fixed in Karnovsky’s fixative and osmium tetroxide,dehydrated in an ascending ethanol series, transferred to propylene

Figure 1. (A) Preparation of truncated cavities of uniform dimensions inradicular dentin. As slanted preparations are not amendable to push-out tests,a minidrill press was used for the preparation of a pilot hole and subsequentlythe attachment of rotary nickel titanium instruments to ensure that all cavitieswere created perpendicular to the tooth slice. Two tapered cavities each wereprepared in the coronal (C), middle (M), and apical (A) thirds of a root slice.(B) Fiberoptic light-illuminated push-out testing device. A plunger (P), con-nected to a 10-kg load cell (L), is set up over the cylindrical well ofa custom-built Plexiglas stage. The stage has a side channel (open arrowhead)for the insertion of a fiberoptic light guide. (C) Light illumination greatlysimplifies the task of plunger (pointer) alignment with the center of an invertedtruncated cavity in the tooth slice.

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oxide as a transitional fluid, and finally embedded in epoxy resin at60 �C for 48 hours (27). The other two slabs were processed for exam-ination of nanoleakage within the sealer-dentin interface by firstimmersing in 50 wt% ammoniacal silver nitrate tracer solution (28)for 48 hours. This was followed by conversion of the diamine silverion complexes into metallic silver grains within the interface beforethe commencement of the epoxy resin–embedding protocol. Unstained,90- to 120-nm thick nondemineralized sections were prepared andexamined using a JEM-1230 TEM (JEOL, Tokyo, Japan) operated at110 kV.

ResultsPush-out strengths of EndoREZ (in MPa) at the coronal, middle,

and apical locations were 8.7� 4.3, 9.1� 2.9, and 7.6� 2.1, respec-tively. Push-out strengths of MetaSEAL at the coronal, middle, and apicallocations were 18.6 � 4.8, 18.2 � 4.8, and 16.1 � 4.6, respectively.Push-out strengths of RealSeal SE at the coronal, middle, and apicallocations were 12.6 � 4.3, 14.9 � 5.5, and 14.4 � 8.1, respectively.

The push-out strength results (Fig. 2A) were significantly influ-enced by sealer type (p < 0.001) but not by dentin location (p =0.297). There was no significant interaction between these two factors(p = 0.364). MetaSEAL has a higher overall push-out strength thanRealSeal SE (p < 0.05), which, in turn, has a higher overall push-outstrength than EndoFREZ (p < 0.05). MetaSEAL failed predominantlyin the mixed failure mode (76.6%), whereas RealSEAL SE and EndoREZfailed predominantly by adhesive failure (55% and 75%, respectively)(Fig. 2B).

Interfacial ultrastructure and nanoleakage results are shown inFigure 3. Irrespective of whether the sealer is nonetching or self-etching, the use of EDTA as the final rinse resulted in a 1- to 1.5-mmthick zone of partially demineralized dentin (ie, hybrid layer) alongthe dentin surface (Fig. 3B, D, and F). The occurrence of gaps alongthe sealer-dentin interface and the extent of resin infiltration (ie, nano-leakage) within the hybrid layers could be readily discerned after silverimpregnation. EndoREZ exhibited extensive silver deposits in the gapsbetween the sealer and dentin (Fig. 3A). Extensive nanoleakage wasseen throughout the hybrid layer and peritubular cuff in RealSeal SE(Fig. 3C). Large interfacial gaps were less readily apparent in MetaSEAL;potential gaps and incomplete sealer infiltration within the hybrid layercould be identified as a discontinuous line of silver deposits along thehybrid layer surface and as a fine reticular pattern of silver depositswithin the hybrid layer (Fig. 3B).

DiscussionThe goal for obturation of a tooth is to obtain an adequate seal

between the root canal and the periradicular tissues. Thus, thereappears to be no direct clinical correlation between sealer bondstrengths and apical leakage. However, with continued efforts to developsealers that bond to radicular dentin, examination of the dentin-sealerinterface has become more relevant. Disruption of the established sealis a primary concern. This disruption may occur because of mechanicalstresses caused by tooth flexure or post space preparation. In this sense,the mechanical properties of the interface between dentin and sealer areimportant factors to consider.

The analysis of failure modes correlates well with the results of thepush-out tests. As the resistance to dislocation increases, disruption ofthe sealer-dentin interface becomes less likely and failure is more likelyto occur within the sealer itself. Thus, MetaSEAL, with its higher push-out strength, had a higher component of cohesive failure than did theother two sealers. Based on the results of this study, MetaSEAL providedincreased resistance to dislocation and may provide a more imperme-

580 Babb et al.

able seal to coronal leakage. This has to be confirmed in future clinicaltrials.

Although testing designs that use natural canal spaces have prag-matic appeals to clinicians, they have severe limitations from a materialsscience perspective (22). The application of a compressive load overa thermoplastic material, which has the tendency to deform duringtesting (29), generates erroneous results that could have been respon-sible, in part, for the report that sealers tested in thin films were consid-erably weaker than when they were tested in bulk by eliminating thecompliant core material from the canal space (24). To minimize thisshortcoming, the largest plunger that corresponds to the size of thecore material is usually selected (30). Although this is a legitimatecompromise, the procedure requires the use of different diameterplungers for the different root sections created from a tapered canal.Because the contact surface areas of the plungers are different, nonstan-dardized data generated from different parts of the canal walls renderstatistical comparisons difficult. The presence of calcospherites-con-taining noninstrumented regions in instrumented oval-shaped canals(31) would also have increased undercut retention and contact surfaceareas. These regions may unpredictably alter the sealer rankings unlessthey are meticulously quantified for adjustments in the push-outstrength results using more robust statistical methods that incorporatethe contribution of covariates in accessing treatment outcomes.

Figure 2. (A) A bar chart showing the effects of sealer type and radiculardentin location on push-out strength. For each methacrylate resin–basedsealer, dentin location subgroups that are connected by the same line abovethe colored bars are not significantly different in their push-out strengths(p > 0.05). Sealers labeled with different letters (A, B, and C) are significantlydifferent in their overall push-out strengths (p < 0.05). (B) Failure modedistributions (adhesive, cohesive, or mixed failure) in cavities filled with thetwo self-etching, self-adhesive sealers (MetaSEAL and RealSeal SE) and thenonetching sealer (EndoREZ).

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Figure 3. Transmission electron microscopy of sealer-dentin interfaces created by the three sealers. Specimens that had been immersed in ammoniacal silvernitrate are shown at low magnifications to illustrate the extent of silver nanoleakage in the sealer-dentin interfaces (A, C, and E). To avoid redundancy, specimensthat had not been immersed in silver nitrate are shown only at high magnifications (B, D, and F) to demonstrate the 1- to 1.5-mm thick partially demineralizedhybrid layers (between open arrows). S, sealer; M, mineralized radicular dentin; T, resin tag. (A) Nanoleakage in EndoREZ appeared as heavy silver deposits (openarrowheads) on top of the partially demineralized hybrid layer, creating gaps between the sealer and the top of the hybrid layer. (B) High magnification of theEndoREZ interface showing apatite crystallite remnants (open arrowhead) within the partially demineralized hybrid layer. As EndoREZ is nonetching in nature,this partially demineralized zone is created by the use of 17% EDTA as the final rinse. F, electron-dense fillers. (C) Nanoleakage in MetaSEAL occurred asfine, reticular silver deposits in the hybrid layer (open arrowheads). (D) High magnification of the MetaSEAL interface showing a partially demineralized hybridlayer containing apatite crystallite remnants (open arrowhead). Asterisk: spherical zirconium oxide fillers; arrow: silica fillers. (E) Nanoleakage in RealSeal SE wasfound throughout the entire hybrid layer (asterisk) and the subsurface peritubular cuff of partially demineralized dentin (pointer). Although no gap was apparentbetween the sealer and the hybrid layer, there was poor infiltration of the sealer into the partially demineralized zone created by EDTA. (F) High magnification of theRealSEal SE interface showing apatite crystallite remnants (open arrowhead) within the partially demineralized hybrid layer. F, electron-dense fillers.

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Under identical cleaning and shaping conditions, the push-out

strengths of all three sealers are independent of the location of theradicular dentin. These results suggest that variations in tubular densityor sclerotic dentin along the canal walls are unlikely to alter theirmechanical retention. The use of EDTA for smear layer removal resultedin the consistent appearance of a 1- to 1.5-mm thick zone of partiallydemineralized root dentin in the resin-dentin interfaces of all threesealers, including the nonetching EndoREZ sealer (6, 32). Theuniversal use of EDTA instead of NaOCl as the active final rinse furtherensured that differences in the overall push-out strengths of the threesealers to radicular dentin were not influenced by the adverse oxidizingeffect of NaOCl on methacrylate resin polymerization (33).

Under identical cleaning and shaping conditions, push-outstrengths of the two self-adhesive sealers are significantly higher thanthe nonetch sealer. Although a variable extent of sealer resin monomerinfiltration for MetaSEAL (Fig. 3C) and RealSeal SE (Fig. 3E) could havecontributed to the difference in their push-out strengths, the true self-etching potential (17) of these two self-adhering resin-based sealerscould not be assessed with the present study design in which the manu-facturers’ recommended use of EDTA as the final rinse was strictly fol-lowed. The ability of these self-adhering sealers to etch through thickcanal wall smear layers to create micromechanical retention andachieve gap-free sealing of the canal walls has important clinical impli-cations because debris and smear layer removal may be incomplete inthe apical thirds of the canal walls with many of the contemporary irri-gation protocols (34, 35). Thus, it is worthwhile to examine in futurestudies the self-etching potential of these self-adhesive sealers to ensurethat they are aggressive enough to etch through smear layers and demin-eralize the intact radicular dentin without the adjunctive use of EDTA.This may be tested by bonding the self-adhesive sealers to dentin withthick smear layers and dentin with minimal smear layers. The lattermay be created by polishing dentin with diamond pastes of successivelyfiner particle sizes. This will provide important information, both toclinicians as well as manufacturers, on whether these self-adhesiveresin-based sealers are capable of bonding to the most inaccessibleand challenging sites within the instrumented root canal spaces, therebyachieving strong dislocation resistance as well as a fluid-tight seal.

AcknowledgmentsThe EndoREZ and MetaSEAL kits used in this study were

generous gifts from Ultradent Products Inc. and Parkell Inc., respec-tively. The authors are grateful to Mrs Michelle Barnes for her secre-tarial support.

References1. Schwartz RS. Adhesive dentistry and endodontics. Part 2: bonding in the root canal

system-the promise and the problems: a review. J Endod 2006;32:1125–34.2. Eldeniz AU, Erdemir A, Belli S. Shear bond strength of three resin based sealers to

dentin with and without the smear layer. J Endod 2005;31:293–6.3. Hammad M, Qualtrough A, Silikas N. Extended setting shrinkage behavior of

endodontic sealers. J Endod 2008;34:90–3.4. Zmener O, Pameijer CH, Serrano SA, et al. Significance of moist root canal dentin

with the use of methacrylate-based endodontic sealers: an in vitro coronal dyeleakage study. J Endod 2008;34:76–9.

5. Tay FR, Loushine RJ, Monticelli F, et al. Effectiveness of resin-coated gutta-perchacones and a dual-cured, hydrophilic methacrylate resin-based sealer in obturatingroot canals. J Endod 2005;31:659–64.

582 Babb et al.

6. Bergmans L, Moisiadis P, De Munck J, et al. Effect of polymerization shrinkage onthe sealing capacity of resin fillers for endodontic use. J Adhes Dent 2005;7:321–9.

7. Perdigao J, Lopes MM, Gomes G. Interfacial adaptation of adhesive materials to rootcanal dentin. J Endod 2007;33:259–63.

8. De-Deus G, Namen F, Galan J Jr. Reduced long-term sealing ability of adhesive rootfillings after water-storage stress. J Endod 2008;34:322–5.

9. Cotton TP, Schindler WG, Schwartz SA, et al. A retrospective study comparing clinicaloutcomes after obturation with Resilon/Epiphany or Gutta-Percha/Kerr sealer.J Endod 2008;34:789–97.

10. Salz U, Zimmermann J, Salzer T. Self-curing, self-etching adhesive cement systems.J Adhes Dent 2005;7:7–17.

11. Al-Assaf K, Chakmakchi M, Palaghias G, et al. Interfacial characteristics of adhesiveluting resins and composites with dentine. Dent Mater 2007;23:829–39.

12. Watanabe I, Nakabayashi N, Pashley DH. Bonding to ground dentin by a phenyl-Pself-etching primer. J Dent Res 1994;73:1212–20.

13. Tay FR, Pashley DH. Aggressiveness of contemporary self-etching systems. I: depth ofpenetration beyond dentin smear layers. Dent Mater 2001;17:296–308.

14. Pinna L, Brackett MG, Lockwood PE, et al. In vitro cytotoxicity evaluation of a self-adhesive, methacrylate resin-based root canal sealer. J Endod 2008;34:1085–8.

15. Belli S, Ozcan E, Derinbayi O, et al. A comparative evaluation of sealing ability ofa new, self-etching, dual-curable sealer: Hybrid Root SEAL (MetaSEAL). Oral SurgOral Med Oral Pathol Oral Radiol Endod 2008;106:e45–52.

16. Radovic I, Monticelli F, Goracci C, et al. Self-adhesive resin cements: a literaturereview. J Adhes Dent 2008;10:251–8.

17. Monticelli F, Osorio R, Mazzitelli C, et al. Limited decalcification/diffusion of self-adhesive cements into dentin. J Dent Res 2008;87:974–9.

18. Peumans M, Hikita K, De Munck J, et al. Bond durability of composite luting agentsto ceramic when exposed to long-term thermocycling. Oper Dent 2007;32:372–9.

19. Frankenberger R, Lohbauer U, Schaible RB, et al. Luting of ceramic inlays in vitro:marginal quality of self-etch and etch-and-rinse adhesives versus self-etch cements.Dent Mater 2008;24:185–91.

20. Mazzitelli C, Monticelli F, Osorio R, et al. Effect of simulated pulpal pressure on self-adhesive cements bonding to dentin. Dent Mater 2008;24:1156–63.

21. Tay FR, Gutmann JL, Pashley DH. Microporous, demineralized collagen matricesin intact radicular dentin created by commonly used calcium-depleting endodonticirrigants. J Endod 2007;33:1086–90.

22. Huffman BP, Mai S, Pinna L, et al. Dislocation resistance of ProRoot Endo Sealer,a calcium silicate-based root canal sealer from radicular dentine. Int Endod J2009;42:34–46.

23. Tatsuta CT, Morgan LA, Baumgartner JC, et al. Effect of calcium hydroxide and fourirrigation regimens on instrumented and uninstrumented canal wall topography.J Endod 1999;25:93–8.

24. Jainaen A, Palamara JE, Messer HH. Push-out bond strengths of the dentine-sealerinterface with and without a main cone. Int Endod J 2007;40:882–90.

25. Kallas MN, Koss DA, Hahn HT, et al. Interfacial stress state present in a ‘‘thin-slice’’fibre push-out test. J Mater Sci 1992;27:3821–6.

26. Chandra N, Ghonem H. Interfacial mechanics of push-out tests: theory and exper-iments. Compos Part A Appl Sci Manuf 2001;32:578–84.

27. Tay FR, Moulding KM, Pashley DH. Distribution of nanofillers from a simplified-stepadhesive in acid-conditioned dentin. J Adhes Dent 1999;1:103–17.

28. Tay FR, Pashley DH, Yoshiyama M. Two modes of nanoleakage expression in single-step adhesives. J Dent Res 2002;81:472–6.

29. Williams C, Loushine RJ, Weller RN, et al. A comparison of cohesive strength andstiffness of Resilon and gutta-percha. J Endod 2006;32:553–5.

30. Gesi A, Raffaelli O, Goracci C, et al. Interfacial strength of Resilon and gutta-perchato intraradicular dentin. J Endod 2005;31:809–13.

31. Peters OA. Current challenges and concepts in the preparation of root canal systems:a review. J Endod 2004;30:559–67.

32. Morris MD, Lee KW, Agee KA, et al. Effects of sodium hypochlorite and RC-prep onbond strengths of resin cement to endodontic surfaces. J Endod 2001;27:753–7.

33. Garcıa-Godoy F, Loushine RJ, Itthagarun A, et al. Application of biologically-orienteddentin bonding principles to the use of endodontic irrigants. Am J Dent 2005;18:281–90.

34. Carvalho AS, Camargo CH, Valera MC, et al. Smear layer removal by auxiliary chem-ical substances in biomechanical preparation: a scanning electron microscopestudy. J Endod 2008;34:1396–400.

35. Khedmat S, Shokouhinejad N. Comparison of the efficacy of three chelating agents insmear layer removal. J Endod 2008;34:599–602.

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