Push-out bond strength and SEM evaluation of new polymericroot canal fillingsEmel Olga Onay, DDS, PhD,a Mete Ungor, DDS, PhD,b Hale Ari, DDS, PhD,c

Sema Belli, DDS, PhD,d Ersin Ogus, PhD,e Ankara and Konya, TurkeyBASKENT UNIVERSITY AND SELCUK UNIVERSITY

Objective. The objective of this study was to compare the interfacial strengths and failure modes of new polymericendodontic obturation systems consisting of different material combinations.Study design. Extracted human single-rooted teeth (n � 105) were instrumented using HERO Shaper rotaryinstruments and obturated with different combinations of core and sealer as follows: group 1, RealSeal/Resilon; group2, RealSeal/Herofill; group 3, Hybrid Root Seal/Resilon; group 4, Hybrid Root Seal/Herofill; group 5, MM-Seal/Resilon;group 6, MM-Seal/Herofill; group 7 (control). Failure modes of root slices (1.00 � 0.05-mm thick) after push-outtesting were examined with stereomicroscope and scanning electron microscopy.Results. Hybrid Root Seal/Resilon combination had significantly greater bond strength than all the other groups (P �.001); RealSeal/Resilon combination proved to have the second highest bond strength (P � .001). Bond failure wasmainly mixed failure in both adhesive and cohesive modes at the dentin/sealer interface.Conclusion. The push-out bond strengths of methacrylate-based sealers (Hybrid Root Seal and RealSeal) andthermoplastic synthetic-polymer-based core material (Resilon) combinations were higher than epoxy-resin�basedsealer (MM-Seal) and gutta-percha (Herofill) combination. (Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2009;

107:879-885)

The bond strength of root canal sealers to dentin isimportant for maintaining the integrity of the seal inroot canal filling in both static and dynamic situa-tions.1-3 Additionally, increased adhesive properties todentin may lead to greater strength of the restoredtooth, which may provide greater resistance to rootfracture and clinical longevity of an endodonticallytreated tooth.4-6

Improvements in adhesive technology have fosteredattempts to incorporate adhesive dentistry in endodon-tics by introducing obturation systems with a specificfocus on obtaining a “mono-block” in which core ma-

This paper was presented at the International Association of DentalResearch (IADR) Pan European Federation (PEF) Conference inLondon, UK, on September 2008.aAssistant Professor, Department of Endodontics, Baskent UniversitySchool of Dentistry, Ankara, Turkey.bProfessor, Department of Endodontics, Baskent University Schoolof Dentistry, Ankara, Turkey.cAssociated Professor, Department of Endodontics, Selcuk Univer-sity, School of Dentistry, Konya, Turkey.dProfessor, Department of Endodontics, Selcuk University, School ofDentistry, Konya, Turkey.eAssistant Professor, Department of Biostatistics, Baskent UniversitySchool of Medicine, Ankara, Turkey.Received for publication Oct 12, 2008; returned for revision Dec 21,2008; accepted for publication Jan 12, 2009.1079-2104/$ - see front matter© 2009 Published by Mosby, Inc.

doi:10.1016/j.tripleo.2009.01.023

terial, sealing agent, and root canal dentin form a singlecohesive unit.7

The recent introduction of Resilon (Resilon ResearchLLC, Madison, CT) as an alternative root filling mate-rial offers the promise of adhesion to root dentin.6-8

This thermoplastic synthetic polymer is presented as aroot canal filling material having bonding ability tomethacrylate-based resin sealers through the inclusionof dimethacrylate monomers.9 This core material cancouple to a variety of dentin adhesives and resin typesealers, including RealSeal (SybronEndo, Glendora,CA) and Hybrid Root Seal (Sun Medical Co., Ltd.,Shiga, Japan). The RealSeal root canal sealant is adual-cured, methacrylate-based dental resin compositesealer. The RealSeal/Resilon system is used in conjunc-tion with a self-etching primer, which induces a chem-ical interaction that leads to the formation of a singleresin block and adheres to the root canal walls.

Based on a similar approach, a new material (HybridRoot Seal) that contains 4-methacryloxyethyl trimelli-tate anhydride (4-META) has been introduced on themarket. Hybrid Root Seal is a dual-cured and a self-etching sealer, which does not require any additionalpriming or acid etching to the root canal dentin. Themanufacturer claims that it has the advantage of form-ing a hybrid layer that creates a bond to the dentinalwalls as well as the Resilon and gutta-percha.

Epoxy-resin sealers have been used because of their

reduced solubility,10 apical seal,11 and micro-retention

879

OOOOE880 Onay et al. June 2009

to root dentin.1 MM-Seal root canal sealer (Micro-Mega, Besançon, France) is an epoxy-resin�based rootcanal sealer packaged in a dual syringe that is used withgutta-percha. Thermoplasticized gutta-percha tech-niques have been demonstrated to provide superioradaptation to canal walls and better filling of lateralcanals when compared to lateral compaction.12,13 Hero-fill Soft-Core obturators (Micro-Mega) consist of plas-tic carriers coated with thermoplastic �-phase gutta-percha. The central carrier has an ISO standard size andthe system provides a size verifier that ensures adequatepreparation of the canal and the selection of the corre-sponding Herofill Soft-Core obturator.

Using the push-out test method to test the dentinbond strength, we previously reported that Epiphanysealer and Resilon core combination was not superior tothat of the AH Plus sealer (Dentsply De Trey GmbH,Konstanz, Germany) and gutta-percha core combina-tion.14 Based on the previous observed push-out testvalues, the null hypothesis tested was that there are nodifferences in the bond strength of the RealSeal/Resiloncombination and all the other groups tested.

The purpose of this study was to assess the interfacialstrengths and failure modes of new polymeric endodon-tic obturation systems consisting of different materialcombinations.

MATERIALS AND METHODSA total of 105 extracted human single-rooted teeth

were used. Each tooth was placed in 5.25% sodiumhypochlorite (NaOCl) for 2 hours for surface disinfec-tion, and then stored in distilled water until used. Thecrown of each tooth was sectioned at the cemento-enamel junction using a water-cooled diamond disk.The working length was determined visually by sub-tracting 1 mm from the length of a size 10 K-file(MANI Inc., Tochigi, Japan) at the apical foramen. Themiddle and coronal thirds were prepared using ISO size1, 2, 3, 4, and 5 Gates Glidden drills (Produits DentairesS.A., Vevey, Switzerland). The preparations in the rootportions were standardized with a depth of 5 mm and adiameter of 1.3 mm.

All teeth were instrumented with a crown-down tech-nique, using HERO Shaper rotary instruments (Micro-Mega) as follows. A #30 file with a 0.06 taper wasinitially introduced in two thirds of the working length.Shaping was completed with a #30 file with 0.04 taperat the working length. One milliliter of 1.25% NaOClwas used for irrigation between each instrument. Alubricant (Glyde File Prep., Dentsply, Montigny-le-Bretonneux, France) was used throughout the cleaningand shaping of the root canal. The smear layer wasremoved during the instrumentation with 5 mL of 17%

EDTA (pH 7.4). Finally, the root canal was flushed

with 5mL of 5.25% NaOCl, 5 mL of 17% EDTA, and3 mL distilled water respectively and dried with paperpoints before filling.

The roots were then divided randomly into 7 groups(6 experimental groups of 15 roots each and 1 controlgroup of 15 roots) and obturated as follows:

● Group 1: After instrumentation, a self-etchingprimer (RealSeal Primer; SybronEndo, Glendora,CA) was placed into the canal with a syringe. Theprimer was allowed to remain for 30 seconds and theexcess was removed with paper points. Roots werefilled with RealSeal root canal sealer and Resiloncore material (SybronEndo, Glendora, CA) using thewarm vertical compaction technique. The RealSealroot canal sealer was introduced into the root canalusing a paper point. A Resilon point, previouslytried-in with tug back, was lightly coated with Re-alSeal root canal sealer and placed into the canal toworking length. Down-packing was performed byusing System B (SybronEndo, Orange, CA) set at150°C to within 4 to 5 mm of working length. Back-filling of Resilon pellets was performed using anObtura II unit (Obtura Spartan, Fenton, MO) set at140°C. After backfilling, the coronal surface of theroot filling was light-cured (Hilux Expert, Benlioglu,Ankara, Turkey) for 40 seconds to create an imme-diate coronal seal according to manufacturer’s rec-ommendations.

● Group 2: The roots were treated with the primer andthen filled with RealSeal root canal sealer. A size 30Herofill obturator was heated for 35 seconds in theHerofill oven, and then it was seated to the workinglength. The handle of the obturator and excess ma-terial were removed after cooling with a small roundbur. The canal orifice was light-cured for 40 seconds.

● Group 3: The roots were filled with Hybrid RootSeal and Resilon core material using the warm ver-tical compaction technique. As specified by the man-ufacturer’s instructions, 3 drops of liquid and 1 scoopof powder were dispensed onto a mixing pad with aspatula. The homogeneous texture was introducedinto the root canal using a paper point. The root canalfilling was completed in the same manner as in group1. The coronal surface of the root filling was light-cured for 20 seconds to create an immediate coronalseal according to manufacturer’s recommendations.

● Group 4: The roots were filled with Hybrid RootSeal and Herofill core material in the same manner asin group 2. The canal orifice was light-cured for 20seconds.

● Group 5: The roots were filled with MM-Seal root

canal sealer and Resilon core material using the

ups (P

OOOOEVolume 107, Number 6 Onay et al. 881

warm vertical compaction technique. As specified bythe manufacturer’s instructions, the base and catalyst(2:1 wt. ratio) were dispensed onto a mixing plate,and then placed into the canal with paper points. Theroot canal filling was completed in the same manneras in group 1.

● Group 6: The roots were filled with MM-Seal rootcanal sealer and Herofill core material in the samemanner as in group 2.

● Group 7 (control group): The roots were filledHerofill core material without any sealer.

All 105 roots were stored in gauze dampened withsterile saline and enclosed in sealed tubes for 7 days toallow the sealer to set.

Preparation of roots for push-out bond strengthtesting

Each slice was marked on its coronal side with anindelible marker. The coronal portion just below thecemento-enamel junction of each root was sectionedperpendicular to its long axis to create 1.00 � 0.05-mm-thick slices using a water-cooled diamond blade onan Isomet machine (Buehler, Lake Bluff, IL). If thesample contained filling material of a noncircular cross-sectional shape, it was discarded, as this would result innonuniform stress distribution15 during testing and in-accurate measurements.16,17

After measuring the thickness of each slice withdigital calipers, the filling material was loaded with a1-mm-diameter cylindrical plunger that provided themost extended coverage over the filling material with-out touching the canal wall. Loading was performed ona universal testing machine (Instron, Canton, MA) at aspeed of 1 mm/min until bond failure occurred. Theload was applied to the apical aspect of the root sliceand in an apical-coronal direction, so as to push thefilling material toward the larger part of the root slice,thus avoiding any limitation to the filling materialmovement. The bond strength was determined by using

Table I. Mean push-out bond strengths (MPa) (� SD

Material (n � 15) Mpa (Mean) � S

Group 1 RealSeal/Resilon 3.18 � 2.16b

Group 2 RealSeal/Herofill 1.42 � 1.25a

Group 3 Hybrid Root Seal/Resilon 3.43 � 2.07c

Group 4 Hybrid Root Seal/Herofill 1.33 � 1.47a

Group 5 MM Seal/Resilon 1.95 � 0.84d

Group 6 MM Seal/Herofill 2.66 � 1.51e

aGroups identified by the same superscript letter were not significanb,c,d,eDifferent superscript letters identify significantly different gro

a real-time computer software program, which plotted a

load/time curve during compression testing. Bond fail-ure load was noted when a sharp decline was observedon the graph and/or complete dislodgement of the rootfilling material.

To express the bond strength in MPa, the load atfailure recorded in Newtons was divided by the area ofthe bonded interface as calculated by the followingformula18: A � 2�r � h, where � is the constant 3.14,r is the root canal radius, and h is the thickness of theslice in millimeters.

Analysis of failure modesAfter adhesion testing, the sections were split longi-

tudinally in buccolingual direction using a diamonddisk and dentin surfaces were examined under a ste-reomicroscope (Leica MZ12, Leica Microsystems,Wetzlar, Germany) at �40 magnification to determinethe nature of bond failure: 1, adhesive failure at thefilling material/dentin interface; 2, cohesive failurewithin the filling material; 3, mixed failure in both thefilling material and dentin.

Scanning Electron Microscope evaluationFour randomly selected samples from each group

were prepared for scanning electron microscope (SEM)examination. Each specimen was sectioned longitudi-nally, and then mounted on stubs, sputter coated withgold, and examined under SEM (JSM-6400; JEOL,Tokyo, Japan).

Statistical analysisNonparametric statistical analysis (Kruskal-Wallis)

followed by a multiple comparison test (Dunn’s test)indicated significant differences among the groups. Thestatistical significance was set at a P less than .05.

RESULTSThe respective mean push-out test values for groups

1 to 7 were: 3.18 � 2.16 MPa, 1.42 � 1.25 MPa, 3.43

failure modes for the experimental groupsFailure mode, n (%)

Adhesive Cohesive Mixed

2 (13.3) 7 (46.7) 6 (40)2 (13.3) 5 (33.3) 8 (53.4)1 (6.6) 8 (53.4) 6 (40)0 (0) 5 (33.3) 10 (66.7)2 (13.3) 4 (26.7) 9 (60)2 (13.3) 2 (13.3) 11 (73.4)

rent (P � .05).� .001).

) and

D

tly diffe

� 2.07 MPa, 1.33 � 1.47 MPa, 1.95 � 0.84 MPa, 2.66

OOOOE882 Onay et al. June 2009

� 1.51 MPa, and 0.1 � 0.07 MPa (Table I). Significantdifference (P � .001) occurred among the groups.Multiple comparisons (Dunn’s test) revealed that group3 (Hybrid Root Seal/Resilon) had significantly (P �.001) greater bond strength than all the other groups;group 2 (RealSeal/Herofill) and group 4 (Hybrid RootSeal/Herofill) were not significantly different from eachother (P � .05), but both had significantly (P � .001)lower bond strength than all the other experimentalgroups. RealSeal/Resilon (group 1) combinationproved to have second highest bond strength (P �.001).

Failure modesStereomicroscopic examination of 90 samples re-

vealed that the bond failure to be mainly mixed failure(55.6%) in both adhesive and cohesive modes at thefilling material/dentin interface (Table I). Cohesive fail-ures within the filling material were mostly observed inHybrid Root Seal/Resilon combination (group 3)(53.4%). Mixed failures in both adhesive and cohesivemodes at the filling material/dentin interface weremostly observed in MM-Seal/Herofill combination(group 6) (73.4%). Most of the groups failed 13.3% inadhesion between filling material and dentin interface,whereas no instances were found of adhesive failure forthe samples of Hybrid Root Seal/Herofill combination(group 4) (Table I).

When samples were examined under SEM, resinparticles often obscured tubule orifices (Figs. 1 and 2),and clear dentin surfaces were also found where sometags had sheared off but remained in the dentinal tu-

Fig. 1. Remnants of RealSeal could be observed on thedebonded dentin surfaces of the RealSeal/Resilon group.

bules (Fig. 3).

DISCUSSIONAll 7 of the groups tested showed measurable adhe-

sive properties. The Hybrid Root Seal/Resilon combi-nation (group 3) had the highest bond strength. Re-alSeal/Resilon (group 1) combination proved to havesecond highest bond strength. According to the resultsof this study we have to reject the null hypothesis thatthere are no differences in bond strength of the Re-alSeal/Resilon combination (group 1) and all the othergroups tested.

Bond-strength testing has become a popular methodfor determining the effectiveness of adhesion betweenendodontic materials and tooth structure. There are

Fig. 2. Residual sealer material covering the surface in theHybrid Root Seal/Resilon group. Some finer filler particlesclogged or partially occluded dentinal tubules.

Fig. 3. The dentin surface of the pulpal wall of a canalobturated with MM-Seal/Herofill showed a clean dentin wallwith resin tags partially or completely pulled out.

many methods for measuring the adhesion of endodon-

OOOOEVolume 107, Number 6 Onay et al. 883

tic root canal sealers, but none has yet been widelyaccepted.19 The tensile strength test is sensitive, withthe result that small alterations in the specimen or instress distribution during load application have a sub-stantial influence on the results.20 On the other hand, amajor problem with shear testing is that it is difficult toclosely align the shear-loading device with the bondinterface. The load is offset at some distance from thebonded interface, resulting in unpredictable torqueloading on the specimen.21

In this study, the push-out test method was used totest the dentin bond strengths of different root canalsealers. The model has been shown to be effective andreproducible.22 Another advantage of this method isthat it allows root canal sealers to be evaluated evenwhen bond strengths are low.

Development of molecules that will adhere to toothstructure is a high-priority area of dental research.Although dentin presents formidable obstacles to adhe-sive mechanisms, there is substantial progress in theformulation of agents that are already altering certainprocedures.

The materials and techniques currently used for ad-hesive bonding to dentin in restorative dentistry havebeen developed over many years. The latest generationdentin-bonding systems achieve high bond strength andreduced microleakage by micromechanical bonding orby forming a hybrid layer between the dentin and theresin.23 Studies have examined the potential of adhe-sive resins as root canal filling materials.24,25

4-META adhesive resins have been reported in thedental literature for more than 20 years. 4-META oc-curs in nature as a white crystalline structure with amelting point of 95°C to 96°C. It can be prepared by thecondensation of 2-hydroxyethyl methacrylate and trim-ellitic anhydride chloride in the presence of pyridine toremove hydrogen chloride. It consists of hydrophobicand hydrophilic groups and promotes the infiltration ofmonomers into the hard tissue.26 In 1982, Nakabayashiet al.27 demonstrated that the addition of 4-META tothe monomer of a luting agent increased adhesion of anacrylic rod to etched enamel and dentin.

Nakabayashi et al.28 first described a “hybrid” layerwhen working with 5% 4-META in methyl methacry-late, initiated by partially oxidized tri-n-butyl borane inthe presence of poly-methyl methacrylate powder onvital human dentin. This transitional or hybrid layerwas assumed to be durable, “of resin-reinforced dentincreated by the impregnation, co-mingling, and envel-opment of collagen bundles, and encapsulation of hy-droxylapatite crystals.”28, p135

In the present study, both 4-META containing Hy-brid Root Seal and RealSeal showed superior bonding

ability to root dentin with Resilon core material. The

mostly cohesive failures at the filling material/dentininterface and occluded dentinal tubule formation (Figs.1 and 2) in these groups corresponded to the goodbonding of Hybrid Root Seal and RealSeal to dentin. Itis also interesting to note in the current study that inaddition to a greater bonding ability, both the groupshad a much greater standard deviation than the otherexperimental groups. Although the manufacturer’s di-rections for placement of these materials were followedprecisely, two potential reasons may have led to incon-sistent results. Obturation techniques that use heat ap-plication could have affected the adaptation and thepenetration of the methacrylate-based sealers. A studyby Lawson et al.29 using the push-out test design andSEM analysis has shown the patent dentinal tubules andthe absence of resin-rich zones in samples obturatedwith MetaSEAL (Hybrid Root Seal; Parkell Inc., Farm-ingdale, NY) and gutta-percha combination by warmvertical compaction technique. The authors also con-cluded that the evaporation of the liquid resin compo-nent of the sealer by heat generated during down-packing with the System B heat source could haveresulted in a highly viscous sealer that had a limitedflow capacity into the patent dentinal tubules.

An even more challenging concern is that of thegeometric variables in the use of adhesive sealers. Theconfiguration factor (C-factor) defined as the ratio ofbonded to unbonded surface areas of cavities, in a rootcanal is highly unfavorable and contributes to maximiz-ing the polymerization stress of resin-based materialsalong the root canal walls.30 Previous studies havedescribed that the extremely high C-factors encoun-tered in long, narrow root canals tended to cause aresistance in the relieving of shrinkage stresses duringpolymerization.18,30 Focusing on this issue becomesmore critical as the resin sealer is light-cured to createan immediate coronal seal, because this prevents stressrelief by resin flow and the resin sealer may detachfrom dentin walls, thus creating interfacial gaps anddecreasing the interfacial strength.18

The design of this in vitro study might also havesome limitations, making it difficult to compare theresults with the clinical situation. Beside the adverseeffects of the filling techniques that use heat applicationand C-factor, the unpredictable variation in tooth mor-phology such as continued dentin deposition in olderteeth31 may also explain the relatively high standarddeviations where the push-out bond strength variedconsiderably within groups for all sealer types.32

Resilon is the central component of the EpiphanySoft Resin Endodontic Obturation System (PentronClinical Technologies, LLC), and RealSeal(SybronEndo). The mean push-out bond strength val-

ues for Resilon and RealSeal (or Epiphany) combina-

OOOOE884 Onay et al. June 2009

tion reported in the literature vary within wide range:from 0.32 MPa to 5 MPa.14,16,17,33-38 These differentresults can be related to the variety of samples andprotocols to test the bond strength in these variousstudies.

Using the cold lateral condensation technique andpush-out test method to compare the bond strength ofdifferent material combinations, we reported the Epiph-any sealer and Resilon core combination was not supe-rior to that of the epoxy-resin�based AH Plus sealerand gutta-percha core combination.14 In the presentstudy, the RealSeal/Resilon combination performedbetter and had higher bond strength when comparedwith the previous study. This might be attributable tothe advantage of the continuous wave of condensationtechnique that creates a homogeneous mass of Resilonthat corresponds to an increase in Resilon density whencompared with cold lateral condensation technique.This result is consistent with findings of Ureyen Kayaet al.36 who compared the interfacial strength of differ-ent root canal filling materials consisting of differentobturation techniques and material combinations. Theauthors showed that the Epiphany/Resilon used withcombined warm vertical compaction had significantlyhigher bond strength value than when used in combi-nation with cold lateral condensation technique.

Numerous investigations have shown that the epoxy-resin�based sealers had higher bond strength than mostother sealers.39,40 Although the specimen evaluation bySEM revealed clear dentin surface where some resintags had sheared off but remained in the dentinal tu-bules (Fig. 3), the MM-Seal/Herofill combination maybe viewed as having intermediate bond strength in thepresent study when compared with other root canalsealer and core material combinations. To our knowl-edge, no study investigating the MM-Seal/Herofillcombination appeared in the literature and so the resultsfound in the present study could not be compared.

The current study showed that the newly developedHybrid Root Seal and Resilon core material combina-tion had the greatest push-out bond strength of all of thematerials tested. Nevertheless, adhesive strength is onlyone aspect of the quality of root canal sealer. Furtherinvestigation of other features of root canal sealers isrequired.

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Reprint requests:

Emel Olga Onay, DDS, PhDDepartment of EndodonticsBaskent University School of Dentistry11. sok. No: 26 06490Bahcelievler, Ankara, Turkey

eonay@baskent.edu.tr