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Behavior of resin-based endodontic sealer cements in thinand thick films

Epita S. Panea,b, Joseph E.A. Palamaraa, Harold H. Messera,∗

a Melbourne Dental School, University of Melbourne, Melbourne, Victoria, Australiab Faculty of Dentistry, University of Sumatera Utara, Medan, Indonesia

a r t i c l e i n f o

Article history:

Received 2 October 2011

Received in revised form

20 March 2012

Accepted 16 April 2012

Keywords:

Sealer cement

Epoxy resin

Methacrylates resin

Film thickness

Tensile bond strength

Shear bond strength

Corner effects

a b s t r a c t

Objectives. For root canal fillings, a thin layer of sealer cement is generally recommended.

However, with resin-based sealers, lower bond strength to dentin has been shown in thin

layers compared to thick, contrary to typical behavior of adhesive layers between two adher-

ents. The aim of this study was to evaluate tensile and shear bond strength of thin and thick

films of three resin-based sealers (one epoxy-based and two methacrylate-based) materials

and to investigate corner effects of one methacrylate-based resin sealer.

Methods. Freshly mixed sealer cements were placed between metal-to-metal surfaces of

plano-parallel stainless steel aligned rods with diameter 4.7 mm. Ten samples were prepared

for each type, thickness (0.1 and 1.0 mm) of sealer and test. Tensile and shear strengths were

measured after 48 h for the methacrylate-based materials and after 7 days for the epoxy-

based material using a universal testing machine at a crosshead speed of 1 mm/min. Corner

effects were investigated using one methacrylate-based resin material.

Results. Film thickness had a highly significant influence on both tensile and shear strengths.

For methacrylate resin-based sealers, thin films had higher bond strength than thick

(p < 0.001 for both tensile and shear bond strength). With the epoxy-based sealer either no

difference (shear) or lower bond strength in thin films (tensile; p < 0.05) was found, and

appeared to result from numerous voids created during mixing. The methacrylate based

sealer demonstrated typical engineering behavior for an adhesive material, with corner

effects shown as a material property and in good agreement with the tensile bond strength

results.

Significance. The higher tensile and shear bond strength of resin-based sealer in thin films is

the opposite of that previously reported for bonding to dentin. The substrate clearly has an

important role in failure behavior.

emy

reasons, most root canal obturation techniques have been

© 2012 Acad

1. Introduction

Most root canal sealer cements shrink during setting, leav-ing gaps that potentially serve as pathways for leakage [1].

∗ Corresponding author at: Melbourne Dental School, University of MelbTel.: +61 3 9341 1472; fax: +61 3 9341 1595.

E-mail address: hhm@unimelb.edu.au (H.H. Messer).0109-5641/$ – see front matter © 2012 Academy of Dental Materials. Puhttp://dx.doi.org/10.1016/j.dental.2012.04.012

of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Sealers also gradually dissolve in tissue fluid. For these

ourne, 720 Swanston Street, Melbourne 3010, Australia.

developed to minimize the thickness of the sealer, in orderto enhance the sealing ability of the root filling. Thin layersleak less than thick in experimental studies of most types of

blished by Elsevier Ltd. All rights reserved.

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ealers. However, some studies have shown that epoxy-basedealer leaked less in thick layers [2–4] or did not show sig-ificant differences between thin and thick layers [5]. Of theesin-based sealers, the epoxy-based type has been showno produce higher bond strength to root dentin comparedith both non resin-based and methacrylate resin-based seal-

rs [6–9]. The possibility of better sealing, bonding and roottrengthening with the use of methacrylate-based sealers10,11] using adhesive technologies, however, has not resultedn the anticipated benefits [1,12].

Adhesives are also used in thin layers in luting cementsfor posts, crowns and other indirect restorations). Character-stics of adhesion of thin layers have received little attention inentistry. Resin-based endodontic sealers have been found tohow higher bond strength to dentin in thick than in thin lay-rs, using both push-out and micro-shear bond tests [13,14].he suggested explanation for the effect was based on thextensive penetration of sealers into dentinal tubules. Becauseller particles tend to be too large to enter tubules, the resinatrix material is selectively drawn into the tubules, leav-

ng a particle-enriched but resin-depleted layer on the canalall when the cement layer is thin. In contrast, resin luting

ements used for cementing fiber posts in root canals showedo difference in bond strength in relation to thickness or aariable affect of increasing, and then decreasing strengthith increasing thickness [15].

The behavior of resin composite in different thicknessesonded between two parallel impervious metal plates wastudied by Alster et al. They found that tensile bond strengthas inversely related to the thickness [16], in contrast to

he finding with sealer cements when the dentin serves ashe bonding surface [13,14]. At about the same time, anngineering study of adhesively bonded butt joints [17] alsoocumented that bond strength was inversely related to thick-ess of the adhesive layer. The authors attributed the effect

o interface corner stresses (‘corner effects’) which are influ-nced by the geometry of the test system. Neves et al. reportedhe same phenomenon using finite element modeling of adhe-ion of resin composites to dentin, although they modeledentin as an impervious adherent surface [18].

This study was undertaken to investigate bond strengthn relation to film thickness of endodontic sealer cements,

hen bonding occurred between impervious metal surfacesather than in contact with dentin. Both epoxy resin-based and

ethacrylate resin-based sealers were evaluated in the firstart of the study. The null hypothesis tested that tensile andhear bond strength of resin-based sealer cements were notffected by film thickness. In the second part of the study, thempact of corner effects was investigated more systematicallysing one methacrylate resin-based cement.

. Materials and methods

.1. Sealer cements

hree resin-based sealer cements were used in thistudy (Table 1), one epoxy resin-based sealer (AH Plus®)nd two methacrylate resin-based sealers (EndoREZ® andealSeal®). Each sealer cement was mixed according to the

0 1 2 ) e150–e159 e151

manufacturer’s instructions. AH Plus was mixed using AHPlus Jet® mixing system, EndoREZ with an Ultra-Mixer® andRealSeal was mixed with an auto-mix syringe. The mixedsealer was expressed onto a mixing pad, and the initiallyextruded material was discarded to ensure uniform mixingof the materials.

2.2. Test system

Both tensile and shear bond strengths were measured usingstainless steel rods in pairs, with sealer cement placedbetween the ends of concentrically mounted rods with a pre-set gap (0.1 or 1.0 mm). The setup consisted of 10 pairs ofcylindrically shaped stainless steel rods, 5 cm long and with adiameter of 4.7 mm. The rods were milled to produce an accu-rate plano-parallel bonding surface perpendicular to the longaxis of the rod. Before bonding procedures, the surfaces weresandblasted with alumina particles (average grain size 50 �m)for 3 s from a 3 cm distance perpendicular to the surface. Allsurfaces then were cleaned with acetone and dried for 10 min.

An adjustable alignment device was used to mount andsecure the rods in a precisely aligned position. Spacers wereused to establish a precise gap between the rods, which werethen fixed in position. The rods were then separated, the bond-ing surface of each rod was coated with sealer cement andthen the rods were repositioned at the pre-measured thick-ness. Excess sealer was carefully removed with a cementspatula. For the dual-cured methacrylate resin-based sealers,no light curing procedure was performed and all samples werekept in a nitrogen chamber for 2 h to prevent oxygen inhi-bition of polymerization. All samples were stored in a 37 ◦Cincubator with 95% humidity for 48 h before testing, except forthe epoxy-based sealer, for which all specimens were storedfor 7 days before testing. A pilot study found that maximumstrength was not achieved until this time.

Before the bond strength tests, any excess cement aroundthe periphery was carefully removed with a scalpel blade. Amagnified image of the material thickness was recorded at16× magnification (Leica DMEP Microsystem, Wetzlar GmbH,Germany) and the thickness of the layer between the stainlesssteel rods was measured using Image Tool software (UTHSCSAImage Tool for Windows version 3.00) after calibration. Bothtensile and shear testing were performed without removingthe specimens from the alignment device to avoid prematurefailure.

For tensile tests, each sample was carefully mounted in auniversal testing machine (Instron model 5544, Instron Corp,Canton, MA, USA) with one end held rigidly and the otherattached to a universal joint to prevent any shear compo-nent during testing. For shear tests, one rod was preciselypositioned (under magnification) in a metal holder at the junc-tion between the sealer cement and rod surface, with therod at right angles to the load direction. A shear load wasapplied using a blade with a circular aperture placed over thesealer–rod interface. Specimens were subjected to a tensile orshear load at a constant cross-head speed of 1 mm per minute.

The force (N) required to fracture the bond was recorded andused to calculate the bond strength (MPa).

Modes of failure were evaluated visually and withlight microscopy at 16× magnification. After the tests,

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Table 1 – Endodontic sealer cements used in this study.

Sealercement

Manufacturer Batch Composition

AH Plus Dentsply/MailleferDeTrey, Konstanz,Germany

LOT 0905004007 Epoxide pasteBisphenol-A-epoxyresinBisphenol-F-epoxy resinCalcium tungstateIron oxide pigmentsZirconium oxideAerosilPigment

Amine PasteI-adamantane amineN,N′-dibenzyl-5-oxa-nonandiamine-1,9AerosilTricyclodecane-diamineCalcium tungstateZirconium oxideSilicaSilicone oil

EndoREZ Ultradent Product,South Jordan, UT

LOT B47F2 Part 1 Part 2Resin Fillers Dual-cured initiators Resin Fillers Dual-cured

initiatorsTEGDMA Bismuth oxychloride benzoyl peroxide TEGDMA Bismuth

oxychloridep-tolyiminodiethanol

Diurethanedimethylacrylate

Calcium lactatepentahydrate

Diureathanedimethacrylate

Calcium lactatepentahydrate

Phenyl bis(2,4,6-trimethylbenzoyl)phosphate oxide

Bisglyceroldimethacrylatephosphate

Silica Bisglyceroldimethacrylatephosphate

Silica

RealSeal Sybron Endo, OrmcoCorporation, Orange,CA

LOT 172286 ResinsPEGDMAEBPADMAUDMABisGMA

AminesSilane-treated bariumborosilicate glassesBarium sulphateSilicaCalcium hydroxideBismuth oxychloridewith aminesPeroxidePhotoinitiatorStabilizersPigments

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mpressions of the fractured surfaces were taken witholyvinyl siloxane impression material (Elite® HD+, Zhermacklinical, Zhermack SpA, Italy). Epoxy resin replicas (EpofixTM,truers, Copenhagen) were prepared, mounted on stubs, sput-er coated with gold and examined using a scanning electron

icroscope (Quanta FEG SEM, FEI Co, Oregon, USA).For data analysis, the level of significance was 5%. Two-way

NOVA was conducted separately for the tensile and shearond data, with post hoc tests (Bonferroni test) comparinghick vs. thin films for each sealer type.

.3. Investigation of corner effects

dditional samples using only one methacrylate resin-basedaterial (EndoREZ®) were subjected to tensile testing, using aider range of film thicknesses (0.05–1.3 mm). The experimen-

al set-up was identical to that in the first part of the study,ith the gap pre-set using spacers of appropriate thickness.he interface corner stress intensity factor (Kf) and interfaceorner fracture toughness (I–C Kfc) were calculated accordingo Reedy and Guess, using literature values for Young’s mod-lus and Poisson’s ratio for methacrylate resin [19].

To confirm whether interface corner stress occurred as pre-icted, a calculation was done based on Reedy and Guess [17].irstly the I–C Kfc was calculated with parameter Poisson’s ratiof 0.4 for polymethyl methacrylate resin material and basedn the Poisson’s ratio value, the stress singularity (� − 1) waselected [20].

. Results

he tensile and shear bond strengths of thin and thick layersf three types of sealer cements are shown in Fig. 1. Over-ll, there were variations among the epoxy and methacrylateealers results. Similar results for tensile and shear tests in theethacrylate resin-based sealers were found, with the mean

ensile and shear bond strengths in thin layers greater than inhick layers. A highly significant effect of film thickness wasoted. With the epoxy-based sealer, the shear bond strengthas not significantly (p = 0.124) affected by thickness, while

he tensile bond strength was slightly higher in the thick layerroup (p < 0.05)

SEM of replicas of the fractured surfaces of the epoxy-basedealer (Fig. 2) showed extensive areas of voids covering mostf the surface, when the sealer was tested in a thin layer. Theresence of extensive voids rather than areas of adhesive fail-re was confirmed by the mirror-image distribution of the darkreas on both sides of the fractured surface, and by higherowered views of the edge of voids showing a smooth edgeather than a fracture line.

Areas of cohesive failure demonstrated the differences inller particle size and shape among the three materials. Thepoxy-based sealer had polyhedral particles of varying sizesp to 10 �m embedded within a smooth continuous matrix.

®

ndoREZ had a diffuse appearance with few filler particleslearly visible, while RealSeal® contained large flat filler par-icles up to 50 �m across, with resin matrix clumped aroundhe edges of particles (Fig. 3).

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3.1. Corner effects

Tensile bond strength in relation to adhesive film thickness(0.05–1.3 mm) for EndoREZ® is shown in Fig. 4. The interfacecorner stress intensity factor (Kf) and interface corner frac-ture toughness (I–C Kfc) were calculated according to Reedyand Guess, using the literature values for Young’s modulusand Poisson’s ratio for methacrylate resin [20].

The I–C Kfc data were not influenced by layer thickness(Fig. 5), indicating that it is a material property of the sealercement. The combined I–C Kfc value and the measured tensilestrength were in good agreement (Fig. 4). SEM of the fracturesurface (Fig. 6) demonstrated the fracture pattern describedby Reedy and Guess of adhesive failure along a small segmentof the periphery, progressing to cohesive failure within theinterior of the joint.

4. Discussion

Bond strength testing is considered the best measure of adhe-sion of endodontic sealer cements to dentin [21]. Adhesionto both root dentin and core material is considered a desirableproperty of root canal sealer cements [22] to resist dislodgmentof the filling during restoration [7], to eliminate any space thatallows percolation of fluid through the canal system [21], andto stabilize the apical seal during post-space preparation [23].In addition, root-filled teeth are subjected to occlusal loads,which are transmitted throughout the root structure. Theresultant axial and non-axial forces could generate complextensile and shear stresses across the sealer–dentin interface.Under compression (vertical loading), the effect of Poisson’sratio will also generate stress at the interface. Ultimately,stress to the root filled area may produce adhesive failure [24].

Measuring the bond strength of sealer cements is challeng-ing. The use of an adhesive layer as a thin film is commonin dentistry, in the form of luting cements and root canalsealers. The bonding surfaces are complex, in contrast to thesimple butt joints that are commonly found in engineeringapplications [17]. Measuring bond strengths under experimen-tal conditions, however, often involves simplified setups withtwo flat bonding surfaces. The importance of adhesive layerthickness and corner effects has received very little attentionin studies of dental materials [16,18]. Most previous work onbond strengths has focussed on a single interface between anadhesive material and dentin or enamel. From studies on theeffect of geometry and specimen size on bond strength of asingle interface, it was stated that smaller samples will havesmaller flaw size and higher bond strength [25,26].

Although the tensile test is a preferred method for measur-ing bond strength, both tensile and shear tests were conductedin this study because clinically obturation material will beexposed to more shear load than tensile in the clinical sit-uation. Both tensile and shear tests have been stated to besensitive to small variations amongst specimens and stress

distribution during load application [27]. There is a strong ten-dency to develop a bending moment in the shear test and withcorrect alignment the tensile test should produce more uni-form stress [8,28] but usually yields lower strength value than

e154 d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) e150–e159

Fig. 1 – Tensile and shear bond strength of three resin-based sealers in thin and thick layers. Higher tensile bond strengthwas noted in thick layers of epoxy-based sealer (p < 0.05) with mean strength between thin and thick mean (SD) 16.1 (4.07)vs. 22.3 (2.64) MPa. However, although higher mean shear bond strength was found in thin layer of epoxy-based group (17.5(5.65) vs. 14.1 (3.63) MPa), the difference was not significant statistically (p = 0.124). Higher bond strengths in thin layers ofsealers were noted in methacrylate-based groups (p < 0.05) with comparison between thin and thick of tensile EndoREZ 27.1(3.82) vs. 14.0 (1.9) MPa, shear strength 30.6 (2.76) vs. 6.4 (2.3) MPa; RealSeal: tensile strength 17.3 (2.59) vs. 12.0 (0.98) MPa,

shear strength 18.6 (3.17) vs. 9.4 (1.07) MPa.

the shear test [7]. In this study, higher shear strength than thetensile was noted only in thin layer of sealers.

Mean tensile and shear strengths of thin layers ofmethacrylate resin-based sealers used in this study werehigher than the thick, and therefore the null hypothesis ofthis study was rejected (Fig. 1). This result is similar to thatfound previously [16] for chemically cured resin compositematerial. The difference was explained on the basis of pos-sibility of increased flaw occurrences in thicker layers anddifferent stress distribution in thin and thick layers during cur-ing and under load [16]. Alster et al., however, did not addresscorner effects as the source of the difference. Although allmethacrylate resin specimens were kept in a nitrogen cham-ber to prevent oxygen inhibition during setting, a thin layer ofnon-cured material was still present in the outer surface area.This non-cured layer has also been observed in some otherstudies [29], but not all [30]. Oxygen interaction with the unsat-urated carbon bonds of composite material might compromisethe polymerization of the outer layer of sealer [29].

The epoxy resin-based sealer behaved differently fromthe methacrylate groups, with slightly higher tensile bond

strength of the thicker layer than the thin and a non-significant difference between the shear bond strengths ofthin and thick layers. Although the materials were manipu-lated according to manufacturers’ instructions, multiple small

voids were discovered in all set sealers; this is a consistentfinding in set epoxy-based sealers [31,32]. With the epoxy-based sealer, extensive voids covered more than half of thebonding surface in the thin layer group and only in thisgroup (Fig. 2). A similar appearance was previously shown ina published stereomicrograph of a fractured surface of AHPlus® in a thin layer [33]. Moreover, the initial expansion ofAH Plus (0.62–1.3%) [29,34] followed by the slow setting timeand limited relief of ultimate shrinkage by the presence oflarge particles [4,5], may enhance the size of voids in thismaterial.

The fracture pattern shown in the SEM images of methacry-late resin-based materials was consistent with failure initiatedby high interface corner stresses [17]. SEM images of fracturedsurfaces showed that adhesive failure appeared to initiate in alocalized site at the periphery and progressed to cohesive fail-ure toward the center (Fig. 6). This appearance could be seenclearly in thicker layer specimens. The differences betweenthe elastic properties of the adherent surfaces and the adhe-sive may result in stress concentration at the outer perimeterof the specimens and is called ‘stress singularity area’ [18].

With FEA study, it was shown that increase in material thick-ness caused a significant increase in stress concentration atthe free edge and in the length of the stress-affected area. Thefailure was said to be initiated along a small segment of the

d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) e150–e159 e155

Fig. 2 – Images of fractured surfaces of AH Plus after tensile test. Typical irregular distribution of material in thin layerspecimens (a) and (b) were seen. The positive image of the contralateral surface and the round edges showed an areasuspected to be voids. Differences between type of failure in visual evaluation and SEM images were noticed. Under lightmicroscopy 16× magnification, the failure was classified as mixed. SEM evaluation of the same surface (c) showed a smoothedge of void rather than fracture (d), area with cohesive failure was confirmed (d) but thin layer of material was present atthe area with suspected adhesive failure (f) (1000× SEM magnification).

e156 d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) e150–e159

Fig. 3 – SEM images with 2000× (a) and 5000× (b) magnification of resin-based sealer’s fractured cohesive surfacesdisclosing different form and sizes of particles. (1) Typical AH Plus surfaces with spherical shaped particles imbedded inresin matrix. Cohesive failure exposed the continuous relation between particles and resin. (2) Typical EndoREZ fracturedsurfaces with abundant resin with no obvious filler particles. (3) Images of RealSeal surfaces showed large flat plate-likeparticles, up to 0.05 mm across.

d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) e150–e159 e157

Rela�onship be twee n te nsile bon d strength in different thick nes s andinterf ace corner fractur e tough ness-based es�mate of

methacrylate resin -based sealer ce ment

0

5

10

15

20

25

30

35

40

45

0.0 0.2 0. 4 0.6 0. 8 1.0 1. 2 1.4Thickness (mm)

Tensile

Bond

Streng

th(M

pa)

Tensile bond st rength in diff erentthi cknesses

Interface co rner fracture tou ghn ess indifferent thickne sses

Interface co rner fracture tou ghn ess-basedes�mate

Fig. 4 – Tensile bond strength and film thickness of methacrylate resin-based sealer EndoREZ in good relation with thei

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d

Fft

nterface corner fracture toughness (I–C Kfc) prediction.

pecimen periphery. Both ‘stress singularity area’ and pres-

nce of flaws at the interface would produce localized areas ofigh stress concentration leading to failure [18].

To confirm that interface corner stress occurred as pre-icted, the interface corner fracture toughness (I–C Kfc) was

Interf ace corner fracture toughnes methacrylate resin -based sea

0

1

2

3

4

5

6

7

0.0 0. 2 0. 4 0. 6 0. 8 Thickn ess (mm )

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ig. 5 – Interface corner fracture toughness (I–C Kfc) of methacrylaunction of thickness. The I–C Kfc average value of EndoREZ is 4.3oughness is not associated with particular thickness, implying

calculated in relation to film thickness [17,20]. The I–C Kfc

results showed that the value is independent of thickness ofthe adhesive layer, and thus is a material property (Fig. 5).When the predicted I–C Kfc value were transferred to the testdata, an agreement of calculated and test results could be seen

s for te nsile bond st rength ofler in di ffere nt thicknes s

1.0 1.2 1.4

Average 4.3

+1 SD

-1 SD

te resin-based sealer between two stainless steel rods as a MPa mm0.35. It is clearly shown that the fracture

that I–C Kfc is a material property.

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Fig. 6 – Visual appearance (a) and SEM images (b) of two mixed modes of failure of EndoREZ fractured surfaces (1 and 2). (a)Images of mixed mode of failure of thick layer of EndoREZ with adhesive failure in the outer circumference area. (b) SEMimages of the surface. The failure seemed to initiate adhesively in a localized periphery area and continued to a cohesivefailure toward the center. Wake hackle lines [35] extending out from the small voids showed the direction of crackpropagation (arrowed). These images were able to show that interface corner stress occurred in fractured tensile strength

inle

r

butt joint of resin-based sealer cements bonded between sta

clearly, showing that the interface corner fracture could bepredicted accurately (Fig. 4).

It can be concluded that resin-based sealer cementsbehaved differently in thin and thick layers, and it is thegeometric factor of the system that affected the result. Thetendency of higher bond strength in thin layers of resin-basedsealers in this study is in contrast to the behavior of bondedresin-based sealers in root canals [13,14]. A resin-depletedlayer at the dentin/sealer interface was described by Jainaenet al. [13] and it was suggested that in thin film, the bond

was lower due to the resin being drawn into dentinal tubulesand leaving a filler particle-enriched interface. The adhesionmechanism of the resin-based sealer cements in differentthicknesses to root dentin needs to be further evaluated.

ss steel rods.

Acknowledgments

This research was supported by Postgraduate Research Fundof Melbourne Dental School, University of Melbourne and inpart by Dentsply Australia, and some materials were donatedby Dentsply Australia and Ultradent.

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