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d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) 797–803

Available online at www.sciencedirect.com

jo ur nal home p ag e: www.int l .e lsev ierhea l th .com/ journa ls /dema

he effect of smear layer on the push-out bond strength ofoot canal calcium silicate cements

hmad M. EL-Ma’aitaa,b,∗, Alison J.E. Qualtrougha, David C. Wattsa,c

School of Dentistry, The University of Manchester, Manchester, UKFaculty of Dentistry, The University of Jordan, Amman, JordanInstitute of Materials Science and Technology, Friedrich-Schiller-University, Jena, Germany

r t i c l e i n f o

rticle history:

eceived 20 February 2013

eceived in revised form

6 April 2013

ccepted 26 April 2013

eywords:

alcium silicate cements

TA

ineral trioxide aggregate

ndodontics

bturation

ush-out bond strength

a b s t r a c t

Introduction. The aim of this study was to evaluate the effect of smear layer removal on the

push-out bond strength between radicular dentin and three calcium silicate cements (CSC)

in comparison with gutta percha and sealer.

Methods. Eighty human anterior extracted teeth were decoronated, cleaned and shaped to

size 50/0.05 apically and randomly divided into 2 major groups: (A) smear layer preserved,

and (B) smear layer removed using irrigation with 17% EDTA. Roots within each major group

were further divided into 4 subgroups according to the obturation material used: (1) ProRoot

MTA, (2) Biodentine, (3) Harvard MTA, (4) Gutta percha and AH-plus sealer. Obturated roots

were stored in synthetic tissue fluid for 7 days to allow maximum setting of the root filling

materials. Three 2-mm-thick slices were obtained from each root at different section levels

(coronal, middle, apical). The canal diameters and slice thickness were measured, and the

adhesion surface area for each slice was calculated. Push-out bond strength test was carried

out using a universal testing machine. The bond failure mode was assessed under an optical

microscope at 40×.

Results. The mean push-out bond strength in groups 1A, 2A and 3A were 7.54 (±1.11), 7.64

(±1.08) and 8.79 (±1.55) MPa respectively, while those for groups 1B, 2B and 3B were 6.58

(±1.13), 6.47 (±1.08), 7.71 (±1.81) MPa, respectively. In the gutta percha and sealer groups the

push-out bond strength means were: 1.98 (±0.48) and 2.09 (±0.51) MPa in the preserved and

removed smear layer groups respectively. The push-out strength values were significantly

reduced when the smear layer was removed in the CSC groups (P < 0.05) while no significant

difference was detected in the gutta percha and sealer groups.

Conclusions. Based on the conditions of this ex vivo study, it can be concluded that smear

layer removal is detrimental to the bond strength between calcium silicate cements and

dentin.

© 2013 Academy

∗ Corresponding author at: Biomaterial Department, School of Dentistrhester M15 6FH, UK. Tel.: +44 0161 275 6660; fax: +44 0161 275 6710.

E-mail address: ahmad.el-ma’aita@manchester.ac.uk (A.M. EL-Ma’a109-5641/$ – see front matter © 2013 Academy of Dental Materials. Puttp://dx.doi.org/10.1016/j.dental.2013.04.020

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

y, The University of Manchester, Higher Cambridge Street, Man-

ita).blished by Elsevier Ltd. All rights reserved.

l s 2

798 d e n t a l m a t e r i a

1. Introduction

The aim of obturation of the root canal space is to providea tight seal against the ingress of microbes and fluids intothe disinfected root canal space [1]. Ideally, obturation mate-rials should form a strong bond with the canal wall and resistdislodgement during function.

Mineral trioxide aggregate (MTA) was first developed atLoma Linda University in 1993 [2]. It was initially proposed asa perforation repair and retrograde filling material in surgicalendodontics. Since then, other products with similar chemicalconstituents have been developed and are available commer-cially under different brand names. It has been proposed that ageneric name be used for this class of materials [3]. “Hydraulicsilicate cements” [3] and “calcium silicate cements” [4] are themost common of those proposed. Calcium silicate cements(CSC) possess several desirable properties such as superiorsealing ability, bioactivity, and the ability to set in the pres-ence of fluids [5]. Clinical studies demonstrated satisfactoryoutcomes of different clinical applications of CSCs [6], andrecently they were used for obturation of the entire root canalspace [7].

ProRoot MTA (Dentsply Maillefer, Ballaigues, Switzerland)is composed of a hydrophilic powder made of calcium sili-cates, which reacts with water and sets into a hard structurethrough a hydration reaction forming calcium hydroxide andcalcium silicate hydrates [8]. Biodentine (Septodont, SaintMaur des Fosses, France) is a calcium silicate-based cementcomposed of a powder (in a capsule) and liquid (in a pipette).The powder consists of tricalcium and dicalcium silicate,calcium carbonate, and zirconium oxide, while the liquid con-tains calcium chloride as an accelerator and a water reducingagent [9]. Biodentine sets in 10 min [9] and it is promotedas a good dentin substitute in direct and indirect pulp cap-ping procedures. Harvard MTA (Harvard Dental InternationalGmbH, Hoppegarten, Germany) is a new encapsulated CSCwith a predetermined powder: liquid ratio. The capsule is acti-vated, mixed in an amalgamator, and the content is ejectedwith its corresponding gun. Harvard MTA has a working timeof 2 min and sets in 40 min (manufacturer’s material safetysheet).

Theoretically, CSC could be used for obturation, but detailsregarding the optimum conditions are not known. It has notbeen clearly demonstrated, for example, whether smear layershould be removed prior to obturation with these cements.The aim of this study was to determine the effect of smearlayer removal on the push-out bond strength between differ-ent CSCs and dentin in comparison with that between guttapercha and sealer and dentin. The null hypotheses are: (a)there is no difference in the push-out bond strength betweendentin and different obturation materials. (b) Smear layerremoval does not affect the push-out bond strength betweenthe obturation material and dentin.

2. Materials and methods

Eighty extracted human teeth with single canals and curva-tures less than 5◦ [10] were used in this study. The crowns wereremoved using a water-cooled diamond wheel saw leaving

9 ( 2 0 1 3 ) 797–803

13 ± 1 mm long roots. After determining the working lengthand establishing a glide path, root canals were cleaned andshaped with a series of ProTaper files (S1–F5) (Dentsply, Maille-fer, Ballaigues, Switzerland) to size 50/05 apically. Irrigationbetween each file was carried out with copious irrigation with1% NaOCl using a 27-gauge monoject needle with a notched tip(Monoject, Kendall, Covidien, Mansfield, Massachusetts, USA)inserted to 1 mm short of the working length. Prepared rootswere randomly divided into two major groups (n = 40). In groupA, no attempt at removal of the smear layer was carried out. Ingroup B, the smear layer was removed by irrigation with 1 ml of17% EDTA (PULPDENT, Watertown, MA, USA) for one minute asrecommended by Teixeira et al. [11] using the same irrigationneedle as for NaOCl. To eliminate the EDTA action, irrigationwith 2 ml of NaOCl was carried out followed by a final flushwith 5 ml of sterile water. Within each major group, roots werefurther divided into four subgroups (n = 10) according to theobturation material used (Table 1). ProRoot MTA (subgroups 1Aand 1B), Harvard MTA (subgroups 2A and 2B) and Biodentine(subgroups 3A and 3B) were used to obturate the root canalsin their respective groups using a manual compaction tech-nique with hand pluggers as described by EL-Ma’aita et al.[12]. In groups 4A and 4B (control groups), gutta percha andAH-plus sealer were used as the control groups. The gutta per-cha was applied into the canals in a thermoplastic injectiontechnique (Obtura Spartan, Algonquin, IL, USA) to allow forstandardization of the technique between the three thirds ofthe canals. The coronal 2 mm of each root were sealed with aglass ionomer filling (AquaCem, DENTSPLY, Surrey, UK). Obtu-rated roots were radiographed in 2 directions; bucco-lingualand mesio-distal, to ensure the canals were densely obtu-rated. The samples were stored at 37◦ C in synthetic tissuefluid (STF) for 7 days to allow for maximum setting of thematerials.

Following the storage period, each root was sectioned hori-zontally at three different levels (namely: coronal, middle andapical) to obtain three slices 2 ± 0.1 mm in thickness (Fig. 1).The greater and lesser root canal diameters and the thicknessof each slice were recorded to the nearest 0.01 mm using a dig-ital caliper. The adhesion surface area was calculated by thefollowing equation:

Adhesion surface area (mm2) =(

D1 + D2

2

)× � × h

where D1 and D2 are the greater and lesser canal diametersrespectively, � is the constant 3.14 and h is the thickness ofthe obturated root slice.

The force required to dislodge the obturation material fromthe root slice was measured using a Universal Testing Machine(Roell Z020, Zwick GmbH & Co. KG, Germany). Each samplewas attached to a metal jig with the coronal side facing down-wards. The metal jig had an adjustable central hole whichwas made slightly bigger than the greater canal diameter toprovide support for the root slice and to allow for unrestrictedmovement of the dislodged obturation material (Fig. 2). Com-pressive force was applied to the obturation material through

a flat metal rod (0.5, 0.7 and 1.0 mm in diameter for the apical,middle and coronal slices respectively) attached to a load celland moving downwards at a crosshead speed of 1 mm/min.The metal rod had a clearance of at least 0.2 mm from the

d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) 797–803 799

Table 1 – The materials used in this study and their manufacturers’ details.

Group Smear layer Material Manufacturer

1A Preserved ProRoot MTA Dentsply Maillefer, Ballaigues, Switzerland1B Removed

2A Preserved Harvard MTA Harvard Dental International GmbH, Hoppegarten, Germany2B Removed

3A Preserved Biodentine Septodont, Saint Maur des Fosses, France3B Removed

4A Preserved Gutta perchaAH-plus sealer

Obtura Spartan, Fenton, MO, USADentsply DeTrey, Konstanz, Germany4B Removed

Fig. 1 – Each obturated root was sectioned at 3 different levels (a). The lesser and greater canal diameters and the heightw

mt(r

ere measured for each root slice.

argins of the root walls to ensure the contact occurred with

he obturation material only. The maximum force in NewtonF-max) at which the dislodgement of the obturation mate-ial occurred was recorded, and the push-out bond strength in

Fig. 2 – Push-out bond strength test diagram.

megapascal (MPa) was calculated for each specimen accordingto the following equation:

Push-out bond strength (MPa) = F-max (N)adhesion surface area (mm2)

The samples were examined under a light microscope ata magnification of 40× to determine the bond failure modewhich was classified into: (a) adhesive (between the obtura-tion material and dentin), (b) cohesive (within the obturationmaterial) or (c) combined.

The means and standard deviations of the push-out bondstrength were calculated for each group and the data were sta-tistically analyzed using the two-way ANOVA test and Tukey’spost hoc test.

Two samples from the ProRoot MTA subgroups (one fromgroup 1A and another from group 1B) were chosen randomlyfor scanning electron microscopy (SEM) assessment to observethe presence/absence of smear layer in the two samples.

3. Results

Statistical analysis of the mean push-out bond strength usingthe one-way ANOVA test revealed no statistically significantdifferences at level of section (i.e.: coronal, middle and apical)

800 d e n t a l m a t e r i a l s 2

Table 2 – The mean and standard deviation push-outstrength values of each group.

Group (n = 30) Mean (MPa) St. deviation

1A 7.54 1.111B 6.58 1.13

2A 7.64 1.082B 6.47 1.08

3A 8.79 1.553B 7.71 1.81

4A 1.98 0.484B 2.09 0.51

within each group. Therefore the values for all thirds withinone group were combined to increase the power of the statis-tical test.

The mean and standard deviation values of the push-outbond strength of each group are presented in Table 2. Statis-tical analysis using the two-way ANOVA and Tukey post hoctests revealed that the push-out bond strength is significantlyinfluenced by the obturation material used and smear layerremoval/preservation (P < 0.01). In the CSC groups (1A, 1B, 2A,2B, 3A and 3B), the push-out bond strength was significantlyreduced following removal of the smear layer (Fig. 3). In thegutta percha and sealer groups, no statistically significant dif-ference was detected in the push-out bond strength betweenthe removed and preserved smear layer subgroups (2.09 and1.98 MPa respectively). Roots filled with Biodentine exhibitedthe highest push-out bond strength which was significantlyhigher than all other groups whether smear layer was removedor preserved (7.71 and 8.79 MPa, respectively). Gutta perchaand sealer demonstrated the lowest adhesion with root canaldentin which was not significantly influenced by the removalof smear layer. There was no statistically significant differencein the push-out bond strength between the ProRoot MTA and

Harvard MTA in both the preserved (ProRoot MTA: 7.54, Har-vard MTA: 7.64 MPa) and removed (ProRoot MTA: 6.58, HarvardMTA: 6.47 MPa) smear layer groups (P = 1).

Fig. 3 – The mean and standard deviation push-out bondstrength values for each group. The P values show thedifference in push-out strength between each two groups ofthe same obturation material.

9 ( 2 0 1 3 ) 797–803

Examination of the roots slices under optical microscope(40×) revealed adhesive bond failure in the majority of theCSC groups except for 1 sample in the ProRoot MTA groups,3 samples in the Harvard MTA groups and 3 samples inthe Biodentine groups where cohesive and combined failureoccurred. In the gutta percha and sealer groups, adhesive bondfailure occurred in all the specimens. The null hypotheseswere rejected according to these results (Fig. 4).

SEM images clearly demonstrated the presence/absence ofsmear layer in their respective groups (Fig. 5). Better adapta-tion was observed between the MTA and root canal dentinwhen the smear layer was preserved.

4. Discussion

The creation of a good seal is one of the major requirementsof root canal obturation materials. The bond with radiculardentin depends in part on the type of material used. CSCrelease calcium hydroxide during their hydration setting reac-tion [13]. In the presence of tissue fluid, an interfacial layerresembling hydroxyapatite in structure and composition isformed between root canal dentin and the CSC [13,14]. A recentstudy showed the formation of intra-tubular tags in conjunc-tion with an interfacial mineral interaction layer referred toas the “mineral infiltration zone” [15]. The results from thesestudies suggest the formation of a chemical bond between theCSC and radicular dentin [14].

Root canal instrumentation produces a layer of organicand inorganic material known as smear layer. It is a 1–5 �m-thick layer made of dentin shavings, necrotic pulp remnants,bacteria and their by-products [16–18]. Smear layer removalprior to obturation of the pulp space remains a controversialissue. On one hand, it is a loosely adherent layer that canprovide a pathway for microbial micro-leakage [19], it poten-tially harbors bacteria and can serve as a reservoir of irritants[20], it can provide a substrate for any remaining bacteria fol-lowing chemo-mechanical disinfection of the pulp space [21],and can prevent the penetration of irrigation solutions andinter-appointment medication into the dentinal tubules, thusjeopardizing the effective disinfection during root canal treat-ment [22]. On the other hand, the smear layer can block thedentinal tubules and alter their permeability which can limitbacterial and toxin penetration [23]. Furthermore, bacteria sur-viving the disinfection protocol can be entombed within thedentinal tubules by the smear layer and the obturation mate-rial [24].

The adhesion of obturation materials to root canal dentincan be influenced by the presence/absence of smear layer[25]. Zinc oxide eugenol sealers have been shown to fail topenetrate the dentinal tubules when smear layer was pre-served [16]. Plastic filling materials (pHEMA and silicone) [26]and sealers penetrated the dentinal tubules to depths of40–60 �m after removal of smear layer [27]. Micro-leakagestudies demonstrated improved sealing ability of gutta per-cha and sealer when smear layer was removed [28–30]. This

was explained by the improved penetration of sealers into thedentinal tubules in the absence of smear layer.

The apical sealing ability of CSC, on the other hand, wassignificantly reduced after smear layer removal [31–33]. This

d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) 797–803 801

Fig. 4 – Modes of bond failure in the calcium silicate groups: adhesive (a and b), cohesive (c) and combined (d). In the guttap d (e)

wtc(tprtd

Cposcbdrs

Fa

ercha and sealer groups adhesive bond failure was detecte

as attributed mainly to the inability of the CSC particleso penetrate the dentinal tubules due to their larger parti-le size (2.44–3.05 �m) [34] compared with dentinal tubules0.9–2.5 �m) [35]. A recent in vitro study used a scanning elec-ron microscope demonstrated that ProRoot MTA failed toenetrate the dentinal tubules at any level [36]. Therefore,emoval of smear layer results in poor adaptation of the CSCo dentin walls, and consequently, reduced bond strength toentin.

The effect of smear layer on the bond strength betweenSC and dentin has not been sufficiently investigated. Theush-out test has been demonstrated to be a reliable methodf assessing bond strength to root canal dentin [37]. In ourtudy, three CSCs were investigated in addition to gutta per-ha and AH-plus sealer. The samples were stored for one weekefore the mechanical testing to allow for the hydration of the

icalcium silicate constituent in the silicate cements, which isesponsible for the increase in compressive strength over theubsequent week following mixing [38]. The effect of smear

ig. 5 – Scanning electron microscopy images of root slices fillednd removed (b). The images suggest better adaptation between

with sealer tags remained attached to the dentin walls (f).

layer removal on the push-out bond strength was investi-gated in the four material groups. The push-out bond strengthvalues in our study are within the range of other recentlypublished papers [39–41]. White ProRoot MTA was reportedto have a push-out bond strength that ranged from 4.8 MPa,5.68–9.46 MPa [40] and 5.95–7.88 MPa [41]. The push-out val-ues for Angelus MTA (Angelus Dental Industry Products,Londrina, Brazil) were erroneously higher and ranged from99.60–118.95 MPa [42]. Therefore, comparison of the push-outbond strength of different CSC was not possible due to thedifferent methodologies used in previous studies.

Analysis of the push-out bond strengths revealed no sta-tistically significant difference between the different thirdswithin each group as the dislodgement forces were directlyproportional to the adhesion surface areas Our results showeda consistent decrease in the push-out bond strength of the

three CSC groups when the smear layer was removed. It seemsthat the smear layer is important in the formation of theinterfacial layer and possibly gets actively involved in the

with ProRoot MTA where the smear layer was preserved (a) the MTA and dentin when the smear layer is preserved.

l s 2

r

802 d e n t a l m a t e r i a

mineral interaction between the CSC and radicular dentin.In the gutta percha and AH-plus sealer groups, sealer rem-nants/tags were detected on the dentin walls in most of thesamples following the push-out test. While this observationneeds to be confirmed by SEM analysis, it suggests that thebond failure occurred at the gutta percha/sealer interface.Therefore, whether smear layer removal improved the bondstrength between the sealer and the radicular dentin couldnot be detected by this test.

The results of this study could have a clinical significanceas root filling materials should resist dislodgement when theyare indirectly subjected to occlusal forces, which could reflecton their sealing ability. It is essential to reiterate that thereare certain drawbacks for obturating the root canal space withCSCs as they are difficult to handle, extremely challengingto retrieve and while their placement is possible in straightwide root canals, it could be exceedingly demanding in curvednarrow canals. Nevertheless, owing to their bioactivity andexcellent sealing abilities, CSCs have a definite place in den-tistry and their use as obturation materials merits furtherresearch.

5. Conclusion

Within the limitations of this ex vivo study, it can be concludedthat smear layer removal is detrimental to the bond strengthbetween calcium silicate cements and root canal dentin.

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