The Root Canal Bonding of Chemical-curedTotal-etch Resin CementsMikako Hayashi, DDS, PhD, Kenji Okamura, DDS, Hongxia Wu, DDS,Yutaka Takahashi, DDS, PhD, Evgeni V. Koytchev, DDS, PhD, Satoshi Imazato, DDS, PhD,and Shigeyuki Ebisu, DDS, PhD

Abstract

Discovering a durable restorative method to reconstructand reinforce pulpless teeth is a vital key to helpprevent root fractures. Complete and firm adhesion ofresin cement in root canal dentin using a post is criticalto achieve it. The null hypothesis in the present studywas that the bond strength of dual-cured and chemical-cured adhesive resin cements to root canal dentin is notaffected by their vertical locations in the root canal. Inthe experiments, extracted human incisors restoredwith fiber-reinforced posts and adhesive resin cementswere subjected to microtensile bond strength testing.Then, the failure modes and the dentin-bonding inter-faces were observed. Self-etch and self-adhesive dual-cured resin cements showed frequent pretesting failuredespite using a silane coupling agent. Chemical-curedtotal-etch adhesive material showed stable bondingperformances throughout the entire post space andthus has an advantage in post-core restorations. (JEndod 2008;34:583–586)

Key Words

Chemical-cured, dual-cured, fiber-reinforced post, mi-crotensile bond strength, resin cement, root canal den-tin, self-etch, silane coupling agent, total-etch

Pulpless teeth restored with a combination of fiber reinforced posts (FRP) and resincores together with adhesive resin cements have shown excellent clinical perfor-

mances in several studies (1– 4). Particularly important for clinicians, those reportssuggest that the incidence of root fractures is low in such treated teeth. However,debonding of the FRP from its post space was the most frequent failure pattern in thoserestorations (3, 5, 6). Therefore, more certain adhesion of post-core materials to rootcanal dentin is essential to improve the fracture resistance of pulpless teeth and preventroot fracture.

One of the most influential factors compromising the bonding may be the intensecontraction stress generated when curing adhesive materials in the post space. Bouil-laguet et al. (7) conjectured that the unstable bonding performance of adhesive mate-rials in the post space may be attributed to the high configuration factor (C-factor). Theyalso argued that when the C-factor is high, the use of slower setting materials couldreduce stress at the bonding interface by allowing the flow of the materials to relievepolymerization stress.

The difficulty in achieving high bond strength throughout an entire root canal hasbeen reported when using modern dentin bonding systems with FRP (7–17). Imperfectcuring of the light-cured adhesives at the apical portions may be the cause of the inferiorbond strengths. Therefore, using dual-cured and chemical-cured materials could havean advantage in root canal adhesion.

The null hypothesis in the present study was that the bond strength of dual-curedand chemical-cured adhesive materials to root canal dentin with FRP was not affected bythe vertical locations in a root canal.

Materials and MethodsA total of 54 human upper incisors, free of caries and fractures, were stored in

Hank’s balanced salt solution at 4°C and used within 6 months of extraction. Theincisors were decoronated using a low-speed diamond saw (Isomet III; Buehler, LakeBluff, IL) under copious water cooling at the cement-enamel junction. A post space wasprepared in the root portions of the teeth with a depth of 10 mm and a diameter of 1.5 mmusing preparation drills (Para Post Drill #6; Coltene/Whaledent, Cuyahoga Falls, OH).

The prepared roots with the post were randomly assigned into one of three groups,and each group was treated with a dual-cured self-etch resin cement (Panavia F 2.0;Kuraray Medical, Tokyo, Japan [PNV]), a dual-cured self-adhesive resin cement (RelyX Unicem; 3M ESPE, St. Paul, MN, USA [RXU]), or a chemical-cured total-etch resinsealer (Super Bond Sealer; Sun Medical, Shiga, Japan [SBS]). Although SBS is anendodontic sealer, we used this material as an adhesive cement because of its advantageof an extended setting time. Each group was further subdivided, and half of the speci-mens were restored with glass FRP (Para Post Fiber White, Coltene/Whaledent) treatedby a silane-coupling agent (Clearfil Ceramic Primer, Kuraray Medical [SCA]), whereasthe other specimens were restored with the FRP without the SCA.

In the group with PNV, a self-etching primer (ED Primer A and B, Kuraray Medical)was applied to the dentin surface in each post space for 20 seconds and dried by a paperpoint and air blowing. Then, a dual-cured resin cement (PNV) and a glass FRP wereinserted into the post space consecutively, and the cement was light cured for 40seconds with an irradiation unit (Elipar Free Light 2, 3M ESPE) from the cervical orifice.

From the Department of Restorative Dentistry and Endod-ontology, Osaka University Graduate School of Dentistry,Osaka, Japan.

Supported in part by Grants-in-Aid for Scientific Research(nos. 19390842 and 19209060) from the Japan Society for thePromotion of Science, by the 21st century COE program enti-tled “Origination of Frontier BioDentistry” at Osaka UniversityGraduate School of Dentistry, and by the Japan-ChinaSasakawa Medical Fellowship.

Address requests for reprints to Dr. Mikako Hayashi, De-partment of Restorative Dentistry and Endodontology, OsakaUniversity Graduate School of Dentistry, 1-8 Yamadaoka,Suita, Osaka 565-0871, Japan. E-mail address: mikarin@dent.osaka-u.ac.jp.0099-2399/$0 - see front matter

Copyright © 2008 by the American Association ofEndodontists.doi:10.1016/j.joen.2008.02.003

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In the group with the dual-cured resin cement, RXU was applied toeach post space without any pretreatment. After that, the FRP was placedinto the post space, and the cement was light cured for 40 seconds.

In the chemical-cured resin sealer group, the prepared post spacewas treated with a 10% citric acid / 3% ferric chloride solution (SuperBond Green Activator, Sun Medical) for 10 seconds followed by copiousrinsing with water and drying using a paper point and by air blowing.Then, 4META/TBB resin sealer (SBS) was applied to the post space, andan FRP was immediately inserted.

All specimens with the bonded fiber posts were stored under con-ditions of 100% humidity at 37°C for 24 hours. Disk-shaped slabs witha thickness of approximately 1.0 mm were sliced by using the low-speeddiamond saw perpendicular to the tooth axis. A total of seven slabs wereobtained from each root, and the vertical locations of the seven slabs inthe root were maintained throughout the testing. Then, one hourglass-shaped specimen was trimmed from each slab. A groove is madethrough the entire canal wall from two sides, leaving only the postmaterial and the luting cement to bear the load during testing. Inci-dences of bonding success or failure occurring during the trimming ofthe specimens were recorded in three categories: successful bondingon both sides, successful bonding on one side, and debonding on bothsides. Distributions of the three different bonding statuses in the groups

with and without SCA were compared by means of a chi-square test at a95% level of confidence.

For the MTBS testing, each hourglass-shaped specimen was fixed to acustom-made metallic jig with cyanoacrylate glue. Then, the testing wasconducted by means of a tabletop material testing machine (EZ test; Shi-madzu, Kyoto, Japan) with a crosshead speed of 1.0 mm/min. The MTBSamong the groups with different vertical locations in the post space and withthe same adhesive materials were compared by means of the one-way anal-ysis of variance and Scheffe’s F test at a 95% level of confidence. The fracturesurfaces were observed by an optical microscope (SMZ-U; Nikon, Tokyo,Japan) at a magnification of�20 to determine the failure modes.

To observe the details of the bonding interfaces, disk-shaped slabswere fabricated with the same methods as for the MTBS testing andtreated with a 50% phosphoric acid solution for 30 seconds and 5%NaOCl for 2 minutes. Then, the bonding interfaces were observed by ascanning electron microscope (SEM) (JSM9-840A; JOEL, Tokyo, Ja-pan) at a magnification of�350 to�750.

ResultsPretesting failure was found less frequently in the groups with SBS

(Table 1). In these groups, the use of the SCA significantly reduced the

TABLE 1. Bonding Statuses of Specimens before Microtensile Bond Strength Testing

Bonding statuses (Both-side bonding/Single-side bonding/Both-side debonding)

Materials PNV RXU SBS

Locations SCA (�) SCA (�) SCA (�) SCA (�) SCA (�) SCA (�)

Cervical1 3/6/0 4/5/0 0/8/1 2/4/3 0/3/6 7/1/12 2/6/1 1/7/1 0/6/3 3/4/2 5/2/2 8/1/03 4/5/0 3/6/0 1/4/4 1/6/2 6/3/0 6/3/04 4/5/0 2/6/1 3/5/1 1/6/2 8/1/0 7/2/05 7/2/0 4/4/1 1/7/1 3/5/1 9/0/0 9/0/06 4/5/0 5/3/1 3/6/0 4/4/1 9/0/0 9/0/07 3/5/1 4/5/0 4/4/1 8/1/0 9/0/0 9/0/0

ApicalTotal

n 27/34/2 23/36/4 12/40/11 22/30/11 46/9/8 55/7/1p � 0.59 p � 0.11 p 0.05

Pretesting failure rate% 63.5 57.1 81.0 65.1 27.0 12.7

Distributions of the specimens with the three different bonding statuses were compared between the groups with and without the SCA by means of a chi-square test at a 95% level of confidence. Pretesting failure

(%)� ([numbers of single-side bonding� numbers of both-side debonding]/total numbers)� 100.

PNV, Panavia F; RXU, Rely X Unicem; SBS, Super Bond Sealer; SCA, silane coupling agent.

TABLE 2. Microtensile Bond Strength of Specimens Successfully Bonded on Both Sides

PNV RXU SBS

Materials SCA (�) SCA (�) SCA (�) SCA (�) SCA (�) SCA (�)

Locations Mean (SD) n Mean (SD) n Mean (SD) n Mean (SD) n Mean (SD) n Mean (SD) n

Cervical1 10.8 (9.6) 3 17.1 (4.1) 4 — (—) 0 7.2 (2.1) 2 — (—) 0 31.7 (17.1)a 72 7.5 (1.5) 2 15.3 (—) 1 — (—) 0 6.7 (4.0) 3 7.7 (2.8)a 5 28.8 (11.4)a 83 12.1 (5.0) 4 8.3 (2.9) 3 11.9 (—) 1 12.5 (—) 1 12.3 (6.0)a,b 6 24.3 (11.9)a 64 13.2 (3.8) 4 10.4 (6.5) 2 11.6 (6.6) 3 19.9 (—) 1 11.7 (4.1)b 8 22.5 (8.3)a 75 11.1 (3.8) 7 11.3 (2.5) 4 9.7 (—) 1 13.6 (3.4) 3 14.2 (6.0)b 9 24.7 (11.3)a 96 8.9 (2.8) 4 12.3 (4.6) 5 9.4 (8.7) 3 10.3 (5.6) 4 12.2 (2.3)b 9 25.2 (11.7)a 97 13.0 (0.4) 3 11.0 (2.4) 4 6.4 (4.6) 4 15.9 (7.5) 8 13.8 (4.3)b 9 24.1 (7.0)a 9

Apical

Statistical analyses, which compared the MTBS among the different vertical locations, were applied only for the SBS groups because insufficient specimens with successful adhesion were obtained in the PNV and

RXU groups. Groups with the same letters (a and b) showed no significant differences in the MTBS by means of the one-way analysis of variance and the Scheffe’s F test at a 95% level of confidence.

SD, standard deviation; PNV, Panavia F; RXU, Rely X Unicem; SBS; Super Bond Sealer; SCA, silane coupling agent.

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incidence of pretesting failure from 27.0% to 12.7%. In the groups withPNV and RXU, the incidences of pretesting failure were 57.1% and65.1%, respectively, despite using the SCA. The SCA did not significantlyimprove the bonding in these groups.

The SBS group with the SCA showed MTBS in the range of 22.5MPa to 31.7 MPa, whereas the MTBS in other groups were below 20MPa (Table 2). The SBS group with the SCA showed that there wereno differences in the MTBS throughout an entire post space.

In the PNV groups, approximately 85% of the specimens werefractured at the interfaces between the dentin and the cement or be-tween the cement and the posts. In the RXU groups, more than two thirdswere fractured at the interfaces between the dentin and the cement. Inthe SBS group without the SCA, 74.5% were fractured at the interfacesbetween the cement and the posts. In the SBS group with the SCA, half ofthe specimens failed at the interfaces between the dentin and the ce-ment, and the other half showed cohesive failure in the posts or mixedfailure including cohesive failure in the posts.

The bonding interfaces of the adhesive materials to root canal dentinshowed that no resin tag formation was found at the adhesive interfaces inthe PNV and RXU (Fig. 1A) groups, whereas stout resin tags extending over100 �m were observed in the SBS groups (Fig. 1B). Intertubular anasto-moses in lateral branches were also found in those resin tags.

DiscussionOur finding of frequent pretesting failures before conducting the

MTBS testing with root canal dentin in post spaces is similar to thatreported in other studies (9, 18, 19). Push-out testing has sometimesbeen recommended as an alternative because of the low incidence ofpretesting failure while preparing the specimens (9, 19). In the presentstudy, only 12.7% of the specimens in the SBS group with the SCAshowed pretesting failure, although all other groups showed frequentincidences. These results indicate that methods of the MTBS testing usedin the present study are superior as sensitive techniques to detect latentfailures when compared with push-out testing. Our results revealed thatSBS with SCA is a reliable bonding material for placing FRP because itshowed the fewest pretesting failures and also provided high bondingstrength throughout the entire post space.

This remarkably superior performance of the adhesion in the rootcanal when using SBS can be explained by the unique characteristics ofthis material. The dentin conditioner that is part of the SBS system,

composed of 10% citric acid/3% ferric chloride solution, removes thesmear layer and has an etching effect that opens the dentinal tubules.This then allows monomers with small molecular size to penetrate theopened tubules, leading to the formation of resin tags of over 100 �m(Fig. 1B). Such long resin tags with intertubular anastomoses in theirlateral branches can counter the stress caused by the polymerizationshrinkage and contribute to enhancing the mechanical bondingstrength (20). Chemical curing may also have an advantage in promot-ing even distribution of the stress caused by the polymerization shrink-age and inducing even bonding strength in the entire post space (21).

The application of the SCA was beneficial in improving the bondstrength in the SBS group because the most frequent failure mode in thisgroup without SCA was adhesive failure at the interface between the postand the sealer. Therefore, it is reasonable to conclude that the inci-dences of pretesting failure were significantly reduced and the MTBSwere also markedly improved by applying the SCA. By contrast, thebonding in the PNA and RXU groups was not improved by the SCAbecause the fractures were found at the interface between the cementsand the dentin in those groups without SCA. Improving the bonding ofPNA and RXU cements to root canal dentin is a priority. Only when suchadhesion is achieved can SCA contribute to improve the bonding of FRPin a post space, as we showed in the SBS groups.

Another problematic consideration in achieving root canal bond-ing is the thick smear layer after the preparation of a post space. Goracciet al. (19) reported that a total-etch resin cement showed greater bond-ing potential than a self-etch cement when luting the FRP to root canaldentin. It may be because acidic monomers responsible for substrateconditioning in the self-etch resin cement were less effective in etchingthrough the thick smear layer. This might have accounted for the sig-nificantly lower bond strength of the FRP to the root canal dentin in thepresent study. Further studies need to be performed to identify theappropriate treatments to control the thick smear layer and to produceproper conditions for self-etch resin cement.

In conclusion, chemical-cured total-etch adhesive materials,which showed stable bonding performances in an entire post space,clearly have an advantage in post-core restorations.

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Figure 1. Bonding interfaces of the adhesive materials to root canal dentin. (A) In the RXU group, no resin tag formation was found at the adhesive interfaces. (B)In the SBS group, stout resin tags extending over 100 �m were found, and the following intertubular anastomoses in lateral branches were observed (a) post, (b)adhesive material, (c) root canal dentin, and (d) resin tag.

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