Basic Research—Technology
The Root Canal Bonding of Chemical-cured Total-etch Resin Cements Mikako 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 reconstruct and reinforce pulpless teeth is a vital key to help prevent root fractures. Complete and firm adhesion of resin cement in root canal dentin using a post is critical to achieve it. The null hypothesis in the present study was that the bond strength of dual-cured and chemicalcured adhesive resin cements to root canal dentin is not affected by their vertical locations in the root canal. In the experiments, extracted human incisors restored with fiber-reinforced posts and adhesive resin cements were subjected to microtensile bond strength testing. Then, the failure modes and the dentin-bonding interfaces were observed. Self-etch and self-adhesive dualcured resin cements showed frequent pretesting failure despite using a silane coupling agent. Chemical-cured total-etch adhesive material showed stable bonding performances throughout the entire post space and thus has an advantage in post-core restorations. (J Endod 2008;34:583–586)
Key Words Chemical-cured, dual-cured, fiber-reinforced post, microtensile bond strength, resin cement, root canal dentin, self-etch, silane coupling agent, total-etch
P
ulpless teeth restored with a combination of fiber reinforced posts (FRP) and resin cores together with adhesive resin cements have shown excellent clinical performances in several studies (1– 4). Particularly important for clinicians, those reports suggest 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 those restorations (3, 5, 6). Therefore, more certain adhesion of post-core materials to root canal dentin is essential to improve the fracture resistance of pulpless teeth and prevent root fracture. One of the most influential factors compromising the bonding may be the intense contraction stress generated when curing adhesive materials in the post space. Bouillaguet et al. (7) conjectured that the unstable bonding performance of adhesive materials in the post space may be attributed to the high configuration factor (C-factor). They also argued that when the C-factor is high, the use of slower setting materials could reduce stress at the bonding interface by allowing the flow of the materials to relieve polymerization stress. The difficulty in achieving high bond strength throughout an entire root canal has been reported when using modern dentin bonding systems with FRP (7–17). Imperfect curing of the light-cured adhesives at the apical portions may be the cause of the inferior bond strengths. Therefore, using dual-cured and chemical-cured materials could have an advantage in root canal adhesion. The null hypothesis in the present study was that the bond strength of dual-cured and chemical-cured adhesive materials to root canal dentin with FRP was not affected by the vertical locations in a root canal.
Materials and Methods From the Department of Restorative Dentistry and Endodontology, 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 the Promotion of Science, by the 21st century COE program entitled “Origination of Frontier BioDentistry” at Osaka University Graduate School of Dentistry, and by the Japan-China Sasakawa Medical Fellowship. Address requests for reprints to Dr. Mikako Hayashi, Department of Restorative Dentistry and Endodontology, Osaka University 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 of Endodontists. doi:10.1016/j.joen.2008.02.003
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A 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. The incisors were decoronated using a low-speed diamond saw (Isomet III; Buehler, Lake Bluff, IL) under copious water cooling at the cement-enamel junction. A post space was prepared in the root portions of the teeth with a depth of 10 mm and a diameter of 1.5 mm using 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 (Rely X Unicem; 3M ESPE, St. Paul, MN, USA [RXU]), or a chemical-cured total-etch resin sealer (Super Bond Sealer; Sun Medical, Shiga, Japan [SBS]). Although SBS is an endodontic sealer, we used this material as an adhesive cement because of its advantage of an extended setting time. Each group was further subdivided, and half of the specimens were restored with glass FRP (Para Post Fiber White, Coltene/Whaledent) treated by a silane-coupling agent (Clearfil Ceramic Primer, Kuraray Medical [SCA]), whereas the 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 paper point and air blowing. Then, a dual-cured resin cement (PNV) and a glass FRP were inserted into the post space consecutively, and the cement was light cured for 40 seconds with an irradiation unit (Elipar Free Light 2, 3M ESPE) from the cervical orifice.
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Basic Research—Technology TABLE 1. Bonding Statuses of Specimens before Microtensile Bond Strength Testing Bonding statuses (Both-side bonding/Single-side bonding/Both-side debonding) Materials
PNV
Locations
RXU
SBS
SCA (ⴚ)
SCA (ⴙ)
SCA (ⴚ)
SCA (ⴙ)
SCA (ⴚ)
SCA (ⴙ)
3/6/0 2/6/1 4/5/0 4/5/0 7/2/0 4/5/0 3/5/1
4/5/0 1/7/1 3/6/0 2/6/1 4/4/1 5/3/1 4/5/0
0/8/1 0/6/3 1/4/4 3/5/1 1/7/1 3/6/0 4/4/1
2/4/3 3/4/2 1/6/2 1/6/2 3/5/1 4/4/1 8/1/0
0/3/6 5/2/2 6/3/0 8/1/0 9/0/0 9/0/0 9/0/0
7/1/1 8/1/0 6/3/0 7/2/0 9/0/0 9/0/0 9/0/0
23/36/4
12/40/11
22/30/11
46/9/8
Cervical 1 2 3 4 5 6 7 Apical Total n
27/34/2 p 0.59
Pretesting failure rate %
63.5
55/7/1
p 0.11 57.1
81.0
p 65.1
27.0
0.05 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.
In the group with the dual-cured resin cement, RXU was applied to each post space without any pretreatment. After that, the FRP was placed into the post space, and the cement was light cured for 40 seconds. In the chemical-cured resin sealer group, the prepared post space was treated with a 10% citric acid / 3% ferric chloride solution (Super Bond Green Activator, Sun Medical) for 10 seconds followed by copious rinsing with water and drying using a paper point and by air blowing. Then, 4META/TBB resin sealer (SBS) was applied to the post space, and an FRP was immediately inserted. All specimens with the bonded fiber posts were stored under conditions of 100% humidity at 37°C for 24 hours. Disk-shaped slabs with a thickness of approximately 1.0 mm were sliced by using the low-speed diamond saw perpendicular to the tooth axis. A total of seven slabs were obtained from each root, and the vertical locations of the seven slabs in the root were maintained throughout the testing. Then, one hourglassshaped specimen was trimmed from each slab. A groove is made through the entire canal wall from two sides, leaving only the post material and the luting cement to bear the load during testing. Incidences of bonding success or failure occurring during the trimming of the specimens were recorded in three categories: successful bonding on both sides, successful bonding on one side, and debonding on both sides. Distributions of the three different bonding statuses in the groups
with and without SCA were compared by means of a chi-square test at a 95% level of confidence. For the MTBS testing, each hourglass-shaped specimen was fixed to a custom-made metallic jig with cyanoacrylate glue. Then, the testing was conducted by means of a tabletop material testing machine (EZ test; Shimadzu, Kyoto, Japan) with a crosshead speed of 1.0 mm/min. The MTBS among the groups with different vertical locations in the post space and with the same adhesive materials were compared by means of the one-way analysis of variance and Scheffe’s F test at a 95% level of confidence. The fracture surfaces 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 slabs were fabricated with the same methods as for the MTBS testing and treated with a 50% phosphoric acid solution for 30 seconds and 5% NaOCl for 2 minutes. Then, the bonding interfaces were observed by a scanning electron microscope (SEM) (JSM9-840A; JOEL, Tokyo, Japan) at a magnification of 350 to 750.
Results Pretesting failure was found less frequently in the groups with SBS (Table 1). In these groups, the use of the SCA significantly reduced the
TABLE 2. Microtensile Bond Strength of Specimens Successfully Bonded on Both Sides PNV Materials Locations Cervical 1 2 3 4 5 6 7 Apical
SCA (ⴚ)
RXU SCA (ⴙ)
SCA (ⴚ)
SBS SCA (ⴙ)
SCA (ⴚ)
SCA (ⴙ)
Mean (SD)
n
Mean (SD)
n
Mean (SD)
n
Mean (SD)
n
Mean (SD)
n
Mean (SD)
n
10.8 (9.6) 7.5 (1.5) 12.1 (5.0) 13.2 (3.8) 11.1 (3.8) 8.9 (2.8) 13.0 (0.4)
3 2 4 4 7 4 3
17.1 (4.1) 15.3 (—) 8.3 (2.9) 10.4 (6.5) 11.3 (2.5) 12.3 (4.6) 11.0 (2.4)
4 1 3 2 4 5 4
— (—) — (—) 11.9 (—) 11.6 (6.6) 9.7 (—) 9.4 (8.7) 6.4 (4.6)
0 0 1 3 1 3 4
7.2 (2.1) 6.7 (4.0) 12.5 (—) 19.9 (—) 13.6 (3.4) 10.3 (5.6) 15.9 (7.5)
2 3 1 1 3 4 8
— (—) 7.7 (2.8)a 12.3 (6.0)a,b 11.7 (4.1)b 14.2 (6.0)b 12.2 (2.3)b 13.8 (4.3)b
0 5 6 8 9 9 9
31.7 (17.1)a 28.8 (11.4)a 24.3 (11.9)a 22.5 (8.3)a 24.7 (11.3)a 25.2 (11.7)a 24.1 (7.0)a
7 8 6 7 9 9 9
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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Basic Research—Technology
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.
incidence of pretesting failure from 27.0% to 12.7%. In the groups with PNV and RXU, the incidences of pretesting failure were 57.1% and 65.1%, respectively, despite using the SCA. The SCA did not significantly improve the bonding in these groups. The SBS group with the SCA showed MTBS in the range of 22.5 MPa to 31.7 MPa, whereas the MTBS in other groups were below 20 MPa (Table 2). The SBS group with the SCA showed that there were no differences in the MTBS throughout an entire post space. In the PNV groups, approximately 85% of the specimens were fractured at the interfaces between the dentin and the cement or between the cement and the posts. In the RXU groups, more than two thirds were fractured at the interfaces between the dentin and the cement. In the SBS group without the SCA, 74.5% were fractured at the interfaces between the cement and the posts. In the SBS group with the SCA, half of the specimens failed at the interfaces between the dentin and the cement, and the other half showed cohesive failure in the posts or mixed failure including cohesive failure in the posts. The bonding interfaces of the adhesive materials to root canal dentin showed that no resin tag formation was found at the adhesive interfaces in the PNV and RXU (Fig. 1A) groups, whereas stout resin tags extending over 100 m were observed in the SBS groups (Fig. 1B). Intertubular anastomoses in lateral branches were also found in those resin tags.
Discussion Our finding of frequent pretesting failures before conducting the MTBS testing with root canal dentin in post spaces is similar to that reported in other studies (9, 18, 19). Push-out testing has sometimes been recommended as an alternative because of the low incidence of pretesting failure while preparing the specimens (9, 19). In the present study, only 12.7% of the specimens in the SBS group with the SCA showed pretesting failure, although all other groups showed frequent incidences. These results indicate that methods of the MTBS testing used in the present study are superior as sensitive techniques to detect latent failures when compared with push-out testing. Our results revealed that SBS with SCA is a reliable bonding material for placing FRP because it showed the fewest pretesting failures and also provided high bonding strength throughout the entire post space. This remarkably superior performance of the adhesion in the root canal when using SBS can be explained by the unique characteristics of this material. The dentin conditioner that is part of the SBS system,
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composed of 10% citric acid/3% ferric chloride solution, removes the smear layer and has an etching effect that opens the dentinal tubules. This then allows monomers with small molecular size to penetrate the opened tubules, leading to the formation of resin tags of over 100 m (Fig. 1B). Such long resin tags with intertubular anastomoses in their lateral branches can counter the stress caused by the polymerization shrinkage and contribute to enhancing the mechanical bonding strength (20). Chemical curing may also have an advantage in promoting even distribution of the stress caused by the polymerization shrinkage and inducing even bonding strength in the entire post space (21). The application of the SCA was beneficial in improving the bond strength in the SBS group because the most frequent failure mode in this group without SCA was adhesive failure at the interface between the post and the sealer. Therefore, it is reasonable to conclude that the incidences of pretesting failure were significantly reduced and the MTBS were also markedly improved by applying the SCA. By contrast, the bonding in the PNA and RXU groups was not improved by the SCA because the fractures were found at the interface between the cements and the dentin in those groups without SCA. Improving the bonding of PNA and RXU cements to root canal dentin is a priority. Only when such adhesion is achieved can SCA contribute to improve the bonding of FRP in a post space, as we showed in the SBS groups. Another problematic consideration in achieving root canal bonding is the thick smear layer after the preparation of a post space. Goracci et al. (19) reported that a total-etch resin cement showed greater bonding potential than a self-etch cement when luting the FRP to root canal dentin. It may be because acidic monomers responsible for substrate conditioning in the self-etch resin cement were less effective in etching through the thick smear layer. This might have accounted for the significantly lower bond strength of the FRP to the root canal dentin in the present study. Further studies need to be performed to identify the appropriate treatments to control the thick smear layer and to produce proper 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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12. Akgungor G, Akkayan B. Influence of dentin bonding agents and polymerization models on the bond strength between translucent fiber posts and three dentin regions within a post space. J Prosthet Dent 2006;95:368 –78. 13. Aksornmuang J, Nakajima M, Foxton RM, Tagami J. Effect of prolonged photoirradiation time of three self-etch systems on the bonding to root canal dentin. J Dent 2006;34:389 –97. 14. Mallmann A, Jacques LB, Valandro LF, Muench A. Microtensile bond strength of photoactivated and autopolymerized adhesive systems to root dentin using translucent and opaque fiber-reinforced composite posts. J Prosthet Dent 2007;97:165–72. 15. Aksornmuang J, Nakajima M, Foxton RM, Tagami J. Mechanical properties and bond strength of dual-cure resin composites to root canal dentin. Dent Mater 2007;23:226 –34. 16. Pirani C, Chersoni S, Foschi F, et al. Does hybridization of intraradicular dentin really improve fiber post retention in endodontically treated teeth? J Endod 2005;31:891– 4. 17. Faria e Silva AL, Casselli DS, Ambrosono GMB, Martins LRM. Effect of the adhesive application mode and fiber post translucency on the push-out bond strength to denin. J Endod 2007;33:1078 – 81. 18. Mallmann A, Jacques LB, Valandro LF, Mathias P, Muench A. Microtensile bond strength of light- and self-cured adhesive systems to intraradicular dentin using a translucent fiber post. Oper Dent 2005;30:500 – 6. 19. Goracci C, Sadek FT, Fabianelli A, Tay FR, Ferrari M. Evaluation of the adhesion of fiber posts to intraradicular dentin. Oper Dent 2005;30:627–35. 20. Hayashi M, Takahashi Y, Hirai M, Iwami Y, Imazato S, Ebisu S. Effect of endodontic irrigation on bonding of resin cement to radicular dentin. Eur J Oral Sci 2005;113:70 – 6. 21. Braga RR, Ferracane JL. Alternative in polymerization contraction stress management. Crit Rev Oral Biol Med 2004;15:176 – 84.
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