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Effect of viscosity of dual-cure luting resin composite core materials on bond strength to fiber posts with various surface treatments
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Journal of Dental Sciences (2014) xx, 1e8
Available online at www.sciencedirect.com
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ORIGINAL ARTICLE
Effect of viscosity of dual-cure luting resin composite core materials on bond strength to fiber posts with various surface treatments Juthatip Aksornmuang a*, Masatoshi Nakajima b, Junji Tagami b,c a
Department of Prosthetic Dentistry, Faculty of Dentistry, Prince of Songkla University, Hat Yai, Songkhla, Thailand b Cariology and Operative Dentistry, Department of Restorative Science, Tokyo Medical and Dental University, Tokyo, Japan c Global Center of Excellence Program, International Research Center for Molecular Science in Tooth and Bone Diseases, Tokyo Medical and Dental University, Tokyo, Japan Received 23 January 2014; Final revision received 21 March 2014
Available online - - -
KEYWORDS bond strength; post-surface treatment; resin composites; viscosity
Abstract Background/purpose: The objective of this study was to evaluate the viscosity of two dual-cure resin composite core materials and their bond strength to fiber posts treated with various surface treatments. Materials and methods: Viscosity at 60e90 seconds after mixing two resin composite core materials, Clearfil DC Core (DC) and Build-It FR (BI), was tested by a rheometer. Eighteen fiber posts (FibreKor) were divided into three groups according to the following post surface treatments: (1) no surface treatment; (2) application of silane coupling agent (Silane); and (3) application of silane followed by Bond-1 adhesive resin (Silane þ Bond-1). Treated posts were cemented into artificial post spaces using DC or BI. After 24-hour storage, each specimen was serially sliced into 12 beams for a microtensile bond strength test. The data were divided into upper, middle, and bottom regions, and statistically analyzed (a Z 0.05). Failure modes were observed using a scanning electron microscope. Results: The viscosity of BI at 60 seconds after mixing was lower than that of DC. Bond strengths were found to be affected by luting resin composites, surface treatment, and region. For DC, bond strength was significantly improved in the group of Silane þ Bond-1 (P < 0.05).
* Corresponding author. Department of Prosthetic Dentistry, Faculty of Dentistry, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand. E-mail address: [email protected] (J. Aksornmuang). http://dx.doi.org/10.1016/j.jds.2014.04.001 1991-7902/Copyright ª 2014, Association for Dental Sciences of the Republic of China. Published by Elsevier Taiwan LLC. All rights reserved.
Please cite this article in press as: Aksornmuang J, et al., Effect of viscosity of dual-cure luting resin composite core materials on bond strength to fiber posts with various surface treatments, Journal of Dental Sciences (2014), http://dx.doi.org/10.1016/j.jds.2014.04.001
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J. Aksornmuang et al For BI, application of Silane significantly improved bond strength (P < 0.05), but application of Silane þ Bond-1 had no advantageous effect on it. Conclusion: Bond strengths between luting resin composites and fiber posts were affected by post surface treatments, depending on the resin composite used. Application of a low-viscosity adhesive resin to the post surface seemed to be beneficial for a high-viscosity luting resin composite. Copyright ª 2014, Association for Dental Sciences of the Republic of China. Published by Elsevier Taiwan LLC. All rights reserved.
Introduction Currently, utilization of fiber posts to provide radicular retention for crown restoration is routinely recommended for the treatment of endodontically treated teeth. Several studies have accordingly supported the benefit of fiber posts in reducing the incidence of root fracture, due to their modulus elasticity, which is comparable to that of dentin.1e3 Moreover, fiber posts provide superior aesthetics, easier removal, and shorter treatment visits compared to cast metal posts and cores.4 In fiber post luting procedures, either a resin cement or a resin composite core material can be used as a luting medium. Dualcure resin composite core materials have been introduced for the placement of fiber posts in post spaces because a resin composite has a modulus of elasticity close to that of dentin and fiber posts, and better mechanical properties than those of a resin cement.5 A recent study revealed significantly higher bond strength when luting the fiber post with dual-cure core build-up materials than with resin cements.6 Even though the utilization of a fiber post can reduce the incidence of root fracture, failure can still occur through decementation of the fiber post from the canal. The flexibility of fiber posts allows them to consensually bend with dentin during function, resulting in debonding of the interface when bond strength is inadequate. Failure can occur either at the resinedentin or at the resinepost interface. A previous study reported that debonding between fiber posts and a luting resin took place when the root canal dentin was efficiently bonded.7 Various surface treatment methods have been proposed to improve the adhesion between post surfaces and the luting resin, either through mechanical interlocking or through chemical bonding at the interface.8 Various techniques for increasing the roughness of post surfaces, by means of hydrogen peroxide etching,9 hydrofluoric acid etching,10 or airborneparticle abrasion with aluminum oxide,11,12 have been reported to be effective for increasing bond strength between fiber posts and the luting resin. However, one study revealed that post surface abrasion produced undesirable surface changes.13 Silane application to post surfaces was found to be a simple method for improving the bonding between post surfaces and the luting resin.10,14 However, there have been conflicting research results in regard to the efficiency of silane as a coupling agent to fiber post surfaces.8 Various fiber posts are available in the dental market. According to manufacturers’ instructions, several
methodsdsuch as no treatment needed, application of silane, or application of silane followed by an adhesive resindhave been suggested as post surface treatments prior to bonding posts in root canals. Application of these chemical agents to post surfaces has been recommended in order to improve the adhesion between fiber posts and the luting resin through the mechanisms of chemical interaction and increased wettability. Low viscosity of an adhesive resin is important for good wettability of post surfaces.15 However, if the luting resin itself has low viscosity, additional application of an adhesive resin may have a negligible effect. Dual-cure resin core materials of various viscosities have been developed. Lower-viscosity resin core materials would be easier to inject into the root canal; however, it would be more difficult to build up a core part without a matrix. By contrast, resin composites with higher viscosity may produce a gap at the surface, although they facilitate the construction of an abutment form. Only a limited number of studies have investigated the effect of viscosity of dualcure luting resin composite core materials on bond strength. Moreover, the effect of surface treatments may be related to the viscosity of the luting medium, an assumption that has not yet been proved. This study was performed to evaluate the viscosity of two dual-cured resin composite core materials used for luting fiber posts and their bond strength to fiber posts treated with various surface treatments. In addition, the correlation between viscosity and bond strength data would be described. The null hypotheses were that the viscosity of the luting agent, types of luting agent, post surface treatments, and regional differences do not affect the bonding between fiber posts and luting resin composite materials.
Materials and methods Two dual-cure resin composite core materials were used for bonding fiber posts: Clearfil DC Core (DC; Kuraray Medical, Tokyo, Japan) and Build-It FR (BI; Pentron Clinical Technologies, Wallingford, CT, USA). After mixing, their viscosity was tested using a rheometer (HAAKE RheoStress RS75; Thermo Electron, Karlsruhe, Germany) at 25 C with a C20/4 (diameter Z 20 mm, angle Z 4 ) sensor. Viscosity of each resin composite was determined twice; the viscosity values in Pa$second were reported at 60 seconds, 70 seconds, 80 seconds, and 90 seconds after mixing. Eighteen 1.48-mm-diameter and 18-mm-length FibreKor nonserrated glass fiber-reinforced composite posts (Pentron
Please cite this article in press as: Aksornmuang J, et al., Effect of viscosity of dual-cure luting resin composite core materials on bond strength to fiber posts with various surface treatments, Journal of Dental Sciences (2014), http://dx.doi.org/10.1016/j.jds.2014.04.001
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Clinical Technologies) were used in this study. The posts were cut to a length of 12 mm from the upper end by a diamond bur (#201; Shofu, Kyoto, Japan) mounted on a high-speed handpiece under water spray. The cut posts were straight in shape. Surfaces of the posts were then cleaned with alcohol. Eighteen artificial post spaces, 8 mm in diameter and 12 mm in depth, were prepared in 1 cm 1 cm 2 cm acrylonitrile butadiene styrene resin blocks.14 Materials used in this study are presented in Table 1. FibreKor posts were randomly divided into three groups according to the following post surface treatments: (1) no surface treatment; (2) application of silane coupling agent (Silane; Pentron Clinical Technologies); and (3) application of Silane followed by an adhesive resin (Bond-1; Pentron Clinical Technologies). All materials were used following the manufacturer’s instructions. After the post surfaces were treated, one of the tested dual-cure resin core materials (DC or BI) was mixed and injected into the post cavity. At 60 seconds after mixing, the remaining resin was placed on the post surface and then the post was carefully placed into the center of the cavity. The upper surface of the specimen was covered with a plastic strip and pressed gently with a glass slide to squeeze out any excess resin. The specimens were exposed to light for 60 seconds using a halogen light source (Optilux 500; Demetron, Danbury, CT, USA), the tip of which was placed at the top of the cavity (Fig. 1). These were then stored in water for 24 hours at 37 C. After 24 hours of storage, each specimen was attached to the arm of a low-speed diamond saw (IsoMet; Buehler, Lake Bluff, IL, USA), and 12 slabs were serially cut perpendicular to the bonded interface under water cooling. Each slab was then transversely sectioned at the middle part of the post into approximately 0.6 mm 0.6 mm-thick beams. The cross-sectional area of each beam was measured using digital calipers (CD15; Mitutoyo, Kawasaki, Japan). One of two interfaces of each beam was randomly selected for testing. The ends of the beam and the remaining interface were glued onto a testing device on a table-top testing machine (EZ Test; Shimadzu, Kyoto, Japan) using cyanoacrylate glue (Zapit; DVA, Corona, CA,
Table 1
USA), and subjected to tensile force at a crosshead speed of 1 mm/minute (Fig. 1). After the specimens had fractured, both the resin side and the post side of the fractured beams were mounted on brass tablets and sputter-coated with gold. Fracture modes were observed using a scanning electron microscope (JSM5310; JEOL, Tokyo, Japan) and were classified as follows: cohesive failure within the post; mixed interfacial failure and cohesive failure within the post; interfacial failure, mixed interfacial failure, and cohesive failure within the resin; and cohesive failure within the resin. Microtensile bond strength (mTBS) data for each post were divided into three regions (upper, middle, and bottom). Each region consisted of four beams per bonded post; 12 beams were obtained from each subexperimental group. Data were analyzed using three-way analysis of variance (ANOVA), and Tukey’s HSD was used as a post hoc test for multiple comparisons. All statistical analyses were performed at a 95% confidence level.
Results Viscosity data after testing the viscosity of DC and BI twice are shown in Table 2 and graphically presented in Fig. 2. At 60 seconds after mixing, the viscosity of BI was lower than that of DC; however, its viscosity increased over time. By contrast, the viscosity of DC decreased from 60 seconds to 90 seconds after mixing. At 90 seconds after mixing, the viscosity of these two core materials became closer than that at 60 seconds after mixing. Three-way ANOVA indicated that mTBS was significantly affected by the luting resin composite material, surface treatment, and region of the post surface (P < 0.0001). There was a significant interaction between the luting resin composite and surface treatment factors (P < 0.0001). The means and standard deviation values of mTBS for each experimental group and each region are shown in Table 3. For DC, the average bond strength values at the middle and bottom regions increased slightly when the post surfaces were treated with Silane compared to no surface treatment (P > 0.05), but no statistically significant
Materials used in this study.
Material
Manufacturer
Composition
FibreKor Clearfil DC Core
Pentron Clinical Technologies (Wallingford, CT, USA) Kuraray Medical (Tokyo, Japan)
Build-It FR
Pentron Clinical Technologies
Silane Bond-1
Pentron Clinical Technologies Pentron Clinical Technologies
30.8% volume of glass fiber, 16.2% volume of filler, 53% volume of resin content (Bis-GMA, UDMA, HDDMA) Catalyst: Bis-GMA, TEGDMA, silanated colloidal silica, barium glass, d,l-camphorquinone, benzoyl peroxide Universal: Bis-GMA, TEGDMA, silanated colloidal silica, barium glass, N,N-diethanol-p-toluidine Bis-GMA, UDMA, HDDMA, barium borosilicate glass fillers, chopped glass fiber, photochemical initiator Organosilane in methyl alcohol PMGDM, HEMA, TMPTA, ethanol, acetone, photoinitiator, amine accelerator, stabilizer
Bis-GMA Z bisphenol-a-glycidyl dimethacrylate; HDDMA Z hexanediol dimethacrylate; HEMA Z 2-hydroxyethyl methacrylate; PMGDM Z pyromellitic glycerol dimethacrylate; TEGDMA Z triethyleneglycol dimethacrylate; TMPTA Z trimethylolpropane triacrylate; UDMA Z urethane dimethacrylate.
Please cite this article in press as: Aksornmuang J, et al., Effect of viscosity of dual-cure luting resin composite core materials on bond strength to fiber posts with various surface treatments, Journal of Dental Sciences (2014), http://dx.doi.org/10.1016/j.jds.2014.04.001
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Figure 1
Schematic illustration of bonding and microtensile bond strength test procedures.
difference was detected. A significant increase of bond strength after silane application was identified only at the upper region (P < 0.05). Bond strengths significantly improved in the Silane þ Bond-1 group for all regions (P < 0.05). For BI, bond strengths of the groups treated with Silane and Silane þ Bond-1 were significantly higher than those of the no surface treatment group for all regions (P < 0.05). No significant difference was observed in bond strengths between the Silane and Silane þ Bond-1 groups (P > 0.05). A comparison of the two luting resin composite materials, DC and BI, showed that BI provided significantly
higher bond strengths than DC in the no surface treatment and silane-treated groups (P < 0.05). However, both resin composites generated similar bond strengths when the surfaces were treated with Silane þ Bond-1 (P > 0.05). Regarding regional differences in bond strengths, significant differences were found only in the group of DC treated by Silane, in which the upper region exhibited higher bond strength than the middle and bottom regions (P < 0.05).
Table 2 Viscosity (Pa$second) of two dual-cure resin composite core materials, DC and BI, at 60e90 seconds after mixing. Time (seconds after mixing)
Test 1
DC Test 2
Test 1
BI Test 2
60 70 80 90
919.6 989.8 899.1 806.7
1001.0 949.3 761.1 650.9
101.2 108.0 207.5 622.7
108.5 113.0 194.0 462.9
BI Z Build-It FR; DC Z Clearfil DC Core.
Figure 2 Viscosity (Pa$second) of DC and BI at 60e90 seconds after mixing. BI Z Build-It FR; DC Z Clearfil DC Core.
Please cite this article in press as: Aksornmuang J, et al., Effect of viscosity of dual-cure luting resin composite core materials on bond strength to fiber posts with various surface treatments, Journal of Dental Sciences (2014), http://dx.doi.org/10.1016/j.jds.2014.04.001
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Table 3 Regional mTBS [mean (sd) in MPa] of FibreKor posts to dual-cure resin composite core materials.a
No surface treatment
Silane
Silane+Bond-1
DC
BI
DC
BI
DC
BI
Upper
19.6(8.1)C
36.6(9.1)A
36.0(10.8)A
50.1(9.0)B
40.5(10.9)A,B
44.4(6.2) A,B
Middle
13.7(6.9)O
33.1(10.4)M
20.9(9.8)O
45.4(9.2)N
38.2(9.6)M,N
42.7(8.0)M,N
Bottom
13.6(7.1)Z
32.3(9.0)X
17.3(9.4)Z
44.2(11.5)Y
34.7(6.8)X,Y
41.9(9.2)X,Y
a
Same superscript letters within each region indicate no significant difference. Brackets indicate significant differences in µTBS between two regions.
BI = Build-It FR; DC = Clearfil DC Core; µTBS = microtensile bond strength; sd = standard deviation.
Fig. 3 presents the percentage of fracture modes for each resin composite and surface treatment. The results revealed that most specimens fractured at the interface between the post and the resin composite (Fig. 4). Several air bubbles were observed on the resin side when DC was used (Fig. 5). Cohesive failure within the bonding substrates and mixed failure were found more frequently when the post surfaces were treated with Silane or Silane þ Bond-1 compared with the no surface treatment group (Fig. 6).
Discussion A viscosity test was performed to evaluate the rheological characteristic of two dual-cure resin composite core materials used for luting fiber posts. Due to the limitations of the rheometer, viscosity data could be reported no earlier than 60 seconds after mixing. The results showed that the viscosity of DC at 60 seconds after mixing was approximately 10 times higher than that of BI. BI is an auto-mixtype dual-cure resin composite. Two cartridges of resin paste were prepared and connected to an auto-mix syringe
Figure 3 DC Core.
tip. Using a mixing gun, the resins in both cartridges were compressed and mixed together in a spiral tube. The resins prepared were therefore of low viscosity to allow them to become entangled easily and be mixed consistently. The viscosity of BI increased four to six times from 60 seconds to 90 seconds after mixing, which might be due to the initiation of the polymer propagation process of the resin. The consistency of the composite resin increased after mixing due to the polymerization reaction that occurred; when the polymer chains grow larger and connect to each other, consistency becomes higher. The degree of conversion cannot be controlled even when the mixing protocols for each mix are the same. Therefore, the viscosity data exhibited some differences between the two times testing. The viscosity experiment was repeated, and both of them showed the same tendency in the alteration of consistency during 60e90 seconds after mixing. By contrast, DC is a conventional hand-mix-type dualcure resin composite, in which resins were equally pressed from two cartridges and mixed by hand spatulation. Resistance force during mixing of DC could be sensed by the
Percentage of failure mode for each surface treatment and each luting resin composite. BI Z Build-It FR; DC Z Clearfil
Please cite this article in press as: Aksornmuang J, et al., Effect of viscosity of dual-cure luting resin composite core materials on bond strength to fiber posts with various surface treatments, Journal of Dental Sciences (2014), http://dx.doi.org/10.1016/j.jds.2014.04.001
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Figure 4 Representative SEM micrographs of the fracture surfaces of dentin and resin sides in the group bonded with BI without surface treatment, showing interfacial failure. On the post side, exposed fibers can be observed with resin matrix in between. On the resin side, parallel impression lines of fibers are present. BI Z Build-It FR; SEM Z scanning electron microscope.
operator due to the thick consistency of the resins. The high viscosity of DC would be an effect of higher filler loading compared to BI. The amount of fillers in BI accounts for 68.2% of its mass, whereas that in DC accounts for 77.8% of its mass.16 A previous study also showed a significantly higher Knoop hardness number of DC than that of BI at both the coronal and the apical regions of the post space.17 However, it was noticed that the viscosity of DC
Figure 5 Representative SEM micrograph of the fracture surface of the resin side, showing interfacial failure in the group bonded with hand-mix resin composite (DC). Several voids are observed. DC Z Clearfil DC Core; SEM Z scanning electron microscope.
decreased slightly from 60 seconds to 90 seconds after mixing, which might be due to the presence of the smallmolecule diluent monomer triethyleneglycol dimethacrylate in DC. The presence of air bubbles within DC could also affect the loadedeformation diagram. It was noticed from the present study results that the two tested resin composites showed different viscosity conversion from 60 seconds to 90 seconds. Bonding fiber posts with different resin composite materials at different times may affect the bond strength. In the present study, the resin composite was placed on the post surface at 60 seconds after mixing to simulate real clinical procedures. Further studies should be conducted to evaluate the viscosity characteristics and bond strength to post surfaces of various resin composite core materials at different bonding times. Statistical analysis using three-way ANOVA indicated that the strength of bonds between luting resin composites and post surfaces was affected by the type of resin composite and surface treatment. The null hypothesis was rejected. In addition, a significant interaction was found between these two independent variables. In other words, the adhesion between luting resin composites and post surfaces was affected by surface treatment methods, depending on the type of resin composite used for luting. In the no surface treatment group, fiber posts luted with the lower-viscosity resin composite (BI) provided significantly higher bond strength than those luted with the higher-viscosity resin composite (DC). This might indicate that the viscosity of the luting resin composites had an effect on their adhesion to post surfaces. BI and DC are
Please cite this article in press as: Aksornmuang J, et al., Effect of viscosity of dual-cure luting resin composite core materials on bond strength to fiber posts with various surface treatments, Journal of Dental Sciences (2014), http://dx.doi.org/10.1016/j.jds.2014.04.001
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Figure 6 Representative SEM micrographs of fracture surfaces showing failures as mixed failure in the group bonded with BI and surface treated with Silane þ Bond-1 and cohesive failure within the post in the group bonded with DC and surface treated with Silane þ Bond-1. For mixed failure, failure within the adhesive resin is indicated by the wavy pattern of adhesive resin on both sides of the fracture beams. For failure within the post, numerous parallel fibers with some resin matrix in between are observed on both sides. BI Z Build-It FR; DC Z Clearfil DC Core; SEM Z scanning electron microscope.
both bisphenol-a-glycidyl dimethacrylate-based resin composites. However, the resin matrix of BI is also composed of urethane dimethacrylate and hexanediol dimethacrylate, which are the same materials contained in FibreKor. The resin part of the fiber posts was supposed to be completely polymerized during postfabrication procedures. Carbon double bonds in the resin matrix of the post were mostly used up. Further polymer chain propagation of urethane dimethacrylate and hexanediol dimethacrylate between the post surface and BI could not occur. The identical resin matrix of BI and FibreKor, therefore, would not be a favorable factor for high bond strength. For DC, bond strengths increased slightly when post surfaces were treated with silane. Statistically significant differences in the bond strength between no surface treatment and silane-treated groups could not be detected at the middle and bottom regions. Due to the high viscosity of DC, no surface treatment, or treatment with silane only, might not have enough potential to improve bond strength. The chemical bond between silane and the silica component of glass fiber appeared to have little effect on increased bond strength, whereas Bond-1 adhesive resin was able to improve bond strength significantly. The lowviscosity resin provided a good wettability to the post surfaces. According to the principle of adhesion, wettability of the adhesive is one of the important factors for good adhesion between bonding substrates. Bond strength of a high-consistency adherend can be improved using a lowviscosity resin.18
On the contrary, for BI the average bond strengths of the experimental groups, Silane and Silane þ Bond-1, were higher than that of the control group; however, no statistically significant difference was detected between the no surface treatment and the Silane þ Bond-1 groups. This might indicate that the application of a low-viscosity resin adhesive was less advantageous when a low-viscosity luting resin composite was used. However, application of silane could only statistically elevate the bond strength; this might be due to the chemical reaction of silane as a coupling agent. The decrease in bond strength when an adhesive resin was applied would be due to the poor mechanical properties of the adhesive itself. Failure analysis revealed that cohesive failure within the resin and mixed failure (including failure within the resin) were more frequently observed when Bond-1 adhesive was applied to post surfaces (Fig. 3). Regarding regional differences, there was no significant difference in bond strength between upper, middle, and bottom regions for all tested groups, except for the group treated with Silane and fixed with DC. It was reported that regions of the post space did not affect bond strength when the post surface was silanized. The results of the present study were in accordance with those of previous reports.14,19 However, in the DC group, which has high consistency, bond strengths at the upper region were found to be higher than those at the lower region when no adhesive resin was applied. A higher compression force at the top surface during compression with a glass slide prior to curing
Please cite this article in press as: Aksornmuang J, et al., Effect of viscosity of dual-cure luting resin composite core materials on bond strength to fiber posts with various surface treatments, Journal of Dental Sciences (2014), http://dx.doi.org/10.1016/j.jds.2014.04.001
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8 might cause DC to come into more intimate contact with the upper region of the post surface, as opposed to the lower region. Fractographic analysis revealed that the majority of failure patterns occurred as interfacial failure between post surfaces and resin composites. More complicated failure patterns such as mixed failure, cohesive failure within the resin, and cohesive failure within the posts were found when bond strengths were higher. Interestingly, in the group of no surface treatment of DC, several voids could be observed on the resin side of fractured beams (Fig. 5). This finding is in agreement with a previous study that evaluated the properties of various dual-cure resin composite core materials,17 which indicated that air bubbles are trapped inside the resin composite bulk during spatulation. Based on the results of the present study, in order to obtain the optimal bond strength between the luting resin composite and the glass fiber-reinforced composite post surface, surface treatment method should be selected depending on the resin composite used. Post surfaces should be treated with a silane coupling agent for BI, whereas application of silane followed by an adhesive resin was suggested for DC. However, previous studies have indicated diverse results in the benefit of silane. Factors such as the type of fiber, quantity of fiber exposed to the surface, type of silane, or method of silane application can affect silane efficiency.20,21 Exposed silica fibers at the surfaces of FibreKor posts could regularly be observed at the interfacial failure surfaces of fractured beams. Silane, therefore, was demonstrated to have a positive effect in this study. For a low-silica-content post surface, silane treatment may be not necessary if a low-viscosity luting resin composite is used, and application of only an adhesive resin may be adequate for a high-viscosity resin composite. Further studies should be performed to confirm this assumption. Within the limitations of this study, it can be concluded that BI had a lower viscosity than that of DC at 60 seconds after mixing. Bond strengths between luting resin composites and post surfaces could be affected by post surface treatment methods, depending on the resin composite used. At the bonding time of 60 seconds after mixing the luting resin composite, application of an adhesive resin to the post surface seemed to be advantageous for bonding the fiber post to a high-viscosity luting resin composite (DC), whereas application of only a silane coupling agent was adequate for treating the post surface when a lowviscosity resin composite (BI) was used.
Conflicts of interest The authors have no conflicts of interest relevant to this article.
Acknowledgments The authors gratefully acknowledge the financial support received from the Thailand Research Fund (TRF) and the Commission on Higher Education, Thailand.
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Journal of Dental Sciences (2014) xx, 1e8
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ORIGINAL ARTICLE
Effect of viscosity of dual-cure luting resin composite core materials on bond strength to fiber posts with various surface treatments Juthatip Aksornmuang a*, Masatoshi Nakajima b, Junji Tagami b,c a
Department of Prosthetic Dentistry, Faculty of Dentistry, Prince of Songkla University, Hat Yai, Songkhla, Thailand b Cariology and Operative Dentistry, Department of Restorative Science, Tokyo Medical and Dental University, Tokyo, Japan c Global Center of Excellence Program, International Research Center for Molecular Science in Tooth and Bone Diseases, Tokyo Medical and Dental University, Tokyo, Japan Received 23 January 2014; Final revision received 21 March 2014
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KEYWORDS bond strength; post-surface treatment; resin composites; viscosity
Abstract Background/purpose: The objective of this study was to evaluate the viscosity of two dual-cure resin composite core materials and their bond strength to fiber posts treated with various surface treatments. Materials and methods: Viscosity at 60e90 seconds after mixing two resin composite core materials, Clearfil DC Core (DC) and Build-It FR (BI), was tested by a rheometer. Eighteen fiber posts (FibreKor) were divided into three groups according to the following post surface treatments: (1) no surface treatment; (2) application of silane coupling agent (Silane); and (3) application of silane followed by Bond-1 adhesive resin (Silane þ Bond-1). Treated posts were cemented into artificial post spaces using DC or BI. After 24-hour storage, each specimen was serially sliced into 12 beams for a microtensile bond strength test. The data were divided into upper, middle, and bottom regions, and statistically analyzed (a Z 0.05). Failure modes were observed using a scanning electron microscope. Results: The viscosity of BI at 60 seconds after mixing was lower than that of DC. Bond strengths were found to be affected by luting resin composites, surface treatment, and region. For DC, bond strength was significantly improved in the group of Silane þ Bond-1 (P < 0.05).
* Corresponding author. Department of Prosthetic Dentistry, Faculty of Dentistry, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand. E-mail address: [email protected] (J. Aksornmuang). http://dx.doi.org/10.1016/j.jds.2014.04.001 1991-7902/Copyright ª 2014, Association for Dental Sciences of the Republic of China. Published by Elsevier Taiwan LLC. All rights reserved.
Please cite this article in press as: Aksornmuang J, et al., Effect of viscosity of dual-cure luting resin composite core materials on bond strength to fiber posts with various surface treatments, Journal of Dental Sciences (2014), http://dx.doi.org/10.1016/j.jds.2014.04.001
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J. Aksornmuang et al For BI, application of Silane significantly improved bond strength (P < 0.05), but application of Silane þ Bond-1 had no advantageous effect on it. Conclusion: Bond strengths between luting resin composites and fiber posts were affected by post surface treatments, depending on the resin composite used. Application of a low-viscosity adhesive resin to the post surface seemed to be beneficial for a high-viscosity luting resin composite. Copyright ª 2014, Association for Dental Sciences of the Republic of China. Published by Elsevier Taiwan LLC. All rights reserved.
Introduction Currently, utilization of fiber posts to provide radicular retention for crown restoration is routinely recommended for the treatment of endodontically treated teeth. Several studies have accordingly supported the benefit of fiber posts in reducing the incidence of root fracture, due to their modulus elasticity, which is comparable to that of dentin.1e3 Moreover, fiber posts provide superior aesthetics, easier removal, and shorter treatment visits compared to cast metal posts and cores.4 In fiber post luting procedures, either a resin cement or a resin composite core material can be used as a luting medium. Dualcure resin composite core materials have been introduced for the placement of fiber posts in post spaces because a resin composite has a modulus of elasticity close to that of dentin and fiber posts, and better mechanical properties than those of a resin cement.5 A recent study revealed significantly higher bond strength when luting the fiber post with dual-cure core build-up materials than with resin cements.6 Even though the utilization of a fiber post can reduce the incidence of root fracture, failure can still occur through decementation of the fiber post from the canal. The flexibility of fiber posts allows them to consensually bend with dentin during function, resulting in debonding of the interface when bond strength is inadequate. Failure can occur either at the resinedentin or at the resinepost interface. A previous study reported that debonding between fiber posts and a luting resin took place when the root canal dentin was efficiently bonded.7 Various surface treatment methods have been proposed to improve the adhesion between post surfaces and the luting resin, either through mechanical interlocking or through chemical bonding at the interface.8 Various techniques for increasing the roughness of post surfaces, by means of hydrogen peroxide etching,9 hydrofluoric acid etching,10 or airborneparticle abrasion with aluminum oxide,11,12 have been reported to be effective for increasing bond strength between fiber posts and the luting resin. However, one study revealed that post surface abrasion produced undesirable surface changes.13 Silane application to post surfaces was found to be a simple method for improving the bonding between post surfaces and the luting resin.10,14 However, there have been conflicting research results in regard to the efficiency of silane as a coupling agent to fiber post surfaces.8 Various fiber posts are available in the dental market. According to manufacturers’ instructions, several
methodsdsuch as no treatment needed, application of silane, or application of silane followed by an adhesive resindhave been suggested as post surface treatments prior to bonding posts in root canals. Application of these chemical agents to post surfaces has been recommended in order to improve the adhesion between fiber posts and the luting resin through the mechanisms of chemical interaction and increased wettability. Low viscosity of an adhesive resin is important for good wettability of post surfaces.15 However, if the luting resin itself has low viscosity, additional application of an adhesive resin may have a negligible effect. Dual-cure resin core materials of various viscosities have been developed. Lower-viscosity resin core materials would be easier to inject into the root canal; however, it would be more difficult to build up a core part without a matrix. By contrast, resin composites with higher viscosity may produce a gap at the surface, although they facilitate the construction of an abutment form. Only a limited number of studies have investigated the effect of viscosity of dualcure luting resin composite core materials on bond strength. Moreover, the effect of surface treatments may be related to the viscosity of the luting medium, an assumption that has not yet been proved. This study was performed to evaluate the viscosity of two dual-cured resin composite core materials used for luting fiber posts and their bond strength to fiber posts treated with various surface treatments. In addition, the correlation between viscosity and bond strength data would be described. The null hypotheses were that the viscosity of the luting agent, types of luting agent, post surface treatments, and regional differences do not affect the bonding between fiber posts and luting resin composite materials.
Materials and methods Two dual-cure resin composite core materials were used for bonding fiber posts: Clearfil DC Core (DC; Kuraray Medical, Tokyo, Japan) and Build-It FR (BI; Pentron Clinical Technologies, Wallingford, CT, USA). After mixing, their viscosity was tested using a rheometer (HAAKE RheoStress RS75; Thermo Electron, Karlsruhe, Germany) at 25 C with a C20/4 (diameter Z 20 mm, angle Z 4 ) sensor. Viscosity of each resin composite was determined twice; the viscosity values in Pa$second were reported at 60 seconds, 70 seconds, 80 seconds, and 90 seconds after mixing. Eighteen 1.48-mm-diameter and 18-mm-length FibreKor nonserrated glass fiber-reinforced composite posts (Pentron
Please cite this article in press as: Aksornmuang J, et al., Effect of viscosity of dual-cure luting resin composite core materials on bond strength to fiber posts with various surface treatments, Journal of Dental Sciences (2014), http://dx.doi.org/10.1016/j.jds.2014.04.001
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Clinical Technologies) were used in this study. The posts were cut to a length of 12 mm from the upper end by a diamond bur (#201; Shofu, Kyoto, Japan) mounted on a high-speed handpiece under water spray. The cut posts were straight in shape. Surfaces of the posts were then cleaned with alcohol. Eighteen artificial post spaces, 8 mm in diameter and 12 mm in depth, were prepared in 1 cm 1 cm 2 cm acrylonitrile butadiene styrene resin blocks.14 Materials used in this study are presented in Table 1. FibreKor posts were randomly divided into three groups according to the following post surface treatments: (1) no surface treatment; (2) application of silane coupling agent (Silane; Pentron Clinical Technologies); and (3) application of Silane followed by an adhesive resin (Bond-1; Pentron Clinical Technologies). All materials were used following the manufacturer’s instructions. After the post surfaces were treated, one of the tested dual-cure resin core materials (DC or BI) was mixed and injected into the post cavity. At 60 seconds after mixing, the remaining resin was placed on the post surface and then the post was carefully placed into the center of the cavity. The upper surface of the specimen was covered with a plastic strip and pressed gently with a glass slide to squeeze out any excess resin. The specimens were exposed to light for 60 seconds using a halogen light source (Optilux 500; Demetron, Danbury, CT, USA), the tip of which was placed at the top of the cavity (Fig. 1). These were then stored in water for 24 hours at 37 C. After 24 hours of storage, each specimen was attached to the arm of a low-speed diamond saw (IsoMet; Buehler, Lake Bluff, IL, USA), and 12 slabs were serially cut perpendicular to the bonded interface under water cooling. Each slab was then transversely sectioned at the middle part of the post into approximately 0.6 mm 0.6 mm-thick beams. The cross-sectional area of each beam was measured using digital calipers (CD15; Mitutoyo, Kawasaki, Japan). One of two interfaces of each beam was randomly selected for testing. The ends of the beam and the remaining interface were glued onto a testing device on a table-top testing machine (EZ Test; Shimadzu, Kyoto, Japan) using cyanoacrylate glue (Zapit; DVA, Corona, CA,
Table 1
USA), and subjected to tensile force at a crosshead speed of 1 mm/minute (Fig. 1). After the specimens had fractured, both the resin side and the post side of the fractured beams were mounted on brass tablets and sputter-coated with gold. Fracture modes were observed using a scanning electron microscope (JSM5310; JEOL, Tokyo, Japan) and were classified as follows: cohesive failure within the post; mixed interfacial failure and cohesive failure within the post; interfacial failure, mixed interfacial failure, and cohesive failure within the resin; and cohesive failure within the resin. Microtensile bond strength (mTBS) data for each post were divided into three regions (upper, middle, and bottom). Each region consisted of four beams per bonded post; 12 beams were obtained from each subexperimental group. Data were analyzed using three-way analysis of variance (ANOVA), and Tukey’s HSD was used as a post hoc test for multiple comparisons. All statistical analyses were performed at a 95% confidence level.
Results Viscosity data after testing the viscosity of DC and BI twice are shown in Table 2 and graphically presented in Fig. 2. At 60 seconds after mixing, the viscosity of BI was lower than that of DC; however, its viscosity increased over time. By contrast, the viscosity of DC decreased from 60 seconds to 90 seconds after mixing. At 90 seconds after mixing, the viscosity of these two core materials became closer than that at 60 seconds after mixing. Three-way ANOVA indicated that mTBS was significantly affected by the luting resin composite material, surface treatment, and region of the post surface (P < 0.0001). There was a significant interaction between the luting resin composite and surface treatment factors (P < 0.0001). The means and standard deviation values of mTBS for each experimental group and each region are shown in Table 3. For DC, the average bond strength values at the middle and bottom regions increased slightly when the post surfaces were treated with Silane compared to no surface treatment (P > 0.05), but no statistically significant
Materials used in this study.
Material
Manufacturer
Composition
FibreKor Clearfil DC Core
Pentron Clinical Technologies (Wallingford, CT, USA) Kuraray Medical (Tokyo, Japan)
Build-It FR
Pentron Clinical Technologies
Silane Bond-1
Pentron Clinical Technologies Pentron Clinical Technologies
30.8% volume of glass fiber, 16.2% volume of filler, 53% volume of resin content (Bis-GMA, UDMA, HDDMA) Catalyst: Bis-GMA, TEGDMA, silanated colloidal silica, barium glass, d,l-camphorquinone, benzoyl peroxide Universal: Bis-GMA, TEGDMA, silanated colloidal silica, barium glass, N,N-diethanol-p-toluidine Bis-GMA, UDMA, HDDMA, barium borosilicate glass fillers, chopped glass fiber, photochemical initiator Organosilane in methyl alcohol PMGDM, HEMA, TMPTA, ethanol, acetone, photoinitiator, amine accelerator, stabilizer
Bis-GMA Z bisphenol-a-glycidyl dimethacrylate; HDDMA Z hexanediol dimethacrylate; HEMA Z 2-hydroxyethyl methacrylate; PMGDM Z pyromellitic glycerol dimethacrylate; TEGDMA Z triethyleneglycol dimethacrylate; TMPTA Z trimethylolpropane triacrylate; UDMA Z urethane dimethacrylate.
Please cite this article in press as: Aksornmuang J, et al., Effect of viscosity of dual-cure luting resin composite core materials on bond strength to fiber posts with various surface treatments, Journal of Dental Sciences (2014), http://dx.doi.org/10.1016/j.jds.2014.04.001
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Figure 1
Schematic illustration of bonding and microtensile bond strength test procedures.
difference was detected. A significant increase of bond strength after silane application was identified only at the upper region (P < 0.05). Bond strengths significantly improved in the Silane þ Bond-1 group for all regions (P < 0.05). For BI, bond strengths of the groups treated with Silane and Silane þ Bond-1 were significantly higher than those of the no surface treatment group for all regions (P < 0.05). No significant difference was observed in bond strengths between the Silane and Silane þ Bond-1 groups (P > 0.05). A comparison of the two luting resin composite materials, DC and BI, showed that BI provided significantly
higher bond strengths than DC in the no surface treatment and silane-treated groups (P < 0.05). However, both resin composites generated similar bond strengths when the surfaces were treated with Silane þ Bond-1 (P > 0.05). Regarding regional differences in bond strengths, significant differences were found only in the group of DC treated by Silane, in which the upper region exhibited higher bond strength than the middle and bottom regions (P < 0.05).
Table 2 Viscosity (Pa$second) of two dual-cure resin composite core materials, DC and BI, at 60e90 seconds after mixing. Time (seconds after mixing)
Test 1
DC Test 2
Test 1
BI Test 2
60 70 80 90
919.6 989.8 899.1 806.7
1001.0 949.3 761.1 650.9
101.2 108.0 207.5 622.7
108.5 113.0 194.0 462.9
BI Z Build-It FR; DC Z Clearfil DC Core.
Figure 2 Viscosity (Pa$second) of DC and BI at 60e90 seconds after mixing. BI Z Build-It FR; DC Z Clearfil DC Core.
Please cite this article in press as: Aksornmuang J, et al., Effect of viscosity of dual-cure luting resin composite core materials on bond strength to fiber posts with various surface treatments, Journal of Dental Sciences (2014), http://dx.doi.org/10.1016/j.jds.2014.04.001
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Table 3 Regional mTBS [mean (sd) in MPa] of FibreKor posts to dual-cure resin composite core materials.a
No surface treatment
Silane
Silane+Bond-1
DC
BI
DC
BI
DC
BI
Upper
19.6(8.1)C
36.6(9.1)A
36.0(10.8)A
50.1(9.0)B
40.5(10.9)A,B
44.4(6.2) A,B
Middle
13.7(6.9)O
33.1(10.4)M
20.9(9.8)O
45.4(9.2)N
38.2(9.6)M,N
42.7(8.0)M,N
Bottom
13.6(7.1)Z
32.3(9.0)X
17.3(9.4)Z
44.2(11.5)Y
34.7(6.8)X,Y
41.9(9.2)X,Y
a
Same superscript letters within each region indicate no significant difference. Brackets indicate significant differences in µTBS between two regions.
BI = Build-It FR; DC = Clearfil DC Core; µTBS = microtensile bond strength; sd = standard deviation.
Fig. 3 presents the percentage of fracture modes for each resin composite and surface treatment. The results revealed that most specimens fractured at the interface between the post and the resin composite (Fig. 4). Several air bubbles were observed on the resin side when DC was used (Fig. 5). Cohesive failure within the bonding substrates and mixed failure were found more frequently when the post surfaces were treated with Silane or Silane þ Bond-1 compared with the no surface treatment group (Fig. 6).
Discussion A viscosity test was performed to evaluate the rheological characteristic of two dual-cure resin composite core materials used for luting fiber posts. Due to the limitations of the rheometer, viscosity data could be reported no earlier than 60 seconds after mixing. The results showed that the viscosity of DC at 60 seconds after mixing was approximately 10 times higher than that of BI. BI is an auto-mixtype dual-cure resin composite. Two cartridges of resin paste were prepared and connected to an auto-mix syringe
Figure 3 DC Core.
tip. Using a mixing gun, the resins in both cartridges were compressed and mixed together in a spiral tube. The resins prepared were therefore of low viscosity to allow them to become entangled easily and be mixed consistently. The viscosity of BI increased four to six times from 60 seconds to 90 seconds after mixing, which might be due to the initiation of the polymer propagation process of the resin. The consistency of the composite resin increased after mixing due to the polymerization reaction that occurred; when the polymer chains grow larger and connect to each other, consistency becomes higher. The degree of conversion cannot be controlled even when the mixing protocols for each mix are the same. Therefore, the viscosity data exhibited some differences between the two times testing. The viscosity experiment was repeated, and both of them showed the same tendency in the alteration of consistency during 60e90 seconds after mixing. By contrast, DC is a conventional hand-mix-type dualcure resin composite, in which resins were equally pressed from two cartridges and mixed by hand spatulation. Resistance force during mixing of DC could be sensed by the
Percentage of failure mode for each surface treatment and each luting resin composite. BI Z Build-It FR; DC Z Clearfil
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Figure 4 Representative SEM micrographs of the fracture surfaces of dentin and resin sides in the group bonded with BI without surface treatment, showing interfacial failure. On the post side, exposed fibers can be observed with resin matrix in between. On the resin side, parallel impression lines of fibers are present. BI Z Build-It FR; SEM Z scanning electron microscope.
operator due to the thick consistency of the resins. The high viscosity of DC would be an effect of higher filler loading compared to BI. The amount of fillers in BI accounts for 68.2% of its mass, whereas that in DC accounts for 77.8% of its mass.16 A previous study also showed a significantly higher Knoop hardness number of DC than that of BI at both the coronal and the apical regions of the post space.17 However, it was noticed that the viscosity of DC
Figure 5 Representative SEM micrograph of the fracture surface of the resin side, showing interfacial failure in the group bonded with hand-mix resin composite (DC). Several voids are observed. DC Z Clearfil DC Core; SEM Z scanning electron microscope.
decreased slightly from 60 seconds to 90 seconds after mixing, which might be due to the presence of the smallmolecule diluent monomer triethyleneglycol dimethacrylate in DC. The presence of air bubbles within DC could also affect the loadedeformation diagram. It was noticed from the present study results that the two tested resin composites showed different viscosity conversion from 60 seconds to 90 seconds. Bonding fiber posts with different resin composite materials at different times may affect the bond strength. In the present study, the resin composite was placed on the post surface at 60 seconds after mixing to simulate real clinical procedures. Further studies should be conducted to evaluate the viscosity characteristics and bond strength to post surfaces of various resin composite core materials at different bonding times. Statistical analysis using three-way ANOVA indicated that the strength of bonds between luting resin composites and post surfaces was affected by the type of resin composite and surface treatment. The null hypothesis was rejected. In addition, a significant interaction was found between these two independent variables. In other words, the adhesion between luting resin composites and post surfaces was affected by surface treatment methods, depending on the type of resin composite used for luting. In the no surface treatment group, fiber posts luted with the lower-viscosity resin composite (BI) provided significantly higher bond strength than those luted with the higher-viscosity resin composite (DC). This might indicate that the viscosity of the luting resin composites had an effect on their adhesion to post surfaces. BI and DC are
Please cite this article in press as: Aksornmuang J, et al., Effect of viscosity of dual-cure luting resin composite core materials on bond strength to fiber posts with various surface treatments, Journal of Dental Sciences (2014), http://dx.doi.org/10.1016/j.jds.2014.04.001
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Figure 6 Representative SEM micrographs of fracture surfaces showing failures as mixed failure in the group bonded with BI and surface treated with Silane þ Bond-1 and cohesive failure within the post in the group bonded with DC and surface treated with Silane þ Bond-1. For mixed failure, failure within the adhesive resin is indicated by the wavy pattern of adhesive resin on both sides of the fracture beams. For failure within the post, numerous parallel fibers with some resin matrix in between are observed on both sides. BI Z Build-It FR; DC Z Clearfil DC Core; SEM Z scanning electron microscope.
both bisphenol-a-glycidyl dimethacrylate-based resin composites. However, the resin matrix of BI is also composed of urethane dimethacrylate and hexanediol dimethacrylate, which are the same materials contained in FibreKor. The resin part of the fiber posts was supposed to be completely polymerized during postfabrication procedures. Carbon double bonds in the resin matrix of the post were mostly used up. Further polymer chain propagation of urethane dimethacrylate and hexanediol dimethacrylate between the post surface and BI could not occur. The identical resin matrix of BI and FibreKor, therefore, would not be a favorable factor for high bond strength. For DC, bond strengths increased slightly when post surfaces were treated with silane. Statistically significant differences in the bond strength between no surface treatment and silane-treated groups could not be detected at the middle and bottom regions. Due to the high viscosity of DC, no surface treatment, or treatment with silane only, might not have enough potential to improve bond strength. The chemical bond between silane and the silica component of glass fiber appeared to have little effect on increased bond strength, whereas Bond-1 adhesive resin was able to improve bond strength significantly. The lowviscosity resin provided a good wettability to the post surfaces. According to the principle of adhesion, wettability of the adhesive is one of the important factors for good adhesion between bonding substrates. Bond strength of a high-consistency adherend can be improved using a lowviscosity resin.18
On the contrary, for BI the average bond strengths of the experimental groups, Silane and Silane þ Bond-1, were higher than that of the control group; however, no statistically significant difference was detected between the no surface treatment and the Silane þ Bond-1 groups. This might indicate that the application of a low-viscosity resin adhesive was less advantageous when a low-viscosity luting resin composite was used. However, application of silane could only statistically elevate the bond strength; this might be due to the chemical reaction of silane as a coupling agent. The decrease in bond strength when an adhesive resin was applied would be due to the poor mechanical properties of the adhesive itself. Failure analysis revealed that cohesive failure within the resin and mixed failure (including failure within the resin) were more frequently observed when Bond-1 adhesive was applied to post surfaces (Fig. 3). Regarding regional differences, there was no significant difference in bond strength between upper, middle, and bottom regions for all tested groups, except for the group treated with Silane and fixed with DC. It was reported that regions of the post space did not affect bond strength when the post surface was silanized. The results of the present study were in accordance with those of previous reports.14,19 However, in the DC group, which has high consistency, bond strengths at the upper region were found to be higher than those at the lower region when no adhesive resin was applied. A higher compression force at the top surface during compression with a glass slide prior to curing
Please cite this article in press as: Aksornmuang J, et al., Effect of viscosity of dual-cure luting resin composite core materials on bond strength to fiber posts with various surface treatments, Journal of Dental Sciences (2014), http://dx.doi.org/10.1016/j.jds.2014.04.001
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8 might cause DC to come into more intimate contact with the upper region of the post surface, as opposed to the lower region. Fractographic analysis revealed that the majority of failure patterns occurred as interfacial failure between post surfaces and resin composites. More complicated failure patterns such as mixed failure, cohesive failure within the resin, and cohesive failure within the posts were found when bond strengths were higher. Interestingly, in the group of no surface treatment of DC, several voids could be observed on the resin side of fractured beams (Fig. 5). This finding is in agreement with a previous study that evaluated the properties of various dual-cure resin composite core materials,17 which indicated that air bubbles are trapped inside the resin composite bulk during spatulation. Based on the results of the present study, in order to obtain the optimal bond strength between the luting resin composite and the glass fiber-reinforced composite post surface, surface treatment method should be selected depending on the resin composite used. Post surfaces should be treated with a silane coupling agent for BI, whereas application of silane followed by an adhesive resin was suggested for DC. However, previous studies have indicated diverse results in the benefit of silane. Factors such as the type of fiber, quantity of fiber exposed to the surface, type of silane, or method of silane application can affect silane efficiency.20,21 Exposed silica fibers at the surfaces of FibreKor posts could regularly be observed at the interfacial failure surfaces of fractured beams. Silane, therefore, was demonstrated to have a positive effect in this study. For a low-silica-content post surface, silane treatment may be not necessary if a low-viscosity luting resin composite is used, and application of only an adhesive resin may be adequate for a high-viscosity resin composite. Further studies should be performed to confirm this assumption. Within the limitations of this study, it can be concluded that BI had a lower viscosity than that of DC at 60 seconds after mixing. Bond strengths between luting resin composites and post surfaces could be affected by post surface treatment methods, depending on the resin composite used. At the bonding time of 60 seconds after mixing the luting resin composite, application of an adhesive resin to the post surface seemed to be advantageous for bonding the fiber post to a high-viscosity luting resin composite (DC), whereas application of only a silane coupling agent was adequate for treating the post surface when a lowviscosity resin composite (BI) was used.
Conflicts of interest The authors have no conflicts of interest relevant to this article.
Acknowledgments The authors gratefully acknowledge the financial support received from the Thailand Research Fund (TRF) and the Commission on Higher Education, Thailand.
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Please cite this article in press as: Aksornmuang J, et al., Effect of viscosity of dual-cure luting resin composite core materials on bond strength to fiber posts with various surface treatments, Journal of Dental Sciences (2014), http://dx.doi.org/10.1016/j.jds.2014.04.001