Corporation, Wallingford, CT, USA) in a low-speed handpiece under ... an auto-mix cartridge and syringe. .... When the p
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Journal of Dentistry (2005) xx, 1–9
www.intl.elsevierhealth.com/journals/jden
Effect of prolonged photo-irradiation time of three self-etch systems on the bonding to root canal dentine Juthatip Aksornmuanga,*, Masatoshi Nakajimab, Richard M. Foxtonc, Junji Tagamib,d a
Department of Prosthetic Dentistry, Faculty of Dentistry, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand b Cariology and Operative Dentistry, Department of Restorative Science, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan c Department of Conservative Dentistry, Floor 25, Guy’s, King’s and St Thomas’ Dental Institute, King’s College London, London Bridge, London SE1-9RT, UK d Center of Excellence Program for Frontier Research on Molecular Destruction and Reconstruction of Tooth and Bone, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan Received 7 June 2005; received in revised form 2 September 2005; accepted 8 September 2005
KEYWORDS Microtensile bond strength; Root canal dentine; Dual-cure resin core material; Photo-irradiation time
Summary Objectives: To evaluate the effect of photo-irradiation time to the adhesive on the regional bond strength of a dual-cure resin core material to root canal dentine using photo and dual-cure adhesives with self-etching primer. Materials and methods: Post spaces were prepared in extracted premolars and then the root canal dentine was treated with one of the following bonding procedures: (1) Clearfil SE Bond Primer/Bond (SE), (2) Nano-Bond Primer/Photocure adhesive (PNB), (3) Nano-Bond Primer/Dual-cure adhesive (DNB). Photoirradiation was performed for 10 or 20 s from a coronal direction. The post spaces were then filled with a dual-cure composite resin (Build-It FR) and light-cured for 60 s. After 24 h storage, each specimen was serially sliced into 8, 0.6!0.6 mmthick beams for the mTBS test. The bond strength data were divided into coronal and apical regions and analysed using three-way ANOVA and Games–Howell multiple comparison (aZ0.05). Results: The mTBS of the photo-cure adhesive resin, SE and PNB, significantly decreased (p!0.05) at the apical region when the photo-irradiation time was 10 s. However, the bond strength of the SE group was significantly improved at both regions when photo-irradiation time was extended to 20 s (p!0.05). There were no differences in mTBS of the photo-cure adhesive resin (PNB) cured for 20 s and dualcure adhesive resin (DNB) (pO0.05).
* Corresponding author. Tel.: C66 74 429874; fax: C66 74 429874. E-mail address:
[email protected] (J. Aksornmuang).
0300-5712/$ - see front matter Q 2005 Published by Elsevier Ltd. doi:10.1016/j.jdent.2005.09.001
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J. Aksornmuang et al. Conclusion: Photo-cure adhesive was effective for application on root canal dentine when the photo-irradiation time was sufficient. Extension of photo-irradiation time to the adhesive improved the bond strength depending on the type of adhesive resin. Q 2005 Published by Elsevier Ltd.
Introduction Recently, utilization of fibre-reinforced composite posts in combination with adhesive resin cement to restore endodontically treated teeth has increased in popularity. The major advantage of fibre posts is their similar elastic moduli to dentine, producing a stress field similar to that of natural teeth and resulting in a reduction of root fractures, whereas metal posts exhibit high stress concentration at the post-dentine interface.1,2 However, in a flared canal, which might result from carious extension, trauma, pulpal pathosis, iatrogenic misadventure, or idiopathic causes,3 an excessively thick layer of resin cement in the coronal region of the post space could be present and may not be strong enough to resist occlusal loading. To overcome this problem, a dual-cure composite resin core material has been introduced as a luting medium because it is stronger than resin cement and has a modulus of elasticity close to dentine and fibre posts.4 Laboratory and clinical studies have found that failures of fibre post and core restorations often occurred following decementation between the fibre post-resin and/or the resin–root dentine interfaces.5–8 Therefore, good adhesion of these interfaces is an important factor for a successful restoration. In our previous studies, evaluation of the bonding of dual-cure composite resin core materials to non-translucent glass and quartz fibre posts, and also translucent quartz fibre posts has been accomplished. It was found that applying a silane coupling bonding agent to the post surface was effective in achieving optimal bond strength between dual-cure composite resin and silica-based fibre posts.9,10 Even though some studies have investigated the adhesion of resin cement to root canal dentine,7,11–14 published research on the adhesion of dual-cure composite resin to root canal dentine using contemporary adhesive systems is still very limited.15 Currently, clinicians can use various ‘total-etch’ or ‘self-etch’ adhesive systems for bonding to root canal dentine. For total-etch adhesive systems, moisture control on the etched dentine subsurface
is necessary prior to bonding because exposed collagen fibrils collapse after drying and prevent penetration of resin monomer into demineralised dentine, resulting in a lower bond strength to dentine.16–19 However, it is difficult to control the surface wetness in a complex cavity such as a deep and narrow post space within root canal dentine. In contrast, self-etch adhesive systems are generally less technique-sensitive because this system simultaneously demineralises and penetrates resin monomer into dentine20,21 and avoids the rinsing and drying steps. Most clinicians generally use dual-cure adhesives for bonding to root canal dentine because of their ability to self-polymerise in the absence of light in deeper regions of post cavity. However, an adverse chemical reaction was reported to occur between chemically activated resin composite and acidic resin monomers.22–24 Self-etching primer contains acidic monomer, and consequently, a high concentration of uncured acidic monomers would be present within the primed dentine surface. Therefore, the compatibility of dual-cure adhesive when chemically polymerised and self-etch primed dentine is questionable. On the other hand, our previous study, which was conducted to evaluate the regional bond strength of both dual-cure and photo-cure adhesive systems to root canal dentine revealed that photo-cure adhesives seemed to be effective in bonding to root canal dentine. However, the bond strength at the apical region was found to be lower than that at the coronal region.25 The recommended photo-irradiation times for the adhesive resin in manufacturers’ instructions may be appropriate for bonding to a flat dentine surface, whereby the adhesive is directly perpendicularly exposed to the light source. In contrast, the accessibility of light energy passing through a deep and narrow post space is restricted. Moreover, the adhesive surface on the root canal dentine is parallel to the light direction. For these reasons, the effectiveness of photo polymerisation of the adhesive resin may decrease. Extending the light exposure time to the adhesive resin may improve the adhesion of dual-cure composite resin to root canal dentine. Therefore, this study was performed to evaluate the effect of photo-irradiation time to the adhesive
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on the regional bond strength of a dual-cure composite resin core material to root canal dentine using self-etching priming adhesive systems.
Materials and methods Preparation of bonded specimens Eighteen single-rooted human premolar teeth, recently extracted from adolescents for orthodontic reasons and stored frozen, were decoronated at the cementoenamel junction using a low speed diamond saw (Isomet, Buehler, Lake Bluff, IL, USA). Pulpal tissue was removed using endodontics files and the post spaces were then prepared using Gates–Glidden drills (Matsutani Seisakusho Co., Ltd, Takanezawa, Japan) and FibreKor drills (Pentron Corporation, Wallingford, CT, USA) in a low-speed handpiece under copious water cooling to a depth of 8 mm and a diameter of 1.5 mm. After post space preparation, the root canals were rinsed with distilled water and dried with paper points. Prior the bonding procedures, the external surfaces of the roots were built up with Clearfil DC Core composite resin (Kuraray Medical Inc, Tokyo,
Table 1
Japan) to make grips for testing and to prevent the effect of external light from the curing tip, which can pass through the thin portion of dentine wall to the adhesive resin during photo curing procedures. The materials used in this study and their chemical compositions are presented in Table 1. The roots were randomly divided into three groups, each consisting of six teeth, and their root canal dentine surfaces were treated with one of the following bonding procedures: (1) Clearfil SE Bond Primer/Bond (SE), (2) Nano-Bond Primer/Photocure adhesive (PNB), (3) Nano-Bond Primer/Dualcure adhesive (DNB). After application of the adhesive, in each group, photo-irradiation was performed from a coronal direction for 10 s to three specimens and for 20 s to the remaining three specimens. The post spaces were then filled with a dual-cure composite resin core material (Build It F.R., Pentron Clinical Technologies, LLC, USA) using an auto-mix cartridge and syringe. The coronal surface of the root was covered with a plastic strip and pressed gently with a glass slide to squeeze out any excess resin. Light exposure was performed for 60 s by placing the tip of the light source (Optilux 500, Demetron, Danbury, USA) at the top
Materials used in this study.
Materials and manufacturer
Composition
Application procedures
Clearfil SE Bond (Kuraray Medical, Inc., Japan)
MDP, HEMA, hydrophilic dimethacrylates, DL-camphorquinone, N,Ndiethanol-p-toluidine, water MDP, Bis-GMA, HEMA, hydrophobic dimethacrylates, DL-camphorquinone, co-photoinitiator, N,Ndiethanol-p-toluidine, silanated colloidal silica Sulfonic acid resin, HEMA, multifunctional methacrylate resins, water, initiator
Apply SE-Primer for 20 s with microtip applicator Gently air dry Apply SE-Bond Gently air dry
SE-Primer, Batch no. 00403A SE-Bond, Batch no. 00545A
Nano-Bond (Pentron Clinical Technologies, LLC, USA)
Primer, Batch no. 84492
Adhesive, Batch no. 84493
Build-It F.R. (Pentron Clinical Technologies, LLC, USA)
Dual Cure Activator, Batch no. 83598 Batch no. 85517
PMGDM, HEMA, Bis-GMA, nanosized fillers, acetone, ethanol, camphorquinone, amine accelerator Benzoyl peroxide, acetone
Bis-GMA, UDMA, HDDMA, silanated bariumborosilicate glass fillers, chopped glass fibre, chemical/ photoinitiator
Apply Primer for 30 s Air dry excess Apply two coats of adhesive for photo-cure (PNB) type or a mixture of adhesive and dual-cure activator for dual-cure (DNB) type Gently air dry
Place the auto-mix syringe tip at the bottom of the post space Dispense material into the post space until fulled
MDP, 10-methacryloxydecyl dihydrogen phosphate; HEMA, 2-hydroxyethyl methacrylate; Bis-GMA, bisphenol-A-glycidyldimethacrylate; PMGDM, pyromellitate of glyceryl dimethacrylate; UDMA, urethane dimethacrylate; HDDMA, hexane diol dimethacrylate.
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of the cavity. Prior to each bonding procedure, the power density of the light source was checked with a digital radiometer (Jetlite light tester, J. Morita, Mason Irvine, CA, USA) to ensure that the power density of the light source ranged between 640 and 660 mW/cm2. The specimens were then stored in water for 24 h at 37 8C.
Bond strength testing After 24 h storage, the bonded specimens were attached to the arm of a low speed diamond saw (Isomet, Buehler, Lake Bluff, IL, USA) and eight 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 cavity into approximately 0.6! 0.6 mm thick beams. There were 24 beams harvested for each bonding system cured by each photo-irradiation time. The cross-sectional area of each beam was measured using digital calipers (Mitutoyo CD15, Mitutoyo Co., Kawasaki, Japan). The mean cross-sectional area was 0.36C 0.02 mm2. 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 in a table-top testing machine (EZ Test, Shimadzu Co., Kyoto, Japan) using cyanoacrylate glue (Zapit, DVA, Anaheim, CA, USA) and subjected to a tensile force at a crosshead speed of 1 mm/min (Fig. 1). The coronal four beams data were considered to represent the coronal portion of the post space corresponding to the coronal third of the root canal, and the apical four beams data were
Fig. 1
considered to represent the apical region corresponding to the middle third of the root canal. The number of samples was 12 for each experimental group and each region.
Fracture analysis After the specimens had fractured, both the resin side and dentine side of the fractured beams were mounted on brass tablets and gold sputter-coated. The fracture modes were observed using a scanning electron microscope (JSM-5310, JEOL, Tokyo, Japan). Fracture mode was classified as: adhesive failure, mixed adhesive/cohesive failure at the resin/dentine interface, cohesive failure within resin, and cohesive failure within dentine.
Statistical analysis The bond strength data were analysed using threeway ANOVA (adhesive resin/irradiation time/region) and Games–Howell was used as a post hoc test for multiple comparisons (aZ0.05), since Levene’s test indicated significant non-homogeneity among the variances. The failure mode data were analyzed using the chi-squared test (aZ0.05).
Results Three-way ANOVA revealed that there were significant effects of adhesive system, photoirradiation time, and region on the mTBS of the dual-cure resin core material and root canal dentine (p!0.0001). Interactions between
Schematic illustration of the bonding and mTBS test procedures.
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Bonding to root canal dentine Table 2
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Microtensile bond strength (MPa) to root canal dentine in each experimental group and each region. SE
PNB
10S Coronal Apical
20S
53.67G14.67 P!0.05 23.80G7.06A
a
DNB
10S b
74.36G9.41 NS 59.58G18.67C
20S
38.24G10.95 p!0.05 23.25G8.47A
a
10S a
40.04G7.59 NS 32.72G8.59A,B
20S a
49.04G7.28 NS 37.63G8.66B,C
47.46G13.91a NS 36.85G8.77B,C
nZ12 for each group. The same superscript letters indicate no significant differences of mTBS.
adhesive system and photo-irradiation time (p! 0.0001), and between adhesive system and region (pZ0.004) were present. The regional bond strengths are summarized in Table 2. When the mTBS between each adhesive resin (SE, PNB, and DNB) was compared, for the 10 s photo-cured groups, multiple comparisons revealed no significant differences in mTBS between each adhesive resin at the coronal region (pO0.05). However, at the apical region, the dual-cure adhesive resin (DNB) provided significantly higher bond strength compared with the photo-cure adhesive resin (SE and PNB) (p! 0.05). Within the groups photo-cured for 20 s, the root canal dentine surface treated with SE presents the significantly higher bond strength than that treated with NB (PNB and DNB) for coronal region (p!0.05). The SE group also showed a significantly higher bond strength than that of the PNB group at the apical region (p! 0.05). However, the post hoc test revealed no significant difference in mTBS between the SE and DNB groups at pZ0.54, although mTBS of the SE group (59.52 MPa) was much higher than that of the DNB group (36.85 MPa).
Regarding regional differences, significant differences in the bond strength between coronal and apical regions were found only in the group treated with photo-cure type adhesive resin, SE and PNB, cured for 10 s (p!0.05), whereas the 20 s photocured and dual-cured adhesive resin groups showed no regional differences (pO0.05). When the photo-irradiation time was considered, it was found that only the group treated with SE provided significantly superior bond strength at both regions when the photoirradiation time extended from 10 to 20 s (p! 0.05). Even though the bond strength at the apical region of the PNB group cured for 20 s was higher than that of 10 s, no significant difference was detected (pZ0.280). Fig. 2 presents the percentage of fracture modes for each experimental group. Fracture analysis indicated that the majority of failures were adhesive failure (Fig. 3). There were no significant differences between the failure modes in each group except the 20 s-cured SE group, in which approximately 40% failed cohesively within dentine or within resin (Fig. 4).
**
100% 90% 80%
Cohesivefailure within dentin
70% 60%
Cohesivefailure within resin 50% 40%
Mixed adhesive/cohesive failure at resin/dentin interface
30%
Adhesivefailure
20% 10% 0% 10 S
20 S SE
Fig. 2 mode.
10 S
20 S PNB
10 S
20 S DNB
Classification of failure mode. Double asterisks (**) indicate the group showing significant difference in failure
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Fig. 3 SEM photographs represent the adhesive failure surfaces of the 10 s photo-cured SE group. Exposed dentinal tubules were observed on the dentine side (A), whereas resin tags were pulled out and observed on the resin side (B).
Discussion Utilization of adhesive systems with composite resin material for cementation of endodontic posts has been found to be advantageous in improving post retention26 and fracture resistance of the teeth3,27. There are many factors that can influence the success of the restored endodontically treated tooth. Adhesion at the post-resin and resin–dentine interfaces is one factor, which was found to be a common cause of failure.28 Dentine mechanical properties and structure are dependent on the region.29–31 It was reported that tubule density in the coronal region of root dentine is higher than that of the apical region, and the diameter of the tubules decreases in an apical direction.12 However, Liu et al.30 found that dentine location did not affect the microtensile strength of bovine root dentine. Additionally, it was revealed that bond strength to root canal dentine was not influenced by dentine depth and tubule density when a self-etch adhesive
system was used.29 Therefore, it is speculated that in the present study with self-etch adhesive systems, resin/dentine bond strengths may not have been dependent on the density of dentinal tubules. On the other hand, the regional bond strength of a photocured adhesive system to root canal dentine would be affected by the photo energy, which decreases due to the depth of the post cavity, because polymerization of photo-cured adhesive resin is dependent upon the energy of photo-irradiation.3,32 In the present study, the mTBS obtained from photo-cured adhesive resin (SE and NB) significantly decreased at the apical region when photo-curing was performed for only 10 s. Photo-irradiation time for 10 s according to the manufacturer’s instructions might be enough to reach the optimal light energy for bonding to the flat dentine in which photo-irradiation can be directly performed in the direction perpendicular to the adhesive surface. However, due to the limitation of light accessibility through the root canal and the parallel
Fig. 4 SEM photographs represent the cohesive failure within dentine surface (A) and cohesive failure within adhesive resin surface (B) of the 20 s photo-cured SE group.
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Bonding to root canal dentine photo-exposure direction to the adhesive surface, 10 s photo-irradiation wound be insufficient for bonding to the apical region of post cavity. Indeed, when the adhesive resin was cured for 20 s, the bond strength of SE significantly improved at both coronal and apical regions, and the bond strength of NB increased at the apical region. Extension of the photo-irradiation time might result in an increase of total light energy inside the root canal. The conversion of double bonds during polymerization, which is critical for the optimisation of mechanical properties of the adhesive resin, might therefore be enhanced. Moreover, the improved bond strength at the apical region indicated that the depth of cure of the adhesive resin increased when the irradiation time was prolonged. This agreed with the previous studies, which revealed that extending photoirradiation time could increase the depth of cure of composite resin.33,34 Extending the photo-irradiation time could improve the bond strength of SE Bond to root canal dentine at both the coronal and apical regions, while for Nano-Bond, longer irradiation time increased the bond strength at the apical region, but the bond strengths at the coronal region were similar between 10 and 20 s groups. This indicates that the response to increased light energy was different between these two adhesives. It has been reported that the polymerization rate of photo-cure adhesive monomer is dependent on the type and concentration of the photoinitiators.35,36 Camphorquinone, which is a common photoinitiator in composite resin and adhesive resin, could produce a synergistic effect when combined with another co-initiator and result in an improvement in polymerization conversion.36 NB contains camphorquinone as a photoinitiator, whereas SE contains camphorquinone and another co-photoinitiator. The concentration of photoinitiator in these two adhesives might also be different since the responses to extended photo exposure time were different between SE Bond and Nano-Bond. Both the photo-cure and dual-cure versions of Nano-Bond adhesive were employed in this study. It was found that the mTBS of the photo-cure adhesive resin (PNB) and the dual-cure adhesive resin (DNB) groups was similar except for the apical region’s bond strength of 10 s-cured PNB, which showed lower bond strength than that of DNB. On the other hand, superior bond strength has been reported when photo-cure adhesive resin was used to bond root canal surface compared with the dual-cure adhesive resin when using Clearfil Liner Bond 2V self-etching priming adhesive system (Kuraray Medical, Inc., Tokyo, Japan).15,25 The adhesive
7 resin of Clearfil Liner Bond 2V contains phosphoric acid monomer, MDP (10-methacryloxydecyl dihydrogen phosphate). A previous study using Optibond Solo Plus (Kerr, Orange, CA, USA), which contains the phosphoric acid monomer, GPDM (glycerophosporic acid dimethacrylate), also reported significantly lower bond strength when the dual-cure type of adhesive was employed.37 This might be due to the acidity of phosphoric acid monomer, which deactivated the tertiary amine in chemically polymerised process of the adhesive resin.24 Even though aromatic sulfinate salts are included in both Clearfil Liner Bond 2V and Optibond Solo Plus to overcome the incompatibility between the acidic monomer and chemically polymerised composite resin, the rate and extent of polymerisation were still inferior compared with adhesive resin without acidic monomer.24,37 On the contrary, PMGDM (pyromellitate of glyceryl dimethacrylate) is used as the adhesive monomer in Nano-Bond, and a previous study conducted using this adhesive monomer found that dual-cure type adhesive provided better bond strength than the light-cure adhesive.38 The acidity of the adhesive monomer might be a factor that possibly predicts the bonding quality when dual-cure type adhesive resin is used. In this experiment, the post spaces were prepared in root canals, which had not been endodontically treated or treated with NaOCl in order not to alter the biomechanical behaviour of the root dentin. It has been reported that the bond strength to root canal dentin is affected by the chemical irrigant depending on the type of bonding agent.11,39,40 Moreover, eugenol, which could remain in the drilled post space, was found to have an affect on bond strength.41 In order to control these variables, the root canals were therefore not obturated and irrigated only with distilled water after drilling. From the results of the present study, it appears that photo-cure adhesive resin is effective for the application to root canal dentine if the photoexposure time is sufficient. Additionally, clinicians may find it financially beneficial to use only one type of adhesive resin to bond to both coronal and radicular dentine without worrying about inefficient polymerization of the adhesive resin in deeper regions.
Conclusion Photo-cure bonding resin was effective for application to root canal dentine if the photo-irradiation time was sufficient. Extension of photo-irradiation
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Acknowledgements The authors gratefully acknowledge the financial support received from Center of Excellence Program for Frontier Research on Molecular Destruction and Reconstruction of Tooth and Bone, Tokyo Medical and Dental University.
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38.
39.
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