Effect on Push-out Bond Strength of Glass-fiber Posts Functionalized with Polydopamine Using Different Adhesives Qian Chena / Qing Caib / Yan Lic / Xu-Yi Weid / Zhi Huange / Xin-Zhi Wangf Purpose: To evaluate the push-out bond strengths of prefabricated glass-fiber posts (Beijing Oya Biomaterials) with polydopamine functionalized to root dentin using two different resin cements (Paracore and RelyX Unicem) in different root regions (cervical, middle, and apical). Materials and Methods: Forty extracted human, single-rooted teeth were endodontically treated and a 9-mm post space was prepared in each tooth with post drills provided by the manufacturer. Specimens were then randomly assigned into four groups (n = 10 per group), depending on the adhesive system and post surface treatment used: group IA (Paracore + polydopamine); group IB (Paracore + control); group IIA (RelyX Unicem + polydopamine); group IIB (RelyX Unicem + control). Following post cementation, the specimens were stored in distilled water at 37ºC for 7 days. The push-out test was performed using a universal testing machine (0.5 mm/ min), and the failure modes were examined with a stereomicroscope. Data were statistically analyzed using twoway ANOVA (p = 0.05). Results: Bond strengths (mean ± SD) were: 7.909 ± 3.166 MPa (group IA), 4.675 ± 2.170 MPa (group IB), 8.186 ± 2.766 MPa (group IIA), 4.723 ± 2.084 MPa (group IIB). The bond strength of polydopamine groups was significantly higher than one of the control groups (p < 0.0001). No significant difference was found in the micro push-out bond strengths between the two resin cement groups or the root regions (p > 0.05). Stereomicroscopic analysis showed a higher percentage of adhesive than cohesive failures in all groups. Conclusion: Surface polydopamine functionalization was confirmed to be a reliable method for improving the bond strength of resin luting agents to fiber posts. The bond strength of Paracore to fiber posts was not significantly different from that of RelyX Unicem, and considering its convenient application, Paracore can be recommended. Keywords: glass-fiber posts, polydopamine, push-out bond strength, resin cement, adhesive. J Adhes Dent 2014; 16: 177-184. doi: 10.3290/j.jad.a31810
a
Postgraduate Student, Department of Prosthodontics, Peking University School and Hospital of Stomatology, Beijing, P. R. China. Main operator in the experiments, wrote the paper.
b
Professor, State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing, P. R. China. Supervised the research project.
c
Postgraduate Student, State Key Laboratory of Organic-inorganic Composites, Beijing University of Chemical Technology, Beijing, P. R. China. Assisted in the polydopamine treatment of glass-fiber posts.
d
Postgraduate Student, Department of Prosthodontics, Peking University School and Hospital of Stomatology, Beijing, P. R. China. Performed the experiments in partial fulfillment of a degree, performed statistical evaluation, co-wrote manuscript.
e
Doctor of Stomatological Science, Department of Prosthodontics, Peking University School and Hospital of Stomatology, Beijing, P. R. China. Conducted experiments, prepared root canals for treatment.
f
Professor, Department of Prosthodontics, Peking University School and Hospital of Stomatology, Beijing, P. R. China. Supervised the research project.
Correspondence: Professor Xin-Zhi Wang, Department of Prosthodontics, Peking University School and Hospital of Stomatology, 22 Zhongguancun Nandajie, Haidian District, Beijing, P. R. China 100081. Tel: Tel: +86-010-82195348, Fax: +86-010-6213-0611. e-mail: [email protected]
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Submitted for publication: 03.12.12; accepted for publication: 02.07.13
T
he restoration of root canals of severely damaged teeth with relatively complete ferrules often requires the placement of a post to ensure adequate retention of the core.18,45 Prefabricated fiber posts have been rapidly adopted by dentists in preference to cast metal posts, because of the advantageous properties of the former in having an elastic modulus similar to that of dentin, which produces a stress field similar to that of natural teeth.14,44 Clinical studies of fiber posts have demonstrated high success rates without the occurrence of root fractures.3,26 Debonding is the most common kind of failure of prefabricated fiber-reinforced composites following restoration,19 as prefabricated posts do not fit well into the flared canals that result from carious extension and trauma.12 However, with the development of the CAD/ CAM one-piece glass-fiber post and core, the retention of fiber posts has improved. The other factor involved in debonding is the poor surface adhesive properties of 177
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prefabricated fiber posts, where failure generally occurs at the post/adhesive junction.6,49 In vivo data have shown that bonds at the resin cement/post interface play an important role in the clinical success of a post-retained restoration.40 To improve the bond strength between the prefabricated fiber post and the resin cement, many surface pretreatment procedures for fiber posts have been investigated involving mechanical and/or chemical treatment. Hydrogen peroxide,34 hydrofluoric acid,52 and sandblasting with aluminum oxide5,10,13,24,43,48,54 have all been shown to significantly increase the bond strength. However, these treatments may modify the post shape and consequently decrease its ability to fit snugly within the root canal.5,10,13,24,43,48,54 They may also reduce the strength of the post,33,41 because of the absence of a chemical union between the methacrylate-based resin composite and the epoxy resin matrix of fiber posts. Chemical coupling has been proposed to enhance the bond strength. However, the inorganic fibers which are enveloped by the epoxy resin cannot covalently bind with a coupling agent that does not contain silicate. This makes it difficult to form a strong chemical bond.5,10,13,24,33,38,41,43,48,51,54 Dopamine, when polymerized steadily, forms an ultrathin active layer and chemical structure that can support the matrix surface.30 The adhesive properties of many different materials, including PTFE, have been improved by dopamine treatment.29 Polymerized dopamine has been found to improve the hydrophilicity and biocompatibility of polyethylene, PTFE, silicone rubber, and glass.28 Therefore, the purpose of this study was to evaluate the effect of polydopamine functionalized surface modification on the bond strength of prefabricated glass-fiber posts. The tested null hypotheses were: (1) the bond strength between the post surface and the resin cement cannot be increased by polydopamine functionalized post surface modification; (2) the choice of resin cement used for bonding has no significant effect on the post/cement interfacial strength.
MATERIALS AND METHODS Tooth Preparation For this study, teeth with single, straight root canals at least 9 mm long which were extracted from patients for periodontal and/or prosthetic reasons were used. All teeth were selected on the basis of no visible root cracks or caries, or with caries only in the 2 mm coronal to the cementoenamel junction. Radiographic images were obtained from the mesiodistal and buccolingual perspectives and used to exclude teeth with calcifications, anatomical abnormalities, the lack of a single root canal, or previous endodontic treatment. Using this selection process, 40 teeth were chosen. The selected teeth were cleaned of both calculus deposits and soft tissues, and stored in 5% chloramine T aqueous solution at 4°C. The crowns were sectioned horizontally 2 mm coronal to the cementoenamel junction (CEJ) using a 0.4-mm-thick precision saw (Isomet 1000, Buehler; Lake Bluff, IL, USA) with distilled water as the coolant. All roots 178
were cut to 14 mm as a standardized length. The working length was set 1 mm shorter than this (wl = 13 mm). The pulpal tissues were removed with a #0 barbed broach (Mani; Tokyo, Japan). Roots were irrigated using 2 ml of 5.25% sodium hypochlorite (NaClO) and Prep-Rite RC root canal lubricant (Pulpdent; Watertown, MA, USA), prepared using Protaper rotary nickel titanium instruments (SX, S1, S2, F1, F2, F3; Dentsply; York, PA, USA) with a low-speed rotary endodontic handpiece (X-smart; Dentsply), followed by ultrasonic rinsing with distilled water. Specimens were dried with paper points (Gapadent; Beijing, China), then obturated with gutta-percha cones (Dayading; Beijing, China) and an endodontic sealer (Septodent; Paris, France) using a lateral condensation technique. Cervical root canal cavities were filled with the light-curing composite resin AP-X (Kuraray; Osaka, Japan). After endodontic treatment, the teeth were stored at 37°C in 0.9% NaCl solution for 7 days. Prefabricated Glass-fiber Post Preparation Forty prefabricated glass-fiber posts (Beijing Oya Biomaterials; Beijing, China) with a maximum diameter of 1.4 mm were selected. The coronal two-thirds is parallel to columnar, and the apical third is tapered. It is composed of 60% glass fiber. The glass fibers are SE8400LS (Owens Corning; Toledo, OH, USA) and the resin matrix is Epoxy-828 (Shell; den Hague, Netherlands). The posts were randomly divided into two groups (group A and group B, with each group containing 20 posts) according to the post surface conditioning. Group A, 20 posts: The post surface was ultrasonically cleaned in ethanol before use (50 Hz, 0.5 hrs) and conditioned with 2 mg/ml dopamine in 100 ml Tris, pH 8.5 for 14 h at 25°C under continuous stirring.18,54 The posts were then retrieved and washed with deionized water to remove excess monomer and dried under nitrogen applied with a nozzle. Group B, 20 posts (control group): The post surface received no conditioning. Post Space Preparation and Post Cementation Gutta-percha was removed with #1-2 Peeso Reamers (Mani), and the post space of each specimen was enlarged with a standard drill system from the corresponding fiber post system (Beijing Oya Biomaterials) to create a 9-mm post space with at least 4 mm of filling material in the root apex. The post space was rinsed with distilled water and dried with paper points. Forty root specimens were randomly divided into four groups (n = 10 per group) as follows. Group IA: Paracore dual-curing resin cement (Coltène-Whaledent; Altstätten, Switzerland) + polydopamine-treated prefabricated glassfiber post. Group IB: Paracore dual-curing resin cement (Coltène-Whaledent) + untreated glass-fiber post. Group IIA: RelyX Unicem (3M ESPE; St Paul, MN, USA) + polydopamine-treated prefabricated glass-fiber post. Group IIB: 3M ESPE RelyX Unicem + untreated glass-fiber post. Fiber posts for each group were cemented to the root canals with adhesive resin cement, according to the manufacturer’s instructions. All specimens were stored at 37°C in distilled water for 7 days. The Journal of Adhesive Dentistry
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Table 1 Mean (MPa) and standard deviation (SD) of micro push-out bond strength values Post surface modification
Resin cement for bonding
Polydopamine conditioning No conditioning
Table 2
7.9 ± 3.2 4.7 ± 2.2
8.2 ± 2.8 4.7 ± 2.1
Results of two-way ANOVA with two factors (cement and treatment) SSq
DF
MSq
F
p
Intercept Cement Treatment
4879.279 0.880 338.176
1 1 1
4879.279 0.880 338.176
651.862 0.118 45.180
< 0.0001 0.734 < 0.0001
Results of repeated measures ANOVA test for the factor “regions of
Source
SSq
DF
MSq
F
p
Region
18.147
2
9.074
2.187
0.119
SEM Analysis Before performing the push-out test, one slice from each group was selected and examined under a scanning electron microscope (SEM). Sections were polished with wet silicon carbide paper of increasing grit (600, 800, 1000), rinsed with deionized water, ultrasonically cleaned in deionized water, and air dried. The slices were gold-sputtered and imaged using a scanning electron microscope (EVO 18 Special Edition Scanning Electron Microscope, Zeiss; Jena, Germany). Micro Push-out Test All specimens were cut perpendicular to the long axis of the tooth and 1.0-mm slices were made from the cervical, middle, and apical regions of the root using a water-cooled, 0.4-mm-thick precision saw with an accuracy of 0.1 mm, speed of 400 rpm, and pressure of 50 g (Isomet 1000, Buehler). Two slices from each region were selected and marked. The coronal post radius R (mm), apical post radius r (mm) and thickness of the specimen H (mm) of each slice was measured with electronic digital-display vernier calipers to an accuracy of 0.01 mm. The micro push-out test was performed with a universal testing machine (Instron 1121; Norwood, MA, USA) at a crosshead speed of 0.5 mm/min until failure. The maximum failure load was recorded in N and converted into MPa. The circular post slices were considered as truncated cones in the following calculations. The micro push-out bond strength value m (MPa) was calculated using the following formula:23
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RelyX Unicem resin cement Mean ± SD (MPa)
Source
Table 3 root”
Debond stress =
Paracore resin cement Mean ± SD (MPa)
Debonding force (N) area of the post/dentin surface (A)
A = / (R + r) √ H2 + (R – r)2 ≈ / (r + R) H where / = 3.14, R = coronal post radius, r = apical post radius, and H = root slice thickness. Statistical Analysis Statistical analysis was performed using SPSS software (SPSS 13.0, SPSS; Chicago, IL, USA) and the level of significance was set at 0.05. Data were analyzed using two-way and repeated measurements ANOVA.
RESULTS Interfacial Strength The mean (± SD) micro push-out bond strength values and the differences within the groups are presented in Tables 1 to 4. The results of two-way ANOVA revealed that the surface polydopamine treatment had a significant influence on the micro push-out bond strength of the adhesive resin cement to the glass-fiber post (p < 0.05). Group A had higher values than group B (p < 0.05). There were no significant differences between groups 1 and 2 (p > 0.05; Tables 1 and 2). The bond strength values did not vary significantly between groups by root region (p > 0.05; Table 3). Group A showed consistently higher push-out bond strength values than Group B for all root sections. Debonded specimens were examined using a stereomicroscope (Kestrel, Vision Engineering; Surrey, UK) at 50X magnification, and the failure modes were classified as follows: adhesive failure between the cement 179
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Table 4
Mean (MPa) and standard deviation (SD) of micro push-out bond strength values in groups by root region
Resin cement (roots) Paracore resin cement (20)
RelyX Unicem resin cement (20)
Post surface conditioning (No. of posts used)
Region (No. of slices used)
Failure mode (number of slices) Adhesive
Cohesive
Mixed
Polydopamine conditioning (10)
Cervical Middle Apical
(20) (20) (20)
7.1 ± 2.0 7.1 ± 1.6 9.6 ± 4.2
18 18 19
2 2 1
0 0 0
No conditioning (10)
Cervical Middle Apical
(20) (20) (20)
4.9 ± 1.4 4.1 ± 1.9 4.9 ± 1.8
16 17 17
2 1 2
2 3 1
Polydopamine conditioning (10)
Cervical Middle Apical
(20) (20) (20)
8.7 ± 3.2 7.9 ± 1.9 8.0 ± 2.6
16 16 15
3 0 1
1 4 4
No conditioning (10)
Cervical Middle Apical
(20) (20) (20)
3.9 ± 1.1 5.0 ± 1.6 5.2 ± 2.2
18 17 18
1 3 0
1 0 2
and the post, adhesive failure between the dentin and the cement, cohesive failure within the cement, cohesive failure within the post, and mixed failure. The numbers of each type of adhesive failure are presented in Table 4. Adhesive failure was the most frequent type of failure in all groups (85%). SEM Observations Post-cement surfaces were examined under a scanning electron microscope (SEM) to observe morphological changes in the cross-sectional interface following post surface treatment. The SEM images in Figs 1a, 1b, 2a, and 2b show no relevant changes after polydopamine treatment, probably because of the low degree of physical change on the surface of the post.
DISCUSSION The first null hypothesis of this study was not confirmed, since the bond strength between the glass-fiber posts and resin cement was statistically significantly higher after surface conditioning with polydopamine. However, the second hypothesis was confirmed, as the bond strength of the resin cement to prefabricated posts was not affected by the type of resin cement used. The choice of resin cement used for bonding made no difference in the post/cement interfacial strength. Chemical coupling agents can be used to create a chemical bond at the interface between the glass-fiber posts and the resin cement. Silane coupling agents may improve the surface wettability of posts by creating a chemical union between the resins and OH-covered inorganic substrates,37 or between resin-based materials and glass fibers,2,22 thereby enhancing the bond strength.22 Some studies have explored the effect of silanization on fiber post surfaces.53 The ability of silanization to increase bond strength is currently under debate.2,22,37,53 Given that epoxy resin is a chemically 180
Bond strength values (mean ± SD)
stable polymer, the chemical combination is thought to be relatively weak. In 2005, researchers found that an adhesive protein used by marine mussels contained high levels of a catecholamine50 whose chemical name is 3,4-dihydroxy-Lphenylalanine (DOPA). This adhesive protein secreted by the mollusk’s byssus is suitable for bonding. The higher the dopamine content, the stronger the adhesive ability of the compound.1 The research by Messersmith expounded that the adhesive force between dopamine and titanium dioxide is four times stronger than that between avidin and biotin, which is the strongest chemical force known in any ecosystem without an interatomic electronic intervention.20 At present, the adhesive mechanism of dopamine binding to objects has not been fully elucidated. However, it is known that the structure and properties of objects are not destroyed by adhesion with dopamine.17 In our study, changes in topography on the post surface functionalized with polydopamine (Fig 3b) were evident, compared to the untreated post surface (Fig 3a). The surface of the fiber post was smoother after treating with polydopamine. In recent years, polydopamine has found applications in many fields.27,28,56 However, to date no study has investigated the effects of polydopamine surface modification on the structure and function of fiber posts in dentistry. Fiber posts were soaked in dopamine solution and mildly stirred at room temperature resulting in a polydopamine layer on the surface via the oxidative polymerization of dopamine.28 It has been assumed that polydopamine modification of glass-fiber surfaces introduces carboxyl, hydroxyl, and amino groups, which further react with organic functional monomers, thereby improving hydrophilicity and enhancing chemical combination.56 The push-out bond strength of the polydopamine-functionalized glass fiber to resin cement was found to be significantly improved (p < 0.0001) in our study. Resin cement has low shrinkage, which contributed to the high chemical bond strength between cement and post. To the best of our knowledge, The Journal of Adhesive Dentistry
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Fig 1a Representative SEM micrograph of a cross-sectional interface between a post surface with no conditioning and Paracore resin cement.
Fig 1b Representative SEM micrograph of cross-sectional interface between a post surface after polydopamine functionalization and Paracore resin cement.
Fig 2a Representative SEM micrograph of a cross-sectional interface between a post surface with no conditioning and RelyX Unicem resin cement.
Fig 2b Representative SEM micrograph of a cross-sectional interface between a post surface after polydopamine functionalization and RelyX Unicem resin cement.
Fig 3a
Fig 3b At low magnification, exposure of the glass fibers can be seen as a result of dopamine modification.
Glass-fiber post surface with no conditioning.
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the present study is the first to supply evidence that surface conditioning of glass-fiber posts with polydopamine significantly increases bond strength between the post and resin cement, regardless of cement type. Effective methods are needed to modify fiber posts. Nevertheless, dopamine functionalization is an effective and clinically feasible method to mass-produce modified posts. The fiber posts can be stored, disinfected, and rinsed before use. Whether the stability of the bonds is maintained over the long term requires further study. The manufacturer of Paracore cement claims it to be effective for both luting and core buildup and more suited for ParaPost fiber systems than other types of posts. It is increasingly used in clinical practice due to its good bonding properties and simple handling requirements. The RelyX Unicem system has a relatively wide range of clinical applications. However, using this self-etching adhesive resin cement for bonding, the infiltration depth into dentin is not as deep as when using etch-and-rinse adhesive resin cement; in addition, formation of the mixed layer is poor, and can create a layer of acidic monomers which may reduce the interfacial bond strength between resin cement and posts. These cements were selected for this study by virtue of their previously reported adaptation to post surfaces.4,9,42 This study aimed to compare and select a better cement with high boding strength and easy clinical application. From the bond strength results, the two resin cements did not differ significantly not only in the control group without surface treatment but also in the treatment group. This could be explained by the fact that only by using the cement recommended by the fiber post’s manufacturer can the best compatibility between the resin matrix of the posts and the resin cement be achieved. Thus, the use of Paracore cement is suggested, based on its ease of clinical application and more comprehensive clinical effects. In contrast to similar studies, the present study found the Paracore adhesive to have a relatively lower push-out bond strength. Although the data for the control group are in agreement with the previous studies,23 it is probable that the bond strength was affected by the material of the post.42 The glass-fiber post commonly used in previous studies (ParaPost Fiber White) is composed of 42% glass fiber, 29% resin, and 29% filler, while the glass-fiber post used in this study is composed of 60% glass fiber. In the case of different experiment conditions and materials, the bond strength may not be completely comparable. In this randomized controlled experiment, fiber posts were used in both groups and the focus was on the impact of polydopamine treatment on the bond strength of posts; therefore, the relatively low bond strength is comparable to previous, similar studies. Future study is needed to determine if the bond strength of resin cements to posts modified with polydopamine is affected by the material of the post. As for the bond strength in the three different root regions examined in this study, some studies suggest that bond strength decreases significantly from the cervical to the apical root canal regions.21,25,46 Bond strength values in cervical root canal regions were the highest in one study, while the other two regions showed no noticeable difference between each other.47 In the 182
present study, no significant difference was found in micro push-out bond strengths between the three root regions (p > 0.05), which was in agreement with a previous report.23 Groups 1A and 2B revealed higher bond strengths in the apical third, which agrees with some previous studies, probably because applied pressure is supposed to be highest near the apex.6,7,35,57 These controversial findings suggest that it is not easy to control the application procedure of bonding within the narrow, deep root canal. Adhesive failure was the most frequent failure type in all groups (85%) and involved two areas: failure between the cement and the post, and failure between the dentin and the cement. This result is similar to a previous study.8 This demonstrates the relative weakness of the post-composite bond, which appears to be weaker than the resin cement or post itself. Excluding the influence of the bonding interface between the cement and dentin, the data exactly reflect the bond strength between posts with polydopamine and cement. The two resin cements selected in this study were self-etching adhesives. Compared to etch-and-rinse adhesives,34 the handling of dentin has been simplified, resulting in a greater chance of failure between the dentin and the cement. Most studies testing the bond strength of posts to endodontically treated roots were performed shortly after cementation.11,16,36 However, after a period of years, restorations may start to experience problems due to several factors, such as fluctuations in temperature and dynamic mechanical loading.32 In this study, storage for 7 days in distilled water may have yielded push-out bond strength values more similar to those found in real clinical conditions, since water, as the main element in the oral environment, can interfere with long-term durability of the bond. For this reason, water storage is considered as an in vitro accelerated aging test.50 Future in vitro studies should employ long-term wet storage conditions to evaluate the durability of bonding provided by polydopamine functionalization, where the resin cement undergoes hydrolysis, affecting the bond strength at the interface. Creating chemical bonds is one of the most critical ways of improving retention between the fiber post and cement. Changes in the surface structure of fiber posts in this study resulted in a better interface for chemical union.
CONCLUSIONS Surface polydopamine functionalization was found to be a reliable method for improving the bond strength of resin luting agents to fiber posts. The bond strength of Paracore to fiber posts was no different than that of RelyX Unicem. Considering the convenience of clinical application, preference should be given to Paracore.
ACKNOWLEDGMENTS The authors gratefully acknowledge Coltène-Whaledent, 3M ESPE, and Oya Biomaterials for providing materials. We thank the staff of the State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Statistical Center of Peking Uni-
The Journal of Adhesive Dentistry
Chen et al versity First Hospital for their support during the preparation of this manuscript. The authors acknowledge the financial support from the National High Technology Research and Development Program of China (2011AA030102) and Program for New Century Excellent Talents in University (NCET-11-0556).
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comparison between microtensile and push-out bond strength measurements. Eur J Oral Sci 2004;112:353-361. Grandini S, Chieffi N, Cagidiaco MC, Goracci C, Ferrari M. Fatigue resistance and structural integrity of different types of fiber posts. Dent Mater J 2008;27:687-694. Hayashi M, Okamura K, Wu H, Takahashi Y, Koytchev EV, Imazato S, Ebisu S. The root canal bonding of chemical-cured total-etch resin cements. J Endod 2008;34:583-586. Huang Z, Chen Q, Liu P. Investigation of material properties of one-piece glass fiber post-and-core affecting biomechanical responses of the restorative system. Advanced Materials Research 2011;160:1691-1698. Jiang JH, Zhu LP, Li XL. Surface modification of PE porous rnembranes based on the strong adhesion of polydoparnine and covalent immobilization of heparin. J Membr Sci 2010;364:194-202. Ku SH, Ryu J, Hong SK, Lee H, Park CB. General functionalization route for cell adhesion on non-wetting surfaces. Biomaterials 2010;31: 2535-2541. Lee H, Dellatore SM, Miller WM, Messersmith PB. Mussel-inspired surface chemistry for multifunctional coatings. Science 2007;318: 426-430. Li B, Liu WP, Jiang Z, Dong X, Wang B, Zhong Y. Ultrathin and stable active layer of dense composite membrane enabled by poly(dopamine). Langmuir 2009;25:7368-7374. Lopes GC, Baratieri CM, Baratieri LN, Monteiro S Jr, Cardoso Vieira LC. Bonding to cervical sclerotic dentin: effect of acid etching time. J Adhes Dent 2004;6:19-23. Mentink AG, Creugers NH, Meeuwissen R, Leempoel PJ, Kayser AE. Clinical performance of different post and core systems-results of a pilot study. J Oral Rehabil 1993;20:577-584. Monticelli F, Toledano M, Tay FR, Cury AH, Goracci C, Ferrari M. Postsurface conditioning improves interfacial adhesion in post/core restorations. Dent Mater 2006;22:602-609. Monticelli F, Toledano M, Tay FR, Sadek FT, Goracci C, Ferrari M. A simple etching technique for improving the retention of fiber posts to resin composites. J Endod 2006;32:44-47. Muniz L, Mathias P. The influence of sodium hypochlorite and root canal sealers on post retention in different dentin regions. Oper Dent 2005;30:533-539. Nergiz I, Schmage P, Özcan M, Platzer U. Effect of length and diameter of tapered posts on the retention. J Oral Rehabil 2002;29:28-34. Park SJ, Jin JS. Effect of silane coupling agent on interphase and performance of glass fibers/unsaturated polyester composites. J Coll Inter Sci 2001;242:174-179. Perdigao J, Gomes G, Lee IK. The effect of silane on the bond strengths of fiber posts. Dent Mater 2006;22:752-758. Perez BE, Barbosa SH, Melo RM, Zamboni SC, Özcan M, Valandro LF, Bottino MA. Does the thickness of the resin Ccment affect the bond strength of a fiber post to the root dentin? Int J Prosthodont 2006;19:606-609. Sahafi A, Peutzfeldt A, Asmussen E, Gotfredsen K. Bond strength of resin cement to dentin and to surface-treated posts of titanium alloy, glass fiber, and zirconia. J Adhes Dent 2003;5:153-162. Sahafi A, Peutzfeld A, Asmussen E, Gotfredsen K. Effect of surface treatment of prefabricated posts on bonding of resin cement. Oper Dent 2004;29:60-68. Sahafi A, Peutzfeldt A. Bond strength of resin cement to dentin and to surface-treated posts of titanium alloy, glass fiber, and zirconia. J Adhes Dent 2003;5:53-162. Sahafi A, Peutzfeldt A, Ravvnholt G. Resistance to cyclic loading of teeth restored with posts. Clin Oral Invest 2005;9:84-90. Schmitter M, Huy C, Ohlmann B, Gabbert O, Gilde H, Rammelsberg P. Fracture resistance of upper and lower incisors restored with glass fiber reinforced posts. J Endod 2006;32:328-330. Soares CJ, Soares PV, de Freitas Santos-Filho PC, Castro CG, Magalhaes D, Versluis A. The influence of cavity design and glass fiber posts on biomechanical behavior of endodontically treated premolars. J Endod 2008;34:1015-1019. Teixeira CS, Silva-Sousa YT, Sousa-Neto MD. Bond strength of fiber posts to weakened roots after resin restoration with different lightcuring times. J Endod 2009;35:1034-1039. Tuncdemir AR, Yıldırım C, Güller F, Özean E, Usumez A. The effect of post surface treatments on the bond strength of fiber posts to root surfaces. Lasers Med Sci 2013;28:13-18. Valandro LF, Bottino MA. Microtensile bond strength between quartz fiber post and resin cement: Effect of post surface treatment. J Endodont Mater 2003;4:271-274.
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Chen et al 49. Valandro LF, Yoshiga S, de Melo RM, Galhano GA, Mallmann A, Marinho CP, Bottino MA. Microtensile bond strength between a quartz fiber post and a resin cement: effect of post surface conditioning. J Adhes Dent 2006;8:105-111. 50. Vichi A, Vano M, Ferrari M. The effect of different storage and conditions and duration on the fracture strength of three types of translucent fiber posts. Dent Mater 2008;24:832-838. 51. Waite JH, Qin X. Polyphosphoprotein from the adhesive pads of Mytilus edulis. Biochemistry 2001;40:2887-2893. 52. Wolf DM, Powers JM, O’Keefe KL. Bond strength of composite to etched and sandblasted porcelain. Am J Dent 1993;6:155-158. 53. Wrbas KT, Altenburger MJ, Schirrmeister JF, Bitter K, Kielbassa AM. Effect of adhesive resin cements and post surface silanization on the bond strengths of adhesively inserted fiber posts. J Endod 2007;33:840-843. 54. Xible AA, de Jesus Tavarez RR, de Araujo Cdos R, Bonachela WC. Effect of silica coating and silanization on flexural and composite-resin bond strengths of zirconia posts: An in vitro study. J Prosthet Dent 2006;95:224-229. 55. Xie J, Michael PL, Zhong S, Ma B, MacEwan MR, Lim CT. Mussel inspired protein-mediated surface modification to electrospun fibers and their potential biomedical applications. J Biomed Mater Res A 2012;100:929–938.
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56. Xi ZY, Xu YY, Zhu LP. A facile method of surface modification for hydrophobic polymer membranes based on the adhesive behavior of polyDOPA and polydopamine. J Membr Sci 2009;327:244-253. 57. Zicari F, Couthino E, De Munck J, Poitevin A, Scotti R, Naert I, Van Meerbeek B. Bonding effectiveness and sealing ability of fiber-post bonding. Dent Mater 2008;24:967-977.
Clinical relevance: Surface conditioning of glassfiber posts with polydopamine significantly increases bond strength between the post and resin cement, regardless of the cement type. Functionalization of dopamine may represent an effective and clinically feasible method by which to mass-produce modified posts for root canal restoration.
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a
Postgraduate Student, Department of Prosthodontics, Peking University School and Hospital of Stomatology, Beijing, P. R. China. Main operator in the experiments, wrote the paper.
b
Professor, State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing, P. R. China. Supervised the research project.
c
Postgraduate Student, State Key Laboratory of Organic-inorganic Composites, Beijing University of Chemical Technology, Beijing, P. R. China. Assisted in the polydopamine treatment of glass-fiber posts.
d
Postgraduate Student, Department of Prosthodontics, Peking University School and Hospital of Stomatology, Beijing, P. R. China. Performed the experiments in partial fulfillment of a degree, performed statistical evaluation, co-wrote manuscript.
e
Doctor of Stomatological Science, Department of Prosthodontics, Peking University School and Hospital of Stomatology, Beijing, P. R. China. Conducted experiments, prepared root canals for treatment.
f
Professor, Department of Prosthodontics, Peking University School and Hospital of Stomatology, Beijing, P. R. China. Supervised the research project.
Correspondence: Professor Xin-Zhi Wang, Department of Prosthodontics, Peking University School and Hospital of Stomatology, 22 Zhongguancun Nandajie, Haidian District, Beijing, P. R. China 100081. Tel: Tel: +86-010-82195348, Fax: +86-010-6213-0611. e-mail: [email protected]
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Submitted for publication: 03.12.12; accepted for publication: 02.07.13
T
he restoration of root canals of severely damaged teeth with relatively complete ferrules often requires the placement of a post to ensure adequate retention of the core.18,45 Prefabricated fiber posts have been rapidly adopted by dentists in preference to cast metal posts, because of the advantageous properties of the former in having an elastic modulus similar to that of dentin, which produces a stress field similar to that of natural teeth.14,44 Clinical studies of fiber posts have demonstrated high success rates without the occurrence of root fractures.3,26 Debonding is the most common kind of failure of prefabricated fiber-reinforced composites following restoration,19 as prefabricated posts do not fit well into the flared canals that result from carious extension and trauma.12 However, with the development of the CAD/ CAM one-piece glass-fiber post and core, the retention of fiber posts has improved. The other factor involved in debonding is the poor surface adhesive properties of 177
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prefabricated fiber posts, where failure generally occurs at the post/adhesive junction.6,49 In vivo data have shown that bonds at the resin cement/post interface play an important role in the clinical success of a post-retained restoration.40 To improve the bond strength between the prefabricated fiber post and the resin cement, many surface pretreatment procedures for fiber posts have been investigated involving mechanical and/or chemical treatment. Hydrogen peroxide,34 hydrofluoric acid,52 and sandblasting with aluminum oxide5,10,13,24,43,48,54 have all been shown to significantly increase the bond strength. However, these treatments may modify the post shape and consequently decrease its ability to fit snugly within the root canal.5,10,13,24,43,48,54 They may also reduce the strength of the post,33,41 because of the absence of a chemical union between the methacrylate-based resin composite and the epoxy resin matrix of fiber posts. Chemical coupling has been proposed to enhance the bond strength. However, the inorganic fibers which are enveloped by the epoxy resin cannot covalently bind with a coupling agent that does not contain silicate. This makes it difficult to form a strong chemical bond.5,10,13,24,33,38,41,43,48,51,54 Dopamine, when polymerized steadily, forms an ultrathin active layer and chemical structure that can support the matrix surface.30 The adhesive properties of many different materials, including PTFE, have been improved by dopamine treatment.29 Polymerized dopamine has been found to improve the hydrophilicity and biocompatibility of polyethylene, PTFE, silicone rubber, and glass.28 Therefore, the purpose of this study was to evaluate the effect of polydopamine functionalized surface modification on the bond strength of prefabricated glass-fiber posts. The tested null hypotheses were: (1) the bond strength between the post surface and the resin cement cannot be increased by polydopamine functionalized post surface modification; (2) the choice of resin cement used for bonding has no significant effect on the post/cement interfacial strength.
MATERIALS AND METHODS Tooth Preparation For this study, teeth with single, straight root canals at least 9 mm long which were extracted from patients for periodontal and/or prosthetic reasons were used. All teeth were selected on the basis of no visible root cracks or caries, or with caries only in the 2 mm coronal to the cementoenamel junction. Radiographic images were obtained from the mesiodistal and buccolingual perspectives and used to exclude teeth with calcifications, anatomical abnormalities, the lack of a single root canal, or previous endodontic treatment. Using this selection process, 40 teeth were chosen. The selected teeth were cleaned of both calculus deposits and soft tissues, and stored in 5% chloramine T aqueous solution at 4°C. The crowns were sectioned horizontally 2 mm coronal to the cementoenamel junction (CEJ) using a 0.4-mm-thick precision saw (Isomet 1000, Buehler; Lake Bluff, IL, USA) with distilled water as the coolant. All roots 178
were cut to 14 mm as a standardized length. The working length was set 1 mm shorter than this (wl = 13 mm). The pulpal tissues were removed with a #0 barbed broach (Mani; Tokyo, Japan). Roots were irrigated using 2 ml of 5.25% sodium hypochlorite (NaClO) and Prep-Rite RC root canal lubricant (Pulpdent; Watertown, MA, USA), prepared using Protaper rotary nickel titanium instruments (SX, S1, S2, F1, F2, F3; Dentsply; York, PA, USA) with a low-speed rotary endodontic handpiece (X-smart; Dentsply), followed by ultrasonic rinsing with distilled water. Specimens were dried with paper points (Gapadent; Beijing, China), then obturated with gutta-percha cones (Dayading; Beijing, China) and an endodontic sealer (Septodent; Paris, France) using a lateral condensation technique. Cervical root canal cavities were filled with the light-curing composite resin AP-X (Kuraray; Osaka, Japan). After endodontic treatment, the teeth were stored at 37°C in 0.9% NaCl solution for 7 days. Prefabricated Glass-fiber Post Preparation Forty prefabricated glass-fiber posts (Beijing Oya Biomaterials; Beijing, China) with a maximum diameter of 1.4 mm were selected. The coronal two-thirds is parallel to columnar, and the apical third is tapered. It is composed of 60% glass fiber. The glass fibers are SE8400LS (Owens Corning; Toledo, OH, USA) and the resin matrix is Epoxy-828 (Shell; den Hague, Netherlands). The posts were randomly divided into two groups (group A and group B, with each group containing 20 posts) according to the post surface conditioning. Group A, 20 posts: The post surface was ultrasonically cleaned in ethanol before use (50 Hz, 0.5 hrs) and conditioned with 2 mg/ml dopamine in 100 ml Tris, pH 8.5 for 14 h at 25°C under continuous stirring.18,54 The posts were then retrieved and washed with deionized water to remove excess monomer and dried under nitrogen applied with a nozzle. Group B, 20 posts (control group): The post surface received no conditioning. Post Space Preparation and Post Cementation Gutta-percha was removed with #1-2 Peeso Reamers (Mani), and the post space of each specimen was enlarged with a standard drill system from the corresponding fiber post system (Beijing Oya Biomaterials) to create a 9-mm post space with at least 4 mm of filling material in the root apex. The post space was rinsed with distilled water and dried with paper points. Forty root specimens were randomly divided into four groups (n = 10 per group) as follows. Group IA: Paracore dual-curing resin cement (Coltène-Whaledent; Altstätten, Switzerland) + polydopamine-treated prefabricated glassfiber post. Group IB: Paracore dual-curing resin cement (Coltène-Whaledent) + untreated glass-fiber post. Group IIA: RelyX Unicem (3M ESPE; St Paul, MN, USA) + polydopamine-treated prefabricated glass-fiber post. Group IIB: 3M ESPE RelyX Unicem + untreated glass-fiber post. Fiber posts for each group were cemented to the root canals with adhesive resin cement, according to the manufacturer’s instructions. All specimens were stored at 37°C in distilled water for 7 days. The Journal of Adhesive Dentistry
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Table 1 Mean (MPa) and standard deviation (SD) of micro push-out bond strength values Post surface modification
Resin cement for bonding
Polydopamine conditioning No conditioning
Table 2
7.9 ± 3.2 4.7 ± 2.2
8.2 ± 2.8 4.7 ± 2.1
Results of two-way ANOVA with two factors (cement and treatment) SSq
DF
MSq
F
p
Intercept Cement Treatment
4879.279 0.880 338.176
1 1 1
4879.279 0.880 338.176
651.862 0.118 45.180
< 0.0001 0.734 < 0.0001
Results of repeated measures ANOVA test for the factor “regions of
Source
SSq
DF
MSq
F
p
Region
18.147
2
9.074
2.187
0.119
SEM Analysis Before performing the push-out test, one slice from each group was selected and examined under a scanning electron microscope (SEM). Sections were polished with wet silicon carbide paper of increasing grit (600, 800, 1000), rinsed with deionized water, ultrasonically cleaned in deionized water, and air dried. The slices were gold-sputtered and imaged using a scanning electron microscope (EVO 18 Special Edition Scanning Electron Microscope, Zeiss; Jena, Germany). Micro Push-out Test All specimens were cut perpendicular to the long axis of the tooth and 1.0-mm slices were made from the cervical, middle, and apical regions of the root using a water-cooled, 0.4-mm-thick precision saw with an accuracy of 0.1 mm, speed of 400 rpm, and pressure of 50 g (Isomet 1000, Buehler). Two slices from each region were selected and marked. The coronal post radius R (mm), apical post radius r (mm) and thickness of the specimen H (mm) of each slice was measured with electronic digital-display vernier calipers to an accuracy of 0.01 mm. The micro push-out test was performed with a universal testing machine (Instron 1121; Norwood, MA, USA) at a crosshead speed of 0.5 mm/min until failure. The maximum failure load was recorded in N and converted into MPa. The circular post slices were considered as truncated cones in the following calculations. The micro push-out bond strength value m (MPa) was calculated using the following formula:23
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RelyX Unicem resin cement Mean ± SD (MPa)
Source
Table 3 root”
Debond stress =
Paracore resin cement Mean ± SD (MPa)
Debonding force (N) area of the post/dentin surface (A)
A = / (R + r) √ H2 + (R – r)2 ≈ / (r + R) H where / = 3.14, R = coronal post radius, r = apical post radius, and H = root slice thickness. Statistical Analysis Statistical analysis was performed using SPSS software (SPSS 13.0, SPSS; Chicago, IL, USA) and the level of significance was set at 0.05. Data were analyzed using two-way and repeated measurements ANOVA.
RESULTS Interfacial Strength The mean (± SD) micro push-out bond strength values and the differences within the groups are presented in Tables 1 to 4. The results of two-way ANOVA revealed that the surface polydopamine treatment had a significant influence on the micro push-out bond strength of the adhesive resin cement to the glass-fiber post (p < 0.05). Group A had higher values than group B (p < 0.05). There were no significant differences between groups 1 and 2 (p > 0.05; Tables 1 and 2). The bond strength values did not vary significantly between groups by root region (p > 0.05; Table 3). Group A showed consistently higher push-out bond strength values than Group B for all root sections. Debonded specimens were examined using a stereomicroscope (Kestrel, Vision Engineering; Surrey, UK) at 50X magnification, and the failure modes were classified as follows: adhesive failure between the cement 179
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Table 4
Mean (MPa) and standard deviation (SD) of micro push-out bond strength values in groups by root region
Resin cement (roots) Paracore resin cement (20)
RelyX Unicem resin cement (20)
Post surface conditioning (No. of posts used)
Region (No. of slices used)
Failure mode (number of slices) Adhesive
Cohesive
Mixed
Polydopamine conditioning (10)
Cervical Middle Apical
(20) (20) (20)
7.1 ± 2.0 7.1 ± 1.6 9.6 ± 4.2
18 18 19
2 2 1
0 0 0
No conditioning (10)
Cervical Middle Apical
(20) (20) (20)
4.9 ± 1.4 4.1 ± 1.9 4.9 ± 1.8
16 17 17
2 1 2
2 3 1
Polydopamine conditioning (10)
Cervical Middle Apical
(20) (20) (20)
8.7 ± 3.2 7.9 ± 1.9 8.0 ± 2.6
16 16 15
3 0 1
1 4 4
No conditioning (10)
Cervical Middle Apical
(20) (20) (20)
3.9 ± 1.1 5.0 ± 1.6 5.2 ± 2.2
18 17 18
1 3 0
1 0 2
and the post, adhesive failure between the dentin and the cement, cohesive failure within the cement, cohesive failure within the post, and mixed failure. The numbers of each type of adhesive failure are presented in Table 4. Adhesive failure was the most frequent type of failure in all groups (85%). SEM Observations Post-cement surfaces were examined under a scanning electron microscope (SEM) to observe morphological changes in the cross-sectional interface following post surface treatment. The SEM images in Figs 1a, 1b, 2a, and 2b show no relevant changes after polydopamine treatment, probably because of the low degree of physical change on the surface of the post.
DISCUSSION The first null hypothesis of this study was not confirmed, since the bond strength between the glass-fiber posts and resin cement was statistically significantly higher after surface conditioning with polydopamine. However, the second hypothesis was confirmed, as the bond strength of the resin cement to prefabricated posts was not affected by the type of resin cement used. The choice of resin cement used for bonding made no difference in the post/cement interfacial strength. Chemical coupling agents can be used to create a chemical bond at the interface between the glass-fiber posts and the resin cement. Silane coupling agents may improve the surface wettability of posts by creating a chemical union between the resins and OH-covered inorganic substrates,37 or between resin-based materials and glass fibers,2,22 thereby enhancing the bond strength.22 Some studies have explored the effect of silanization on fiber post surfaces.53 The ability of silanization to increase bond strength is currently under debate.2,22,37,53 Given that epoxy resin is a chemically 180
Bond strength values (mean ± SD)
stable polymer, the chemical combination is thought to be relatively weak. In 2005, researchers found that an adhesive protein used by marine mussels contained high levels of a catecholamine50 whose chemical name is 3,4-dihydroxy-Lphenylalanine (DOPA). This adhesive protein secreted by the mollusk’s byssus is suitable for bonding. The higher the dopamine content, the stronger the adhesive ability of the compound.1 The research by Messersmith expounded that the adhesive force between dopamine and titanium dioxide is four times stronger than that between avidin and biotin, which is the strongest chemical force known in any ecosystem without an interatomic electronic intervention.20 At present, the adhesive mechanism of dopamine binding to objects has not been fully elucidated. However, it is known that the structure and properties of objects are not destroyed by adhesion with dopamine.17 In our study, changes in topography on the post surface functionalized with polydopamine (Fig 3b) were evident, compared to the untreated post surface (Fig 3a). The surface of the fiber post was smoother after treating with polydopamine. In recent years, polydopamine has found applications in many fields.27,28,56 However, to date no study has investigated the effects of polydopamine surface modification on the structure and function of fiber posts in dentistry. Fiber posts were soaked in dopamine solution and mildly stirred at room temperature resulting in a polydopamine layer on the surface via the oxidative polymerization of dopamine.28 It has been assumed that polydopamine modification of glass-fiber surfaces introduces carboxyl, hydroxyl, and amino groups, which further react with organic functional monomers, thereby improving hydrophilicity and enhancing chemical combination.56 The push-out bond strength of the polydopamine-functionalized glass fiber to resin cement was found to be significantly improved (p < 0.0001) in our study. Resin cement has low shrinkage, which contributed to the high chemical bond strength between cement and post. To the best of our knowledge, The Journal of Adhesive Dentistry
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Fig 1a Representative SEM micrograph of a cross-sectional interface between a post surface with no conditioning and Paracore resin cement.
Fig 1b Representative SEM micrograph of cross-sectional interface between a post surface after polydopamine functionalization and Paracore resin cement.
Fig 2a Representative SEM micrograph of a cross-sectional interface between a post surface with no conditioning and RelyX Unicem resin cement.
Fig 2b Representative SEM micrograph of a cross-sectional interface between a post surface after polydopamine functionalization and RelyX Unicem resin cement.
Fig 3a
Fig 3b At low magnification, exposure of the glass fibers can be seen as a result of dopamine modification.
Glass-fiber post surface with no conditioning.
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the present study is the first to supply evidence that surface conditioning of glass-fiber posts with polydopamine significantly increases bond strength between the post and resin cement, regardless of cement type. Effective methods are needed to modify fiber posts. Nevertheless, dopamine functionalization is an effective and clinically feasible method to mass-produce modified posts. The fiber posts can be stored, disinfected, and rinsed before use. Whether the stability of the bonds is maintained over the long term requires further study. The manufacturer of Paracore cement claims it to be effective for both luting and core buildup and more suited for ParaPost fiber systems than other types of posts. It is increasingly used in clinical practice due to its good bonding properties and simple handling requirements. The RelyX Unicem system has a relatively wide range of clinical applications. However, using this self-etching adhesive resin cement for bonding, the infiltration depth into dentin is not as deep as when using etch-and-rinse adhesive resin cement; in addition, formation of the mixed layer is poor, and can create a layer of acidic monomers which may reduce the interfacial bond strength between resin cement and posts. These cements were selected for this study by virtue of their previously reported adaptation to post surfaces.4,9,42 This study aimed to compare and select a better cement with high boding strength and easy clinical application. From the bond strength results, the two resin cements did not differ significantly not only in the control group without surface treatment but also in the treatment group. This could be explained by the fact that only by using the cement recommended by the fiber post’s manufacturer can the best compatibility between the resin matrix of the posts and the resin cement be achieved. Thus, the use of Paracore cement is suggested, based on its ease of clinical application and more comprehensive clinical effects. In contrast to similar studies, the present study found the Paracore adhesive to have a relatively lower push-out bond strength. Although the data for the control group are in agreement with the previous studies,23 it is probable that the bond strength was affected by the material of the post.42 The glass-fiber post commonly used in previous studies (ParaPost Fiber White) is composed of 42% glass fiber, 29% resin, and 29% filler, while the glass-fiber post used in this study is composed of 60% glass fiber. In the case of different experiment conditions and materials, the bond strength may not be completely comparable. In this randomized controlled experiment, fiber posts were used in both groups and the focus was on the impact of polydopamine treatment on the bond strength of posts; therefore, the relatively low bond strength is comparable to previous, similar studies. Future study is needed to determine if the bond strength of resin cements to posts modified with polydopamine is affected by the material of the post. As for the bond strength in the three different root regions examined in this study, some studies suggest that bond strength decreases significantly from the cervical to the apical root canal regions.21,25,46 Bond strength values in cervical root canal regions were the highest in one study, while the other two regions showed no noticeable difference between each other.47 In the 182
present study, no significant difference was found in micro push-out bond strengths between the three root regions (p > 0.05), which was in agreement with a previous report.23 Groups 1A and 2B revealed higher bond strengths in the apical third, which agrees with some previous studies, probably because applied pressure is supposed to be highest near the apex.6,7,35,57 These controversial findings suggest that it is not easy to control the application procedure of bonding within the narrow, deep root canal. Adhesive failure was the most frequent failure type in all groups (85%) and involved two areas: failure between the cement and the post, and failure between the dentin and the cement. This result is similar to a previous study.8 This demonstrates the relative weakness of the post-composite bond, which appears to be weaker than the resin cement or post itself. Excluding the influence of the bonding interface between the cement and dentin, the data exactly reflect the bond strength between posts with polydopamine and cement. The two resin cements selected in this study were self-etching adhesives. Compared to etch-and-rinse adhesives,34 the handling of dentin has been simplified, resulting in a greater chance of failure between the dentin and the cement. Most studies testing the bond strength of posts to endodontically treated roots were performed shortly after cementation.11,16,36 However, after a period of years, restorations may start to experience problems due to several factors, such as fluctuations in temperature and dynamic mechanical loading.32 In this study, storage for 7 days in distilled water may have yielded push-out bond strength values more similar to those found in real clinical conditions, since water, as the main element in the oral environment, can interfere with long-term durability of the bond. For this reason, water storage is considered as an in vitro accelerated aging test.50 Future in vitro studies should employ long-term wet storage conditions to evaluate the durability of bonding provided by polydopamine functionalization, where the resin cement undergoes hydrolysis, affecting the bond strength at the interface. Creating chemical bonds is one of the most critical ways of improving retention between the fiber post and cement. Changes in the surface structure of fiber posts in this study resulted in a better interface for chemical union.
CONCLUSIONS Surface polydopamine functionalization was found to be a reliable method for improving the bond strength of resin luting agents to fiber posts. The bond strength of Paracore to fiber posts was no different than that of RelyX Unicem. Considering the convenience of clinical application, preference should be given to Paracore.
ACKNOWLEDGMENTS The authors gratefully acknowledge Coltène-Whaledent, 3M ESPE, and Oya Biomaterials for providing materials. We thank the staff of the State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Statistical Center of Peking Uni-
The Journal of Adhesive Dentistry
Chen et al versity First Hospital for their support during the preparation of this manuscript. The authors acknowledge the financial support from the National High Technology Research and Development Program of China (2011AA030102) and Program for New Century Excellent Talents in University (NCET-11-0556).
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Clinical relevance: Surface conditioning of glassfiber posts with polydopamine significantly increases bond strength between the post and resin cement, regardless of the cement type. Functionalization of dopamine may represent an effective and clinically feasible method by which to mass-produce modified posts for root canal restoration.
The Journal of Adhesive Dentistry
