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ORIGINAL ARTICLE |
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Year : 2013 | Volume
: 3
| Issue : 2 | Page : 112-118 |
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Resin push-out bond between fiber post and root dentin: Evaluation of different cementation approaches
Yvana Goes1, Luiz F Valandro2, Rodrigo O. A. Souza3, Adalúcia C Correia1, Leticia B Jacques2, Andre Mallmann2
1 Department of Restorative Dentistry, Development Foundation from Bahia, Salvador, Brazil 2 Department of Restorative Dentistry, Division of Prosthodontics, Federal University of Santa Maria, Santa Maria, Brazil 3 Department of Restorative Dentistry, Division of Prosthodontics, Federal University of Paraiba, Joao Pessoa, Brazil
Date of Web Publication | 13-Dec-2013 |
Correspondence Address: Rodrigo O. A. Souza Federal University of Paraíba, School of Dentistry, Department of Restorative Dentistry, Rua Praia de Guajirú, 9215, Ponta Negra, CEP: 59.092-220, Natal/ Rio Grande do Norte Brazil
Source of Support: None, Conflict of Interest: None | Check |
DOI: 10.4103/2278-9596.122929
Background: Resin push-out bond between fiber post and root dentin can be achieved in various ways. This study evaluates the bond strength of glass fiber posts cemented into root canal using different luting techniques. The null hypothesis tested was that no difference would be observed among the strategies for post cementation. Materials and Methods: The canals of 50 roots were prepared with the custom drill of glass fiber post and divided into five groups (n = 10), according to the strategy for post cementation: G1- conventional adhesive procedure (single-bottle etch and rinse light-cured adhesive + resin cement (Relyx ARC)); G2- conventional adhesive procedure (single-bottle etch and rinse light-cured adhesive + resin cement (AllCeram); G3- self-adhesive resin cement (Relyx Unicem, 3M/ESPE); G4- self-adhesive resin cement (Relyx Unicem, 3M/ESPE) with previous dentin acid etching; and G5- resin glass ionomer cement (Relyx Luting 2, 3M/ESPE). After post cementation, the specimens were kept for 24 h in 100% humidity. Afterwards, each root was sectioned into 2 mm slices to be pushed-out. The data were submitted to one-way analysis of variance (ANOVA) and Tukey's test (5%). Failure analyses were followed after testing. Results: The self-adhesive resin cement with dentin acid pretreatment (G4: 22.3 ± 2.8) had the highest bond strength values (MPa) (G3: 16.3 ± 4.4 > G1: 7.0 ± 3.9 > G2: 4.9 ± 2.5 > G5: 3.7 ± 2.2) (P < 0.05). This was followed by the same cement without phosphoric acid etching. The other three groups obtained the significantly lowest mean values and were similar to each other. Conclusion: Simplified self-adhesive resin cement promoted the highest bond strength when compared to conventional adhesive cementation and glass ionomer. Keywords: Bond strength, fiber posts, resin cements
How to cite this article: Goes Y, Valandro LF, Souza RO, Correia AC, Jacques LB, Mallmann A. Resin push-out bond between fiber post and root dentin: Evaluation of different cementation approaches. Arch Int Surg 2013;3:112-8 |
How to cite this URL: Goes Y, Valandro LF, Souza RO, Correia AC, Jacques LB, Mallmann A. Resin push-out bond between fiber post and root dentin: Evaluation of different cementation approaches. Arch Int Surg [serial online] 2013 [cited 2024 Mar 19];3:112-8. Available from: https://www.archintsurg.org/text.asp?2013/3/2/112/122929 |
Introduction | | |
The enamel acid etching and dentin hybridization of hard dental tissues has allowed for more conservative restorative procedures and minimal intervention. [1] When losing hard dental tissues, pulpless teeth need the root anchorage to restore the coronal part by using root post. [2] The adhesively cemented prefabricated fiber post allows maximum preservation of dental structures, consequently the risk of root fracture has been reduced. [3],[4],[5],[6] The use of the root posts with biomechanical properties (such as modulus of elasticity) close to the dentin's properties has been contributed for reducing the percentage of root fracture. [3],[4],[5],[6]
Even clinical trials have demonstrated promising results with teeth restored with fiber posts (fiber reinforced composite posts (FRC)), [7],[8],[9] debonding of the cement - FRC - core set (loss of retention) have been found. [7],[8] It may be caused by failure of adhesion between adhesive materials and root dentin walls and it may be associated to some factors including chemical incompatibility between some adhesive systems and resin cements, [10] heterogeneous dentinal substrate [1],[11],[12] unsure hybridization in dentinal walls, [13] access to the adhesive procedures, [13],[14] and mechanical properties of the fiber post. [15] An increased factor of cavity design is an inherent factor that can impair the bond to root dentin, [16],[17] since the higher cavity design increases the polymerization contraction stress of the resinous materials. [18],[19] Thus, the use of a resin cement with low shrinkage for post cementation could improve the retention of fiber posts by improving the friction to root dentin walls. [20],[21]
The bond to dental substrate traditionally combines the use of resin cement and both etch and rinse of the dentin and dentin adhesives, [22] utilizing phosphoric acid to completely dissolve the dentin smear layer and then create a zone of partially demineralized dentin. [23],[24] After acid etching, the primers and adhesives are applied to the demineralized dentin to obtain micromechanical adhesion. Mallmann, et al., [17] verified that, for fiber reinforced resin posts adhesively cemented into root dentin, it is possible to use either light- or chemically-activated adhesive systems, when used in association with dual-cure resin cement.
Recently, a one-step technique for cementing root posts, inlays, onlays, and fixed partial dentures has been marketed, named 'self-adhesive resin cement' (Relyx Unicem, Relyx U100, Relyx U200, 3M/ESPE). For this kind of material, the organic matrix consists of multifunctional phosphoric acidic methacrylates, which contributes to the adhesion to tooth tissue. The inorganic fillers are of basic nature and are able to undergo a cement reaction with the acidic groups of the functional monomers. Due to the reactions of that cement, the pH-value of the material increases from 1 to 6 during the setting reaction. [24],[25] The dominant setting reaction starts with free radical polymerization, which can be initiated both by light or by a redox system, such as with the reactions of dual curing composite materials. Additionally, phosphoric acidic methacrylates in the monomer mixture can react with the basic fillers and the hydroxyapatite of the tooth hard tissue. Water is released in this reaction, which accelerates the neutralization reaction. [20],[25],[26],[27] As it is well-known, the phosphoric acid removes the smear layer, improves the surface area to adhere the adhesive materials for micromechanical retention. It is hypothesized that the dentin root acid etching before cementation with self-adhesive cement may optimize the resin bond strength of fiber post to root dentin.
Thus, the aim of the current study was to evaluate the push-out bond strength between fiber post and root dentin, in function of different strategies for post cementation (conventional adhesive approach, self-adhesive resin cement with and without previous phosphoric acid etching on root dentin, and resin-modified glass ionomer cement). The null hypothesis tested was that no difference would be observed among the strategies for post cementation.
Materials and Methods | | |
Sample selection and preparation
This research was validated by The Ethics Commission of Medicine and Public Health Bahiana School. Fifty single rooted human teeth (central incisors, laterals, and canines) were selected. All of them were healthy, being free of caries and restorations. Immediately after extraction, the teeth were cleaned and stored in physiological serum, which was changed weekly until utilization. The coronal portions of teeth were transversally sectioned to 1 mm above the cementoenamel junction using a high-speed diamond bur (KG Sorensen) with water cooling.
Endodontic treatment
Root canals were manually instrumented with Kerr type files (Maillefer-Dentsply, Ballaigues, Switzerland), beginning with #15 and going to #40 up to a distance 1 mm from the apex, which was considered the working length. Mechanical instrumentation was performed with #2, 3, and 4 Largo type drills (Maillefer-Dentsply, Ballaigues, Switzerland) at low speed. During all procedures, root canals were irrigated with distilled water and 0.5% sodium hypochlorite (Formula and Action Drugstore, Sao Paulo-SP, Brazil).
Root canals were filled with gutta-percha points (Dentsply-Maillefer, Petropolis, RJ, Brazil) using the lateral condensation technique using endodontic resin cement (Sealer 26, Dentsply-Maillefer, Petropolis, RJ, Brazil). One operator carried out all steps of endodontic treatment.
Root preparation
After 7 days of storage in 100% ambient humidity, the root canals were prepared using #2 preparation drill of fiber post system (White Post DC, FGM, Joinvile, SC, Brazil; double-tapered translucent glass fiber post #2 (Ψ coronal = 1.8 mm and Ψ apical = 1.05 mm). The drill corresponding to the post diameter was used at low speed, with intermittent pressure and water cooling, to prepare the canal space at a standardized length of 12 mm. The posts were then tested into the root canal.
Embedment of roots in a resin base
The apical portion of each root (4 mm) was embedded in a chemically cured acrylic resin base using an adapted surveyor intended to keep roots and cemented posts as perpendicular as possible to the horizontal plane. For this, #2 preparation drill of the post system was placed inside the prepared root canal and attached to an adapted surveyor. The adapted surveyor staff with drill-root was then lowered, introducing 4 mm of root in the acrylic resin, enclosed by an A-silicone mold (Express 3M-ESPE, St. Paul, MN, USA). After resin acrylic polymerization, specimens were immersed in distilled water.
Post cementation
The 50 specimens were randomly allocated into five groups (n = 10), as described in [Table 1]. Before post cementation, root canals were irrigated with 1% sodium hypochlorite (10 ml) and then rinsed with distilled water (10 ml) using disposable syringes and dried with paper points. All posts were etched with 35% phosphoric acid (3M-ESPE, St. Paul, MN, USA), followed by a rinse with 20 ml of distilled water, air dried, and a silane coupling agent was applied (Monobond-S; Ivoclar Vivadent; Schaan, Liechtenstein) with a clean brush in one layer and allowed to sit for 5 min. During the luting process, the roots were wrapped with A-silicone to avoid light penetration through the external surface of the root during light activation of adhesive systems and cements. After cementation, specimens were stored in distilled water at 37°C for 24 h.
Production of disk specimens for push-out testing
Specimens were fixed with cyanoacrylate (Super Bonder Gel, Loctite, Sao Paulo, SP, Brazil) on a metallic base of a machine and sectioned with a diamond disc under cooling spray (KG Sorensen, Sao Paulo, SP, Brazil) into approximately 2 mm slices. The first cervical slice (approximately 1 mm) was discarded, given that the excess cement in that region could influence adhesive resistance and to standardize the parallelism between slices. Four other sections per root were produced, resulting in 40 slices per group.
Push-out test
Each specimen was positioned on a metallic device with a central opening with Ψ = 2.5 mm larger than the canal diameter. The most coronal portion of the specimen was placed downward. For push-out testing, a metallic cylinder (Ψ end = 1 mm) was attached to the machine's superior portion that coincided with the post center. The universal testing machine induced a load in the apical to coronal direction of the post without applying any pressure to the cement or dentin. Considering that the specimens were embedded into the epoxy resin parallel to the root axis and sectioned perpendicular to that axis, the post was submitted to parallel pressure to the greatest extent in relation to the root axis. The test was performed in a universal testing machine (EMIC, Sao Jose dos Pinhais, Brazil) at a speed of 1 mm/min.
The bond strength (σ) in MPa was obtained with the formula σ = F/A , where F = load for specimen rupture (N) and
A = bonded area (mm 2 ).
The formula to calculate the lateral area of a circular straight cone with parallel bases was used. The formula used is: A = p * g * (R 1 + R 2 ), where p = 3.14, g = trunk generatrix, R 1 = smaller base radius, and R 2 = larger base radius. To determine the slant height, the following was used: g 2 = h 2 + [R 2 - R 1 ] 2 , where h = section height. [28],[29],[30]
R 1 and R 2 were obtained by measuring the internal diameters of the smaller and larger base, respectively; which corresponded to the internal diameter between the root canal walls. Diameters and section height were measured with a digital caliper after the push-out test. An average of four specimens of each root (n = 10) were calculated, resulting in 50 averages.
Type of failure evaluation
All the specimens tested in push-out were analyzed under an optical microscope (magnification of ×40) in order to evaluate the type of fracture of the samples as following: Adhes DC, adhesive failure between dentin and cement; Adhes PC adhesive failure between post and cement; Cohes P, cohesive failure of the post; Cohes D, cohesive failure of dentin; and M, mixed: Adhesive failure at the DC or PC interface and cohesive failure of the cement.
Both the extruded posts and the split root segments were analyzed in the microscope to classify the type of failure. The failure was classified as adhesive when the fracture occurred only between the substrates, that is, no fracture was observed neither in the cement layer, dentin nor fiber post; whereas in the cohesive failure it was observed fracture in one of these substrates (cement layer, dentin, or fiber post).
The percentage failure after push-out test for each group was calculated, considering that 100% corresponds to 40 slices for each group.
Statistical analysis
Statistical analysis was performed using Statistix for Windows (Analytical Software Inc., version 8.0, 2003, Tallahase, FL, USA). After determining that the results showed "sample normality" (Shapiro-Wilk test, P = 0.078) and homogeneity (Cochran's homogeneity), the data were submitted to one-way analysis of variance (ANOVA) and Tukey's test for multiple comparisons (5%).
Results | | |
The bond strength results were significantly affected by cementation strategies (one-way ANOVA, P = 0.0001).
The Tukey's test [Table 2] demonstrated that simplified self-adhesive resin cement with phosphoric acid treatment obtained the highest mean bond strength statistically significant followed by the same cement without phosphoric acid etching. The other three groups obtained the significantly lowest mean values and were similar to each other. Thus, the null hypothesis was rejected, since the conventional adhesive cementation promoted lower push-out bond strength.
An analysis of the type of failure of the specimens [Table 3] showed that adhesive fracture between dentin cement was most observed in Groups 5 (100%) and 2 (70%). In Groups 3 (45%) and 4 (61.54%), a predominance of dentin cohesive fractures was observed. Nevertheless, Group 1 presented a failure proportion that was distributed in adhesives (37.5%), cohesive of the dentin (20%), and mixed (42.5%). Mixed failure happened in Groups 3 (32.5%), 4 (33.34%), and 5 (20%).
Discussion | | |
Several studies have been developed to evaluate the bond strength of materials that are applied to root canal posts' cementation. Conventional resin cements, with previous phosphoric acid etching and adhesive system application are the most applied and tested. [24],[30],[31]
Studies have shown the effectiveness of some self-adhesive resin cements in coronal dentin [1] and root dentin. [20],[24] However, other researches on dentin related lower bond strength values for self-adhesive cements than conventional resin cements associated to previous acid etching and adhesive systems. [32],[33] Moreover, resin-modified glass ionomer cement has been used when the adhesive cementation approach is difficult. [34] Retention of posts cemented with glass ionomer can be related to frictional retention allowed by hygroscopic expansion occurring after cement cure. [35]
With this intent, several tests such as tensile, microtensile, and push-out tests have been used to evaluate resin cement's effectiveness into intraradicular dentin, however the push-out test is the most commonly mentioned. It is because, according to Goracci, et al., [36] this test present a low or null percentage of loss during specimen preparation before testing, opposite to the microtensile methodology.
The null hypothesis from this study was rejected, since a statistically significant difference was found between self-adhesive resin cement with and without previous phosphoric acid treatment, where pretreatment resulted in higher bond strength. These results are different from those verified by Goracci, et al., [37] and De Durβo, et al., [38] who found high values with conventional resin systems (Variolink II) or with cementation systems that employ self-conditioning primers (Panavia 21) when compared to self-adhesives resin cement (RelyX Unicem), like those used in the current study. Nevertheless, failure type in the study by De Durβo, et al., [38] was mainly found in the adhesive interface between cement and post, meaning underestimation of the bond results probably, whereas in the present study fractures were mainly found in the adhesive interface between cement and dentin, which represent a real bond evaluation. It can be explained by the different surface treatments of the fiber posts between the studies. [39] De Durβo, et al., [38] just cleaned the glass fiber posts with ethanol and silanized after, for 60 s and left to air-dry. In the present study, the posts were etched with 35% phosphoric acid, rinsed with water, air dried, and silanized.
The current study found statistically similar results between conventional resin cements (RelyX ARC and AllCem) and resin-modified glass ionomer cement (RelyX Luting 2). These results disagree with those observed by Bonfante, et al., [34] and De Durβo, et al., [38] where conventional resin cements showed higher bond strength values than resin modified glass ionomer cements with statistically significant results, in tensile and push-out test, respectively.
The low bond strength for G1 and G2 (single-bottle etch and rinse adhesive system + two dual-cured resin cements) can be explained by several factors such as poor photoactivation of the adhesive, leading to a poor degree of conversion and incompatibility between single-bottle adhesives and dual-cured resin cements. [28] According to dos Santos Alves Morgan, et al., [40] and Giachetti, et al., [41] even without the post, the luminous intensity inside the canal seems to decrease to levels that are insufficient for adhesive and cement polymerization, especially in the apical third, what contributes to the low bond strength between dentin/cement. In relation to the incompatibility between adhesive and cement, the low adhesive pH value produces a chemical match with the polymerization chemical process of the dual-cure resin cement. It can be explained by the higher concentration of acidic resin monomers of some single-bottle adhesives and single-step self-etch adhesives, which retard the polymerization of chemical and dual-cured composites that are initiated via peroxide-amine type binary redox catalysts. [42] Interaction between acidic adhesive resin monomers and the basic composite tertiary amines results in the consumption of the latter in acid-base reactions, depriving their capacity of generating free radicals in subsequent redox reactions. [42]
In the present study, the self-adhesive resin cement RelyX Unicem presented bond strength values significantly higher when compared to other investigated materials. According to the manufacturers, no pretreatment is necessary, such as etching, priming, bonding, or silanization, on the enamel or dentin substrates when using self-adhesive cements. The chemical bond mechanism is by forming an ionic change between polyalkenoic acid carboxylic group and hydroxyapatite calcium component with the formation of polymers as the product of the reaction. [20],[43],[44] These resin cements have acid monomers in high concentration that confer acidity to the cements, promoting certain conditioning to the enamel dentin that simultaneously demineralize and infiltrate smear covered dentin.
The results of this study are in agreement with the findings from Bitter, et al., [24] that showed significantly high bond strengths to self-adhesive resin cement RelyX Unicem tested with and without thermocycling, applying push-out test, and then compared to dual-cured resin cements. Moreover, Amaral, et al., [20] found higher retention of fiber post cemented with simplified self-adhesive resin cement. According to the literature, the humidity tolerance of RelyX Unicem due to water formation during neutralization reaction of methacrylate phosphoric acid, of basic sealers and hydroxyapatite, can be one reason for good performance in the present study, since it is difficult to control the humidity index after root canal rinsing because of poor visibility. [24]
The lower bond strength obtained with RelyX Unicem without phosphoric acid treatment found in the present study, compared to the same cement with previous acid etching, can be explained by the methacrylate phosphoric esters, responsible for substrate etching with this adhesive, which are not so efficient as phosphoric acid in dissolving a thick dentin smear layer created in root canal walls during post-hole preparation. [44],[45] The previous acid etching could have improved the contact of the cement to dentin and micromechanical retention, improving the retention of the fiber post.
The used self-adhesive resin cement expressed promised bond results. The evaluated resin cement appears to have low shrinkage due to its viscoelasticity properties, leading to a better intimate contact of the resin cement with the root canal walls and higher frictional resistance. [45],[46],[17] In hard scenario for resin adhesion such as high C-factor, slow setting, and self-cured resin cements are indicated as capable of providing viscoelasticity parameters for maintaining bond integrity under extreme conditions. [37],[47] Additional studies using others self-adhesive resin cements should be developed, once that the bond strength can be affected by the composition of the cement.
Also, further in vitro long-term studies using mechanical fatigue tests of teeth restored with bonded fiber post and randomized controlled clinical trials are a must to state this approach as substitute or alternative to the conventional adhesive cementation (three-steps etch and rinse adhesives). Moreover, additional studies using other types of fiber posts must be developed.
Conclusion | | |
This study shows that previous etching with phosphoric acid into root dentin enhanced the bond strength of self-adhesive resin cement. In addition, the self-adhesive cement presented higher bond strength when compared to the dual-cured resin cement + single-bottle etch and rinse adhesive, and the resin-modified glass ionomer cement, which promoted similar bond strength. The simplified self-adhesive resin cement seems to be best choice to cement fiber posts to root canal.
Acknowledgement | | |
This study is based on a master thesis submitted to the Dental School, Development Foundation from Bahia, Salvador, Brazil, as part of the requirements for the M.S.D. degree.
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[Table 1], [Table 2], [Table 3]
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