Untitled 1http://dx.doi.org/10.20396/bjos.v17i0.8652648 Volume 17 2018 e18127 Original Article 1 DDS. Department of Dental Materials and Prosthodontics, Araçatuba School of Dentistry, São Paulo State University – UNESP, Araçatuba, São Paulo, Brazil; 2 DDS, MS, PhD Student. Department of Restorative Dentistry, Araçatuba School of Dentistry, São Paulo State University – UNESP, Araçatuba, São Paulo, Brazil; 3 DDS, MS, PhD, Post-doctoral Student. Department of Dental Materials and Prosthodontics, Araçatuba School of Dentistry, São Paulo State University – UNESP, Araçatuba, São Paulo, Brazil; 4. Associate Professor, Department of Dental Materials and Prosthodontics, Araçatuba School of Dentistry, São Paulo State University – UNESP, Araçatuba, São Paulo, Brazil. 5. Associate Professor, Department of Restorative Dentistry, Araçatuba School of Dentistry, São Paulo State University – UNESP, Araçatuba, São Paulo, Brazil. Corresponding Author: Paulo Henrique Dos Santos Address: José Bonifácio St, 1193. Araçatuba-SP, Brazil. Zip Code: 16015-050 Phone number: 5518 3636-2802 Facsimile number: 5518 3636-3245 E-mail: paulosantos@foa.unesp.br Received: November 17, 2017 Accepted: February 15, 2018 Microshear bond strength of conventional and self-adhesive resin cements to feldsphatic ceramic Estéfani Maminis Soares Couto1, Bruna de Oliveira Reis2, Thaís Yumi Umeda Suzuki3, Wirley Gonçalves Assunção4, André Luiz Fraga Briso5, Paulo Henrique dos Santos4. Aim: The aim of this study was to verify the microshear bond strength of conventional and self-adhesive resin cements to feldsphatic ceramic. Methods: Twenty discs of Starlight ceramic (Degudent) were made (15mm x  2mm). The bonding procedure was accomplished by insertion of resin cements into tubes  of 0.7mm internal diameter in contact with the ceramic. The resin cements used were: RelyX ARC (3M Espe), Panavia F (Kuraray), RelyX Unicem (3M Espe) and seT (SDI). Total of six tubes of each material on each ceramic disc.  The specimens were tested for microshear in a universal testing machine, 24hours and 6months after bonding procedure.  Values of microshear bonding strength (MPa)  were subjected to ANOVA and Fisher PLSD test (p<0.05). Results: For 24hours analysis, RelyX ARC  showed the highest microshear bonding strength without statistical difference to RelyX Unicem and seT.  Panavia F showed the lowest values of microshear bonding strength in both periods. In the analysis after 6months there was a significant increase in the microshear bonding strength values for all cements compared to 24hours. Only for RelyX ARC, the increase was not statistically significant. Conclusion: In conclusion, the bond strength depends mainly on the type of resin cement used, and the self-adhesive cements behave similarly to conventional resin cement. Keywords: Resin cements. Ceramics. Dentin-Bonding Agents. Shear Strength. 2 Santos et al. Introduction The restorative dentistry has been changed in the last years, with significant improve- ment in the esthetic issue, biocompatibility and strength of direct and indirect restor- ative materials1. Dental ceramics has been the choice material for aesthetic resto- rations  because of their optical properties.  The chemical stability and resistance to wear make the ceramic the indicated for both anterior and posterior areas, in which aesthetic demand  and strength are necessary2. Traditionally, dental porcelain is composed by two distinct phases: crystalline phase, responsible for strength, and a glass phase, primarily responsible for the translucency of the material. Polycrystalline ceramics are basically characterized by having a larger amount of crystalline phase as alumina, leucite, lithium disilicate and zirconia3. Improvements in the clinical performance of ceramic restorations are closely related with the use of adhesive resin materials for luting procedures, including a photoactiva- tion method and efficiency of the adhesive system4. The use of resin cements is pre- ferred because they offer many advantages, as the ability to micromechanical bond to tooth structure, low solubility and greater strength to wear compared to conventional dental cements5. The resin cements are divided into conventional and self-adhesive materials and involve two bonding interfaces: tooth structure/resin cement and resin cement/restorative material6. The conventional resin cements require prior treatments of the tooth surface. The bonding of these materials to the tooth is dependent of an adhesive system, following the same principles of direct composite resin restorations. The bonding of the resin cement to the restoration depends on the type of ceramic material and the treatments performed on the internal surface which include rough- ness with a diamond bur, sandblasting with aluminum oxide, etching with hydrofluoric acid, ammonia bifluoride or acidulated phosphate fluoride, silanization and ultrasonic cleaning7. This total-etch adhesive protocol of conventional resin cements is complex, require multiple steps and may make the effectiveness of bonding vulnerable8. The self-adhesive resin cements were developed to simply the bonding procedures, once do not require any pre-treatment of dental substrate, reduces clinical steps and the “win- dow of contamination”8,9. The cementation can be accomplished in a single step8. The organic matrix of these materials consists of a multifunctional monomer (generally a phos- phoric acid ester methacrylate). This can react with the filler of the resin cement as well as hydroxyapatite of dentin and enamel10.  Following the trend to simplify the technique of luting, many manufacturers launched their versions for the self-adhesive cements. Usually most of scientific papers that involve the study of indirect restorative materials such as dental ceramics and their adhesive procedure focus in the investigation of the effectiveness of these adhesive materials to bond in tooth substrates, with few studies focusing the other bonding interface between resin materials and dental ceramics. Thus, the present study aimed to evaluate the microshear bonding strength between conven- tional and self-adhesive resin cements to dental ceramics, 24 hours and 6 months after the bonding procedure. Two null hypotheses were tested: (1) different resin cements did not have difference in the bonding strength to dental ceramics and (2) the bonding strength of resin cements to dental ceramics measured after 6 month of bonding pro- cedure did not have difference for the measurements done after 24 hours. 3 Santos et al. Materials and Methods Twenty discs of Starlight feldspathic ceramic (DeguDent Gmbh, Hanau-Wolfgang, Germany)  were made using a silicone matrix, measuring 15mm in diameter and 2mm in thickness. A Centurion furnace (Degussa-Ney Dental, Yucaipa, CA, USA) was used for the ceramics sintering, following the time-temperature scale recom- mended by the manufacturer. After sintering, the samples were finished and polished with Ninja points (Talladium do Brasil, Curitiba, PR, Brazil), #104023 diamond points flame-shaped and #35 inverted cone-shape. The measurements of the samples were checked with a digital caliper (Mitutoyo, Suzano, SP, Brazil). One of the ceramic surfaces was etched with 10% hydrofluoric acid (Porcelain con- ditioner, Angelus, Londrina, PR, Brazil)  during 2 minutes. After the acid etching, the specimens were washed with air-water spray during 60 seconds and dried with com- pressed air for 30 seconds, and then submitted to the silanizing agent application (Ceramic Primer, 3M Espe, St. Paul, MN, USA) during 1 minute. The discs were divided into four groups, according to the resin cement tested (n=5). The bonding process of resin cements to ceramic  was performed by the insertion of resin cements in Tygon®-Microbore (TGY-030, Small Parts Inc., Miami Lakes, FL, USA) tubes with 0.7mm of internal diameter, 2.1mm of external diameter and 1.0mm of height, located on the prepared surface of the ceramic. Four resin cements were used: RelyX ARC (3M Espe) and Panavia F (Kuraray, Kurashiki, Japan) conventional resin cements, RelyX Uni- cem (3M Espe) and seT (SDI, Bayswater Victoria, Australia) self-adhesives resin cements. Six tubes of each cement were placed in each ceramic specimen. The photoactivation of cement was performed using Ultraled II light-curing unit (Dabi Atlante, Ribeirão Preto, SP, Brazil), with 700 mW/cm2 light output intensity for 40 seconds, through the ceramic surface. The photoactivation process was conducted in a darkened environment, so the external ambient light has not influenced the degree of conversion of the resin cement. The specimens were stored in deionized water at 37°C for 24 hours. The specimens  were submitted to microshear bonding test in a universal testing machine (DL 3000, Emic, São José dos Pinhais, SP, Brazil) at a crosshead speed of 1.0 mm/min. For the test, each specimen was horizontally positioned in a metal glove and an orthodontic wire with 0.3mm forming a loop surrounding the cylinder of resin cement composite has performed the tensile stress resulting in a microshear bonding test. The microshear bonding strength (MPa) was calculated by: F/A where: F was the force applied (N); and A, the area of bonding (mm²). To perform the microshear bonding strength after 6 months, the same ceramic discs were worn with no. 80-, 320- and 600-grit silicon carbide paper (Extec Corp, Enfield, CT, USA) and then submitted to same surface treatment  with hydrofluoric acid and silane application. Resin cements were positioned in Tygon®-Microbore tubes as the same manner as above described. The specimens were stored in deionized water at 37ºC for 6 months, changed weekly, until the microshear bonding test. Data were submitted to the Kolmorov-Smirnov normality test and the averages com- pared by ANOVA and Fisher´s PLSD test (p=0.05). Two factors were studied: material (RelyX ARC, Panavia F, RelyX Unicem and seT) and time (24 hours and 6 months).  4 Santos et al. Results ANOVA (Table 1) showed that there was a statistically significant difference between the cements studied (p<0.001) as well as between the evaluated times (p<0.001). The interaction between the groups was not statistically significant (p=0.94). At 24 hours, RelyX ARC showed the highest microshear bonding strength without sta- tistical difference to RelyX Unicem and seT (p>0.05). The lowest values were obtained for Panavia F with significant difference for the other cements (p<0.001). These data are shown in Table 2. The same phenomenon occurred to the specimens analyzed 6 months after the bonding procedure, in which Panavia F showed the lowest value of microshear bonding strength, compared to the other cements studied. A significant increase in the values of microshear bonding strength was found 6 months after the bonding procedure compared to the 24 hour for all resin cements studied (Table 2). Just for RelyX ARC, this increase was not statistically significant (p=0.134).  Discussion The success of the luting procedure  is dependent of a strong and durable bonding strength in both interfaces between resin cement to ceramics and between resin cement to tooth11. In this study, the focus was to evaluate the bonding strength in the resin cement-ceramic interface. Currently, the bonding process between feldspathic ceramic to resin cements is provided by etching the ceramic surface with hydrofluoric acid, followed by silane agent application. Both have the property to increase the wet- ting of the cement on the surface, facilitating the contact between the materials. Fur- Table 2.  Means of microshear bonding strength (MPa) between the resin cements and feldspathic ceramic, 24 hours and 6 months after the bonding procedure. Microshear bonding strength (MPa) RelyX ARC Panavia RelyX Unicem Set 24 hours 21.13 ± 4.77 A a 8.40 ± 3.75 A b 20.86 ± 0.43 A a 19.12 ± 1.98 A a 6 months 26.54 ± 5.45 A a 13.18 ± 2.92 B b 27.11 ± 2.81 B a 25.60 ± 2.06 B a Distinct letters, uppercase in the column and lowercase letter in the line, show a statistically significant difference between them (p<0.05). Table 1.  ANOVA results for microshear bonding strength (MPa). DF Sum of squares Mean Square F-Value P-Value Lambda Power Groups 3 1191.924 397.308 34.491 <.0001 103.472 1.000 Time 1 318.310 318.310 27.633 <.0001 27.633 1.000 Groups * time 3 4.499 1.500 .130 .9414 .391 .071 Residual 31 357.099 5 Santos et al. thermore, the silane performs a chemical bonding between the silica contained in ceramic and the organic matrix of resin cements,  through siloxane bonding12,13.  For this reason, all ceramic specimens received the same surface treatment,  based on application of hydrofluoric acid followed by silanization. Based on the results of this study, it was found that Panavia F showed the lowest value of microshear bond strength in both periods studied (Table 2). In the other hand, RelyX ARC showed the highest value of microshear bond strength after 24 hours and 6 months, without significant difference to RelyX Unicem and seT cements. The differ- ence in the behavior of conventional cements could be related to the formulation and their viscosity.  The difference in composition is inherent to the materials and could not be compensated by the polymerization, although an increased viscosity of the material could restrict the photoactivation process14. The conventional cements con- tain in their basic composition,  resin monomers as BIS-GMA (bisphenol A glycidyl methacrylate) or UDMA (urethane dimethacrylate). Self-adhesive resin cements have in their composition phosphoric acid monomers as MDP or META15. Kern and Thomp- son16  (1995) related in their studies that MDP-modified resin cements, as Panavia F, has better adhesion to aluminized surfaces than conventional resin cements based on BIS-GMA (as RelyX ARC), because of the existence of chemical bonding between MDP and alumina. Furthermore,  phosphate ester groups of MPD-modified resin cement have been described in the literature as able to bond directly to metal oxides17. However, this benefit may be limited when it bonds to feldspathic ceramics, as used in this study. The high viscosity shown by Panavia F could not allow sufficient flow, neg- atively affecting the bonding to ceramics, as shown also in dentin interface18.  This could probably explain the lower values found for Panavia F in this study. Regarding the self-adhesive resin cements, RelyX Unicem and seT did not show signif- icant differences between them, but showed higher values of bonding strength than Panavia F in both analyzed periods (Table 2). One of the factors that could explain the performance of the self-adhesive is the way of commercial presentation.  They are available in the market in capsules, which require pre-activation, and need a mechan- ical device for mixing procedure,  with no manual mixing.  The mechanical mixing enables greater effectiveness for the cement, once the manual manipulation  could lead the formation and entrapment of air bubbles creating voids in the adhesive inter- face and therefore, interference in the performance of the material19. The analysis of the microshear bonding strength after 6 months showed a statisti- cally significant increase in the values compared to the values after 24 hours for all cements studied, except for RelyX ARC (Table 2). The late polymerization may be the main responsible for the increase of the bonding strength over time,  once the photoactivation provides the generation of free radicals responsible  for the induc- tion of chemical polymerization20. According to Arrais et al.21 (2009), the initial expo- sure to light may cause a change  in the viscosity of dual-cured materials,  making difficult the migration of active radicals, which could lead to a delay in the process of polymerization. This study has showed some limitations  as the difficulty of load standardization applied during the resin cement insertion inside the microtubes representing the pressure during the luting procedure,  the fixation of the tube so the cement could 6 Santos et al. be adequately inserted and the removal of the Tygon®-Microbore  tubes  before the microshear bonding test. Other aging process as thermal and mechanical cycling, are also necessary to guarantee the longevity of the bonded interface studied 22, since in the present study, only the storage in water was realized. However, it could be seen that the inherent properties of the resin cement affects the final performance of the bonding interface. 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