Microshear bond strength of conventional and self-adhesive resin cements to feldsphatic ceramic

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.


Introduction
The restorative dentistry has been changed in the last years, with significant improvement in the esthetic issue, biocompatibility and strength of direct and indirect restorative materials 1 .Dental ceramics has been the choice material for aesthetic restorations 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 necessary 2 .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 zirconia 3 .
Improvements in the clinical performance of ceramic restorations are closely related with the use of adhesive resin materials for luting procedures, including a photoactivation method and efficiency of the adhesive system 4 .The use of resin cements is preferred 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 cements 5 .The resin cements are divided into conventional and self-adhesive materials and involve two bonding interfaces: tooth structure/resin cement and resin cement/restorative material 6 .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 roughness with a diamond bur, sandblasting with aluminum oxide, etching with hydrofluoric acid, ammonia bifluoride or acidulated phosphate fluoride, silanization and ultrasonic cleaning 7 .This total-etch adhesive protocol of conventional resin cements is complex, require multiple steps and may make the effectiveness of bonding vulnerable 8 .
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 "window of contamination" 8,9 .The cementation can be accomplished in a single step 8 .The organic matrix of these materials consists of a multifunctional monomer (generally a phosphoric acid ester methacrylate).This can react with the filler of the resin cement as well as hydroxyapatite of dentin and enamel 10 .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 conventional 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 procedure did not have difference for the measurements done after 24 hours.

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 recommended 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 conditioner, 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 compressed 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 Unicem (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/cm 2 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 compared 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).

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 statistical 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 tooth 11 .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 wetting of the cement on the surface, facilitating the contact between the materials.Fur- thermore, the silane performs a chemical bonding between the silica contained in ceramic and the organic matrix of resin cements, through siloxane bonding 12,13 .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 difference 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 process 14 .The conventional cements contain 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 META 15 .Kern and Thompson 16 (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 oxides 17 .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, negatively affecting the bonding to ceramics, as shown also in dentin interface 18 .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 significant 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 mechanical 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 interface and therefore, interference in the performance of the material 19 .
The analysis of the microshear bonding strength after 6 months showed a statistically 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 induction of chemical polymerization 20 .(2009), the initial exposure 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 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.

Conclusion
Based on the results, it can be observed that time had a directly influence in the bonding strength between resin cement and feldsphatic ceramic.For all resin cements, except for RelyX ARC, there was an increase of microshear bonding strength over time.Panavia F resin cement showed the lowest value of microshear bonding strength compared to the other resin cements in both times analyzed.The characteristics of these materials have influenced its short and long-term performance.

Table 2 .
Means of microshear bonding strength (MPa) between the resin cements and feldspathic ceramic, 24 hours and 6 months after the bonding procedure.