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Brazilian Journal of Oral Sciences, Vol. 10, No. 2, Apr-Jun, 2011, pp. 124-129 Influence of ceramic primers on microshearbond strength between resin cements andzirconia-based ceramic Valéria Bisinoto Gotti1, Saturnino Calabrez Filho2, Marcos Massao Shimano3, Gilberto Antônio Borges3, Luiz Henrique Borges3, Luciano de Souza Gonçalves3 1DDS, Graduate Student, Dental School, University of Uberaba, Brazil Received for publication: December 08, 2010 Accepted: April 29, 2011 Code Number: os11025 Abstract Aim: This study evaluated the effect of a coupling agent ceramic primer (CP) on the microshear bond strength (ìsbs) between luting cements and zirconia-based ceramic. Keywords: zirconia, dual luting cement, bond strength. IntroductionZirconia-based ceramics present optimal mechanical properties, which are important for the long-term performance of these restorative materials1-3. The clinical Luciano de Souza Gonçalves success of all-ceramic restorations is dependent on the cementation process. Zirconia restorations can be cemented to the tooth with non-adhesive cements. However, the choice of a resin-cement agent would be justified due to the bond to the dental structure, with higher retention and better marginal adaptation4-6. As there is no silica in its composition and it lacks a vitreous phase, conventional hydrofluoric acid etching and silanization procedures are incapable of modifying the zirconia surface6-9. Previous studies have recommended the use of airborne particle abrasion or silica coating for improvingthe bond strength to zirconia10-11. However, controversial results have been found12-15 with regard to the efficacy of these alternative treatment procedures. It has been shown that airborne particle a brasion alone does not provide adequate bond strength to zirconia-based ceramics becauseit promotes only mechanical retention on the surface16. Silica coating combined with silanization provides a chemical interaction with the resin luting cement and appears to be amore appropriate treatment17-18. However this treatment is expensive for the prosthetic laboratories and dentist. Different resin luting cements with various blends of monomers have been developed to improve these materialsand their chemical affinity with the ceramic surface. Higherchemical affinity would be attained with the use of resincements containing phosphate monomers, such as 10-methacryloyloxydecyl dihydrogen phosphate (MDP),promoting higher bond strength4, or by using additional bondagents, called primers19-20, which also have these monomers in their composition. Nevertheless, another study has shownthat the use of the primers without airborne particle abrasion promoted a non-durable bond to zirconia21. Therefore, the combination of airborne particle abrasion with primersrequires further investigation. According to previous studies4,20-21, the association of resin luting cements with primers promoted a better interaction with the ceramic surface due to the increase in cement wetting. This wetting favors the adhesion processand improves the chemical interaction between resin cement and the zirconia surface. These ceramic primers usually contain silane and a functional phosphated monomer. ClearfilEsthetic Cement (Kuraray) is composed of the functionalmonomer 10-MDP, 3-methacryloxypropyltrimetoxisilano (3-MPS) as silane and ethanol. Conventional silane is noteffective on zirconia due to the absence of silica in its composition10,13,19. However, when a silane primer (3-MPS)reacts with 10-MDP, the interaction of the primer with thesubstrate and resin cement is promoted, forming cross links with the OH groups from ceramic and cement methacrylates.This reaction can be induced and sustained by the acidity ofthe ceramic treated with the coupling solution4. Primers containing other phosphate monomers, such as 6-MHPA (6methacryloyloxyhexyl phosphonoacetate)9 and MTU-6 (6methacryloyloxyhexyl 2-thiouracil-5-carboxylate)22 have been evaluated with less favorable results when comparedwith 10-MDP. Addition of multifunctional methacrylate acids directlyto cements that do not have primers, called self-adhesivecements, can also promote this higher interaction between the material and substrate4,23. However, ResiCems manufacturer, Shofu Inc., does not disclose the full composition of the primer, omitting some components. Inaddition, no phosphate monomer is specified in the cementor primer formulations stated in the Material Safety Data Sheet (MSDS) (Table 1). Therefore, the aim of this study was to investigate theeffect of primers on the bond strength between resin luting cements and zirconia-based ceramic. The failure mode analysis was also examined after the test. The hypothesis tested in this study was that the application of a ceramicprimer on airborne particle abraded zirconia surfaces increases the bond strength of resin cements. Material and methodsThe materials used in this study are presented in Table 1. Forty zirconia discs (20 mm in diameter and 2 mm thick) were made from pressed blocks (Cercon, DeguDent GmbH,Hanau/Wofgang, Germany). After sintering, the finishing and polishing procedures were performed with 320-, 400-, 600and 1200-grit silicon carbide papers. After the trimming, the zirconia discs were submitted to airborne particle abrasion with 50 µm (Al2O3), performed perpendicularly to the ceramic surface for 10 s at a distance of 10 mm. After airborne particle abrasion, the zirconia discs were cleaned in an ultrasonic bath (Odontobras Ind. & Com.de Equipamentos Médico Odontológicos LTDA., RibeirãoPreto, SP, Brazil) immersed in ethanol for 5 min, immersed in distilled water for 5 min and dried with oil-free air blast. Pretreated zirconia discs were then divided into four groups(n=10) according to the bond procedure and the resin luting cement (Table 2). Adhesion procedures were performed at room temperature and controlled humidity (23 ± 2ºC and 50 ± 5%,respectively) according to the ISO/TS 11405/2003 Standard. Customized 0.5-mm-thick polyvinyl siloxane molds(Vigodent S.A. Indústria e Comércio, Rio de Janeiro, RJ,Brazil) with five cylinder-shaped orifices (0.8 mm in diameter)were placed on the ceramic discs to allow delimitation ofthe bond area. Resin luting cements were prepared accordingto the manufacturers instructions and inserted in the mold orifice with a #5 explorer (Duflex - SS White do Brasil, Riode Janeiro, RJ, Brazil). Excess cement was removed with a #24 spatula (Duflex). The orifices were filled with each of the resin luting cements, and a transparent polyester stripwas placed over the filled orifices. Before polymerization, aconstant and uniform 0.454 kgF load was applied for 1 min,using a custom-made device. Samples were photo activatedin continuous mode with a LED Radii Cal (SDI. Victoria®, Australia) appliance with 1,400 mW/cm2 irradiance, as verified with a power meter (Ophir Optronics Ltda.®, Jerusalém, Israel) for 20 s for each orifice individually. After activation, samples were stored at 37 ºC and 100% relative humidity for 24 h, protected from light. The µsbs test was performed in a universal testing machine (EMIC DL 3000®; (EMIC, São José dos Pinhais,PR, Brazil), using a knife-edged blade at a crosshead speedof 0.5 mm/min until fracture. The µsbs of each zirconia disc was obtained by calculating the mean value of five repetitions on the surface of each disc, as shown in Figure 1.This procedure was performed to submit a larger area of thedisc to bond strength test, increasing the reliability of thevalues of each specimen. The µsbs data were submitted to two-way ANOVA (p=0.05). Failure mode analysis was performed with a stereomicroscope at 40x magnification and classified as: cohesive incement (C), adhesive (A) and mixed (M). In addition,representative fractured specimens were sputter-coated withgold and examined with a scanning electron microscope(JSM5600LV; JEOL Inc., Peabody, MA, USA). ResultsThe µsbs test results are shown in Table 2. Two-way ANOVA showed that no significant differences in bond strength were detected. The studied factors revealed no statistically significant difference among the resin luting cements used in this study (p = 0.179). The same occurred with regard to the ceramic surface treatments with primer (p= 0.609) and the interaction between the factors (p = 0.742). Failure mode analysis showed 100% of adhesive failures, SEM micrographs of fractured specimens are shown in Figure 2. DiscussionThe influence of different surface treatments and ceramic primers on the bond strength of zirconia-based ceramics hasbeen extensively investigated. The majority of studies have demonstrated that chemical or mechanical modification was shown to positively influence bond strength to resin luting cements12-15,24. However the increase in bond strength was not always achieved, with contradictory results being shown2,6,12-15,20,25. In the present study, no statisticallysignificant difference was found in the bond strength betweenthe resin luting cements and zirconia discs, irrespective ofthe bond agent applied to the ceramic (Table 2). The similarity between the bond strength values of the two agents, ResiCem (Shofu) and Clearfil Esthetic Cement (Kuraray),probably occurred because of the absence of phosphatemonomers in their compositions as reported in the manufacturers MSDS of the respective products. Both resin cements have only inorganic particles in the organic matrixof methacrylates, as shown in Table 1. It is known that monomers, such as MDP or 4-methacryloxyethyl trimelliticanhydride (4-META), present chemical bond to metaloxides13,26. Previous studies have shown that the chemical interaction of these functional monomers can improve thebond strength of crystalline ceramics such as zirconia andalumina, as well as the long-term bond stability, when compared with conventional methacrylates26-27. A previous study reported that the functional monomers are able to form chemical bonds with the metal oxides of zirconia, secondary bonds such as van der Waals, or hydrogen bonds at the ceramic-resin interface, increasing the surface wettability21. This increase in bond strength is more evident when functional monomers are used in combination with mechanical retention by means of surface pretreatment, such as airborne particle abrasion or silica coating4. In the present study, the application of primer to ceramic had no influence on the µsbs of the cementation agents to the zirconia discs treated with 50 µm airborne particle abrasion. Nevertheless, the majority of studies have shown better bond strength results when associated with primer application4,12,20. However, previous studies have shown thateven in the presence of functional monomers, it is importantto use different surface treatments to increase resin cement bond strength to zirconia-based ceramics4,9. The combination factors, such as the use of resin cements containing adhesive phosphate monomers, airborne particle abrasion pressure andprimers containing MDP, could promote a durable long-termbond to zirconia20-22,28. The use of primers without aluminaairborne particle abrasion resulted in no long-terminterlocking with the zirconia surface20-21. In a previous study,when airborne particle abrasion was combined with primers containing MDP, higher tensile bond strength values allied to low percentages of adhesive failures were observed21. Therefore, the absence of phosphate monomers in the composition of the cement and primer of ResiCem may have not affected the bond strength. Perhaps, there should be phosphate monomer in the resin cement and primer. The combined action of the two materials would increase the presence of phosphate-radicals, and could lead to a better bond between the zirconia and resin cement. Therefore, in the present study, the effect of primers, irrespective of the presence of MDP probably had less influence than retentions created by the Al2O3 airborne particle abrasion, resulting insimilar µsbs values for both groups. The failure mode analysis also indicated that the bondbetween the resin cement and zirconia surface was not improved by the action of the primers. All specimens showed adhesive failures, and no increase in µsbs values was found. The appearance of resin cement debris (e.g. mixed failure) would indicate a better bond of cement to parts of the zirconia surface, forcing the stress into the cement bulk, inducing points of cohesive failure. Figure 2 shows the adhesive failure mode for Clearfil Esthetic Cement with ceramic primer (a and a') and ResiCem without the use of primer (b and b'). At higher magnification, details of the adhesive failure showthe zirconia surface with grooves formed by the Al2O3 airborne particle abrasion without residual cement in the bond area(dotted line). Some authors have pointed out that the use of primer forceramic, especially those that contain MDP, promoted a more resistant bond in the short term. Silica coating associated with application of the primer would be the treatment of choice to increase bond stability. However, degradation of this bond wasfound even in the presence of the functional monomer with along-chain carbonyl contained in Clearfil Ceramic Primer4. The bond to zirconia remains an insightful and challenging procedure to the clinician. Therefore, with the increase in clinical use of ceramics in Dentistry, more evidence related to the adhesive cementation of zirconia based ceramic restorations is necessary to establish the most reliable technique, since the zirconia surface cannot be conditioned by conventional pretreatment methods. Thepresent study showed that primers do not necessarily improve the bond strength to zirconia. Therefore, further studies are required to develop more efficient primers and cements, or more reliable alternative methods to improve the bond strength to zirconia-based ceramics. The aging process of resincements must be analyzed to establish a more durable and reliable chemical and mechanical bond strength on the cementation process. Based on the limitations of this study it may be concluded that: no difference in µsbs was found between the resin luting cements used in this study; application of the ceramic primer to the ceramic did not improve the bond strength of the resincements to zirconia-based ceramic surfaces. References
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