BONDING TO DENSELY SINTERED ALUMINA-AND GLASS INFILTRATED ALUMINUM / ZIRCONIUM-BASED CERAMICS

1D.D.S., M.Sc. Assistant Professor, Dept. of Restorative Dentistry, Federal University of Santa Maria, Brazil. Graduate student (Ph.D. degree) in Prosthodontic, São Paulo State University at São José dos Campos/SP (UNESP), Brazil and University of Bologna, Italy. 2D.D.S., Ph.D. Associate Professor, School of Dentistry, Foundation to the Science Development, Salvador/BA, Brazil. 3D.D.S., MMedSci, Ph.D. Professor and Research Coordinator, School of Dentistry, University of Passo Fundo, Brazil. 4D.D.S., Ph.D. Professor, Dept. of Dental Materials and Prosthodontics, São Paulo State University at São José dos Campos, Brazil.


INTRODUCTION
The metal-free ceramic systems appeared when the content of aluminum oxide was increased in the feldsparbased ceramic 18 , increasing its mechanical properties and allowing the ceramic restorations to be used in the posterior teeth.
Several studies have reported that the bonding mechanism between feldspar ceramics (silica-based ceramics with low content of Al 2 O 3 ) and resin cements starts by the etching with hydrofluoric acid and is optimized by the silane coupling agent.The etching and silanization increase the surface energy and the wettability of the ceramic substrate and decrease the contact angle between the ceramic surface and the resin cement 24 .Besides, the silane agents present bi-functional characteristics, i.e., they allow bonding between the silica of the feldspar ceramic and the organic matrix of the resinous materials through siloxane bridges 7,13,24 .
The tribochemical silica coating in the ceramic surface seems to be one of the best methods to promote the bonding of acid-resistant ceramics to resin cements 1,2,10,30 .However, no other study has investigated the effect of surface treatments of different acid-resistant ceramics on the bonding to a resin cement.
Thus, the purpose of this study was to test the following hypotheses: (1) tribochemical silica coating affects the bond strength between acid-resistant ceramics and a resin cement; (2) bond strength is affected by the type of ceramic (with vitreous phase and without vitreous phase).
Two samples of each ceramics (one for each treatment) were treated and sputter-coated with gold-palladium for 3 min in a Hummer II Sputter Coater (21020, Technics Inc., Alexandria, VA, USA) at a current of 10mA, and vacuum of 130mTorr, and the surface topography was examined using a scanning electron microscope (JSM 6400, Jeol Ltd., Tokyo, Japan).
Each ceramic block was cemented to the composite block using a resin cement (Dual-Cure Dental Adhesive System Panavia F, Kuraray Med Inc., Okayama, Japan, batch # 51.133), which was manipulated according to the manufacturer's specifications, and then applied onto the treated ceramic surface using a Centrix syringe (DFL, Rio de Janeiro, Brazil).The ceramic-cement-composite set was placed on a press with the interface (cementation surface) perpendicular to a vertical load of 750g/10min 16,20,25,30 .For this time, the excesses were removed and each free block face was cured for 40s using XL 3000 curing light (light intensity = 500mW/cm 2 ; distance = 0mm) 8 .Oxyguard was applied onto the interface margins, and after 10 minutes the blocks were removed from the press, washed with air-water spray, and stored in distilled water at 37°C for 7 days.Four groups containing 5 cemented blocks (Figure 1A) were therefore constituted: (G1) ZR + GB; (G2) ZR + SC; (G3) PR + GB; (G4) PR + SC.
The blocks were bonded with cyanoacrylate glue (Super Bonder gel, Loctite Ltda, São Paulo, Brazil) to an adapted metallic base, which was coupled to a cutting machine (Figure 1B).Slices were obtained using a slow-speed diamond wheel saw ( # 7016, KG Sorensen, Barueri, Brazil) under cooling.The first slice of each block -approximately 1mm in thickness -was discarded because the results could be influenced by the excess or absence of resin cement on the interface 17,26 .Two slices (1 ± 0.1mm thickness each) were then obtained per block (Figure 1C).Each slice was rotated (90 o ) and bonded onto the metallic base (Figure 1D).The first bar specimen was also disregarded for the same reasons described above.Three other cuts were made.Non-trimmed rectangular shape samples (bar specimens) with approximately 0.6 mm 2 of bonding area.This same procedure was performed in the other slice.Six samples per cemented block were obtained (Figure 1E).Therefore, only the internal samples were used (Figure 1F), so each experimental group was composed of 30 samples 3,7,20,25 .The adhesive area ("A") of each sample was measured using a digital caliper (Mitutoyo, Tokyo, Japan) prior to the test.
For the microtensile test, the extremities of each sample were bonded to an adapted caliper using cyanoacrylate glue.The sample was glued parallel to the long axis of the caliper, thus minimizing the bending forces in the adhesive zone.This apparatus was coupled to a universal testing machine (EMIC DL-1000, EMIC, São José dos Pinhais, Brazil), and tensile stress was applied to it (crosshead speed of 1mm.min - 1 ) 3,7,20,25 .
The bond strength (σ = L/A) was calculated."L" is the load in the moment of rupture (Kgf) and "A" is the bonding area of the sample (mm 2 ).The results were submitted to ANOVA (two-way analysis of variance) and Tukey test (α = 0.05).
The fractured surfaces of the samples were analyzed in a light microscope -50x magnification (Zeiss MC 80 DX, Zeiss, Jena, Germany) -to assess the failure mode (adhesive, cohesive or mixed).

RESULTS
The mean bond strength values and the standard deviation of the experimental groups are presented in Table 1.It was possible to note that: (1) the interaction effect of the two variables: ceramic and surface treatment (F df(1,116) = 9.08; p = 0.003); (2) the ceramic effect (F df(1,116) = 30.74;p = 0.001); (3) the surface treatment effect (F df(1,116) = 81,44; p = 0.001).According to the Tukey test, the bond strength of G2 was statistically higher than the other groups.G1 and G4 were statistically similar.
The mean values of bond strength of the surface  treatment factor are presented in Table 1 and in Figure 2.
When the treatment factor is analyzed in Table 1, one can observe that the CoJet System (tribochemical silica coating) presented statistically higher bond strength than the Al 2 O 3treatment.
The mean bond strength values of the ceramic factor are described in Table 1 and Figure 3. Analyzing this Table, one can conclude that the In-Ceram Zirconia (ZR) ceramic presented higher bond strength than that of the Procera ceramic, but that is dependent on the surface treatment.
The SEM images of the two ceramics submitted to surface treatments are shown in Figure 4.The topographic analyses of these SEM micrographs suggest that aiborne abrasion with the SiO x particles (Figures 4B and 4D) promoted silica coating (SC) on the surface.The topographic patterns for 4B and 4D are different when compared to blasting with 110-mm aluminum oxide particles (Figures 4A and 4C).The micro-retentive pattern observed after blasting with Al 2 O 3 particles (GB) seems to contribute to the micromechanical bond when compared to the topographic pattern observed in the treatment with airborne abrasion with SiO x particles (silica coating) (SC), albeit the SC have allowed higher bond strength.
All samples tested were analyzed in light microscope (50x magnification).Notwithstanding the groups investigated, all samples (100%) presented adhesive failure mode in the adhesive zone (interface resin cement / ceramic or resin cement).

DISCUSSION
In Table 1, it is possible to observe that G2 (In-Ceram Zirconia ceramic treated with chairside tribochemical silica coating) presented the highest value of bond strength.G1 (ZR + GB) presented bond strength similar to G4 (PR + SC).
These results may be explained by the following phenomena: (1) there is a chemical bond between coated silica, silane agent, and resin cement 15,21 ; (2) there is a chemical bond of the MDP monomers-phosphate of the resin cement to the aluminum-and zirconium-oxides 29,30 ; (3) the presence of a vitreous phase in the In-Ceram Zirconia ceramics facilitates the silica coating, and therefore increases the bond strength.
Considering the ceramic factor (Table 1), notwithstanding the surface treatment, one can observe that the In-Ceram Zirconia ceramics presented higher bond strength than the Procera AllCeram ceramic.ZR is composed of 67% aluminum oxide, 13% zirconium oxide, and 20% vitreous phase composed of lanthanum oxide 14 , whereas Procera is composed of 99.9% alumina, without vitreous phases 1 .Although there are no other comparative studies available, our study corroborates with the trend that there is higher bond strength between resin cement and glass infiltrated aluminous ceramic treated with tribochemical systems 14,20,21,25 .Thus, one can assume that the presence of a vitreous phase in the In-Ceram Zirconia (ZR) ceramic assists the silica coating on the ceramic surface.
The dense microstructure of the Procera AllCeram ceramic restricts a reliable bonding to the resin cement without monomer-phosphate, when this ceramic is treated with conventional methods (hydrofluoric acid or blasting with Al 2 O 3 ) 1,2 .However, Friederich and Kern 10 observed better bond strength results when using a resin cement with monomer-phosphate.
According to Wegner, et al. 29 , the yttrium-oxide-partiallystabilized zirconia ceramic (Y-TZP) contains high crystalline content (94.9% zirconium oxide stabilized by 5.1% yttrium oxide).It was reported that this ceramic presented an adhesive performance similar to the Procera AllCeram ceramic.For instance, when Y-TZP is blasted with Al 2 O 3 and is cemented to a resin cement containing monomerphosphate, a higher and stable bond strength is obtained 29,30 .
Although some studies have evaluated ceramics with different microstructures, it can be said that the Y-TZP ceramic and the densely sintered alumina ceramic (PR) are compact materials without vitreous content 10,29,30 , therefore characterized as acid-resistant ceramics and resistant to silica coating.In this study, we were able to observe this tendency because we compared one densely ceramic without vitreous phase (PR) and one ceramic with vitreous phase (ZR).
Regardless the ceramic being studied, our results confirm that the chairside tribochemical silica coating systems presented a statistically higher bond strength when compared to the blasting with Al 2 O 3 .
The bonding to the ceramic substrate has typically been based in the relationship between the silica of the ceramic and silane agent 24 (acid-sensitive).The silane presents a bifunctional characteristic, i.e., it allows bonding between the silica of the ceramic and the organic matrix of the resin cements through covalent bridges (siloxane bonds).Besides, silane coupling agents increase the surface energy and wettability, improving the microscopic interaction between ceramics and resin cements 7,13,24 .The bonding capacity of silanes with silica is very well established in the acidsensitive ceramics (feldspar-, leucita-, and lithium disilicatebased ceramics).The basic chemical reaction between the silane agent and the ceramics is obtained by the reaction between the y-methacryloxypropyltrimethoxy-silane (y -MPTS) and the siliceous oxide present in the surface, i.e., the silanes promote a chemical bond via cross-link with methacrilate grouping of the resinous materials 13 .This silicasilane chemical bond can also occur with acid-resitant ceramics, provided that the silica coating of the ceramic surface is used 16 .
The topographic analyses of the treated ceramic surfaces suggest that the airborne abrasion with SiO x particles (Figures 5B and 5D) promoted silica coating on the surface, allowing chemical bond between coated silica -silaneresin cement.This topographic pattern is different from that obtained by blasting with Al 2 O 3 particles alone (Figures 5A  and 5C).This is corroborated by the work of Kern and Thompson 16 .They observed an increase of the silica content in the surface of the In-Ceram ceramic (15.8% -19.7%) after treatment with the 110µm SiO x particles (Rocatec-Plus).The samples treated with Al 2 O 3 particles (Rocatec-Pre) did not present an increase in the silica content.The authors suggested that the silicated surface by SiO x particles (Rocatec System) could develop a better bond strength between the In-Ceram ceramic and the resin cement, which was later observed in the studies by Kern and Thompson 14 , Özcan, et al. 21, and Valandro, et al. 25 .
Hence, it is essential to consider the following: a surface treatment can improve greater bond strength to some allceramic systems, but not to all.Although the tribochemical silica coating process may be considered an important mechanism to promote bonding 14,21,25 , the current study observed that this conditioning method promoted a larger increase in bond strength in the acid-resistant ceramic with vitreous phase (ZR), when compared to densely sintered alumina ceramic without vitreous phase (PR).
The results confirm the hypotheses initially proposed: (1) tribochemical silica coating increased the bond strength and, (2) the glass-infiltrated alumina/zirconium ceramic (ZR) presented higher bond strength than the densely sintered alumina ceramic (PR) without vitreous phase.
The storage and thermocycling always should be considered to evaluate the bond strength between ceramic and resin cement, because these conditions contribute to the hydrolytic degradation of the resin cement / ceramic interface and the degradation of resin cement due to failure between fillers and matrix.These experimental conditions can contribute to bond strength decrease 14,30 .
As to the test method, the goal of bond strength in in vitro tests (tensile and shear) is the load application in the samples, i.e., to produce stress specifically in the interface between the materials being tested.Thus, for the test to reproduce the real bond strength between an adhesive system and a dental-, metallic-, ceramic-, or polymericsubstrate, it is important that the stress is homogeneously distributed in the bonding interface, regardless of the test method employed.Shear tests have thus been criticized due to the development of non-homogeneous stress in the bonding interface -stressing more the substrate than the interface.This phenomenon prevents an accurate interfacial bond strength measurement and limits further improvements in the bonding systems (underestimated and misinterpreted results), for the failure takes place in the substrate and not in the adhesive zone 4,28 .The failure mode and fractographic analyses 3,6,7,19 reduce the risk of data misinterpretation.
Although conventional tensile tests also present some limitations, such as the difficulty of sample alignment in the universal testing machine and the tendency of a heterogeneous stress distribution at the adhesive interface 17,27 , this at kind test may be employed because it provides information of global bond strength 3,5,12 .However, the microtensile test allows (1) appropriate alignment of the samples, (2) more homogeneous distribution of stress, and (3) a more sensitive comparison or evaluation of bond strengths that are similar 22,23 .

FIGURE 1 -FIGURE 2 -TABLE 1 -
FIGURE 1-(A) Cemented ceramic and polymeric blocks.(B) A slice cut of from the block fixed to the cutting machine.(C) Internal slices to be cut again.(D) Samples being obtained.(E) Internal samples obtained and used in this study (F)