INTRODUCTION

Dental ceramic restorations are widely used because of their durability, esthetics, and excellent biocompatibility1). The development of glass ceramics reinforced by lithium disilicate or leucite crystals aimed to improve the strength and durability over those of conventional dental ceramics2). Computer-aided design and computer-aided manufacturing (CAD/CAM) are among the recent advances in dental technology, used for the fabrication of ceramic restorations3). CAD/CAM ceramic blocks, reinforced with lithium disilicate or leucite crystals, are made from highly uniform quality ceramic and do not exhibit the inevitable material variations seen in laboratory fabricated restorations4). This results in increased longevity of the restorations, as was recently demonstrated in a clinical study5).

Another important requirement for the success of ceramic restorations is the achievement of adequate adhesion between the ceramic and tooth substrate6).The selection of an appropriate adhesive system plays a crucial role in the achievement of clinical success7). It is recommended that lithium disilicate and leucite glass ceramics are pretreated with hydrofluoric acid and silane coupling agents in order to improve bonding performance8). The hydrofluoric acid attacks the glassy phase of the ceramics, creating micro porosities and dissolving the surface to a depth of a few micrometers9). The silane coupling agents present bi-functional characteristics, promoting a chemical interaction between the silica of the glass phase and the methacrylate groups of the resin materials by forming siloxane bonds10).

Recently, a novel family of bonding systems (universal adhesives), were introduced into the market11). These adhesive systems can not only be used in direct or indirect restorations, but are also capable of bonding with various substrates including resin composite, ceramics, zirconia, and metal alloys with no need for additional primers12). Some universal adhesives contain silane and a functional monomer, like 10-methacryloyloxydecyl dihydrogen phosphate (MDP), which helps in the adhesion of resin to ceramics. The versatility of universal adhesives provides a new, simplified approach to bonding between resins and ceramics. Some studies have investigated the efficacy of a universal adhesive in bonding resin to zirconia13). However, the effectiveness of universal adhesives on lithium disilicate and leucite glass ceramics, or on ceramics with contaminated surfaces, has not been thoroughly investigated.

The try-in procedure of ceramic restorations causes contamination of the bonding surface of the restoration with saliva, blood, or fitting indicator remnants such as silicone oil14). Saliva contains organic materials such as salivary proteins, enzymatic molecules, bacteria, food debris, and inorganic compounds in water solution15). Saliva contamination is one of the main reasons for decreased bond strength of restorations to tooth substrate16). However, it is almost impossible to avoid during the try-in procedure14,15). Therefore, attempts should be made to eliminate any inorganic or organic contaminants prior to adhesive cementation17). However, there is no consensus regarding the best type of cleaning method for improvement of bonding between saliva contaminated lithium disilicate and leucite glass ceramics and universal adhesives.

The wettability of the surface is important for

Influence of surface treatment of contaminated lithium disilicate and leucite glass ceramics on surface free energy and bond strength of universal adhesivesFumi YOSHIDA1, Akimasa TSUJIMOTO1,2, Ryo ISHII1, Kie NOJIRI1, Toshiki TAKAMIZAWA1, Masashi MIYAZAKI1 and Mark A. LATTA2

1 Department of Operative Dentistry, Nihon University School of Dentistry, 1-8-13 Kandasurugadai, Chiyoda-Ku, Tokyo 101-8310, Japan2 Department of General Dentistry, Creighton University School of Dentistry, 2500 California Plaza, Omaha, NE 68178, USACorresponding author, Akimasa TSUJIMOTO; E-mail: tsujimoto.akimasa@nihon-u.ac.jp

This study investigates the influence of surface treatment of contaminated lithium disilicate and leucite glass ceramic restorations on the bonding efficacy of universal adhesives. Lithium disilicate and leucite glass ceramics were contaminated with saliva, and then cleaned using distilled water (SC), or 37% phosphoric acid (TE), or hydrofluoric acid (CE). Specimens without contamination served as controls. The surface free energy was determined by measuring the contact angles formed when the three test liquids were placed on the specimens. Bond strengths of the universal adhesives were also measured. Saliva contamination and surface treatment of ceramic surfaces significantly influenced the surface free energy. The bond strengths of universal adhesives were also affected by surface treatment and the choice of adhesive materials. Our data suggest that saliva contamination of lithum disilicate and leucite glass ceramics significantly impaired the bonding of the universal adhesives, and reduced the surface free energy of the ceramics.

Keywords: Lithium disilicate, Leucite glass ceramics, Universal adhesive, Surface free energy, Bond strength

Color figures can be viewed in the online issue, which is avail-able at J-STAGE.Received Apr 15, 2015: Accepted Jun 19, 2015doi:10.4012/dmj.2015-123 JOI JST.JSTAGE/dmj/2015-123

Dental Materials Journal 2015; 34(6): 855–862

Table 1 Materials used in this study

Material Code Lot No. Main components Manufacturer

Scotchbond Universal SU 541424MDP, Bis-GMA, HEMA, Vitrebond copolymer, polyethelene glycol, water, initiator, colloidal silica, aluminiam oxide

3M ESPE, St. Paul, MN, USA

G-Premio Bond GB 00038BMDP, 4-MET, MEPS methacrylate monomer, acetone, water, initiator,silica filler

GC, Tokyo, Japan

All-Bond Universal AB 1312131MDP, Bis-GMA, HEMA, ethanol,Water, initiator, silanated colloidal silica

Bisco, Schaumburg, IL, USA

IPS e.max CAD EM P23546SiO2, Li2O, K2O, P2O5, ZrO2, ZnO, other oxides, coloring oxides

Ivoclar Vivadent, Schaan, Lichtenstein

IPS empress CAD EP P14738 SiO2, Al2O3, K2O, Na2O, other oxides, pigments Ivoclar Vivadent

Total Etch TE P14739 phosphoric acid, water, silica thickener Ivoclar Vivadent

IPS Ceramic Etching Gel CE P48566 hydrofluoric acid, fumed silica, dye colorant Ivoclar Vivadent

Ceramic Primer Ⅱ CP 1402101 silane, MDP, ethanol GC

Porcelain Primer PP 14000393c silane, ethanol, acetone Bisco

MDP: 10-methacryloyloxydecyl di-hydrogen phosphate, Bis-GMA: 2,2-bis[4-(2-hydroxyl-3-methacryloyloxypropoxy)phenyl]propane, HEMA: 2-hydroxyethyl methacrylate, 4-MET: 4-methacryloyloxyethyl trimellitate, MEPS: methacryloyloxyalkyl thiophosphate methylmethacrylate

the bonding of ceramics, regardless of the bonding mechanism used (chemical, mechanical interlocking, or a combination of both)18). The strength of the bond between the ceramic and the resin depends on several factors, including the surface treatment of ceramics and the ability of the resin to wet the bonding surface6). Measurement of the contact angle formed with the bonding surface provides information about surface free energy, which is associated with the bonding characteristics of the solids19). To date, neither the effect of saliva contamination of lithium disilicate and leucite glass ceramics nor the impact of different cleaning methods on surface free energy have been investigated.

The purpose of this study was to examine the influence of various surface treatment methods on the bonding efficacy of universal adhesives to contaminated lithium disilicate and leucite glass ceramics. The null hypothesis tested was that surface free energy and bond strength of universal adhesives were not affected by saliva contamination of lithium disilicate and leucite glass ceramics, and that there were no differences seen in the same on use of different surface treatment methods prior to the application of universal adhesives.

MATERIALS AND METHODS

Adhesives usedThe materials tested in this study are summarized in Table 1. The universal adhesives used were: Scotchbond Universal (SU, 3M ESPE, St. Paul, MN, USA), G-Premio Bond (GB, GC, Tokyo, Japan), and All-Bond

Universal (AB, Bisco, Schaumburg, IL, USA). From the manufacturers’ instructions, SU does not require silane coupling treatment, on the other hand GB and AB require silane treatment for ceramics bonding.

A visible-light curing unit (Optilux 501, Demetron Kerr, Danbury, CT, USA) was used. The light intensities (800 mW/cm2) of the curing unit were confirmed using a dental curing radiometer (Model 100, Demetron Kerr), prior to fabrication of the specimens.

Specimen preparation As presented in Table 1, the lithium disilicate (EM, IPS e.max CAD, Ivoclar Vivadent, Schaan, Liechtenstein) and leucite glass ceramic (EP, IPS empress CAD, Ivoclar Vivadent) plates were cut from CAD/CAM ceramic blocks, using a water-cooled precision diamond saw (Isomet 1000 Precision Saw, Buehler, Lake Bluff, IL, USA), to produce specimens that were 10×10×2 mm thick. All the ceramic plates were crystallized in a ceramic furnace (Proframat S1, Ivoclar Vivadent). Each ceramic specimen was then mounted in cold-cure acrylic resin (Trey Resin II, Shofu, Kyoto, Japan) and placed in tap water to reduce the temperature rise caused by the exothermic polymerization reaction of acrylic resin. All specimens were then ground with 600-grit silicon carbide (SiC) paper, and cleaned in an ultrasonic water bath for 10 min.

The cleaned samples were then divided into four groups. The specimens were then used to measure bond strength and surface free energy.

Group 1 (control): The specimens were not

856 Dent Mater J 2015; 34(6): 855–862

Table 2 Surface free energy and their components values in the present study liquids

Liquid Lot No. Manufacturer γL γLd γL

p γLh

1-bromonaphthalene ALH4513 Wako Pure Chemical Industries 44.8 44.6 0.2 0.0

Diiodomethane ALL2310 Wako Pure Chemical Industries 50.8 46.8 4.0 0.0

Distilled water — — 72.8 29.1 1.3 42.4

Unit: mN/m.γL: surface free energy, γL

d: dispersion force, γLp: polarity force, γL

h: hydrogen bonding force

Fig. 1 Drop Matser DM500 apparatus fitted with a charge-coupled device camera (a) allowing automatic measurement of the contact angles to be made (b).

(a) (b)

contaminated, but were abraded and cleaned as mentioned above. Group 2 (SC): After saliva contamination, the surface was rinsed with distilled water for 30 s and air dried for 10 s. Group 3 (TE): After saliva contamination, the surface was treated with Total Etch (Ivoclar Vivadent) for 30 s, rinsed with distilled water for 30 s, and air dried for 10 s. Group 4 (CE): After saliva contamination, the surface was treated with IPS Ceramic Etching Gel (Ivoclar Vivadent) for 60 s, rinsed with distilled water for 30 s, and dried for 10 s.

To achieve saliva contamination, saliva was collected from one healthy male donor (the principal investigator) who had refrained from eating and drinking for 2 h before the collection process. The specimens were placed in saliva at 37°C for 60 s, and then rinsed with tap water for 15 s and air dried for 30 s. The study protocol was approved by the ethics committee of Nihon University, School of Dentistry (#2011-19).

Surface free energy measurementThe surface free energies of the lithium disilicate and leucite glass ceramic specimens were determined by measuring the contact angle formed with the surface by the three test liquids: 1-bromonaphthalen, diiodomethane and distilled water, each of which have known surface free energy parameters (Table 2). The surface free energy of five specimens per treatment

group was determined by measuring their contact angle (Drop Master DM500, Kyowa Interface Science, Saitama, Japan). The apparatus was fitted with a charge-coupled device camera which allowed automatic measurement of the contact angle (Fig. 1). For each test liquid, the equilibrium contact angle (θ) was measured in five specimens per treatment group using the sessile-drop method at 23±1°C20). The surface free energy parameters of the solids were then determined based on the fundamental concepts of wetting.

The Young-Dupré equation describes the work of adhesion (W) between a solid (S) and a liquid (L) in contact as follows:

WSL=γL+γS−γSL=γL(1+cosθ)

Here, γSL is the interfacial free energy between the solid and liquid, γL is the surface free energy of the liquid, and γS is the surface free energy of the solid. By extending the Fowkes equation, γSL can be expressed as follows:

γSL=γL+γS−2(γLdγS

d)1/2−2(γLpγS

p)1/2−2(γLhγS

h)1/2

γL=γLd+γL

p+γLh, γS=γS

d+γSp+γS

h

where γLd, γL

p, and γLh are components of the surface free

energy (γ) arising from the dispersion force, the polar (permanent and induced) force, and the hydrogen-

857Dent Mater J 2015; 34(6): 855–862

Table 3 Application protocols of universal adhesive to ceramics

Adhesive system Application protocols

Scotchbond UniversalAdhesive applied to air-dried ceramic surface with rubbing action for 20 s. Gentle stream of air applied over the liquid adhesive for 5 s and until adhesive no longer moved and the solvent has completely evaporated. Adhesive light cured for 10 s.

G-Premio Bond+

Ceramic Primer Ⅱ

Ceramic primer applied in a thin layer to ceramic surface and dried with oil free air. Adhesive applied to primed ceramic surface with rubbing action for 10 s. Dry thoroughly for 5 s with oil free air under maximum air pressure, until adhesive became a thin film with the appearance of frosted glass and which did not visibly move under the air pressure. Light cured for 10 s.

All-Bond Universal+

Porcelain Primer

Brushed on thin coats of ceramic primer to ceramic surface and waited for 30 s. Dried with warm air syringe. Applied 1 coat of adhesive and evaporated excess solvent by thoroughly air-drying with an air syringe for 10 s, until there was no visible movement of the adhesive. Light cured for 10 s.

bonding force, respectively. Surface free energy (γ) values were determined for the three test liquids, and the surface energy parameters of the treated dentin surfaces were calculated based on these equations, using add-on software and an interface measurement and analysis system (FAMAS, Kyowa Interface Science).

Bond strength measurementA total of 180 lithium disilicate and 180 leucite glass ceramics were prepared as described earlier. The universal adhesives were applied according to the manufacturers’ instructions. The application protocols suggested by each manufacturer for bonding to silica-based ceramics are listed in Table 3. The specimens were clamped in a bonding jig (Ultradent Bonding Jig, Ultradent Products, South Jordan, UT, USA), and plastic molds (2.4 mm in internal diameter, 2.5 mm in height) were used to form and hold the resin composite on the adhesive treated ceramic surface. The resin composite (Clearfil AP-X, Kuraray Noritake Dental, Okayama, Japan) was condensed into the mold and light cured for 30 s. The finished specimens were transferred to distilled water and stored at 37°C for 24 h.

Fifteen specimens used were tested in shear mode using a bond-testing, notched-blade, semi-circular apparatus (Ultradent Products) in a universal testing machine (Type 5500R, Instron, Norwood, MA, USA) at a crosshead speed of 1 mm/min. Bond strength values (MPa) were obtained from the peak load at failure divided by the surface area of the specimen. After testing, the specimens were examined under an optical microscope (SZH-131; Olympus, Tokyo, Japan) at a magnification of 10× to determine the location of bond failure. The type of failure was classified, based on the percentage of substrate-free material, into the following groups: adhesive failure, cohesive failure in the ceramic, or cohesive failure in the composite.

Statistical analysisResults were analyzed using a commercial statistical

software package (SigmaStat Version 3.1, SPSS, Chicago, IL, USA). Since the data was normally distributed (Kolmogorov-Smirnov test), two-way analysis of variance (ANOVA) was used to analyze the surface treatment and the adhesive systems that were used. Multiple comparisons were then conducted using the Tukey-Kramer test, with a significance level of 0.05.

Scanning Electron Microscopy Microstructural observation of the ceramic surface was performed by scanning electron microscopy (SEM). All SEM specimens were dehydrated in increasing concentrations of tert-butanol (50% for 20 min, 75% for 20 min, 95% for 20 min, and 100% for 2 h) and then transferred to a critical-point dryer for 30 min. Surfaces were coated with a thin film of gold in a vacuum evaporator (Quick Coater Type SC-701, Sanyu Denshi, Tokyo, Japan), and observed by SEM (ERA 8800FE, Elionix, Tokyo, Japan) at an accelerating voltage of 10 kV. This procedure was performed for all specimens.

RESULTS

The surface free energies and their components for each cured adhesive are shown in Table 4. Total surface free energy (γS=γS

d+γSp+γS

h) of lithium disilicate and leucite glass ceramics significantly decreased after saliva contamination compared with that of the controls. However, these values increased significantly after surface treatment compared with those of the SC group. For all surfaces, the estimated γS

d values remained relatively constant and in the range of 39.8–40.2 mN/m. The γS

p and γSh values of the SC group significantly

decreased. The γSp and γS

h values of the TE and CE groups were significantly and substantially higher than those of the SC group, but γS

p and γSh values of the TE

group were lower than those of the CE group. The influence of surface treatment on the shear bond

strength of universal adhesives to lithium disilicate and leucite glass ceramics is shown in Table 5. Two-way

858 Dent Mater J 2015; 34(6): 855–862

Table 4 Influence of surface treatment of saliva contaminated ceramics on surface free energy and their components

γS γSd γS

p γSh

EM

Control 70.4 (3.8) 40.1 (1.1) 7.4 (2.8) 22.9 (2.3)

SC 52.3 (4.2) 39.8 (1.8) 1.5 (2.2) 11.0 (3.1)

TE 69.1 (3.4) 40.2 (1.2) 6.7 (2.9) 22.2 (2.3)

CE 74.6 (3.3) 40.0 (0.7) 9.1 (2.2) 25.5 (2.3)

EP

Control 70.7 (3.6) 40.1 (0.9) 7.3 (2.6) 23.3 (2.3)

SC 51.8 (3.0) 40.1 (1.0) 0.9 (1.3) 10.8 (2.5)

TE 69.7 (3.5) 40.1 (0.8) 6.9 (2.4) 22.7 (2.1)

CE 76.4 (3.3) 39.9 (0.7) 9.9 (2.2) 26.6 (2.3)

Unit: mN/m, values in parenthesis are standard deviations (n=5).Values connected by vertical lines indicate no significant difference (p>0.05).

Table 5 Influence of surface treatment of contaminated EM and EP on bond strength of universal adhesive

CeramicSurface treatment

Control SC TE CE

EM

SU 8.0 (3.2)a,A

[15/0/0]3.8 (2.7)a,B

[15/0/0] 7.0 (3.7)a,A

[15/0/0]13.2 (3.2)a,C

[14/1/0]

GB+CP13.5 (2.8)b,A

[14/1/0]6.6 (3.4)b,B

[15/0/0]12.9 (3.8)b,A

[14/1/0]18.3 (3.2)b,C

[13/2/0]

AB+PP13.3 (3.5)b,A

[14/1/0]6.8 (3.4)b,B

[15/0/0]12.3 (3.9)b,A

[13/2/0]18.0(3.0)b,C

[12/3/0]

EP

SU 7.8 (3.2)a,A

[15/0/0]3.4 (3.7)a,B

[15/0/0] 8.3 (3.7)a,A

[15/0/0]12.2 (3.2)a,C

[15/0/0]

GB+CP13.4 (2.8)b,A

[14/1/0]6.2 (3.4)b,B

[15/0/0]12.8 (3.8)b,A

[14/1/0]18.6 (3.1)b,C

[12/3/0]

AB+PP13.21 (3.5)b,A

[15/0/0]6.0 (3.8)b,B

[15/0/0]12.7 (3.9)b,A

[14/1/0]19.0 (3.0)b,C

[12/3/0]

Unit: MPa, values in parenthesis are standard deviations (n=15).Same small letter in vertical columns indicates no significant difference (p>0.05).Same capital letter in horizontal columns indicates no significant difference (p>0.05).[ ]: failure mode [adhesive failure/ cohesive failure/ mixed failure]

ANOVA revealed that both, surface treatment and the type of adhesive significantly influenced bond strength to EM and EP, although there was no significant interaction between the two factors. The bond strength of universal adhesives to EM and EP in the SC group was significantly lower than those of the control, TE and CE groups, and the bond strength of universal adhesives in the CE group was significantly higher than those of the control and TE groups. Failure type was not associated with bond strength and the predominant type of failure seen was adhesive failure.

SEM observations are shown in Fig. 2. Relatively rough surfaces created by SiC paper were observed in the control group (Figs. 2a and b). The surface contaminated with saliva was covered by a thin film of amorphous deposits (Figs. 2c and d). In contrast, the surface in the TE group appeared similar to that in the control group (Figs. 2e and f). The surface in the CE group showed the presence of elongated crystals, after partial disintegration of the silica matrix (Figs. 2g and h).

859Dent Mater J 2015; 34(6): 855–862

Fig. 2 Representative SEM photomicrographs of the surface of lithium disilicate and leucite glass ceramics (Original magnification, ×5,000).

(a) Control of EM, (b) Control of EP, (c) Saliva contaminated EM, (d) Saliva contaminated EP, (e) Phosphoric acid etched EM, (f) Phosphoric acid etched EP, (g) Hydrofluoric acid etched EM, (h) Hydrofluoric acid etched EP.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

DISCUSSION

Surface free energy has three component forces: the dispersion (γS

d), polarity (γSp), and hydrogen-bonding

(γSh) forces. These forces play an important role in saliva

adsorption, which in turn changes these forces21,22). The interaction between the superficial layer of the surface of a biomaterial and its environment is a highly dynamic process. Important factors that contribute to the total interaction include electrostatic force, co-adsorption of small ions, dispersion forces, and changes in the hydrophilic properties, including the polarity and hydrogen bonding forces of both the surface and the protein23).

One of the key factors for success with universal adhesives is the chemical bonding capacity of their functional monomers to various substrates14) and the polarization of the ceramic surface improves these chemical reactions. Moreover, these adhesives have relatively hydrophilic characteristics. Therefore, the hydrophilicity of the ceramic is important in order to enable the universal adhesive to spread across its entire surface and establish optimum adhesion. Hence, investigating the degree of contamination by saliva as well as the chemical bonding between the adhesives and ceramics based on measurement of surface free energy may provide novel insights into the bonding mechanism between the two. This study investigated the influence of surface treatment of contaminated lithium disilicate and leucite glass ceramics on the bonding performance of universal adhesives.

Generally, the surface free energy of organic

substances (γS) is defined as the sum of the dispersion (γS

d), polarity (γSp), and hydrogen-bonding (γS

h) forces21,22). The dispersion force represents London interactions between apolar molecules, whereas the polar (non-dispersion) force represents electrostatic, metallic, and dipolar interactions. Further, the hydrogen-bonding force of these two parameters of γS was also calculated, which is related to the water and hydroxyl components. Because the tendency of the ceramic surface is to bond with universal adhesives, polar interactions and hydrogen-bonding characteristics must be accurately determined in order to clarify the interactions between the adhesives and ceramic surfaces.

The γSd values of the lithium disilicate and leucite

glass ceramic surfaces remained relatively constant (39.8–40.2 mN/m) regardless of the adhesive used, and there were no significant differences seen in γS

d between the light intensity groups. It has been reported that the γS

d values of oxidized surfaces measured using this method were approximately 40 mN/m18,20), whereas those of surfaces coated with polytetrafluoroethylene were approximately 20 mN/m. The total surface free energy, γS

p, and γSh values of the SC group were significantly

lower than those of the control, TE and CE groups. The γSp

value, which reflects polar interactions, and the γSh value,

which relates to the water and hydroxyl components, together measure hydrophilic interactions. Hydrophilic interactions are important since the CAD-CAM ceramic blocks used in this study contained approximately 70% SiO2, which may form hydroxyl groups on the surface, and saliva contains more than 99% water along with small amounts of proteins, glycoprotein sugars, amylase,

860 Dent Mater J 2015; 34(6): 855–862

and inorganic particles24). On saliva contamination, non-covalent adsorption of salivary proteins occurs on the surfaces of restorative materials, creating an organic coating that cannot be removed by rinsing with water25). A possible explanation for the decrease in γS

p and γSh

values after saliva contamination is that this thin residual organic film, which was observed in SEM (Figs. 2c and d), covers the hydroxyl sites of the ceramic surface and reduces its potential for hydrophilic interactions.

Compared with the controls, the bond strength of the universal adhesives to ceramics significantly decreased after saliva contamination. These results suggest that saliva contamination prevents chemical bonding between the two and interferes with adhesion, resulting in a decrease in bond strength. Hence, the null hypothesis that saliva contamination has no influence on surface free energy or on the bond strength of the universal adhesive to ceramics may be rejected. Some previous studies demonstrated that if a ceramic surface is contaminated by saliva, hydrofluoric acid etching is an effective way to re-activate the ceramic surface16,17). However, hydrofluoric acid is a poisonous and caustic liquid that is extremely irritating to the skin and lungs, and concerns about its use intraorally have been reported26). Thus, in intraoral repair, phosphoric acid is preferred for etching and decontamination of bonding surfaces27). Additionally, phosphoric acid has previously been proposed as a ceramic surface cleaning agent based on the assumption that it is a good organic solvent27). Therefore, the effectiveness of both hydrofluoric acid and phosphoric acid treatments were investigated in the present study.

Total surface free energy value of control and TE groups were not significantly different, suggesting that phosphoric acid effectively cleaned the contaminated surface. This was observed in SEM (Figs. 2a, b, e and f). However, the surface free energy values of the CE group were significantly higher than those of the control and TE groups. Cleanliness of the bonding surface is important for successful bonding between ceramics and adhesives. Therefore, residual organic contaminants should be removed before the bonding procedure28). These results suggested that phosphoric acid and hydrofluoric acid etching would work well for decontamination of ceramics. Hydrofluoric acid etching acts on the microstructure of lithium disilicate and leucite glass ceramics by dissolving the glassy phase of these ceramics. This phase was partially dissolved to create a microstructure that increased surface area and wettability for bonding, and this was observed in SEM (Figs. 2g and h). Therefore, the high total free energy value in the CE group was observed due to the decontamination, increase of surface area and increase in wettability of ceramics.

A chi-squared test revealed no significant differences in failure mode between the adhesives or surface treatment. A weak trend to more cohesive failure was observed with increasing bond strength, but it was not statistically significant. Accordingly, these results do not allow us to say anything substantial about the

relationship between bond strength and failure mode.The bond strength of universal adhesives in the SC

group was significantly lower than those of the control, TE and CE groups, and the bond strength of universal adhesives in the CE group was significantly higher than those of the control and TE groups. Therefore, we can reject the null hypothesis that there are no differences in the surface free energy and bond strength of universal adhesives with different methods of surface treatments prior to adhesive application. Cleaning of saliva contaminated lithium disilicate and leucite glass ceramics with phosphoric acid and hydrofluoric acid improved bond strength, and also re-established the same or higher bond strength as that in the control group. The results of this study also reveal that GB and AB, which require an additional silanating step prior to application of a universal adhesive, show higher bond strength than SU, regardless of the method of etching. This result suggests that a silanating agent included in the universal adhesive, as in SU, may not be effective in optimizing the ceramic-resin bond.

CONCLUSION

The results of this study indicated that a thin layer of contaminants remained on the lithum disilicate and leucite glass ceramic surface after exposure to saliva, significantly impending the bonding of the universal adhesives and reducing the surface free energy of the ceramics. Hydrofluoric acid and phosphoric acid etching may be effective methods of removing the contaminants and creating an effective surface for bonding with universal adhesive.

ACKNOWLEDGMENTS

This work was supported, in part, by a Grant-in-Aid for Scientific Research (C) 26462896 and a Grant-in-Aid for Young Scientists (B) 10608409 from the Japan Society for the Promotion of Science. This project was also supported, in part, by the Sato Fund and by a grant from the Dental Research Center of the Nihon University School of Dentistry, Japan.

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