Dental Materials Journal 2010; 29(3): 316–323

Combination treatment of tribochemical treatment and phosphoric acid ester monomer of zirconia ceramics enhances the bonding durability of resin-based luting cementsKenichiro TAKEUCHI1,2, Akihiro FUJISHIMA1, Atsufumi MANABE2, Soichi KURIYAMA1,2, Yasuhiro HOTTA1, Yukimichi TAMAKI1 and Takashi MIYAZAKI1

1Department of Oral Biomaterials and Technology, School of Dentistry, Showa University, 1-5-8 Hatanodai, Shinagawa-ku, Tokyo 142-8555, Japan2Division of Aesthetic Dentistry, Showa University School of Dentistry, 2-1-2 Kitasenzoku, Ohta-ku, Tokyo 145-8515, JapanCorresponding author, Kenichiro TAKEUCHI; E-mail: takep@dent.showa-u.ac.jp

The aim of the present study was to evaluate the bonding durability of resin-based luting cement to partially stabilized tetragonal zirconia (Y-TZP) achieved by combination treatment of tribochemical (TBC) treatment and two different phosphate acid ester monomers. Two phosphate acid ester monomers (EP: Epricord opaque primer, AZ: AZ primer) were applied to each surface modification followed by application of resin-based luting cement (Rely-X ARC). Bonding specimens were placed in deionized water at 37°C and stored for 24 h. The other groups were subjected to 30,000 cycles of a thermal stress for the durability test. Shear bond tests were done using a universal testing machine at 1 mm/min. Shear bond strengths of combination treatments using EP and AZ on TBC treatment after thermal stress showed no significant difference (p>0.05) compared with those of storage after 24 h. Combination treatment using phosphoric acid ester monomer could achieve a durable bond.

Keywords: Zirconia ceramics, Bonding durability, EPMA analysis

INTRODUCTION

Partially stabilized tetragonal zirconia (Y-TZP) ceramics are frequently applied as framework materials of ceramic crowns and fixed partial dentures1-4). The clinical success of ceramic restorations is dependent upon not only on optimal marginal adaptation of frameworks, but also bonding durability.

To achieve excellent bonding durability between the luting agent and the ceramic surface, the surface area of the bonding surface must be increased and an active surface produced5,6). Many surface-treatment methods have been attempted for dental ceramics, including etching7-10), alumina sandblasting (ASB)5,6), laser etching11) and plasma etching12). In particular, etching for dental ceramic restorative materials treated with several acid solutions is believed to increase surface area because these solutions selectively dissolve the glassy component of the ceramic7-10). Etching is not available for Y-TZP because of the higher crystalline content13-16). ASB is a suitable method to increase surface area and produce an active surface for dental materials. These rough surfaces contribute to produce micromechanical interlocks of the bonding interface5,6,17,18). Tribochemical (TBC) treatment by Rocatec™ is also available for ceramics (including Y-TZP). Silica-coated alumina particles are sandblasted at low pressure, and ceramic surfaces chemically modified with silica by the TBC mechanism. TBC treatment has been reported to be effective for the bonding of ceramics because the silica-modified layer reacts chemically with resin-based luting (RBL) cement through a silane coupling agent (SCA)14,19,20).

Primer treatment has also been used to increase bonding strength. A SCA is effective for conventional

silica-based glass ceramics such as feldspathic porcelain because it forms a chemical reaction on the silica surface21,22). Several studies have reported that a functional monomer of 10-methacrylixydecyl dihydrogen phosphate (MDP) is superior to SCAs for high-strength core ceramics such as alumina and Y-TZP5,6,13-18). In particular, a combination of MDP monomer and ASB treatment is expected to generate a stable and durable bond strength of RBL cement to Y-TZP5,6,23,24). The bonding durability of combined treatment to Y-TZP was not satisfactory, particularly after thermal cycling25-28). A novel treatment to increase the bonding durability of Y-TZP is therefore needed.

We reported that combination treatment which applied a functional monomer with TBC treatment was effective in increasing the bonding durability of silica-based glass ceramics29). The effect of combination treatment for Y-TZP has not been investigated. The aim of the present study was to evaluate the bonding durability of RBL cement to Y-TZP achieved by combination treatment of TBC treatment and two different phosphate acid ester monomers for a primer after thermal cycling.

MATERIALS AND METHODS

Specimen preparationTable 1 shows the materials used in the present study. Plate specimens of Y-TZP (14×14×3.5 mm) were cut from pre-sintered green zirconia blocks (Katana, Noritake Dental, Aichi, Japan) using a low-speed diamond cutting saw (Isomet, Buehler, Lake Bluff, IL, USA). Specimen surfaces were then polished by a 1.0-µm alumina abrasive paper (P/N AFP 081-25 LOT11018; South Bay Technology Incorporated, CA,

Received Oct 15, 2009: Accepted Jan 23, 2010doi:10.4012/dmj.2009-099 JOI JST.JSTAGE/dmj/2009-099

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USA) to achieve surface roughness of 0.15 µm. After polishing, specimens were sintered at 1,350°C for 2 h. Specimens were cleaned ultrasonically in acetone solution for 10 min, and then dried by air blasting.

Treatment and modification of the surfaceThree surface treatments were used: (1) the above-mentioned sintering flat specimens were used as a non-modification (NMD) surface; (2) using an ASB treatment device (Rocatec™ Junior, 3M ESPE, Seefeld, Germany), alumina powder with a mean particle size of 25 µm (WA25, Heraeus Kulzer, Hanau, Germany) was blasted onto the bonding surface of zirconia specimens at a pressure of 0.40 MPa (13 s/cm2) at a distance of 10 mm; and (3) TBC treatment using silica-coated alumina particles with a mean particle size of 30 µm (Rocatec™ Soft, 3M ESPE) enabled these particles to be blasted onto the bonding surface of zirconia specimens at a pressure of 0.28 MPa (13 s/cm2) and a distance of 10 mm. This was followed by application of a SCA (ESPE-sil, 3M ESPE) applied for 5 min to the silica-modified surface, which was then dried with air.

Primer and luting cementsWe used one RBL cement (Rely-X ARC, 3M ESPE) and two phosphate acid ester monomers (EP: Epricord opaque primer, Kuraray Medical, Osaka, Japan; AZ: AZ primer, Shofu, Kyoto, Japan) to each surface modification.

Surface roughnessThe mean roughness of the Y-TZP specimens with each surface modification was evaluated by a surface texture-measuring instrument (Surfcom, 480A, Tokyo Seimitsu, Tokyo, Japan).

Preparation of shear bond specimensA sintered zirconia specimen was placed onto the center of a double-sided tape (Sekisui Tape, Sekisui,

Tokyo, Japan) which was fixed onto a glass plate. Cold curing resin (Palapress Vario, Heraeus Kulzer, Hanau, Germany) was poured into the acrylic tube to invest the zirconia specimen. Vinyl tape (50 µm thickness with a 6 mm hole; Vinyl Patches, Kokuyo, Osaka, Japan) was fixed to the bonding surface of the zirconia test specimen to define the bonding area.

For the bonding body test specimens, JIS grade 2 cp titanium cylindrical specimens (KS-50, Kobelco, Kobe, Japan) were used (8 mm diameter, 2.0 mm height). The surface of the titanium bonding body was blasted with alumina powder with a mean particle size of 250 µm using a sandblasting device (Combilabor CL-FSG 3, Heraeus Kulzer) at 0.40 MPa (blast condition: 13 s/cm2) at a distance of 10 mm. The titanium bonding body was cleaned ultrasonically in acetone solution for 10 min, and then dried with air. MDP primer was then applied to the bonding surface of titanium bonding body specimens.

Bonding procedureMixed RBL cement paste was applied to the zirconia surface in the area of the tape, and then pressed onto the titanium bonding body. The specimen was immediately loaded at 2 kgf by a constant loading device, and excess cement paste removed. Cement was irradiated from four directions for 20 s for a total exposure time of 80 s using a light curing unit (Cure Master, Yoshida, Tokyo, Japan). The bonding specimen was placed in deionized water at 37°C and stored for 24 h. The other groups were subjected to 30,000 cycles of a thermal stress durability test, with immersion in deionized water at 5°C and 60°C for 1 min.

Shear bond testThe device used for the shear bond test is shown schematically in Figure 1. The shear bond test was carried out using a universal testing machine (1125-5500R, Instron, Kawasaki, Japan) at a crosshead speed

Brand name Composition Batch number Manufacturer

KATANA Zirconia ZrO2, Y2O3 KT10M Noritake dental supply, Aichi, Japan

Rely-X ARC Bis-GMA, TEGDMA, zirconia filler, silica filler, dimethacrylate-polymer, functionalized, amine, initiator, peroxide, BP, pigment

20080327 3M ESPE, Seefeld, Germany

Espesil (SCA) 3-methacryloxypropyltrimethoxysilane, ethanol 257075 3M ESPE, Seefeld, Germany

Epricord opaque primer (EP)

10-methacrylixydecyl dihydrogen phosphate (MDP), acetone

0148BA Kuraray, Osaka, Japan

AZ primer (AZ) 6-methacryloyloxyhexyl phosphonoacetate (6-MHPA), acetone

20701 Shofu, Kyoto, Japan

Table 1 Materials for the bonding test

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of 1.0 mm/min. Shear bond strength was measured in each bonding evaluation parameter for surface treatment.

Statistical analysesMean values of shear bond strengths were analyzed by two-way ANOVA and Tukey’s multiple comparison tests to determine no significant difference (p>0.05) between surface modifications (n=7).

Field emission-scanning electron microscopy (FE-SEM) observationThe surface of zirconia ceramic specimens with and without surface modification before bonding was observed using a scanning electron microscope (S-4700, Hitachi, Tokyo, Japan). The fractured surface of zirconia ceramic specimens after the bonding test was also observed after sputtering with gold alloy.

Electron probe micro analyzer (EPMA) analysis The distribution of Zr and Si on the fractured surface of zirconia specimens after thermal cycles was evaluated using electron probe micro analysis (EPMA; EPMA1610, Shimadzu Company, Kyoto, Japan). Measurement parameters were under the following conditions: accelerating voltage at 15 kV, beam current at 100 nA, beam size at 1 µm, and sampling time at 0.01 s.

RESULTS

The results of shear bond strength tests for the three surface-modification procedures and the storage conditions are listed in Table 2. All of fracture surface of titanium bonding body after bonding test were covered with RBL cements completely. Two-way

ANOVA showed that the shear bond strengths were significantly affected by surface treatment (F=115.0; p<0.01), thermal cycling (F=56.7; p<0.01) and these interaction (F=3.3; p<0.01).

Shear bond strength of the non-primer (NP) on the NMD surface was the lowest (5.7 MPa). During 30,000 cycles of the thermal stress durability test, all specimens of this group debonded spontaneously. Shear bond strength of the primers (AZ and EP) on the NMD surface was significantly higher (p<0.05) than NP on NMD.

Shear bond strengths of the primers (AZ and EP) on the ASB surface increased significantly (p<0.05) compared with those on the NMD surface and NP-ASB surface. Shear bond strengths of all specimens of the NMD surface and ASB surface after thermal stress of 30,000 cycles were significantly lower (p<0.05) than those after storage for 24 h.

Combination treatment using EP after TBC treatment resulted in the highest value (49.4 MPa). This was significantly higher (p<0.05) than that of the NMD and ASB surface. The shear bond strength of AZ on the TBC surface showed no significant difference (p>0.05) compared with that on the ASB surface. Shear bond strengths for combination treatment using EP and AZ on the TBC surface were not significantly different (p>0.05) after thermal stress.

The mean surface roughness of the Y-TZP

Fig. 1 Testing device for shear bond test (schematic).

24 hours 30,000 cyclesMean (S.D.)

NP 5.7 (5.1)D –NMD AZ 15.6 (5.3)C

a 7.5 (5.9)Cb

EP 15.1 (4.7)Ca 6.7 (4.2)C

b

NP 19.3 (4.1)Ca 9.3 (4.5)C

b

ASB AZ 34.9 (5.2)Ba 27.5 (3.7)B

b

EP 41.2 (6.3)Ba 33.6 (3.4)B

b

NP 37.6 (5.4)Ba 26.7 (5.3)B

b

TBC AZ 34.1 (7.3)Ba 32.0 (8.1)B

a

EP 49.4 (3.3)Aa 44.8 (4.5)A

a

Mean values of left side indicate shear bond strength of after storing 24 hours and those of right side indicate shear bond strength of after 30,000 cycles of a thermal stress. The bold mean values indicate using combination treatment. Within the same column, means with the same upper case superscript letters are not statistically different (p>0.05). Within the same row, means with the same lower case subscript letters are not statistically different (p>0.05).

Table 2 Shear bond strength of zirconia ceramics after treatment under different conditions (means and standard deviations in MPa (n=7))

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specimens with NMD, ASB and TBC were 0.146±0.011, 0.325±0.032, and 0.408±0.026 µm, respectively. A statistically significant difference was observed between these values of surface roughness (p<0.01) by one-way ANOVA.

Figure 2 shows the SEM images of zirconia ceramics after each surface modification. ASB and TBC showed a modified surface texture with the formation of micromechanical grooves.

Figure 3 shows the SEM images of fractured surfaces by a shear bond test after 30,000 thermal cycles. EP on the ASB surface showed mixed failure which was similar to the pre-bonding state, and micro grooves could be observed from ASB treatment. For fractured surfaces NP and AZ, EP on TBC was clearly different compared with the pre-bonding surface. NP and AZ on TBC could be observed embedded in the resin luting cement. However, these could not be

Fig. 2 SEM images (original magnification×600) of the bonding surfaces of Y-TZP. A: Sintered surface (non-treatment). B: Alumina sandblasted surface (mean particle size: 25 µm). C: Tribochemical-treated surface (mean particle size: 30 µm).

Fig. 3 SEM images (magnification×600) of the fractured surface of Y-TZP after the shear bond test after 30,000 thermal cycles. A: MDP primer on ASB treatment surface. B: TBC treatment surface. C: 6-MHPA primer on TBC treatment surface. D: MDP primer on TBC treatment surface.

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observed cohesive failure. The fractured surface of EP on TBC could be clearly observed as cohesive failure because a large amount of resin luting cement was on this surface (Fig. 3D).

Figures 4 and 5 show the back-scattered electron (BSE) images and mapping images of the distribution of Zr and Si by EPMA analysis on the fractured surface of EP on ASB and EP on TBC after thermal cycles. In the mapping image of EP on ASB, in general, Si element could not be observed (Fig. 4C). In the mapping image of EP on TBC, Si element could be observed (Fig. 5C).

DISCUSSION

Y-TZP is a useful material for fabrication of an aesthetic prosthesis to withstand high loading. A key requirement to obtain good clinical outcome is durable bonding to tooth substrates or abutment structures with luting materials3,4). Not only luting materials but also the bonding interface between the functional

monomer as a primer for Y-TZP is closely related to bond durability.

Y-TZP is a less advantageous material for bonding to RBL cements compared with dental alloys because of low surface energy and wettability20). Y-TZP therefore requires treatment or modification of its surface to achieve durable bonding.

It is well known that producing a silicate layer on the ceramic surface increases wettability. In particular, treatment with hydrofluoric acid and application of a SCA has a pronounced effect on conventional dental ceramics (e.g., leucite-reinforced silica-based ceramics, feldspathic porcelain, and lithium disilicate ceramics) to enhance bonding strength. This is because hydrofluoric acid selectively dissolves glass-phase or crystalline components from the ceramic surface (which increases surface roughness for micromechanical retention) and the SCA mediates between RBL cement and hydrophobic silica-based ceramics7-10). The SCA used in the present study, γ-methacryloxypropyltrimethoxysilane (γ-MPTS), has

Fig. 4 BSE image and EPMA analysis of MDP primer on ASB treatment surface of Y-TZP. A: BSE image. B: Zr mapping image. C: Si mapping image.

Fig. 5 BSE image and EPMA analysis of MDP primer on TBC treatment surface of Y-TZP. A: BSE image. B: Zr mapping image. C: Si mapping image.

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three methoxy groups bonded to silicon within the molecule. It specifically attaches to the silicon molecule on the bonding surface, and a siloxane network comprising covalent bonds is formed by dehydration and condensation. This is known to produce a strong bond to silica5,11,12). Y-TZP is a polycrystal zirconia ceramic without silica on the surface, so surface modification of Y-TZP is required before SCA application13,16,30).

Conventional silica-based ceramics treated by sandblasting with alumina particles were not applied to conventional restorations because this method produced poor marginal adaptation and chipping. ASB treatment is an advantageous method for certain dental ceramics for long-term durable bonding because the inner surface of computer-aided design and computer-aided machining (CAD/CAM)-fabricated Y-TZP is relatively smooth. SEM observation and increase in surface roughness revealed that the Y-TZP surface after treatment with ASB and TBC clearly improved surface texture, which helped to provide micromechanical retention for the luting agent (Fig. 2). These surface modifications appear to be necessary to achieve high bond strengths and long-term durable bonds. TBC treatment has been reported to provide not only micromechanical retention, but to also produce a silica layer on the bonding surface31). TBC treatment conveys the mechanical energy of sandblasting to the treated surface in the form of kinetic energy. In general, silication does not produce a temperature rise, and its effects are influenced at the atomic and molecular levels21). SCAs react with the residual silica layer, and a siloxane network is formed by hydrolysis and cross-linking. TBC treatment has been reported to provide good bonding with durability for dental materials such as metals, resins, and ceramics5,32). In the present study, NP on the TBC surface showed improvement of bonding strength as compared with those of NMD and ASB surfaces. TBC treatment without a primer therefore seemed to be an effective bonding method for Y-TZP. NP on the TBC surface after 30,000 thermal cycles was significantly decreased compared with those after storage for 24 h. We therefore suggest that application of only SCAs to the TBC surface was not sufficient to form a durable bonding interface such as a siloxane network.

In the present study, we used MDP and 6-methacryloyloxyhexyl phosphonoacetate (6-MHPA) for primer treatment. Several studies reported that MDP showed excellent bonding to not only non-precious metals, but also ceramics because the hydrogen group in this monomer reacted chemically with the hydrogen layer21,22). The application of MDP monomer and ASB has been expected to generate a stable and durable bond strength between Y-TZP and RBL cements5,6,23,24). Although these studies used self-adhesive cement containing a functional monomer, we suspected that this experimental model could not exclude the mechanical properties of luting materials because the volume of the filler and the substance of the matrix

resin were different. Therefore, we selected a Rely-X ARC to standardize cement selections for RBL cements which did not contain a functional monomer, and to evaluate the bonding interface between Y-TZP and functional primer independently.

We also investigated the effect of 6-MHPA besides MDP because a commercial AZ primer containing 6-MHPA has been developed for Y-TZP, but there were few reports of its usefulness. The present study revealed that the shear bond strength of EP on ASB was significantly higher (p<0.05) than that of ASB without primer at 24 h. These results suggested that ASB treatment contributed not only to an increase in mechanical retention, but also to the cleaning and activation effects for Y-TZP33-35). ASB treatment alone barely achieved high bond strength after 30,000 thermal cycles. EP and AZ have a phosphoric ester group with a different number of methylene chains. We hypothesized that chemical bond situation of the hydrophilic group affects bonding durability rather than the molecular configuration of the functional monomer, and that these chemical reactions occurred due to the weak hydrogen bond.

Shimakura et al.29) reported that combination treatment using MDP after TBC treatment enhanced bond strength and bond durability for silica-based glass ceramics. In the present study, we investigated the effect of the combination treatment using two phosphoric acid ester monomers to Y-TZP. Table 2 shows that shear bond strength of EP on the TBC surface was significantly higher than that of the ASB surface, whereas a predicted result was not provided for AZ on TBC surfaces. EP and AZ on the TBC surface after 30,000 thermal cycles were slightly reduced compared with those after storage for 24 h. The fact that no difference in bond strengths was observed suggested that combination treatment contributed to the long-term durable bond strength for RBL materials. In particular, the shear bond strength of EP on the TBC surface was highest in this experimental model. SEM observation of this fracture surface showed that a large amount of residual cement was present. However, SEM images provided limited information, so we evaluated the fractured surface using EPMA analyses. Compared with EP on ASB and EP on TBC in terms of distribution of Si, the former could not generally observed Si element and concentration of Zr element was higher than EP and BSE image was showed plateau contrast. Si element was observed on EP on TBC. We suggest that this Si element was a filler of the internal residual resin luting cement, and was not broken TBC particles because, in the SEM images of the original TBC treatment surface, residual particles of TBC treatment were not observed (Fig. 2C). Therefore, on the fractured surface of EP on TBC, cohesive failure occurred, and combination treatment enabled formation of a durable bonding interface. The influence of AZ on shear bond strength with combination treatment was lower than that seen for EP. Nevertheless, combination treatment with AZ may

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contribute to reinforcement of the primer layer.SCAs produce a siloxane bond between hydroxyl

and methoxy groups on the ceramic surface. In the initial stage of this chemical reaction, the SCA must be activated to promote hydrolysis by an acidic environment36). In the present study, acidic monomers such as MDP and 6-MHPA were used to promote this environment and hydrolyze the SCA37). The latter could subsequently react with hydroxyl groups on the silica of the ceramic surface through the formation of siloxane bonds. Acetone in the functional monomer simultaneously activated the condensation reaction of the SCA. The durable bond strength produced by combination treatment could be because these factors contributed to reinforce the bonding interface of molecular microstructures. Combination treatment using MDP markedly increased bond strength compared with that observed using AZ. These results suggested that MDP monomer could penetrate the silane coupling layer and selectively adsorb the hydroxyl groups of the Y-TZP surface.

The findings of the present study suggest that combination treatment using a functional monomer was sufficient to produce durable bond strength for Y-TZP. Further study is needed to analyze the bonding interface chemically and to evaluate the behavior of functional monomers.

CONCLUSIONS

After completing the present study, the following conclusions could be drawn:

1. Treatment by ASB and TBC roughened the zirconia ceramic surface and provided an activation surface.

2. ASB using a phosphoric acid ester monomer was an effective method for increasing shear bond strength, but this method could not produce a durable bond.

3. Combination treatment using a phosphoric acid ester monomer demonstrated durable bond strength. Using a MDP monomer showed the highest bonding strength. The phosphoric acid ester monomer can act in concert with a SCA.

ACKNOWLEDGMENTS

The authors thank Noritake Dental Supply Company for providing the zirconia blocks and for helpful discussions. We also wish to thank Dr. Kazuhiro Debari, EM Laboratory, Showa University, for his help to analyze EPMA.

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