journal of the mechanical behavior of biomedical materials 32 (2014) 300 –309
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Research Paper
Evaluation of four surface coating treatments for resin to zirconia bonding Dan Liu, Edmond H.N. Pow, James Kit-Hon Tsoi, Jukka P. Matinlinnan Dental Materials Science, Faculty of Dentistry, The University of Hong Kong, Hong Kong SAR, PR China
ar t ic l e in f o
abs tra ct
Article history:
Objectives: To compare the effects of four surface coating methods on resin to zirconia
Received 25 August 2013
shear bond strength.
Received in revised form
Material and methods: Eighty pre-sintered zirconia discs were prepared and randomly
9 December 2013
divided into five study groups according to the corresponding methods of surface treat-
Accepted 10 December 2013
ments as follows: group C (control group, fully sintered without any surface treatment),
Available online 24 December 2013
group S (fully sintered and then sandblasted with silica coated alumina powder), group G
Keywords:
(fully sintered and then coated with glazing porcelain followed by acid etching), group Si
Surface treatment
(pre-coated with silica slurry then fully sintered), and group Z (coated with zirconia
Adhesion
particles and then fully sintered). The observation of surface morphology and elemental
Surface coating
composition analysis were conducted by SEM and EDX. Self-adhesive resin cement stubs
Resin cement
(diameter 3.6 mm and height 3 mm) were then bonded on the zirconia discs with a
Zirconia
cylindrical shape. Both initial and artificial aged (including 30-day water storage, thermal
Sandblasting
cycling for 3000 and 6000 cycles) shear bond strengths were then evaluated. Results: All the tested coating methods showed significantly higher shear bond strengths than the control group, in both dry and aged conditions. Group S produced the strongest initial zirconia/resin bonding (19.7 MPa) and the control group had the lowest value (8.8 MPa). However, after thermal cycling, group Z exhibited the highest mean value. All the samples in the control group failed in the thermal cycling. Both different coating methods (po0.001) and various aging treatments (po0.001) produced significant influence on resin–zirconia shear bond strength. Conclusions: A reliable and durable resin zirconia bonding is vital for the longevity of dental restorations. Silica coating might be a reliable way in enhancing adhesion between resin and zirconia. & 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
Nowadays, the application of zirconia as a base material in the production of all-ceramic restorations has become one of the
major foci in dental research. Such increase in the interest is ascribed to its high mechanical strength and exceptional biocompatibility (Liu et al., 2012; Manicone et al., 2007; Piconi and Maccauro, 1999). The success of zirconia-based all-ceramic
n Correspondence to: Dental Materials Science, Faculty of Dentistry, University of Hong Kong, 34 Hospital Road, Sai Ying Pun, Hong Kong, PR China. Tel.: þ852 28590380; fax: þ852 25489464. E-mail address: [email protected] (J.P. Matinlinna).
1751-6161/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jmbbm.2013.12.011
journal of the mechanical behavior of biomedical materials 32 (2014) 300 –309
restorations is highly dependent on the establishment of a strong adhesion between zirconia and luting cement. Compared with traditional luting cements, such as zinc phosphate, glass ionomer cement etc, resin cement has some irreplaceable advantages including higher mechanical strength and better esthetic properties (Piwowarczyk et al., 2005). However, without any surface treatment, the resin zirconia integration was found to be susceptible to aging conditions (Özcan et al., 2008). Meanwhile, the conventional bonding approaches, such as acid etching followed by the application of silane coupling agents, could not effectively improve the bond strength between zirconia copings and resin cement due to the chemical inertness of zirconia (Ho and Matinlinna, 2011a, 2011b). There is no inherent glass content in the matrix of zirconia abutment. Thus, zirconia base structures cannot be etched with commonly used mineral (or organic) acids, such as HF and H3PO4, for adding the surface roughness. Furthermore, it is also very cumbersome to form a strong chemical integration between zirconia coping and resin cement by using solely the conventional silane coupling agents (Kern and Wegner, 1998; Komine et al., 2012; Özcan and Vallittu, 2003). Air-abrasion with alumina particles followed by an appropriate chemical bonding process was recommended to achieve long-term retention to zirconia (Kern, 2009). The incorporation of 10-methacryloxydecyldihydrogenphosphate (MDP) in primers or resin cements was a vital factor in producing durable resin zirconia bonding which has already been confirmed in the related clinical trials (Abou Tara et al., 2011; Sasse and Kern, 2013). Other surface treatments, such as the tribochemical method, selective infiltration etching, heating with a hot etching solution, laser surface treatment, plasma treatment and surface fluorination, have been developed for enhancing resin zirconia bonding (Heikkinen et al., 2009). Among these methods, sandblasting with silica coated alumina particles, combined with the application of MDP containing primer/resin cement has been recommended as one of the most effective methods (Atsu et al., 2006; Tanaka et al., 2008). This tribochemical method has been proven not only to increase the values of surface roughness, but also to add silicon (Si) content on zirconia surface. Silicon content is vital for activating the functions of silane coupling agents. Both mechanical interlocking and chemical integration between resin cement and zirconia have thus been enhanced (Peutzfeldt and Asmussen, 1988). However, there are still concerns about the influence of sandblasting on mechanical properties and long-term stability of zirconia base because sandblasting has been reported to induce some flaws on the surface (Zhang et al., 2004). The generation of such flaws might produce some detrimental effects on the reliability of zirconia substructures (Kosmač et al., 1999). For the other methods, some of them are time-consuming and not suitable for clinical application. Some protocols use very aggressive acids that may be detrimental to the health of operators. Further research has being performed for developing innovative approaches with more convenient and safe procedures. Coating treatments on zirconia surface might also be regarded as one of the solutions to the current problems. For instance, it was reported that the application of a socalled glaze-on technique which is composed of the veneering of a thin layer of glazing porcelain on zirconia surface and
301
the follow-on acid etching could result in the enhanced resin to zirconia shear bond strength. The addition of Si containing porcelain provided the necessary foundation for the establishment of chemical integration (Everson et al., 2011). Another study stated that the attachment of silicon dioxide (SiO2) layer on zirconia surface with vapor-phase deposition method could provide functional sites for the use of silane coupling agents and improve resin zirconia adhesion subsequently (Piascik et al., 2009). Coating with zirconia particles also has helped to improve the resistance to the aging effects of water content at the zirconia/resin interface (Aboushelib, 2012; Aboushelib et al., 2009). The aim of this study was to evaluate the differences between four direct zirconia surface coating methods and the effects on the shear bond strength between zirconia and resin cement. The hypothesis of the study was that the tested surface coating methods would produce the same effects on resin to zirconia bonding.
2.
Materials and methods
2.1. Preparation and grouping of pre-sintered zirconia specimens Eighty pre-sintered zirconia discs were cut from commercial dental zirconia ingots (Cercon base, DeguDent GmbH, Hanau, Germany) using a low speed diamond saw (Microslice, Metal Research limited company, England) under running water. Each disc was 25 mm in diameter and 1.5 mm in thickness. All the zirconia specimens were then polished with 1000-grit SiC abrasive paper on a polishing platform (Lumn Major, Struers, Denmark). They were randomly divided into five groups (C, S, G, Si, and Z) according to the corresponding methods of surface treatment: Group C (control): the specimens in this group were densely sintered in a furnace (Cercon heat, DeguDent GmbH, Hanau, Germany) according to the manufacturer0 s instructions (maximum temperature of 1350 1C, and a sintering cycle of 6 h). They were not treated with any surface treatment. Group S (sandblasting): in this group, the zirconia specimens were also densely sintered (using the same sintering program as group C) and followed by sandblasting. The process of sandblasting was carried out by an air-abrasion machine (Shofu Pen-Blaster, Shofu Dental Corporation, Kyoto, Japan) with 110-μm silica coated alumina powder (Rocatecs, 3M ESPE, Seefeld, Germany) at a constant pressure of 3.5 bar for 10 s. The working distance was 10 mm and directly perpendicular to zirconia surface. Group G (glazing porcelain coatingþacid etching): the slurry of glazing porcelain (Cercon Ceram Kiss Glaze, DeguDent, Germany) was prepared by mixing the glazing powders with corresponding modeling liquid. The zirconia specimens were fully sintered (using the same sintering program as group C) and then one thin layer of porcelain slurry was applied on the surfaces of each fully sintered zirconia with a clean brush and lightly vibrated. The specimens were then fired in a porcelain furnace according to the manufacturer0 s instructions. After being cooled to room temperature, each glazed zirconia surface was treated with an acid etching gel
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(Vita Ceramics Etch, Vita Zahnfabrik, Germany) containing 5 wt% HF and 10 wt% H2SO4 for 120 s and then rinsed thoroughly with deionized water. Group Si (silica particle coating): before final sintering, a slurry comprised of 0.02 g silica powder (5–12 μm) with 0.08 g resin monomer (UEDMA, Esstech, Essington, USA), and it was distributed evenly on each pre-sintered zirconia disc. All the specimens were then fully sintered according to the instructions of manufacturer (using the same sintering program as group C). Group Z (zirconia particle coating): each pre-sintered zirconia disc was coated with the mixture which was prepared by mixing 0.032 g Y-TZP powders (the same zirconia material in a amorphous powder form) with 0.08 g UEDMA monomer and then fully sintered (using the same sintering program as group C).
2.2.
Measurement of surface roughness
Twenty four specimens in each group were randomly selected and measured the surface roughness values after surface treatment. The mean value of each group was then calculated. The measurement was carried out after calibration on a flat surface using a profilometer (Surtronic 3þ, Taylor Hobson, UK) with the cut-off value set at 0.8 mm. The value of Ra was chosen as the indication of surface roughness. Rougher surface was indicated by the higher Ra value. Each sample was measured for three times and the mean value was calculated. Thus, there were twenty-four measurements of surface roughness in each group.
After curing, the mold was then removed with care. Each cured resin stub was 3.6 mm in diameter and 3 mm in thickness. Five resin stubs were adhered on each zirconia disc. All the specimens were kept at room temperature for 24 h before further process. There were 80 zirconia/resin composite specimens in each group of surface treatment.
2.5.
In each group, eighty specimens were further divided into four sub-groups (there were 20 samples in each sub-group): one was tested with initial shear bond strength, one was evaluated after being stored in 37 1C water for 30 days, one was tested after thermal cycling (5–55 1C) for 3000 cycles, and the last one was treated with 6000 thermal cycles. In the testing of shear bond strength, each sample was mounted on a universal testing machine (ElectroPuls™ E3000, Instron, USA) with a special designed metal jig and loaded until failure happened. The speed of crosshead was 1 mm/min. The shear bond strength of each specimen was calculated by dividing the failure load with the bonding area. The failure mode in the current study was defined and classified according to the appearance of detached zirconia/resin interface: (1) adhesive failure, less than 20% resin cement was left on zirconia surface; (2) mixed mode of failure, the remaining resin was more than 20% and less than 80%; and (3) cohesive failure, more than 80% resin cement remained. The assessment was carried out by using an optical microscope (Leica MZ6, Leica Microsystems, USA).
2.6. 2.3.
Statistical analysis
Scanning electron microscopy (SEM) observation
A scanning electron microscope (Hitachi S-3400 N, Hitachi High-Technologies Corporation, Tokyo, Japan) was used to evaluate the changes in surface morphology of zirconia samples treated with different methods. In each group, one additional zirconia sample was prepared for SEM examination. After gold sputtering, the examination was conducted at a voltage of 40 kV with 1000 magnification. The elemental composition of zirconia surfaces was investigated and analyzed with energy dispersive X-ray spectrometry (EDX) technique.
2.4.
Testing of resin zirconia shear bond strength
Preparation of zirconia/resin composite specimens
After surface treatment, all the specimens were ultrasonically cleansed in 70% ethanol solution for 15 min and then rinsed with deionized water. They were dried with clean air flow and kept in a desiccator for further process. A solution of silane coupling agent (RelyX™ Ceramic Primer, 3M ESPE, Seefeld, Germany) was applied on each specimen. They were left to react and dry for 5 min. Before bonding process, a transparent polyethylene mold in inner diameter of 3.6 mm was fixed on the specimen surface. A self-adhesive resin cement (RelyX™ Unicem Aplicap, 3M ESPE, USA) was properly prepared according to the manufacturer0 s instruction and filled into the mold. The curing procedure was performed for 40 s from the top of the mold and then from the lateral side for 40 s using a light curing unit (Elipar™ 2500, 3M ESPE, USA).
The final results of initial and aging shear bond strength were analyzed with statistical software (SPSS 16.0, SPSS Inc, USA). Two-way ANOVA test was carried out for assessing the influence of different surface treatments and aging methods. One-way ANOVA was performed to evaluate the difference among various coating groups under the same aging conditions and also the difference in the same group with and without aging treatment.
3.
Results
3.1.
Surface roughness
Table 1 shows that the control group had the lowest mean value of surface roughness (0.16370.026 μm) and the highest mean value was obtained in group Z (5.02770.914 μm). All the other experimental groups, including: group S (0.4857 0.107 μm), group G (0.80370.197 μm), and group Si (2.1217 0.393 μm) had higher values than that of the control group. However, the difference between control group and group S (p40.1) as well as that between group S and group G (p40.1) were not significant.
3.2.
Shear bond strength test
Tables 2 and 3 and Fig. 1, in the evaluation of initial bond strength, the control group showed the lowest mean value of
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bond strength could not be tested. On the other hand, after thermal cycling, the mean values of shear bond strength in all the other groups were significantly decreased. Group S exhibited the lowest bond strength values after both two thermal cycling treatments and group Z the highest values. Failure mode analysis is shown in Table 4. The predominant failure mode of control group was adhesive failure which indicated the relatively weak adhesion at zirconia/ resin interface. Moreover, since the control group did not survive the thermal cycling and the testing was not conducted, the corresponding failure modes could not be counted. Most of the specimens in group S, group Si, and group Z showed mixed mode of failure no matter whether aging treatments were carried out. In group G, in fact, most of
zirconia/resin shear bond strength (8.873.4 MPa) while the highest bond strengths were obtained in group S (19.77 1.5 MPa) and group Si (19.272.7 MPa), followed by group G which had a moderate mean value of bond strength (15.97 2.4 MPa) and group Z (14.772.2 MPa). Nonetheless, there was no significant difference between group S and group Si (p40.5), and also not between group G and group Z (p40.5). After being stored in de-ionized water for 30 days, the bond strengths of all groups were observed to decrease. In particular, in the control group, only seven specimens could survive after the 30-day water storage, the other thirteen resin stubs were spontaneously detached from zirconia surface. Except for group G, significant difference could be found between initial bond strength and aging mean value in all the other four groups. Furthermore, all the samples of control group failed (i.e. all resin stubs detached) in the both thermal cycling treatments of 3000 and 6000 cycles and hence the Table 1 – Surface roughness (Ra) of zirconia specimens after different coating treatments (n ¼24). Group
Surface treatment
Mean (μm)
Standard deviation
C S G Si Z
Control Sandblasting Glazingþetching Silica coating Zirconia coating
0.163a 0.485a,b 0.803b 2.121c 5.027d
0.026 0.107 0.197 0.393 0.914
Different superscript letters indicate the significant differences (po0.05).
Fig. 1 – Comparison on mean shear bond strengths (SBS) with and without aging treatment (key: see Table 1).
Table 2 – Analysis of two-way ANOVA for the influence of surface treatment and aging method on shear bond strength. Dependent variable: shear bond strength Source
Type III sum of squares
df
Mean square
F
Sig.
Corrected Model Intercept Treatment Aging Treatment * aging Error Total Corrected total
5003.409a 41048.726 2318.119 2676.884 1038.618 2001.796 68671.905 7005.205
17 1 4 3 10 329 347 346
294.318 41048.726 579.530 892.295 103.862 6.084
48.372 6.746E3 95.247 146.651 17.070
0.000 0.000 0.000 0.000 0.000
a
R squared ¼ 0.714 (adjusted R squared ¼ 0.699).
Table 3 – Mean shear bond strengths (SBS) of five experimental groups with and without aging treatment (n ¼ 20). Group
Initial value
Water storage
Thermal cycling 3000
Thermal cycling 6000
C S G Si Z
8.873.4A a 19.771.5A b 15.972.4A c 19.272.7A b 14.772.2A c
*
N/A 10.673.1C a 14.473.0A b 15.271.7Bb 16.072.4A b
N/A 7.972.7D a 8.373.5Ba,b 10.171.7C b 12.572.0Bc
1.270. 7Ba 15.172.0Bb 14.972.8A b 15.372.1Bb 12.471.8Bc
* There were only 7 specimens in this subgroup. Different superscript capital letters in the same row demonstrate the significant differences. Different subscript lowercase letters in the same column indicate the significant differences (po0.05). (key: see Table 1).
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Table 4 – Percentage of different modes of failure in each group (%). Surface treatment
Aging method
Mode of failure Adhesive
Mixed
Cohesive
25 0 N/A N/A
0 0 N/A N/A
Control
Initial Water storage Thermocycling 3000 Thermocycling 6000
75 100 N/A N/A
Sandblasting
Initial Water storage Thermocycling 3000 Thermocycling 6000
0 10 5 25
65 80 95 70
35 10 0 5
Glazingþetching
Initial Water storage Thermocycling 3000 Thermocycling 6000
100 100 95 100
0 0 0 0
0 0 5 0
Silica coating
Initial Water storage Thermocycling 3000 Thermocycling 6000
5 5 10 5
85 80 90 95
10 15 0 0
Zirconia coating
Initial Water storage Thermocycling 3000 Thermocycling 6000
15 25 0 20
75 75 95 75
10 0 5 5
the failures happened at zirconia/porcelain interface under all of the aging conditions. Therefore, the failure mode of this group might not present the real state of zirconia/resin integration.
3.3.
Observation of surface morphology
The surface morphology of zirconia surface after being treated with different methods was shown in Fig. 2. In the control group, only some shallow grooves and pits which were due to the polishing process could be found on the surface. After sandblasting, some of the surface materials were removed and resulted in the formation of irregularities. Some surface flaws could be observed. In group G, a layer of amorphous porcelain structure with some pits and holes was noted. Silica particle coating resulted in the deposition of a rough silica crystalline layer. Zirconia coating also led to the generation of rough surface and in addition, some small cracks in the coating layer were found.
3.4.
Analysis of surface elements
The results of elemental analysis on zirconia surfaces modified with various methods are demonstrated in Fig. 3. On the zirconia surface without any modification, only zirconium, oxygen, and a small amount of hafnium could be found. After sandblasting, aluminum and silicon content derived from silica coated alumina powder were increased. For the group G, the glass compositions, consisting of calcium, potassium, aluminum, and barium were detected. Although being etched with an acid gel, substantial silicon content still remained on the zirconia surface which might be valid for the bonding
with a silane coupling agent. In group Si, a large amount of oxygen and silicon content remained on zirconia surface. The elemental constitution of group Z was similar to that of control group.
4.
Discussion
Application of silane coupling agents is considered to be an important approach for achieving a strong and durable bonding between zirconia base structure and resin cement (Lung and Matinlinna, 2010). The hydrolysis of methoxy groups in silane molecule leads to the formation of silanol groups which subsequently react with the surface hydroxyl groups on silica content. The vinyl groups in silane molecule can also combine with the corresponding organic sites, e.g. methacrylate group, in resin. Thus, the silane coupling agents act as the linkage between inorganic surface and organic resin composite (Lung and Matinlinna, 2012). However, due to the weakening effect of water on siloxane bonds at the bonding interface, resin to zirconia bond strength will be significantly decreased over time in oral environment (Lung and Matinlinna, 2012). Silanization with some newly developed silane chemicals was also reported to promote the resistance of zirconia/resin interface to aging environment (Matinlinna et al., 2012; Matinlinna and Lassila, 2011). The stability of zirconia/resin interface under hydrolytic conditions can be improved by the adoption of phosphate monomer-containing primers or corresponding resin cements. RelyX Unicem self-adhesive resin cement contains 40–50 wt% phosphorulated methacrylates and was reported to produce bond strengths similar to those obtained with MDP-containing resin cements under laboratory conditions (Thompson et al., 2011). It was found in our study that after being subjected to
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Fig. 2 – Results of SEM examination (original magnification 1000 ) on zirconia surfaces modified with different methods: (A) control; (B) sandblasting; (C) glazingþetching; (D) silica coating; and (E) zirconia coating.
aging treatments, the bond strengths in different experimental groups were still higher than 5 MPa which is a minimum requirement set by an ISO standard (ISO, 2004). Nonetheless, according to the results of the control group in the current study, it may still be difficult to get a stable zirconia/resin adhesion without appropriate surface treatment since most of the specimens in control group could not withstand the aging process. The linkage functions of silane coupling agent rely on the existence of adequate reactive hydroxyl groups on zirconia surface, the attachment of silica content on zirconia surface will be one of the effective chemical modifications to facilitate the function of silane coupling agents (Ho and Matinlinna, 2011a, 2011b; Lung et al., 2010). Therefore, some new methods aiming at adding silica content were brought forward in recent years. Sandblasting with 50–110 μm alumina particles was reported to roughen and cleanse zirconia surface effectively. When the appropriate coupling agent and resin cement had been adopted, an improved resin to zirconia bonding could be obtained even with the pressure of sandblasting was reduced
to 0.5 bar (Kern et al., 2009). Tribochemical approach by sandblasting with silica coated alumina powder has also been of intensive interest in the dental research community. It is considered that the attached silica content with free hydroxyl groups on zirconia surface would establish chemical adhesion and produce a positive effect on zirconia/resin cement integration (Della Bona et al., 2007; Heikkinen et al., 2007; Nothdurft et al., 2009) which has also been proven in our study. It was shown in group S that the mean initial shear bond strength has been doubled with this tribochemical approach. However, after aging treatments the mean shear bond value of the sandblasting group was significantly reduced, though still higher than that of the control group. This might be due to the loosening of attached silica and the hydrolytic influence on the siloxane interfacial layer. In fact, the relatively unpredictable resin to zirconia adhesion through such a tribochemical approach was reported in a one-year clinical trial which evaluated the performance of inlay-retained fixed partial dentures. It was found that six out
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Fig. 3 – Results of EDX spectra examination on zirconia surfaces treated with different methods: (A) control; (B) sandblasting; (C) glazingþetching; (D) silica coating; and (E) zirconia coating.
of thirty inlay-retained FPDs debonded after one-year followup (Ohlmann et al., 2008). On the other hand, the influence of sandblasting on mechanical properties of zirconia has also been evaluated in some studies. It was claimed in one clinical investigation on the performance of resin-bonded singleretainer FPDs that the sandblasted zirconia-based restorations still had a 100% survival rate after a 5-year follow-up (Sasse and Kern, 2013). Another in vitro study reported that the application of sandblasting could increase the flexural strength of zirconia (Guazzato et al., 2005). However, the concern about the influence of sandblasting on the stability of zirconia structure should not be neglected since the new flaws on zirconia surface induced by the sandblasting process were discovered (Zhang et al., 2004). When being put into practical use, these surface flaws may become the positions of stress concentration and accelerate the development of failures in the practical applications (Kosmac et al., 2000). The transformation from the tetragonal to monoclinic phase of some surface particles may also significantly influence the long-term reliability of zirconia-based restorations (Monaco et al., 2012). Furthermore, a reduced Weibull modulus of sandblasted zirconia might also indicate the strength degradation and the potential risk of unpredictable failure over time (Karakoca and Yilmaz, 2009; Kosmac et al., 1999). Consequently, we may say that despite the tribochemical technique was proven to be an effective pre-treatment method, some new methods causing less adverse influence on zirconia base structure merit investigation.
Another new coating method combining the veneering of one thin glazing porcelain layer and acid etching has been developed in recent years. The veneered low-fusion porcelain was supposed to provide well bonded glass content as the chemical basis for acid etching along with the chemical functions of silane coupling agents (Cura et al., 2012; Kitayama et al., 2009). However, adding one more bonding interface between zirconia and resin cement may also increase the risk of adhesion failure. Although the zirconia/ glazing porcelain bond strength was supposed to be higher than that between zirconia and resin, the analysis of failure in group G showed that almost all the delamination occurred at the zirconia/glazed layer interface which might indicate the relatively weak bonding between zirconia and glazing porcelain. The delamination might be related to the misfit in their coefficients of thermal expansion (CTE). The CTE of zirconia and glazing porcelain are 10.5 μm/m K and 8.0– 8.6 μm/m K, respectively. Furthermore, unlike in other studies reported, sandblasting before veneering procedure which sought to enhance zirconia/porcelain integration was not adopted in this study since the influence of air-abrasion was intended to be avoided (Liu et al., 2013). In another study, the glazing porcelain was applied after one layer of liner porcelain had been fired on zirconia surface, such treatment significantly enhanced the effects glazing treatment (Valentino et al., 2012). However, veneering of two porcelain layers will lead to the difficulties in thickness control and influence the fitness of zirconia-based
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restorations. Thus, a specially designed porcelain material for zirconia surface coating with improved adhesion ability and better thickness control might help to resolve the problem. Studies on silica coatings were reported and the use of some other approaches, such as sol–gel method, plasma spraying, and vapor deposition was suggested. In one study, the sol–gel method was carried out by hydrolyzing tetraethyl orthosilicate (TEOS) in water with ammonia as a catalyst. The effect of silica deposition by this process was found to be inferior to that through sandblasting treatment (Lung et al., 2013). Despite TEOS generates SiO2 in alkaline conditions, such a negative influence might be due to the weak zirconia/ silica integration. The vapor deposition with silicon tetrachloride (SiCl4) and water was reported to produce a silicalike coating layer on zirconia surface. The generation of such a silica-like coating technique with thickness control also helped to strengthen resin to zirconia bonding (Piascik et al., 2009). Different from the previous studies, in the current study, the silica coating slurry was applied directly on zirconia before the final sintering. The final sintering process was carried out at the temperature of 1350 1C according to the manufacturer0 s instructions and it was supposed to provide sufficient energy for the establishment of strong zirconia/ silica adhesion. However, whether there was a chemical reaction between SiO2 and zirconia surface at this temperature should be further clarified. Compared with the other afore-mentioned coating approaches, the current formulated new method is relatively easier and also time-saving. Whereas, the thickness of silica coating layer was not specified and the influence on the mechanical properties has not been evaluated. Given this, the influence of silica coating materials with different particle sizes is still unknown. Therefore, further study is still necessary to obtain the optimized setting of this coating method and interpreting the possible chemical changes. Coating of zirconia particles was stated not only to strengthen porcelain to zirconia bonding (Teng et al., 2012) but also to increase resin to zirconia bond strength (Aboushelib, 2012). In the present study, the coating procedure was performed by spreading a slurry that contains zirconia particles on pre-sintered zirconia. The following final sintering process would fix the zirconia coating particles on zirconia base structure. By doing this, the surface roughness of zirconia has been greatly increased. Moreover, the binding of coated zirconia particles on zirconia disc was supposed to be superior to that between the two different materials. The improvement in zirconia/resin bond strength was ascribed to the creation of topographical structures on zirconia surface for micro-mechanical interlocking. However, chemical integration seems not substantiated with this treatment. Since only the effects of mechanical mechanism were enhanced, the zirconia surface has not yet been completely optimized. This might explain why the highest values of surface roughness in group Z did not result in the strongest initial shear bond strength. Although after thermal cycling for 3000 cycles, the resultant resin to zirconia shear bond strength was slightly higher than the initial value, but such a difference was in fact not significant. It might still demonstrate the relatively higher resistance of this bonding approach to thermal cycling. Such an improved adhesion might be due to the establishment of stronger micro-retentive interfacial structures. Another study
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evaluated the use of flowable resin cement containing zirconia and silica fillers as coating material on fully sintered zirconia (Chen et al., 2012). It was found that after firing, zirconia surface was covered by a zirconia–silica mixture layer. With the combination of silanization process, the zirconia/resin bond strength and resistance to aging conditions had been significantly increased. The increase in the silica content might play an important role in this process. Such a zirconia–silica coating might be worthy for further investigation. According to the comparison among the initial values of different test groups, it was found that the higher value of surface roughness did not necessarily induce the generation of higher resin to zirconia bond strength which could be explained by the absence of chemical bonding. However, rougher surfaces seemed to improve the resistance to aging effects, especially the influence of thermal cycling. Higher surface roughness was less influenced by the thermal cycling. From the current results of aging treatments, including water storage and thermal cycling, all the experimental groups demonstrated comparable stability in the testing of resin to zirconia bonding except for the samples in control group. Nonetheless, after the thermal cycling for both 3000 and 6000 cycles, group Z had significantly higher shear bond strength compared with all the other groups. The coating with zirconia particles had not only created the micro-structures for mechanical interlocking but also greatly increased the area of actual bonding interface due to the highest Ra achieved. Although chemical bonding might be established by the attachment of silicon content in the other groups, the mean values of surface roughness were much lower than that in group Z and the actual area of bonding interface was smaller. Thus, in group Z, the infiltration of water fully into the resin zirconia bonding interface would become much more difficult. This said, the real situation under practical conditions is still unknown and further investigation is necessary. In the current study four coating methods using different materials and distributing methods have been contrasted and examined. They were proved to enhance resin zirconia bonding to certain extent. However, some remaining problems, such as thickness control, coating quality, and material strengthening should be optimized. The performance of resin zirconia integration under more stringent aging conditions, such as longer water storage up to one year, and more cycles in thermal cycling should be evaluated.
5.
Conclusions
Within the limitations of this laboratory study, the following conclusions were drawn: surface pre-treatment is necessary for producing reliable integration between zirconia and resin cement. All the tested coating methods increased zirconia/resin bond strength and the aging resistance of the adhesive interface.
r e f e r e nc e s
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Available online at www.sciencedirect.com
www.elsevier.com/locate/jmbbm
Research Paper
Evaluation of four surface coating treatments for resin to zirconia bonding Dan Liu, Edmond H.N. Pow, James Kit-Hon Tsoi, Jukka P. Matinlinnan Dental Materials Science, Faculty of Dentistry, The University of Hong Kong, Hong Kong SAR, PR China
ar t ic l e in f o
abs tra ct
Article history:
Objectives: To compare the effects of four surface coating methods on resin to zirconia
Received 25 August 2013
shear bond strength.
Received in revised form
Material and methods: Eighty pre-sintered zirconia discs were prepared and randomly
9 December 2013
divided into five study groups according to the corresponding methods of surface treat-
Accepted 10 December 2013
ments as follows: group C (control group, fully sintered without any surface treatment),
Available online 24 December 2013
group S (fully sintered and then sandblasted with silica coated alumina powder), group G
Keywords:
(fully sintered and then coated with glazing porcelain followed by acid etching), group Si
Surface treatment
(pre-coated with silica slurry then fully sintered), and group Z (coated with zirconia
Adhesion
particles and then fully sintered). The observation of surface morphology and elemental
Surface coating
composition analysis were conducted by SEM and EDX. Self-adhesive resin cement stubs
Resin cement
(diameter 3.6 mm and height 3 mm) were then bonded on the zirconia discs with a
Zirconia
cylindrical shape. Both initial and artificial aged (including 30-day water storage, thermal
Sandblasting
cycling for 3000 and 6000 cycles) shear bond strengths were then evaluated. Results: All the tested coating methods showed significantly higher shear bond strengths than the control group, in both dry and aged conditions. Group S produced the strongest initial zirconia/resin bonding (19.7 MPa) and the control group had the lowest value (8.8 MPa). However, after thermal cycling, group Z exhibited the highest mean value. All the samples in the control group failed in the thermal cycling. Both different coating methods (po0.001) and various aging treatments (po0.001) produced significant influence on resin–zirconia shear bond strength. Conclusions: A reliable and durable resin zirconia bonding is vital for the longevity of dental restorations. Silica coating might be a reliable way in enhancing adhesion between resin and zirconia. & 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
Nowadays, the application of zirconia as a base material in the production of all-ceramic restorations has become one of the
major foci in dental research. Such increase in the interest is ascribed to its high mechanical strength and exceptional biocompatibility (Liu et al., 2012; Manicone et al., 2007; Piconi and Maccauro, 1999). The success of zirconia-based all-ceramic
n Correspondence to: Dental Materials Science, Faculty of Dentistry, University of Hong Kong, 34 Hospital Road, Sai Ying Pun, Hong Kong, PR China. Tel.: þ852 28590380; fax: þ852 25489464. E-mail address: [email protected] (J.P. Matinlinna).
1751-6161/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jmbbm.2013.12.011
journal of the mechanical behavior of biomedical materials 32 (2014) 300 –309
restorations is highly dependent on the establishment of a strong adhesion between zirconia and luting cement. Compared with traditional luting cements, such as zinc phosphate, glass ionomer cement etc, resin cement has some irreplaceable advantages including higher mechanical strength and better esthetic properties (Piwowarczyk et al., 2005). However, without any surface treatment, the resin zirconia integration was found to be susceptible to aging conditions (Özcan et al., 2008). Meanwhile, the conventional bonding approaches, such as acid etching followed by the application of silane coupling agents, could not effectively improve the bond strength between zirconia copings and resin cement due to the chemical inertness of zirconia (Ho and Matinlinna, 2011a, 2011b). There is no inherent glass content in the matrix of zirconia abutment. Thus, zirconia base structures cannot be etched with commonly used mineral (or organic) acids, such as HF and H3PO4, for adding the surface roughness. Furthermore, it is also very cumbersome to form a strong chemical integration between zirconia coping and resin cement by using solely the conventional silane coupling agents (Kern and Wegner, 1998; Komine et al., 2012; Özcan and Vallittu, 2003). Air-abrasion with alumina particles followed by an appropriate chemical bonding process was recommended to achieve long-term retention to zirconia (Kern, 2009). The incorporation of 10-methacryloxydecyldihydrogenphosphate (MDP) in primers or resin cements was a vital factor in producing durable resin zirconia bonding which has already been confirmed in the related clinical trials (Abou Tara et al., 2011; Sasse and Kern, 2013). Other surface treatments, such as the tribochemical method, selective infiltration etching, heating with a hot etching solution, laser surface treatment, plasma treatment and surface fluorination, have been developed for enhancing resin zirconia bonding (Heikkinen et al., 2009). Among these methods, sandblasting with silica coated alumina particles, combined with the application of MDP containing primer/resin cement has been recommended as one of the most effective methods (Atsu et al., 2006; Tanaka et al., 2008). This tribochemical method has been proven not only to increase the values of surface roughness, but also to add silicon (Si) content on zirconia surface. Silicon content is vital for activating the functions of silane coupling agents. Both mechanical interlocking and chemical integration between resin cement and zirconia have thus been enhanced (Peutzfeldt and Asmussen, 1988). However, there are still concerns about the influence of sandblasting on mechanical properties and long-term stability of zirconia base because sandblasting has been reported to induce some flaws on the surface (Zhang et al., 2004). The generation of such flaws might produce some detrimental effects on the reliability of zirconia substructures (Kosmač et al., 1999). For the other methods, some of them are time-consuming and not suitable for clinical application. Some protocols use very aggressive acids that may be detrimental to the health of operators. Further research has being performed for developing innovative approaches with more convenient and safe procedures. Coating treatments on zirconia surface might also be regarded as one of the solutions to the current problems. For instance, it was reported that the application of a socalled glaze-on technique which is composed of the veneering of a thin layer of glazing porcelain on zirconia surface and
301
the follow-on acid etching could result in the enhanced resin to zirconia shear bond strength. The addition of Si containing porcelain provided the necessary foundation for the establishment of chemical integration (Everson et al., 2011). Another study stated that the attachment of silicon dioxide (SiO2) layer on zirconia surface with vapor-phase deposition method could provide functional sites for the use of silane coupling agents and improve resin zirconia adhesion subsequently (Piascik et al., 2009). Coating with zirconia particles also has helped to improve the resistance to the aging effects of water content at the zirconia/resin interface (Aboushelib, 2012; Aboushelib et al., 2009). The aim of this study was to evaluate the differences between four direct zirconia surface coating methods and the effects on the shear bond strength between zirconia and resin cement. The hypothesis of the study was that the tested surface coating methods would produce the same effects on resin to zirconia bonding.
2.
Materials and methods
2.1. Preparation and grouping of pre-sintered zirconia specimens Eighty pre-sintered zirconia discs were cut from commercial dental zirconia ingots (Cercon base, DeguDent GmbH, Hanau, Germany) using a low speed diamond saw (Microslice, Metal Research limited company, England) under running water. Each disc was 25 mm in diameter and 1.5 mm in thickness. All the zirconia specimens were then polished with 1000-grit SiC abrasive paper on a polishing platform (Lumn Major, Struers, Denmark). They were randomly divided into five groups (C, S, G, Si, and Z) according to the corresponding methods of surface treatment: Group C (control): the specimens in this group were densely sintered in a furnace (Cercon heat, DeguDent GmbH, Hanau, Germany) according to the manufacturer0 s instructions (maximum temperature of 1350 1C, and a sintering cycle of 6 h). They were not treated with any surface treatment. Group S (sandblasting): in this group, the zirconia specimens were also densely sintered (using the same sintering program as group C) and followed by sandblasting. The process of sandblasting was carried out by an air-abrasion machine (Shofu Pen-Blaster, Shofu Dental Corporation, Kyoto, Japan) with 110-μm silica coated alumina powder (Rocatecs, 3M ESPE, Seefeld, Germany) at a constant pressure of 3.5 bar for 10 s. The working distance was 10 mm and directly perpendicular to zirconia surface. Group G (glazing porcelain coatingþacid etching): the slurry of glazing porcelain (Cercon Ceram Kiss Glaze, DeguDent, Germany) was prepared by mixing the glazing powders with corresponding modeling liquid. The zirconia specimens were fully sintered (using the same sintering program as group C) and then one thin layer of porcelain slurry was applied on the surfaces of each fully sintered zirconia with a clean brush and lightly vibrated. The specimens were then fired in a porcelain furnace according to the manufacturer0 s instructions. After being cooled to room temperature, each glazed zirconia surface was treated with an acid etching gel
302
journal of the mechanical behavior of biomedical materials 32 (2014) 300 –309
(Vita Ceramics Etch, Vita Zahnfabrik, Germany) containing 5 wt% HF and 10 wt% H2SO4 for 120 s and then rinsed thoroughly with deionized water. Group Si (silica particle coating): before final sintering, a slurry comprised of 0.02 g silica powder (5–12 μm) with 0.08 g resin monomer (UEDMA, Esstech, Essington, USA), and it was distributed evenly on each pre-sintered zirconia disc. All the specimens were then fully sintered according to the instructions of manufacturer (using the same sintering program as group C). Group Z (zirconia particle coating): each pre-sintered zirconia disc was coated with the mixture which was prepared by mixing 0.032 g Y-TZP powders (the same zirconia material in a amorphous powder form) with 0.08 g UEDMA monomer and then fully sintered (using the same sintering program as group C).
2.2.
Measurement of surface roughness
Twenty four specimens in each group were randomly selected and measured the surface roughness values after surface treatment. The mean value of each group was then calculated. The measurement was carried out after calibration on a flat surface using a profilometer (Surtronic 3þ, Taylor Hobson, UK) with the cut-off value set at 0.8 mm. The value of Ra was chosen as the indication of surface roughness. Rougher surface was indicated by the higher Ra value. Each sample was measured for three times and the mean value was calculated. Thus, there were twenty-four measurements of surface roughness in each group.
After curing, the mold was then removed with care. Each cured resin stub was 3.6 mm in diameter and 3 mm in thickness. Five resin stubs were adhered on each zirconia disc. All the specimens were kept at room temperature for 24 h before further process. There were 80 zirconia/resin composite specimens in each group of surface treatment.
2.5.
In each group, eighty specimens were further divided into four sub-groups (there were 20 samples in each sub-group): one was tested with initial shear bond strength, one was evaluated after being stored in 37 1C water for 30 days, one was tested after thermal cycling (5–55 1C) for 3000 cycles, and the last one was treated with 6000 thermal cycles. In the testing of shear bond strength, each sample was mounted on a universal testing machine (ElectroPuls™ E3000, Instron, USA) with a special designed metal jig and loaded until failure happened. The speed of crosshead was 1 mm/min. The shear bond strength of each specimen was calculated by dividing the failure load with the bonding area. The failure mode in the current study was defined and classified according to the appearance of detached zirconia/resin interface: (1) adhesive failure, less than 20% resin cement was left on zirconia surface; (2) mixed mode of failure, the remaining resin was more than 20% and less than 80%; and (3) cohesive failure, more than 80% resin cement remained. The assessment was carried out by using an optical microscope (Leica MZ6, Leica Microsystems, USA).
2.6. 2.3.
Statistical analysis
Scanning electron microscopy (SEM) observation
A scanning electron microscope (Hitachi S-3400 N, Hitachi High-Technologies Corporation, Tokyo, Japan) was used to evaluate the changes in surface morphology of zirconia samples treated with different methods. In each group, one additional zirconia sample was prepared for SEM examination. After gold sputtering, the examination was conducted at a voltage of 40 kV with 1000 magnification. The elemental composition of zirconia surfaces was investigated and analyzed with energy dispersive X-ray spectrometry (EDX) technique.
2.4.
Testing of resin zirconia shear bond strength
Preparation of zirconia/resin composite specimens
After surface treatment, all the specimens were ultrasonically cleansed in 70% ethanol solution for 15 min and then rinsed with deionized water. They were dried with clean air flow and kept in a desiccator for further process. A solution of silane coupling agent (RelyX™ Ceramic Primer, 3M ESPE, Seefeld, Germany) was applied on each specimen. They were left to react and dry for 5 min. Before bonding process, a transparent polyethylene mold in inner diameter of 3.6 mm was fixed on the specimen surface. A self-adhesive resin cement (RelyX™ Unicem Aplicap, 3M ESPE, USA) was properly prepared according to the manufacturer0 s instruction and filled into the mold. The curing procedure was performed for 40 s from the top of the mold and then from the lateral side for 40 s using a light curing unit (Elipar™ 2500, 3M ESPE, USA).
The final results of initial and aging shear bond strength were analyzed with statistical software (SPSS 16.0, SPSS Inc, USA). Two-way ANOVA test was carried out for assessing the influence of different surface treatments and aging methods. One-way ANOVA was performed to evaluate the difference among various coating groups under the same aging conditions and also the difference in the same group with and without aging treatment.
3.
Results
3.1.
Surface roughness
Table 1 shows that the control group had the lowest mean value of surface roughness (0.16370.026 μm) and the highest mean value was obtained in group Z (5.02770.914 μm). All the other experimental groups, including: group S (0.4857 0.107 μm), group G (0.80370.197 μm), and group Si (2.1217 0.393 μm) had higher values than that of the control group. However, the difference between control group and group S (p40.1) as well as that between group S and group G (p40.1) were not significant.
3.2.
Shear bond strength test
Tables 2 and 3 and Fig. 1, in the evaluation of initial bond strength, the control group showed the lowest mean value of
journal of the mechanical behavior of biomedical materials 32 (2014) 300 –309
303
bond strength could not be tested. On the other hand, after thermal cycling, the mean values of shear bond strength in all the other groups were significantly decreased. Group S exhibited the lowest bond strength values after both two thermal cycling treatments and group Z the highest values. Failure mode analysis is shown in Table 4. The predominant failure mode of control group was adhesive failure which indicated the relatively weak adhesion at zirconia/ resin interface. Moreover, since the control group did not survive the thermal cycling and the testing was not conducted, the corresponding failure modes could not be counted. Most of the specimens in group S, group Si, and group Z showed mixed mode of failure no matter whether aging treatments were carried out. In group G, in fact, most of
zirconia/resin shear bond strength (8.873.4 MPa) while the highest bond strengths were obtained in group S (19.77 1.5 MPa) and group Si (19.272.7 MPa), followed by group G which had a moderate mean value of bond strength (15.97 2.4 MPa) and group Z (14.772.2 MPa). Nonetheless, there was no significant difference between group S and group Si (p40.5), and also not between group G and group Z (p40.5). After being stored in de-ionized water for 30 days, the bond strengths of all groups were observed to decrease. In particular, in the control group, only seven specimens could survive after the 30-day water storage, the other thirteen resin stubs were spontaneously detached from zirconia surface. Except for group G, significant difference could be found between initial bond strength and aging mean value in all the other four groups. Furthermore, all the samples of control group failed (i.e. all resin stubs detached) in the both thermal cycling treatments of 3000 and 6000 cycles and hence the Table 1 – Surface roughness (Ra) of zirconia specimens after different coating treatments (n ¼24). Group
Surface treatment
Mean (μm)
Standard deviation
C S G Si Z
Control Sandblasting Glazingþetching Silica coating Zirconia coating
0.163a 0.485a,b 0.803b 2.121c 5.027d
0.026 0.107 0.197 0.393 0.914
Different superscript letters indicate the significant differences (po0.05).
Fig. 1 – Comparison on mean shear bond strengths (SBS) with and without aging treatment (key: see Table 1).
Table 2 – Analysis of two-way ANOVA for the influence of surface treatment and aging method on shear bond strength. Dependent variable: shear bond strength Source
Type III sum of squares
df
Mean square
F
Sig.
Corrected Model Intercept Treatment Aging Treatment * aging Error Total Corrected total
5003.409a 41048.726 2318.119 2676.884 1038.618 2001.796 68671.905 7005.205
17 1 4 3 10 329 347 346
294.318 41048.726 579.530 892.295 103.862 6.084
48.372 6.746E3 95.247 146.651 17.070
0.000 0.000 0.000 0.000 0.000
a
R squared ¼ 0.714 (adjusted R squared ¼ 0.699).
Table 3 – Mean shear bond strengths (SBS) of five experimental groups with and without aging treatment (n ¼ 20). Group
Initial value
Water storage
Thermal cycling 3000
Thermal cycling 6000
C S G Si Z
8.873.4A a 19.771.5A b 15.972.4A c 19.272.7A b 14.772.2A c
*
N/A 10.673.1C a 14.473.0A b 15.271.7Bb 16.072.4A b
N/A 7.972.7D a 8.373.5Ba,b 10.171.7C b 12.572.0Bc
1.270. 7Ba 15.172.0Bb 14.972.8A b 15.372.1Bb 12.471.8Bc
* There were only 7 specimens in this subgroup. Different superscript capital letters in the same row demonstrate the significant differences. Different subscript lowercase letters in the same column indicate the significant differences (po0.05). (key: see Table 1).
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journal of the mechanical behavior of biomedical materials 32 (2014) 300 –309
Table 4 – Percentage of different modes of failure in each group (%). Surface treatment
Aging method
Mode of failure Adhesive
Mixed
Cohesive
25 0 N/A N/A
0 0 N/A N/A
Control
Initial Water storage Thermocycling 3000 Thermocycling 6000
75 100 N/A N/A
Sandblasting
Initial Water storage Thermocycling 3000 Thermocycling 6000
0 10 5 25
65 80 95 70
35 10 0 5
Glazingþetching
Initial Water storage Thermocycling 3000 Thermocycling 6000
100 100 95 100
0 0 0 0
0 0 5 0
Silica coating
Initial Water storage Thermocycling 3000 Thermocycling 6000
5 5 10 5
85 80 90 95
10 15 0 0
Zirconia coating
Initial Water storage Thermocycling 3000 Thermocycling 6000
15 25 0 20
75 75 95 75
10 0 5 5
the failures happened at zirconia/porcelain interface under all of the aging conditions. Therefore, the failure mode of this group might not present the real state of zirconia/resin integration.
3.3.
Observation of surface morphology
The surface morphology of zirconia surface after being treated with different methods was shown in Fig. 2. In the control group, only some shallow grooves and pits which were due to the polishing process could be found on the surface. After sandblasting, some of the surface materials were removed and resulted in the formation of irregularities. Some surface flaws could be observed. In group G, a layer of amorphous porcelain structure with some pits and holes was noted. Silica particle coating resulted in the deposition of a rough silica crystalline layer. Zirconia coating also led to the generation of rough surface and in addition, some small cracks in the coating layer were found.
3.4.
Analysis of surface elements
The results of elemental analysis on zirconia surfaces modified with various methods are demonstrated in Fig. 3. On the zirconia surface without any modification, only zirconium, oxygen, and a small amount of hafnium could be found. After sandblasting, aluminum and silicon content derived from silica coated alumina powder were increased. For the group G, the glass compositions, consisting of calcium, potassium, aluminum, and barium were detected. Although being etched with an acid gel, substantial silicon content still remained on the zirconia surface which might be valid for the bonding
with a silane coupling agent. In group Si, a large amount of oxygen and silicon content remained on zirconia surface. The elemental constitution of group Z was similar to that of control group.
4.
Discussion
Application of silane coupling agents is considered to be an important approach for achieving a strong and durable bonding between zirconia base structure and resin cement (Lung and Matinlinna, 2010). The hydrolysis of methoxy groups in silane molecule leads to the formation of silanol groups which subsequently react with the surface hydroxyl groups on silica content. The vinyl groups in silane molecule can also combine with the corresponding organic sites, e.g. methacrylate group, in resin. Thus, the silane coupling agents act as the linkage between inorganic surface and organic resin composite (Lung and Matinlinna, 2012). However, due to the weakening effect of water on siloxane bonds at the bonding interface, resin to zirconia bond strength will be significantly decreased over time in oral environment (Lung and Matinlinna, 2012). Silanization with some newly developed silane chemicals was also reported to promote the resistance of zirconia/resin interface to aging environment (Matinlinna et al., 2012; Matinlinna and Lassila, 2011). The stability of zirconia/resin interface under hydrolytic conditions can be improved by the adoption of phosphate monomer-containing primers or corresponding resin cements. RelyX Unicem self-adhesive resin cement contains 40–50 wt% phosphorulated methacrylates and was reported to produce bond strengths similar to those obtained with MDP-containing resin cements under laboratory conditions (Thompson et al., 2011). It was found in our study that after being subjected to
journal of the mechanical behavior of biomedical materials 32 (2014) 300 –309
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Fig. 2 – Results of SEM examination (original magnification 1000 ) on zirconia surfaces modified with different methods: (A) control; (B) sandblasting; (C) glazingþetching; (D) silica coating; and (E) zirconia coating.
aging treatments, the bond strengths in different experimental groups were still higher than 5 MPa which is a minimum requirement set by an ISO standard (ISO, 2004). Nonetheless, according to the results of the control group in the current study, it may still be difficult to get a stable zirconia/resin adhesion without appropriate surface treatment since most of the specimens in control group could not withstand the aging process. The linkage functions of silane coupling agent rely on the existence of adequate reactive hydroxyl groups on zirconia surface, the attachment of silica content on zirconia surface will be one of the effective chemical modifications to facilitate the function of silane coupling agents (Ho and Matinlinna, 2011a, 2011b; Lung et al., 2010). Therefore, some new methods aiming at adding silica content were brought forward in recent years. Sandblasting with 50–110 μm alumina particles was reported to roughen and cleanse zirconia surface effectively. When the appropriate coupling agent and resin cement had been adopted, an improved resin to zirconia bonding could be obtained even with the pressure of sandblasting was reduced
to 0.5 bar (Kern et al., 2009). Tribochemical approach by sandblasting with silica coated alumina powder has also been of intensive interest in the dental research community. It is considered that the attached silica content with free hydroxyl groups on zirconia surface would establish chemical adhesion and produce a positive effect on zirconia/resin cement integration (Della Bona et al., 2007; Heikkinen et al., 2007; Nothdurft et al., 2009) which has also been proven in our study. It was shown in group S that the mean initial shear bond strength has been doubled with this tribochemical approach. However, after aging treatments the mean shear bond value of the sandblasting group was significantly reduced, though still higher than that of the control group. This might be due to the loosening of attached silica and the hydrolytic influence on the siloxane interfacial layer. In fact, the relatively unpredictable resin to zirconia adhesion through such a tribochemical approach was reported in a one-year clinical trial which evaluated the performance of inlay-retained fixed partial dentures. It was found that six out
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Fig. 3 – Results of EDX spectra examination on zirconia surfaces treated with different methods: (A) control; (B) sandblasting; (C) glazingþetching; (D) silica coating; and (E) zirconia coating.
of thirty inlay-retained FPDs debonded after one-year followup (Ohlmann et al., 2008). On the other hand, the influence of sandblasting on mechanical properties of zirconia has also been evaluated in some studies. It was claimed in one clinical investigation on the performance of resin-bonded singleretainer FPDs that the sandblasted zirconia-based restorations still had a 100% survival rate after a 5-year follow-up (Sasse and Kern, 2013). Another in vitro study reported that the application of sandblasting could increase the flexural strength of zirconia (Guazzato et al., 2005). However, the concern about the influence of sandblasting on the stability of zirconia structure should not be neglected since the new flaws on zirconia surface induced by the sandblasting process were discovered (Zhang et al., 2004). When being put into practical use, these surface flaws may become the positions of stress concentration and accelerate the development of failures in the practical applications (Kosmac et al., 2000). The transformation from the tetragonal to monoclinic phase of some surface particles may also significantly influence the long-term reliability of zirconia-based restorations (Monaco et al., 2012). Furthermore, a reduced Weibull modulus of sandblasted zirconia might also indicate the strength degradation and the potential risk of unpredictable failure over time (Karakoca and Yilmaz, 2009; Kosmac et al., 1999). Consequently, we may say that despite the tribochemical technique was proven to be an effective pre-treatment method, some new methods causing less adverse influence on zirconia base structure merit investigation.
Another new coating method combining the veneering of one thin glazing porcelain layer and acid etching has been developed in recent years. The veneered low-fusion porcelain was supposed to provide well bonded glass content as the chemical basis for acid etching along with the chemical functions of silane coupling agents (Cura et al., 2012; Kitayama et al., 2009). However, adding one more bonding interface between zirconia and resin cement may also increase the risk of adhesion failure. Although the zirconia/ glazing porcelain bond strength was supposed to be higher than that between zirconia and resin, the analysis of failure in group G showed that almost all the delamination occurred at the zirconia/glazed layer interface which might indicate the relatively weak bonding between zirconia and glazing porcelain. The delamination might be related to the misfit in their coefficients of thermal expansion (CTE). The CTE of zirconia and glazing porcelain are 10.5 μm/m K and 8.0– 8.6 μm/m K, respectively. Furthermore, unlike in other studies reported, sandblasting before veneering procedure which sought to enhance zirconia/porcelain integration was not adopted in this study since the influence of air-abrasion was intended to be avoided (Liu et al., 2013). In another study, the glazing porcelain was applied after one layer of liner porcelain had been fired on zirconia surface, such treatment significantly enhanced the effects glazing treatment (Valentino et al., 2012). However, veneering of two porcelain layers will lead to the difficulties in thickness control and influence the fitness of zirconia-based
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restorations. Thus, a specially designed porcelain material for zirconia surface coating with improved adhesion ability and better thickness control might help to resolve the problem. Studies on silica coatings were reported and the use of some other approaches, such as sol–gel method, plasma spraying, and vapor deposition was suggested. In one study, the sol–gel method was carried out by hydrolyzing tetraethyl orthosilicate (TEOS) in water with ammonia as a catalyst. The effect of silica deposition by this process was found to be inferior to that through sandblasting treatment (Lung et al., 2013). Despite TEOS generates SiO2 in alkaline conditions, such a negative influence might be due to the weak zirconia/ silica integration. The vapor deposition with silicon tetrachloride (SiCl4) and water was reported to produce a silicalike coating layer on zirconia surface. The generation of such a silica-like coating technique with thickness control also helped to strengthen resin to zirconia bonding (Piascik et al., 2009). Different from the previous studies, in the current study, the silica coating slurry was applied directly on zirconia before the final sintering. The final sintering process was carried out at the temperature of 1350 1C according to the manufacturer0 s instructions and it was supposed to provide sufficient energy for the establishment of strong zirconia/ silica adhesion. However, whether there was a chemical reaction between SiO2 and zirconia surface at this temperature should be further clarified. Compared with the other afore-mentioned coating approaches, the current formulated new method is relatively easier and also time-saving. Whereas, the thickness of silica coating layer was not specified and the influence on the mechanical properties has not been evaluated. Given this, the influence of silica coating materials with different particle sizes is still unknown. Therefore, further study is still necessary to obtain the optimized setting of this coating method and interpreting the possible chemical changes. Coating of zirconia particles was stated not only to strengthen porcelain to zirconia bonding (Teng et al., 2012) but also to increase resin to zirconia bond strength (Aboushelib, 2012). In the present study, the coating procedure was performed by spreading a slurry that contains zirconia particles on pre-sintered zirconia. The following final sintering process would fix the zirconia coating particles on zirconia base structure. By doing this, the surface roughness of zirconia has been greatly increased. Moreover, the binding of coated zirconia particles on zirconia disc was supposed to be superior to that between the two different materials. The improvement in zirconia/resin bond strength was ascribed to the creation of topographical structures on zirconia surface for micro-mechanical interlocking. However, chemical integration seems not substantiated with this treatment. Since only the effects of mechanical mechanism were enhanced, the zirconia surface has not yet been completely optimized. This might explain why the highest values of surface roughness in group Z did not result in the strongest initial shear bond strength. Although after thermal cycling for 3000 cycles, the resultant resin to zirconia shear bond strength was slightly higher than the initial value, but such a difference was in fact not significant. It might still demonstrate the relatively higher resistance of this bonding approach to thermal cycling. Such an improved adhesion might be due to the establishment of stronger micro-retentive interfacial structures. Another study
307
evaluated the use of flowable resin cement containing zirconia and silica fillers as coating material on fully sintered zirconia (Chen et al., 2012). It was found that after firing, zirconia surface was covered by a zirconia–silica mixture layer. With the combination of silanization process, the zirconia/resin bond strength and resistance to aging conditions had been significantly increased. The increase in the silica content might play an important role in this process. Such a zirconia–silica coating might be worthy for further investigation. According to the comparison among the initial values of different test groups, it was found that the higher value of surface roughness did not necessarily induce the generation of higher resin to zirconia bond strength which could be explained by the absence of chemical bonding. However, rougher surfaces seemed to improve the resistance to aging effects, especially the influence of thermal cycling. Higher surface roughness was less influenced by the thermal cycling. From the current results of aging treatments, including water storage and thermal cycling, all the experimental groups demonstrated comparable stability in the testing of resin to zirconia bonding except for the samples in control group. Nonetheless, after the thermal cycling for both 3000 and 6000 cycles, group Z had significantly higher shear bond strength compared with all the other groups. The coating with zirconia particles had not only created the micro-structures for mechanical interlocking but also greatly increased the area of actual bonding interface due to the highest Ra achieved. Although chemical bonding might be established by the attachment of silicon content in the other groups, the mean values of surface roughness were much lower than that in group Z and the actual area of bonding interface was smaller. Thus, in group Z, the infiltration of water fully into the resin zirconia bonding interface would become much more difficult. This said, the real situation under practical conditions is still unknown and further investigation is necessary. In the current study four coating methods using different materials and distributing methods have been contrasted and examined. They were proved to enhance resin zirconia bonding to certain extent. However, some remaining problems, such as thickness control, coating quality, and material strengthening should be optimized. The performance of resin zirconia integration under more stringent aging conditions, such as longer water storage up to one year, and more cycles in thermal cycling should be evaluated.
5.
Conclusions
Within the limitations of this laboratory study, the following conclusions were drawn: surface pre-treatment is necessary for producing reliable integration between zirconia and resin cement. All the tested coating methods increased zirconia/resin bond strength and the aging resistance of the adhesive interface.
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