Innovations in bonding to zirconia based ceramics: Part III. Phosphate monomer resin cements

d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) 786–792

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Innovations in bonding to zirconia based ceramics: Part III. Phosphate monomer resin cements Hesam Mirmohammadi a,b,∗ , Moustafa N.M. Aboushelib a,c , Ziad Salameh d , Albert J. Feilzer a , Cornelis J. Kleverlaan a a

Department of Dental Materials Science, Academic Centre of Dentistry Amsterdam (ACTA), University of Amsterdam and Free University, The Netherlands b Department of Restorative Dentistry, School of Dentistry, Esfahan University of Medical Sciences, Esfahan, Iran c Dental Biomaterials Department, Faculty of Dentistry, Alexandra University, Egypt d Nano-Structured Biomaterials Unit for Regeneration Eng.A.B. Research Chair for Growth Factors and Bone Regeneration King Saud University, Riyadh, Saudi Arabia

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a b s t r a c t

Article history:

Purpose. To compare the bond strength values and the ranking order of three phosphate

Received 5 July 2009

monomer containing resin cements using microtensile (␮TBS) and microshear (␮SBS) bond

Received in revised form

strength tests.

22 October 2009

Materials and methods. Zirconia discs (Procera Zirconia) were bonded to resin composite discs

Accepted 23 April 2010

(Filtek Z250) using three different cements (Panavia F 2.0, RelyX UniCem, and Multilink). Two bond strength tests were used to determine zirconia resin bond strength; microtensile bond strength test (␮TBS) and microshear bond strength test (␮SBS). Ten specimens were tested

Keywords:

for each group (n = 10). Two-way analysis of variance (ANOVA) was used to analyze the data

Zirconia

(˛ = 0.05).

Adhesion

Results. There were statistical significant differences in bond strength values and in the

Microshear

ranking order obtained using the two test methods. ␮TBS reported significant differences

Microtensile

in bond strength values, whereas ␮SBS failed to detect such effect. Both Multilink and

Resin cements

Panavia demonstrated basically cohesive failure in the resin cement while RelyX UniCem demonstrated interfacial failure. Conclusion. Based on the findings of this study, the data obtained using either ␮TBS or ␮SBS could not be directly compared. ␮TBS was more sensitive to material differences compared to ␮SBS which failed to detect such differences. © 2010 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Adhesive resins used in dentistry have been significantly improved over recent years [1,2]. Micromechanical entanglements of monomer resins with etched enamel as well as bonding to conditioned dentin by hybridization are considered

key factors for achieving successful bonding and an optimal marginal seal. However, the introduction of zirconia to dentistry added an extra challenge as establishing a durable bond with zirconia has been proven to be a difficult task. Bond strength tests have always gained a lot of interest as a point of research in dental literature [3]. While the aim of these tests is usually to evaluate bond strength of differ-

∗ Corresponding author at: Dental Materials Science, Academic Centre of Dentistry Amsterdam (ACTA), University of Amsterdam and Free University, Louwesweg 1, 1066 EA Amsterdam, Noord Holand, The Netherlands. Tel.: +31 0 20 5188697; fax: +31 0 20 6692726. E-mail address: [email protected] (H. Mirmohammadi). 0109-5641/$ – see front matter © 2010 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2010.04.003

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ent materials, they also allow establishing of a ranking order. The data obtained from different bond strength tests largely depend on the actual test setup used that may differ between different laboratories. Parameters such as specimen geometry, size of the bonded surface area, the type of material, the loading conditions, operation variability, and more, all have a significant influence on the data obtained. It is, therefore not surprising that bond strength data substantially vary among different studies throughout the world. All these interacting variables make direct comparison between different studies a very difficult task [4,5]. Most commonly used bond strength tests rely on subjecting the specimens to tensile or shear stresses in order to evaluate the failure point which is usually referred to as the bond strength of an adhesive system to the substrate material. On the contrary, the specimen may fail by cohesive fracture of either of its components and in that case using this failure load to confer bond strength values may be a little ambiguous. For example, a bond strength value in excess of 20 MPa in a shear test setup would tend to cohesively fracture the specimen meanwhile the bonded interface may remain intact [6]. Therefore, a new test is needed especially for testing specimens with a strongly bonded interface. Microtensile bond strength test (␮TBS) was introduced by Sano and others in 1994 [7]. These authors showed that ␮TBS values are inversely related to the bonded surface area [7–9] and that although much higher bond strength values were measured, most failures still occurred at the interface between tooth substrate and adhesive resin. Other advantages of ␮TBS are that regional bond strength and bonding effectiveness could be applied to small sized specimens as a regional area of a tooth substrate focusing on a carious region [10,11] or for example a localized area of sclerotic dentin [12,13]. The major disadvantages of ␮TBS are that the test is rather labor-intensive, technically demanding, and requires careful handling of the fragile specimens. Special care should be taken to reduce the production of micro-fractures at the interface during specimen preparation which may weaken the bond and thus reduce the actual bond strength [14]. Microshear bond strength test (␮SBS) is used for testing of small areas, which permits precise regional mapping of the surface of interest, e.g. different regions of dentin surface [5]. This method allows for straightforward specimen preparation and gives precise results with relatively small standard deviations [15–17]. For ␮TBS test, trimming of the specimen is an indispensable step especially when making small sized specimens which can easily cause micro-cracks in brittle materials like enamel or ceramics. However, when conducting microshear bond test, the resin specimens could be directly bonded on a flat surface without the need of trimming [18]. A point worth of mentioning is that different bond strength tests result in a characteristic pattern of stress distribution of the applied load in the structure of the specimen. These areas of high stress concentration act as crack initiation sites regardless of the presence of surface or bulk defects in the specimen. Once the stress concentration at a crack tip exceeds the strength of the material, the crack will grow. In this situation, failure of the tested specimen is usually reported as cohesive fracture and the obtained data, in most cases, are used to calculate the mean bond strength value even though

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that the bonded interface may be totally intact which would directly lead to unjustified high bond strength values. In a previous study, Aboushelib et al. utilized finite element analysis to map the stress distribution in ␮TBS test setup and have indicated that stresses are concentrated away from the bonded interface between the bonded microbar and the attachment unit. Such condition would imply a higher tendency for cohesive failures in specimens with strong bonded interfaces. In case of cohesive failure, a correction factor of 2.5× has been suggested by the authors to properly report the failure stress in such cases [19]. A number of studies have been performed to investigate the bonding ability of adhesive systems to tooth structures [20–23] or core materials [18,24], including tensile and shear bond strength tests. Additionally, these tests have also been used to evaluate the bond strength between different components in one structure as the bond between the veneer ceramic and the underlying framework material or the bond between composite luting cement and zirconia ceramic surfaces [25,26]. However, a small number of investigations have compared the results of ␮TBS and ␮SBS [27] which need to be properly evaluated to establish a reliable comparative method and thus enable ranking of different adhesives. Up to date, the combination of airborne particle abrasion and 10-methacryloyloxydecyl dihydrogenphosphate (MDP) monomer is the recommended method of bonding to zirconia frameworks. This method has proven to produce a good bond strength and bond durability after thermo-cycling and long term water storage [28]. In previous parts of this study, MDP monomer of Panavia F 2.0 resin cement was successful in establishing good bond strength with zirconia, which was not influenced by 90 days of water storage. The performance of this bond was enhanced by using new types of adhesion promoters designed to enhance wetting and bonding to ceramic substrates [24,29]. Due to patent rights about the structure of MDP monomer (Fig. 1a), manufacturers have produced new phosphate monomers designed not only to bond to zirconia but have also cross-linking branches for bonding the resin matrix as well (Fig. 1). Unfortunately, not much data are available about the performance of these new adhesive resins. One of the recently developed phosphate monomers (RelyX Unicem) has a characteristic of self-etching phosphorylated methacrylates that is designed to bond directly to both enamel and dentin. With two phosphate groups and at least two double bonded carbon atoms, a good bond strength to zirconia plus adequate crosslinking to the resin matrix is achieved (Fig. 1b). Another new self-etch phosphate monomer (Multilink Automix) characterized by hydrolytic stability has one phosphate terminal and at least two sites capable of bonding to resin matrix through oxygen bond. This molecule has a terminal hydroxyl group as a substituent that gives the monomer stability under water and in acidic conditions (Fig. 1c). The aim of this study was to determine zirconia resin bond strength and the ranking order of three phosphate monomer containing adhesive resins by means of ␮TBS and ␮SBS tests. Fracture surface analysis of the broken specimens was used to classify failure pattern. The null hypothesis predicted no significant differences between the two tested methodologies.

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Fig. 2 – Bilayered zirconia resin disc is cut into microbars for conducting microtensile bond strength test. The bonded interface is subjected to tensile load.

Fig. 1 – Chemical structure of phosphate monomer groups in three bonding resins. (a) 10-Methacryloyloxydecyldihydrogenphosphate, the adhesive monomer in Panavia F 2.0, (b) methacrylated phosphoric ester, the adhesive monomer in RelyX UniCem, and (c) phosphoric acid acrylate, the adhesive monomer of Multilink Automix.

2.

Materials and methods

2.1.

Microtensile bond strength (TBS) test

Fully sintered yttria tetragonal zirconia polycrystal (Y-TZP) discs (11.8 mm in diameter × 3.0 mm thick) were prepared by cutting zirconia milling blocks (Procera zirconia; NobelBiocare, Göteborg, Sweden) using a precision cutting instrument (Isomet 1000; Buehler, Lake Bluff, IL) and a diamond-coated cutting disc (Diamond Wafering Blade, No 11-4276; Buehler). The location of the cuts was controlled using a traveling

stage and a horizontally displaced digital micrometer (IDC1508; Mitutoyo Corp, Utsunomiya, Japan). The sintered discs were polished using a rotating metallographic polishing device (Ecomet; Buehler) under a fixed load (300 g) and water cooling. The specimens were airborne particle abraded with 50 ␮m aluminum oxide particles (P-G 400; Harnisch & Rieth, Winterbach, Germany) at 0.35 MPa pressure (S-U-Alustral; Schuler-Dental, Ulm, Germany) at a distance of 1 cm followed by ultrasonic cleaning in distilled water for 10 min. Composite resin (Filtek Z250, shade A2; 3 M ESPE, St. Paul, MN, USA) discs (11.8 mm in diameter × 3 mm thick) were prepared in a plastic mold and light polymerized at 4 different locations, 60 s each (Elipar FreeLight 2; 3M ESPE). Light intensity, 800 mW/cm2 , was frequently monitored to ensure adequate polymerization of all specimens (Demetron 100; Demetron Research Corp, Danbury, CT). Three different adhesive resin cement systems, Panavia F 2.0 (Kuraray Co. Ltd., Tokyo, Japan), RelyX UniCem (3 M ESPE), and Multilink Automix (Ivoclar-Vivadent, Schaan, Liechtenstein) were used to lute the composite resin discs to the airborne particle abraded zirconia discs, according to the manufacturers’ instructions. These bonding agents were selected because each contains a different phosphate based monomer which is known to enhance bond strength to zirconia based materials. Following manufacturer recommendations, the zirconia surface was prepared and each resin cement was mixed and applied on the surface of the composite resin disc which was seated on top of the zirconia disc and loaded with 50 N for 60 s using a special loading gig, excess cement was wiped off, and the specimen was light polymerized with the same unit at 4 different locations for 60 s each. Properties of all used materials are listed in Table 1. Each bonded specimen was sectioned with a 0.3 mm thick diamond-coated cutting disc (Diamond Wafering Blade, No 11-4254; Buehler) into at least 20 microbars (6 mm × 1 mm × 1 mm). The microbars were examined under a stereomicroscope (SZ; Olympus, Tokyo, Japan) and only intact microbars were selected (Fig. 2). For every group (n = 20), the microbars were tested after 24 h water storage (demineralized water at 37 ◦ C). Each microbar

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was bonded to the attachment unit using a light-polymerized adhesive resin (Clearfil SE; Kuraray Co. Ltd.), taking care to center the zirconia–resin interface at the free space between the two plates of the attachment unit [30]. The zirconia–resin microtensile bond strength (MPa) was measured by applying tensile load to the bonded interface using a universal testing machine (Instron 6022; Instron Corp, High Wycombe, England) at a crosshead speed of 0.5 mm/min [26].

2.2.

Microshear bond strength (SBS) test

Twelve fully sintered zirconia discs (22.2 mm diameter, 0.8 mm height), were prepared using the same materials and methods as previously described. Composite resin (Filtek Z250, shade A2; 3 M ESPE, St. Paul, MN) discs (0.9 mm in diameter × 0.7 mm thick) were prepared by injecting the resin composite into a plastic tube which was held between two glass slides and then was light polymerized for 20 s form the top and for 20 s from the bottom side (Elipar FreeLight 2; 3M ESPE). The specimens were then stored in distilled water at 37 ◦ C for 24 h prior to removal from the tubing. The same previously described adhesives and bonding method were used to lute the composite cylinders to the zirconia discs. Excess cement was removed by using compressed air and micro-brushes, nevertheless the small size of the bonded discs required careful handling. The bonded specimens were gently fixed between two steel plates using a traveling stage micrometer (Mitutoyo Corp, Utsunomiya, Japan). Shear force was applied to the bonded interface at a crosshead speed of 0.5 mm/min until failure occurred taking care to properly align the specimens so that the bonded interface was parallel to the direction of load application (Fig. 3).

2.3.

Analysis of failure mode

The fractured zirconia surface after ␮TBS or ␮SBS was examined under an optical microscope at 30× magnification then at higher magnifications using a scanning electron microscopy (XL 20; Philips, Eindhoven, The Netherlands). Failure mode was classified either as interfacial failure where the crack traveled at the zirconia–resin cement interface with consideration of the area of crack origin or a cohesive failure in the

Fig. 3 – Modified microshear test setup. By pushing the zirconia disc downwards, the bonded interface is subjected to shear stress.

resin cement where the crack originated outside the bonded interface. This classification allowed accurate and easier interpretation of failure mode as from a clinical point of view a debonding failure will originate at the interfacial region [31]. Two-way analysis of variance (ANOVA) with one between group variable (2 bond strength tests) and one within group variable (3 resin adhesives) were used to analyze the data (˛ = 0.05). Statistical analysis was carried out using computer software (SigmaStat Version 3.0, SPSS, Inc., Chicago, USA).

3.

Results

Statistical analysis revealed significant differences in the bond strength values obtained using microtensile bond strength tests. Such significant differences were related to the type of resin cement used (F = 6.4, P < 0.003) on the contrary, microshear test did not detect any significant difference between the tested cements (F = 2.3, P < 0.076). There was also a significant interaction between the resin cement and the bond strength test of choice (F = 7.6, P < 0.001). Additionally, the ranking order of the used resin cements was different for both tests; although ␮SBS did not detect significant differences between

Table 1 – Material properties of resin cements. Material Multilink Automix, Ivoclar-Vivadent, Schaan, Liechtenstein

Panavia F 2.0, Kuraray Co. Ltd., Osaka, Japan

RelyX UniCem, 3M ESPE, St. Paul, Mn, USA

Main composition The monomer matrix is composed of DMA, HEMA, Ba-glass fillers, ytterbium fluoride, spheroid mixed oxide A primer: aqueous solution of initiator B primer: HEMA and phosphoric acid and acrylic acid monomers Metal/zirconia primer: phosphoric acid acrylate and methacrylate cross-linking agents in an organic solution A paste: silica, dimethacrylate monomer, functional acid MDP, photo initiator, accelerator B paste: brown color, barium glass, sodium fluoride, dimethacrylate (DMA) monomer Powder: glass powder, initiator, silica sil., pyrimidine, calcium hydroxide, peroxy compound, pigment Liquid: Methacrylated phosphoric ester, DMA, acetate, stabilizer, initiator

Batch

K54378

41233

288018

790

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Table 2 – Mean bond strength (in MPa) and standard deviation in parentheses after 24 h water storage. Cement Panavia F 2.0 RelyX UniCem Multilink

␮SBS

␮TBS aA

29.1 (7.3) 25.4 (6.4)aB 28.9 (6.7)a

32.5 (1.7)A 24.2 (2.7)bB 23.7 (2.3)b

Small letters show no significant differences between the resin cements for same bond strength test, and capital letters show no significant differences between ␮SBS and ␮TBS for the same resin cement.

Fig. 5 – SEM image, 90×, demonstrating cohesive failure in resin cement observed with Multilink during microtensile bond strength test.

4.

Fig. 4 – SEM image, 50×, demonstrating interfacial failure exposing the zirconia surface (Panavia F 2.0). One of the drawbacks of microshear test is related to difficulty removing excess cement around the periphery of the small disc without disrupting its position. Excess cement may interfere with positioning the loading edge and influence bond strength.

Multilink, Panavia, or RelyX UniCem, ␮TBS ranked Panavia first followed by RelyX UniCem and Multilink. These data summarized in Table 2. Both Multilink and Panavia demonstrated predominantly cohesive failure in resin cement as the surface of zirconia was covered by a layer of resin cement. In contrast, RelyX UniCem revealed predominantly interfacial failure where the surface of zirconia was exposed after fracture (Figs. 4 and 5). The percentage of failure pattern was however different for both tests, Table 3.

Discussion

Although microtensile bond strength test has its characteristic advantages (large number of microbars per specimen, subjects the bonded interface to direct tension, and ease of analysis of fracture surface), the complexity of the technique is a barrier against its widespread use especially when the available preparation and testing resources are limited [18]. On the contrary, shear bond strength test is one of the most widely used bond evaluation methods in many fields and it gained a lot of popularity because of its simplicity and ease of manipulation. Nevertheless, there are concerns that this test does not accurately measure the bond strength value due to the phenomena of uneven stress distribution around the loading knife [32]. To overcome this limitation, several modifications were introduced to allow better stress distribution of the loading stresses. A round shaped knife [33], stress breakers, and wire loops [23,34] were all previously tested. At the present time, there are no available solid guidelines that allow direct comparison of the data obtained using ␮TBS and ␮SBS tests. Thus the data obtained in different studies could not be directly correlated, not to mention other variables involved during preparation and testing of the specimens. Beside bond strength, such limitation prevents also comparison of the ranking order obtained in both tests which could be of significant importance for the daily practitioner during material selection [32].

Table 3 – Failure type and percentage of test groups. Resin cement Panavia F 2.0 RelyX UniCem Multilink

␮SBS 60% Cohesive in resin cement 40% Interfacial 80% Interfacial 20% Cohesive in resin cement 60% Cohesive in resin cement 40% Interfacial

␮TBS 100% Cohesive in resin cement 100% Interfacial 100% Cohesive in resin cement

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The data of the present study revealed significant differences in the bond strength values obtained using microtensile test in addition to difference in the ranking order and failure pattern between the three cements used. Thus the proposed null hypothesis was rejected. In agreement with other studies, MDP monomer resulted in the highest ␮TBS values with zirconia and produced higher bond strength compared to the other two phosphate monomers [35–37]. Recent studies reported that the bond strength between resin cement and zirconia would depend on the resin cement selection rather than on the surface treatment [38,39]. On the other hand a new method to chemically modify the surface of zirconia was described by using a chloro-silane-based vapor-phase pretreatment to increase the bonding sites for the subsequent organo-silane primer for conventional dental adhesive applications [40]. Analysis of failure mode in this study showed basically interfacial failure for RelyX UniCem. Meanwhile this cement demonstrated good bonding to composite resin enhanced by its high cross-linking capacity, it failed to adequately establish a strong bond to zirconia, which could be related to its poor wetablility due to the large filler size and high viscosity of the cement at time of application. On the contrary, Multilink Automix demonstrated cohesive failure pattern, comparable to Panavia F 2.0, which directly reflects the capacity of its monomer for bonding to both zirconia and resin composite. For the ␮TBS, it was shown that the size and method of attachment has an influence on the observed bond strength [30]. A previous study revealed that the highest area of stress concentration in ␮TBS was located away from the bonded interface [19], which can play role in case of relative stiff or weak materials. In this study the bonded interface was not influenced by these remote stresses. It has also to be considered that an interfacial crack may deviate during its propagation and pass through the resin cement giving an impression of cohesive failure. On the other hand one should realize that finite element analysis of the used test setup is necessary to fully understand the magnitude and geometry of the local stresses in the cement layer. Several studies investigated the pattern of stress concentration around the loading blade in shear bond strength test [34,41], and it was reported that very high stresses, several folds higher than the applied load, are generated in this region which could easily damage the bonding resin leading to cohesive failure in the resin cement which makes the value obtained from the test more related to the cohesive strength of the bonding material rather than the bond strength itself. Such influence could be minimized using microshear test as using discs of smaller diameter would require lower loads to failure before the cohesive strength of the material is reached, nevertheless the pattern of stress distribution would remain the same. It has also to be considered that it is recommended to restrict to standardized specimen dimensions in order to avoid changes in pattern of stress distribution which would finally allow comparison of data obtained in different studies [27,41]. Thus the dimensions used in this study could be used as a general guideline for preparing specimens for microtensile or microshear tests. Changing the dimensions would only introduce another uncontrollable variable that would further

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complicate data analysis and comparison between different studies. According to the findings of these studies, the combination of airborne particle abrasion and a phosphate based monomer would produce strong bond to zirconia frameworks, however, long term performance of this bond requires artificial aging to assess bond performance under influence of deteriorating conditions, which will be the focus of future studies.

5.

Conclusion

Within limitations of this study, microtensile bond strength test was able to detect bond strength differences between three phosphate monomer containing resins to zirconia frameworks while microshear test failed to detect such differences.

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