Microtensile bond strength of a resin cement to glass infiltrated zirconia-reinforced ceramic: The effect of surface conditioning

Dental Materials (2006) 22, 283–290

www.intl.elsevierhealth.com/journals/dema

Microtensile bond strength of a resin cement to glass infiltrated zirconia-reinforced ceramic: The effect of surface conditioning ¨ zcanc,*, Marco Antonio Bottinoa, Regina Amarala, Mutlu O Luiz Felipe Valandrob a

˜o Paulo State University, Department of Dental Materials and Prosthodontics, Sa ˜o Jose ´ dos Campos, Brazil Sa b Federal University of Santa Maria, Department of Restorative Dentistry, Santa Maria, Brazil c Faculty of Medical Sciences, Department of Dentistry and Dental Hygiene, Antonius Deusinglaan, University of Groningen, Groningen, The Netherlands Received 6 January 2005; accepted 7 April 2005

KEYWORDS Bond strength; Microtensile test; Silane coupling agent; Silica coating; Surface conditioning methods; Zirconia ceramics

Summary Objectives. This study evaluated the effect of three surface conditioning methods on the microtensile bond strength of resin cement to a glass-infiltrated zirconia-reinforced alumina-based core ceramic. Methods. Thirty blocks (5!5!4 mm) of In-Ceram Zirconia ceramics (In-Ceram Zirconia-INC-ZR, VITA) were fabricated according to the manufacturer’s instructions and duplicated in resin composite. The specimens were polished and assigned to one of the following three treatment conditions (nZ10): (1) Airborne particle abrasion with 110 mm Al2O3 particles C silanization, (2) Silica coating with 110 mm SiOx particles (Rocatec Pre and Plus, 3M ESPE) C silanization, (3) Silica coating with 30 mm SiOx particles (CoJet, 3M ESPE) C silanization. The ceramic-composite blocks were cemented with the resin cement (Panavia F) and stored at 37 8C in distilled water for 7 days prior to bond tests. The blocks were cut under coolant water to produce bar specimens with a bonding area of approximately 0.6 mm2. The bond strength tests were performed in a universal testing machine (cross-head speed: 1 mm/min). The mean bond strengths of the specimens of each block were statistically analyzed using ANOVA and Tukey’s test (a%0.05). Results. Silica coating with silanization either using 110 mm SiOx or 30 mm SiOx particles increased the bond strength of the resin cement (24.6G2.7 MPa and 26.7G 2.4 MPa, respectively) to the zirconia-based ceramic significantly compared to that of airborne particle abrasion with 110-mm Al2O3 (20.5G3.8 MPa) (ANOVA, P!0.05).

* Corresponding author. Tel.:C31 50 363 8528; fax: C31 50 363 2696. ¨ zcan). E-mail address: [email protected] (M. O

0109-5641/$ - see front matter Q 2005 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2005.04.021

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R. Amaral et al. Significance. Conditioning the INC-ZR ceramic surfaces with silica coating and silanization using either chairside or laboratory devices provided higher bond strengths of the resin cement than with airborne particle abrasion using 110 mm Al2O3. Q 2005 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Introduction Etching the inner surfaces of ceramics with glassy matrix using hydrofluoric acid followed by the application of a silane coupling agent is an efficient conditioning method for bonding resin composite [1–6]. However neither etching with this agent nor adding silane resulted in an adequate resin bond to some new high-strength ceramics [7,8]. Particularly high-alumina [9–12] or zirconia-reinforced ceramics [13,14] cannot be roughened by hydrofluoric acid etching since such ceramics do not contain a silicon dioxide (silica) phase. Similarly, cement adhesion to glass-infiltrated zirconia–alumina ceramic (In-Ceram Zirconia-INC-ZR) is also not favorable since this ceramic presents the same characteristics due to its high crystal content (aluminum oxide: G67 wt%; zirconium oxide: G13 wt%) and limited vitreous phase (lanthanum aluminum silicate: G20 wt%) [15]. For this reason, special conditioning systems are indicated for these types of ceramics [16]. Previous investigations revealed that most clinical failures have initiated from the cementation or internal surfaces. Failure rates due to high-strength ceramic fractures have been reported to range between 2.3 and 8% [17–19]. Therefore, the integrity of the luting cement to ceramic surfaces plays a major role in the longevity of the restoration and the failures originated from cementation surfaces identified the need for a reliable conditioning method to strengthen this critical area. Modern surface conditioning methods require airborne particle abrasion of the surface before bonding in order to achieve high bond strength. One such system is silica coating. In this technique, the surfaces are air-abraded with aluminum oxide particles modified with silisic acid [20,21]. The blasting pressure results in the embedding of silica particles on the ceramic surface, rendering the silica-modified surface chemically more reactive to the resin through silane coupling agents. Silane molecules, after being hydrolized to silanol, can form polysiloxane network or hyroxyl groups cover the silica surface. Monomeric ends of the silane molecules react with the methacrylate groups of the adhesive resins by free radical polymerization

process. When a ceramic exhibits chemical states of silicon and oxygen, then siloxane bond will be achieved as these represent the binding sites for the coupling agent to the ceramic surface. Since silane coupling agents do not bond well to alumina, the bond strengths of resin composite to such ceramics could be affected [10]. Air-particle abrasion is a prerequisite for achieving sufficient bond strength between the resins and high-strength ceramics that are reinforced either with alumina or zirconia [22]. The air abrasion systems rely on air-particle abrasion with different particle sizes ranging from 30 to 250 mm [16,23]. The abrasive process removes loose contaminated layers and the roughened surface provides some degree of mechanical interlocking or ‘keying’ with the adhesive. It can be argued that the increased roughness also forms a larger surface area for the bond. While these mechanisms explain some of the general characteristics of adhesion to roughened surfaces, it may also introduce physico-chemical changes that affect surface energy and wettability. Such conditioning systems could be applied either at the laboratory or chairside, using large or small size particles. However, there is limited knowledge as to whether micromechanical retention using large or small particle size increase resin bond to high-strength ceramics of different microstructures and chemical compositions. A high and reliable resin bond to alumina and zirconia ceramics was also achieved with airborne particle abrasion and by using a phosphate monomer (MDP) containing resin composite luting cement. Although there are some studies on bond strength of resin cements to the zirconium-based ceramics [24–26], to the authors’ knowledge, no study has investigated the bond strength of phosphate-monomer based resin cement to zirconium-reinforced ceramics in combination with conditioning methods that rely on chairside conditioning systems. The aim of this study, therefore, was to evaluate the effect of three surface conditioning methods based on airborne particle abrasion, employing three types of sand particles, on the microtensile bond strength of the resin cement to a glassinfiltrated zirconia-reinforced ceramic.

Bond strength of a resin cement to zirconia ceramic

Material and methods Thirty blocks (5!5!4 mm) of zirconia-reinforced alumina-based ceramics [In-Ceram Zirconia-INC-ZR (VITA Zahnfabrik, Bad Sackingen, Germany)] were fabricated according to the manufacturer’s instructions. Ceramic surfaces were ground finished up to 1200-grit silicon carbide abrasive (3M, St. Paul, USA) in a polishing machine (Labpol 8–12, Extec, USA) and cleaned for 10 min in an ultrasonic bath (Quantrex 90, L&R Ultrasonics, Kearny, NJ, USA) containing ethylacetate and air-dried. Each ceramic block was duplicated in composite resin (W3D-Master, Wilcos, Petro ´polis, RJ, Brazil) using a mold made out of silicon impression material (Express, 3M/ESPE, St. Paul, USA). Composite resin layers were incrementally condensed into the mold to fill up the mold and each layer was light polymerized for 40 s (XL 3000-3M/ESPE, St. Paul, USA; light output: 500 mW/cm2). One composite resin block was fabricated for each ceramic block.

Surface conditioning methods Table 1 summarizes the three surface conditioning methods, silane, ceramic and cement used for the experiments. The ceramic blocks (10 blocks per conditioning) were assigned to one of the three following treatment conditions: Chairside Gritblasting (CGB): In this group, airborne particle abrasion was performed using 110 mm grain sized Al2O3 particles using an intraoral air abrasion device (Micro–Etcher, Danville Inc.,

285 San Ramon, CA, USA) at a pressure of 2.8 bars from a distance of approx. 10 mm, for 20 s in circling movements. Laboratory Silica Coating (LSC): Silica coating process was conducted using a laboratory type of air abrasion device (Rocatector Delta device, 3M ESPE) in which the specimens were first conditioned by air-abrasion with 110 mm grain sized Al2O3 particles at a pressure of 2.8 bars with Rocatec Pre abrasive. Then the specimens were air-abraded with Rocatec Plus abrasive, which was 110 mm grain sized SiOx, at 2.8 bars under the same conditions with CGB. Chairside Silica Coating (CSC): Silica coating process was achieved using an intraoral air abrasion device (Micro–Etcher, Danville Inc., San Ramon, CA, USA) filled with CoJetw-Sand (30 mm SiOx particles) (3M-ESPE, Minnesota, USA) under the same conditions with CGB. Following all three surface conditioning methods, the remnants of sand particles were gently air blown, silane coupling agent (ESPEw-Sil, 3M ESPE AG, Seefeld, Germany) was applied and waited for its evaporation for 5 min.

Topographic analyses of conditioned ceramic surface Additional ceramic specimens were conditioned using the three surface conditioning methods in order to observe the topographic surface changes under the Scanning Electron Microscope (SEM) (JEOL-JSM-T330A, Jeol Ltd, Tokyo, Japan).

Table 1 Characteristics of surface conditioning methods, silane, ceramic and cement used for the experiments with codes and manufacturing company names. Conditioning principles, silane, ceramic, cement

Abbreviation

Characteristics

Manufacturer

Chairside Gritblasting

CGB

110 mm Al2O3, (2.8 bars, 10 mm, 20 s)

Laboratory Silica Coating Germany

LSC

Chairside Silica Coating

CSC

Rocatec Pre (110 mm Al2O3)C Rocatec Plus (110 mm SiOx) (both at 2.8 bars, 10 mm, 20 s) CoJetw-Sand (30 mm SiOx) (2.8 bars, 10 mm, 20 s) 3-methacryloxyprophyltrimethoxy silane in ethanol (ESPEw-Sil) (5 min)

Korox, Bego, Bremen, Germany 3M ESPE AG, Seefeld,

Silane coupling agent Germany Ceramic In-Ceram Zirconia Cement Panavia F

INC-ZR

3M-ESPE, Minnesota, USA 3M ESPE AG, Seefeld,

Glass-infiltrated zirconia

Vita Zahnfabrik, Bad Saeckingen, Germany

Filler (78%),10-Methacryloyloxydecyldihydrogenphosphate (MDP), dimethacrylates, chemical and photoinitiators

Kuraray, Okayama, Japan

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Bonding procedure and specimen preparation Each conditioned ceramic block was bonded to a composite block under the load of 750 g using a resin cement system (Panavia F, Kuraray CO., Okayama, Japan). The excess resin cement was removed by means of a brush. The resin cement was then light polymerized (XL 3000) for 40 s from each direction. Oxyguard was applied on the cement layer for 10 min. The blocks were washed with airwater spray and stored in distilled water at 37 8C for 7 days prior to bond tests. The blocks were then bonded with cyanoacrylate glue (Super Bonder Gel, Loctite Ltd, Sa ˜o Paulo, Brazil) to a metal base that was coupled to a cutting machine. Slices were obtained using a slow-speed diamond wheel saw (KG Sorensen, Barueri, Brazil) under cooling. The peripheral slices were disregarded in case the results could be influenced by either the excess or insufficient amount of resin cement at the interface. Three slices (0.8G0.1 mm in thickness) were obtained per block initially. The slices were rotated 908 and bonded onto the metal base again. The peripheral bar specimens were also disregarded for the same reasons described above. Other 3 sectioning were carried out. (Fig. 1(a)-(b)). Twelve non-trimmed bar specimens (with approximately 8 mm in length and 0.6G0.1 mm2 adhesive surface area) were obtained per block (Fig. 2(a)-(c)).

Figure 1 (a) Cutting procedure to obtain slices of cemented ceramic and composite blocks with ca 0.8 mm thickness (3 slices per block); (b) The slices rotated 908 and bonded onto the metal base again for further cutting procedures (0.8 mm-thickness) in order to obtain nontrimmed bar specimens with ca 8 mm in length and 0.6 mm2 adhesive surface area.

order to determine the significant differences between surface conditioning methods. P values less than 0.05 are considered to be statistically significant in all tests.

Microtensile bond strength test The bar specimens were glued parallel to the long axis of an adapted caliper using cyanoacrylate glue. This apparatus was coupled to the universal testing machine (EMIC DL-1000, EMIC, Sa ˜o Jose ´ dos Pinhais, Brazil) and the specimens were loaded in tension to failure at a crosshead speed of 1 mm minK1. Bond strength values were calculated using the formula, sZL/A, where ‘L’ is the load at failure (Kgf) and ‘A’ is the adhesive area (mm2) measured using a digital caliper (Mitutoyo, Tokyo, Japan) prior to the tests.

Statistical analysis Statistical analysis was performed using Statistics 8.0 for Windows (Analytical Software Inc, Tallahassee, FL, USA). The means of the specimens of each blocks were obtained and these values (nZ10) were analyzed by 1-way analysis of variance (ANOVA) and Tukey’s test (aZ05) in

Results The results of the microtensile bond strength tests for three surface conditioning methods are presented in Table 2. Silica coating with silanization either with 110 mm SiOx particles or 30 mm SiOx revealed significantly higher bond strengths of the resin cement (24.6G2.6 MPa and 26.7G2.4 MPa, respectively) to the INC-ZR ceramic compared to that of airborne particle abrasion with 110-mm Al2O3 (20.5G3.8 MPa) (ANOVA, PZ0.0004). There were no significant differences between both silica coating groups (PO0.05) SEM analysis at !2000 magnification, complementary to the bond strength tests, revealed that all three types of sand particles penetrated the substrate surfaces and the ceramic surfaces were covered with abundant sand particles even after air blowing (Fig. 3(a)–(c)).

Bond strength of a resin cement to zirconia ceramic

287 (b)

(c)

(a) not tested

tested

Resin composite

Resin Cement

*

Ceramic *Adhesive zone

Figure 2 (a) Protocol of specimen choice according to cutting procedure (tested-and non-tested regions); (b) Bar specimens with ca 8 mm in length and 0.6G0.1 mm2 bonded surface area; (c) Bonded zone (*) between the ceramic and composite block at the bar specimen.

Discussion In this study, roughening the zirconia-reinforced ceramic surfaces with air particle abrasion and applying silane prior to cementation provided high bond strengths and silica coating followed by silanization evidently enhanced the bond between the luting cement and the ceramic surfaces. The silica layer left by silica coating on the ceramic surface provides a basis for silane to react. In the ceramic-resin bond, silane functions as a coupling agent, which adsorbs onto and alters the surface of the ceramic, thereby facilitating chemical interaction [11,23]. When alumina or zirconia ceramics are glass infiltrated, they are melted together at high temperatures to form a ceramic composite. The chemical components of the ceramics (traces such as Li2O, Na2O, K2O, CaO, MgO) are then bonded to each other by strong covalent bonds with hydroxyl groups at the surface of the ceramic material [27]. When the surface is air abraded, this would generate more hydroxyl groups on the surface and also enhance the micro-mechanical retention. Furthermore, the methoxy groups of silane would react with water to form silanol groups that in turn, will react with the surface hydroxyl groups to form siloxane network. Amphoteric alumina in the ceramic matrix could form chemical adhesion, covalent bridges, through its surface hydroxyl groups with hydrolyzed silanol groups of the silane: –Al–O–Si–[10]. In principle, the presence of the glassy phase in ceramics favors better siloxane bonds. The silanol groups could then react further to form a siloxane (–Si–O–Si–O–) network with the silica on the surface. The In-Ceram ceramic system tested in this study, In-Ceram Zirconia (INC-ZR), is glass infiltrated. Most probably the glass infiltration facilitated better

silane bonding and therefore increased bond strength values were obtained for this ceramic. These findings are in compliance with the study of ¨ zcan and Vallittu [10], even though a different O experimental set up was used where a bis-GMA based resin cement and shear bond test were employed. Material selection and clinical recommendations on resin bonding are based on mechanical laboratory tests that show great variability in materials and methods. One of the most common testingmethod is the shear bond test. However the specific fracture pattern in shear testing may cause cohesive failure in the substrate that may lead to erraneous interpretation of the data while in microtensile tests, stress distribution was reported to be more homogeneous [28–31]. Although, for this reason, microtensile test was employed in this study, similar ceramic-cement performance was ¨ zcan and observed in dry conditions in the study of O Vallittu [10]. Some studies, on the other hand, have evaluated ceramics with different microstructures, reporting Table 2 Microtensile bond strength (MPa) of the resin luting cement, and statistical differences considering the surface conditioning factor after (a) Airborne particle abrasion with 110 mm Al2O3 particles, (b) Silica coating with 110 mm Al2O3 and 110-mm SiOx particles, (c) Silica coating with 30 mm SiOx particles. Groups

s* (MPa) (SD)

1-INC ZIRC-CGB 2-INC ZIRC-LSC 3-INC ZIRC-CSC

20.5a (3.8) 24.6b (2.7) 26.7b (2,4)

For abbreviations, see Table 1. Different superscripted letters indicate significant differences between the ceramic-surface conditioning combinations (P!0.05). SD, standard deviation.

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Figure 3 (a) Typical SEM micrographs (!2000) of conditioned ceramic surfaces for a-Airborne particle abrasion with 110 mm Al2O3 particles, (b) Silica coating with 110 mm SiOx particles, c-Silica coating with 30 mm SiOx particles. Note that after all three conditioning methods, ceramic surfaces were covered with abundant sand particles.

that high-strength ceramics are compact materials making them difficult to gritblast [13,22]. Interestingly, the results of this study indicate that the silica coating system with small particle size of 30 mm SiOx particles as well as large particle size of 110 mm SiOx produced statistically higher mean bond strength values than with chairside grit blasting using 110-mm grain sized Al2O3 particles. One can expect higher surface roughness created using bigger particle size thereby higher micromechanical retention but this was not achieved in this study. One reason for this could be associated with the phenomenon of less wettability and contact angle [13,32] between the silane coupling agent and the deep grooves on the ceramic surfaces occurred after grit blasting. However this assumption could not be verified for the application of 110 mm SiOx. The reason for this can be explained on the grounds that particle deposition mechanisms differ depending on the substrate characteristics, particle composition, size distribution, quantity and morphology. Although SEM images demonstrated comparable views with agglomerates of sand particles, it is difficult to deduce whether the sand is in contact with the alumina or the glass phase of the ceramic tested in this study. Nevertheless, the results of this study together with some other studies reveal good adhesion of silica particles in the vitreous phases of the glass-infiltrated zirconia ceramics

[33–36]. In a previous study, a significant increase of silica on the surface of the In-Ceram ceramic (15.8–19.7 wt%) was detected after blasting with Rocatec-Plus (SiOx) when compared with the samples blasted only with Rocatec-Pre (Al2O3 particles) suggesting better bond strength between the In-Ceram ceramic and the resin cements due to the increase of silica content and the interaction with the silane agent. Our ongoing studies involve Energy Dispersive X-ray Spectroscopy (EDS) analysis to gain more insight on the interaction between these three sand particles and the ceramic composites [37]. The other reason for lower results obtained after 110 mm grain sized Al2O3 particle deposition could be due to the weak bond between Al–Si–O as reported earlier elsewhere [10]. Although satisfactory bond strength values of resin cement to high-strength ceramics are yet to be determined for clinically successful performance, the bond values obtained for the ceramic tested in this study could be considered sufficient with both conditioning methods. In clinical applications however, when air abrasion will be contemplated by chairside, clinicians should also consider the possible material loss [9] especially at the margins of the restorations that may lead to ditching when bigger grain size particles are used during airborne particle abrasion.

Bond strength of a resin cement to zirconia ceramic Bonding of ceramic to tooth substance is based on the adhesion of luting cement and its bonding resin to the ceramic substrate together with the adhesion of luting cement to enamel and dentine. Future studies should also concentrate on the involvement of the tooth tissues in the test complex. The cement–ceramic adhesion is susceptible to chemical, thermal and mechanical influences under intraoral conditions. One limitation of this study could be the lack of thermocycling although there are controversial reports on the effect of thermocycling in the literature [16]. Some earlier studies reported high and stable bond strength to the zirconia reinforced ceramic after airborne particle abrasion using Al2 O 3 particles in combination with phosphate monomer based resin cement [24–26]. Comparing the results of these studies with this present study, it can be suggested that the silica coating and silanization may allow a better bond strength to the zirconium with this resin cement. The general outcome of this study suggests that relatively recent surface conditioning techniques based on the combination of micromechanical and chemical conditioning should be considered for improved adhesion of resin cements to glass-infiltrated zirconia ceramics. More importantly, these methods seem to offset the importance of the varieties of the substrates and therefore could be applicable to a wide range of high-strength ceramics [31]. The equipments to apply these techniques were sophisticated and expensive during the last two decades but they are recently simplified and brought to the chairside. By employing chairside devices for airborne particle abrasion, contamination during delivery of the restoration from the laboratory to chairside could also be avoided. As long as the available conditioning methods will not be optimized, the development in the high-strength ceramic field is expected to continue experiencing failures.

Conclusions Silica coating either with 110 mm SiOx particles or 30 mm SiOx followed by silanization increased the bond strength of the phosphate monomer-based resin cement to glass infiltrated zirconiareinforced ceramic when compared with airborne particle abrasion using 110 mm Al2 O 3 and silanization.

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Acknowledgements We express our appreciation to the Wilcos Ltd in Brazil (Petro ´polis/RJ, Brazil) and VITA Zahnfabrik (Bad Sa ¨ckingen, Germany) for providing some of the materials used in this study. We also thank Prof. Dr. Ivan Balducci, School of Dentistry, Sa ˜o Paulo State University at Sa ˜o Jose ´ dos Campos, Brazil, for his assistance with statistical analysis.

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