DENTAL-2527; No. of Pages 12
ARTICLE IN PRESS d e n t a l m a t e r i a l s x x x ( 2 0 1 5 ) xxx–xxx
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Influence of surface treatment on the resin-bonding of zirconia ˘ b , Erhan C¸ömlekoglu ˘ b, Seda S¸anlı a , Mine Dündar C¸ömlekoglu Mehmet Sonugelen b , Tijen Pamir c , B.W. Darvell d,∗ a
Ulukent Dental Clinic, Menemen, Izmir, Turkey Department of Prosthodontics, School of Dentistry, Ege University, Izmir, Turkey c Department of Restorative Dentistry, School of Dentistry, Ege University, Izmir, Turkey d Dental Materials Science, Faculty of Dentistry, Kuwait University, Kuwait b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. To compare the effect of various surface treatments on the bonding of luting resin
Received 29 September 2014
cements to zirconia under four-point bending.
Received in revised form
Methods. Bar specimens (n = 200) (2 mm × 5 mm × 25 mm) were prepared from zirconia blocks
8 January 2015
(VITA In-Ceram YZ, Vita Zahnfabrik) with the cementation surface (2 mm × 5 mm) of
Accepted 13 March 2015
groups of 40 treated in one of five ways: airborne particle abrasion with 50 m Al2 O3
Available online xxx
(GB), zirconia primer (Z-Prime Plus, Bisco) (Z), glaze ceramic (Crystall.Glaze spray, Ivoclar Vivadent) + hydrofluoric acid (GHF), fusion glass-ceramic (Crystall.Connect, Ivoclar Vivadent)
Keywords:
(CC), or left untreated as control (C). Within each treatment, bars were cleaned ultrasoni-
Zirconia surface treatment
cally for 15 min in ethanol and then deionized water before bonding in pairs with one of
Adhesive cementation
two luting resins: Panavia F 2.0, (Kuraray) (P); RelyX U-200 (3 M/Espe) (R), to form 10 test
Four-point bending test
specimens for each treatment and lute combination. Mechanical tests were performed and bond strengths (MPa) were subject, after log transformation, to analysis of variance, Shapiro–Wilk and Holm–Sidak tests; also log-linear contingency analysis of failure mode distribution; all with ˛ = 0.05. Fracture surfaces were examined under light and scanning electron microscopy. Results. While the effect of surface treatment was significant (p = 1.27 × 10−9 ), there was no detected effect due to resin (p = 0.829). All treatments except CC (30.1 MPa ×/÷ 1.44)* were significantly better than the untreated control (24.8 MPa ×/÷ 1.35) (p = 3.28 × 10−9 ). While the effect of GB – which gave the highest mean strength (50.5 MPa ×/÷ 1.29) – was not distinguishable from that of GHF (39.9 MPa ×/÷ 1.29) (p = 0.082), it was significantly better than treatment with either CC or Z (33.1 MPa ×/÷ 1.48) (p < 0.05). (* After log transformation for analysis and back; asymmetric error bounds as s.d. in log values.) Significance. The novel test method design, which has good discriminatory power, confirmed the value of gritblasting as a simple and effective treatment with low operator hazard. It gave the highest bond strengths regardless of the cement type. Glaze layer application followed
∗
Corresponding author. Tel.: +44 1225 81 06 71. E-mail address: [email protected] (B.W. Darvell).
http://dx.doi.org/10.1016/j.dental.2015.03.004 0109-5641/© 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: S¸anlı S, et al. Influence of surface treatment on the resin-bonding of zirconia. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.03.004
DENTAL-2527; No. of Pages 12
2
ARTICLE IN PRESS d e n t a l m a t e r i a l s x x x ( 2 0 1 5 ) xxx–xxx
by hydrofluoric acid-etching on zirconia before cementation might be viable for adhesive zirconia cementation, but represents a much greater hazard as well as having problems with thickness control. © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Zirconia, or rather yttria-partially stabilized tetragonal zirconia (YTZP), is increasingly used in dentistry in view of its remarkable strength [1,2]. However, a major impediment to its effective use is its lack of reactivity since it is a non-polar and more or less inert material which becomes very dense and homogeneous on sintering [3]. This means that all the usual means of bonding (in the broad chemical sense of established a covalent bond between adhesive material and the substrate), effective enough for other ceramic systems, do not work [3], and although phosphate-based systems have shown some improvement [3], nothing is yet considered reliable enough. This also includes the mechanical retention afforded by gritblasting, where particle size has to be limited to minimize subsurface damage and hence the risk of cracking [4]. Thus, it is of great interest to develop an effective means of bonding to zirconia to enable long-term retention. For this purpose, a great number of surface treatment methods have been tried, for example to roughen the surface with rotary systems, laser irradiation, or selective infiltration etching, or to modify the surface through silica coating, silane application, hydrofluoric acid etching after fused glass-ceramic application, or nanostructured alumina coating [3,5–7]. The nature of dentistry, in terms of economy, processing time, and the fact that all restorative devices are effectively one-off prototypes, means that the clinical and laboratory procedures to be used for zirconia-based polycrystalline ceramic restorations to achieve adhesive cementation should be practical and easily handled. We may note that while hydrofluoric acid (HF) etching is an easy – if hazardous – surface treatment for silicate-containing ceramics [8], some oxide ceramics, and in particular zirconia, which contain less than 15 mass% silica and little or no glass phase [9], cannot be so treated. However, the application of a fused glass-ceramic on the intaglio surfaces of zirconia restorations, which glaze is then etchable and so capable of adhesive cementation, has been described [5,10,11]. The effectiveness of this bonding depends on intimate contact and micromechanical key with grain boundary topography, rather than chemical interaction as has sometimes been suggested [12], there being no chemical reaction or elemental migration or diffusion at the interface [13]. Some studies suggest that compressive stresses arising from a difference in coefficient of thermal expansion (CTE) between a ceramic and zirconia have a positive correlation with the bond strength [14]. This is a general principle that should apply in the present context. Ordinary silane chemistry also may not improve the affinity of resin cements for zirconia since it is more stable then silica-based ceramics and cannot be hydrolysed easily [3]. Zirconate coupling agents have been introduced with the aim of improving the bonding of zirconia
to resin cements [3]. It has been reported that the phosphate groups of one of these primers, which is organophosphatecarboxylic acid monomer based, bond to metal oxides such as zirconia, while the organic moiety can be co-polymerized with the monomers of filled resins and therefore might be feasible as a surface treatment for zirconia [15]. The great variability in the data obtained from some studies of dental materials stems from a variety of reasons: inappropriate set-ups for the relevant clinical problem, unsuitable test methods, inappropriate specimen geometries, as well as several variables that cannot be standardized [16,17]. Bond strength is commonly treated in direct tension since brittle dental materials are more sensitive to tensile stresses than compression [18], but there are great difficulties in doing this with ceramics because of that brittleness: risk of introduction of surface flaws during the machining of the specimens, and parasitic stresses introduced in mounting – small misalignments are enough to be problematic [17,19]. Tensile strength as such is then often determined by a bending (or flexural) test [20], which is less sensitive to such problems, although alignment of the supports remains critical. Shear testing is associated with much greater difficulties [17,21]. Simple shear test designs, as commonly used in dental research, do not give a uniform stress at the interface, especially with elastic modulus mismatch, and with stress concentrations at contact points [22]. Flexural strength can be determined using three- and four-point loading. However, the two modes commonly yield different results [23]. Although the equations for the two approaches consider the differences in load application and stress distribution, this difference between results is explained by the volume of the test piece affected: in a threepoint test the effective volume of peak stress (or location for a critical flaw) is very small and lies under the center loading point, while in a four-point test the region between the inner span loading points (which may be half the outer span length) is uniformly stressed in pure bending such that the risk of a large critical flaw being present in the critical zone is much greater. However, if a bending test is to be used instead of direct tension for a bonding effectiveness assessment, logically the bonded surface must be central. However, in three-point bending this then requires great (and, routinely, essentially unobtainable) precision with the alignment of the surface directly under the central load roller, but if the ‘substrate-superstructure’ model is used, whether in a threeor four-point test, the specimen is then asymmetrical in modulus of elasticity and so in deflection, and thus the stress state is not accurately represented by the usual equations [24,25]. Furthermore, to place a load singularity on the adhesive layer is itself problematic. Despite this, such asymmetric systems have been used in four-point bending of zirconia-resin-dentin and zirconia-resin [24,26]. Thus, although it could be argued
Please cite this article in press as: S¸anlı S, et al. Influence of surface treatment on the resin-bonding of zirconia. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.03.004
DENTAL-2527; No. of Pages 12
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that concentrated loading at the interface (i.e. in tension on the lower surface) is relevant there would be practical difficulties. This is borne out perhaps by the large coefficients of variation found with this approach, some 20 ∼ 28% [24]. Elementary beam theory is based on the idea that the geometry of the system is unaffected under load. In addition, the shear stress in the specimen between the outer and inner support(s) contributes to the deflection, and if this is large it must be taken into account [20], especially for deflection calculations [27]. Even slight malalignment in the three-point case will thus superimpose a shear stress on the bonded region which will compromise the interpretation of the results, and this applies especially when the bonded region has a definite thickness (as it must), and especially with intermediary layers: the stress causing failure is unknown. In any case, a uniform stress state is required in order to identify the weakest link, and this cannot be attained in three-point bending. This does not apply in the four-point case. To control shear deformation, and thus without correction, a span to depth ratio (S/d) of 16 is considered acceptable for most materials, although some require S/d = 32–64 in four-point tests [27]. For dental ceramics, the recommended ratio is 10:1 or greater to provide consistent strength values [28]. Four-point bending is used for porous [29] materials such as spinel, zirconia and graphite, according to ASTM C1674–11, a standard flexural test method for advanced ceramics with engineered porosity (e.g. honeycomb cellular channels) at ambient temperatures. The goal of laboratory testing ought to be to provide interpretable data through a mode of loading relevant to the service conditions. Certainly, it is difficult to envisage a pure tensile stress state in any oro-dental system (likewise for pure shear, a fortiori). On the other hand, bending, especially at the pontic of a fixed dental prosthesis (FDP), is much more common, and occlusal loads there cause traction forces that have been reported to cause a lever effect on the terminal edges of the restorations [30] – rather similar to the stress state in the load axis plane of a flexural test specimen. This might therefore provide an appropriate model for simulating (or at least representing) intraoral loading conditions. Indeed, a flexural strength test is widely used and proposed for strength testing for ceramics [27], although the symmetrical four-point mode has not previously been used for dental ceramic-resin bond strength testing. In microtensile tests, since the resin, treated surfaces and zirconia have differing elastic moduli, Poisson ratios and strengths, the materials’ differing behaviors mutually interact and a pure, uniform stress field is unattainable [25]. This situation cannot be controlled. In the usual so-called ‘shear’ tests, uncontrolled and unmeasurable parasitic stresses occur since the bending of the material cannot be prevented despite all the precautions taken in test designs [17,21,31]. In contrast, three-point bending and microtensile tests are recommended for homogeneous materials, while four-point bending tests should be preferred for heterogeneous materials, where feasible [25]. The primary motivation for the present work was to be able to study the behavior of the interface between zirconia and resin cement or surface treatments (and, secondarily, between resin cement and surface treatment). The union between adhesive and tooth tissue was not of concern and
3
thus could be eliminated from the test system. This suggested that a symmetrical zirconia-surface treatment-resin cementsurface treatment-zirconia test system could be used. Given then that all interfaces should be loaded identically, and also given the problems of direct tension and shear, four-point bending appears to provide all necessary conditions. Thus, with a view to identifying an effective procedure for bonding zirconia, and a suitable test method free of common problems, the effect of various surface treatments on the strength of the union to luting resins was investigated using a symmetrical specimen, bonding zirconia to zirconia, using four-point bending, enabling easier fabrication and simpler interpretation of results.
2.
Materials and methods
2.1.
Specimen preparation
Presintered zirconia blocks (Vita In-Ceram YZ-40/19; Vita Zahnfabrik, Bad Säckingen, Germany) were cut with a low-speed (400 rpm) diamond saw under water cooling (Isomet 1000; Buehler, Lake Bluff, IL, USA) to give bars with the dimensions of 31.25 mm × 6.25 mm × 2.50 mm (each aspect measured at three places with a digital caliper) using an acrylic jig. These were then sintered at 1530 ◦ C for 7.5 h in a furnace (MOS 160/1; Protherm, Ankara, Turkey) giving final dimensions of 25.0 mm × 5.0 mm × 2.0 mm after shrinkage. These dimensions were decided according to previous remarks on S/d ratio [27,28] for this test method [29], given the size of the zirconia blocks. The zirconia specimens were then ultrasonically cleaned (Sonorex RK102 Transistor; Bandelin, Walldorf, Germany) for 15 min in 96% ethanol (Smyras, Bornova, I˙zmir, Turkey) and 15 min in distilled water to ensure the absence of particulate debris immediately before surface treatment. Calculation indicated that 10 specimens per subgroup would give a power >90%.
2.2.
Surface treatments
The following treatments were used, with 20 pieces per group for n = 10 luted assemblies.
2.2.1.
Control (C)
No surface treatment.
2.2.2.
Gritblasting (GB)
Gritblasting (EasyBlast; Bego, Bremen, Germany) was performed normal to the luting surface with 50 m Al2 O3 (Shera Werkstoff Technologie, Hannover, Germany) for 13 s at a pressure of 2.8 bar from a distance of 10 mm [32].
2.2.3.
Glaze and hydrofluoric acid etching (GHF)
The luting surface received two coats of sprayed glaze (Crystall Glaze spray; Ivoclar Vivadent, Schaan, Liechtenstein) yielding a continuous, thin layer. Glaze firing was conducted with a ceramic furnace (Programat P90; Ivoclar Vivadent), following the manufacturer’s detailed program, but broadly this involved starting at 403 ◦ C, the temperature then being raised
Please cite this article in press as: S¸anlı S, et al. Influence of surface treatment on the resin-bonding of zirconia. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.03.004
DENTAL-2527; No. of Pages 12
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ARTICLE IN PRESS d e n t a l m a t e r i a l s x x x ( 2 0 1 5 ) xxx–xxx
at 60 ◦ C/min to 725 ◦ C, which was held for 1 h before cooling. Glaze that extended beyond the cementation surface was removed, by grinding flat on 600-grit SiC paper under water, to avoid any supportive effect during the fracture test. The glazed surfaces were then treated at room temperature with 9.5% hydrofluoric acid gel (Porcelain Etch; Ultradent Products, South Jordan, UT, USA) for 60 s, rinsed with deionized water for 90 s. They were then neutralized with a slurry of neutralizing powder (CaCO3 , Na2 CO3 ) (IPS Ceramic Neutralizing Powder, Ivoclar Vivadent) for 5 min, washed thoroughly for 20 s with distilled water, and air dried [5].
2.2.4.
Fusion glass-ceramic (CC)
To mix the material (Crystall.Connect; Ivoclar Vivadent), the supplied closed capsule was lightly pressed onto a vibrating plate (Ivomix; Ivoclar Vivadent) for 10 s for agitation. The mixture was applied with a brush tip on the cementation surface, the specimen held against the vibrating plate for 5 s, and then allowed to dry for 60 s. After cleaning away the excess material with a clean brush tip, firing was conducted using a ceramic furnace (Programat P90; Ivoclar Vivadent), again following the manufacturer’s detailed program; broadly, this involved starting at 403 ◦ C, the temperature then being raised at 30 ◦ C/min to 820 ◦ C for 2 min, then to 840 ◦ C for 7 min before cooling. The prepared luting surface was then etched, washed, and dried as above.
2.2.5.
Zirconia primer (Z)
After cleaning the cementation surfaces by rinsing with deionized water and air drying, two coats of primer (Z-Prime Plus; Bisco, Schaumburg, IL, USA) were applied uniformly, wetting the bondable surface, and dried with an air syringe for 3–5 s to remove solvent, according to the manufacturer’s instructions.
2.3.
Luting
The zirconia bars were luted end-to-end on the prepared surfaces with one of two chemically distinct luting resins (Panavia F 2.0: Kuraray, Tokyo, Japan [P]; Rely-X U 200: 3 M ESPE, St. Paul, USA [R]) (Table 1). A custom-made stainless steel mold was used to standardize the luting material thickness to 0.10 ± 0.05 mm. Specimens were placed in the mold end to end having applied the cement to one end of each using a cotton pellet, the mold’s screw was tightened to approximate the pieces and decrease the cement gap thickness to the target value, as indicated by the separation of the mold faces. A 4 mm gap between the mold’s approximating faces was left to allow for the removal of excess cement. Prior to the cementation the length of each half-specimen (∼25 mm) was measured before placing in the mold. The gap between the mold’s edges was measured using a digital caliper to calculate the closure required. After closure, excess cement was removed and the mold face separation rechecked. For both resins, “catalyst” and “base” pastes in equal amounts were mixed according to the manufacturer’s instructions, applied to both luting surfaces of the zirconia bars, which were then brought together. Under magnification (loupe, 20×), excess cement was removed using microapplicators (Disposable micro applicators, fine, Premium Plus International, Hong Kong, China) to avoid dragging the resin
from the interface. For resin P, the material was then covered with an oxygen-inhibiting material (Oxyguard; Kuraray, Tokyo, Japan), and irradiated for 20 s each from the top and the bottom (LED Bluephase C5; Ivoclar Vivadent),. Resin R was similarly irradiated from top and bottom for 10 s each, to prevent movement, then left undisturbed for 5 min (25 ◦ C, 50–65% relative humidity) to complete self-curing.
2.4.
Four-point bending tests
Specimens were mounted with the luted interface centralized between the upper load points. The load was applied through rods of radius 2.0 mm at a crosshead speed of 0.5 mm/min on a computer-controlled universal testing machine (Autograph Model AG-5 kN; Shimadzu, Kyoto, Japan). The load at fracture was recorded. Four-point flexural strength ( 4 ) was calculated from: 4 =
3F(L − Li ) 2wh2
where F is the load at fracture, Li is the distance between the centers of the loading rollers (20.0 mm), L is the distance between the centers of the supporting rollers (40.0 mm), w is the specimen width, and h the mean thickness (2.0 mm) [29], giving S/d = 20.
2.5.
Fractographic analysis
After fracture, the surfaces of the specimens were evaluated under light microscopy (LM) (Eclipse ME600, Nikon, Melville, NY, USA) at 10× magnification. Failure was classified as: adhesive (a) – more than 85% between the framework and the cement; cohesive (c) – more than 85% debonding within the zirconia, lute, or both; and mixed (m) – other combination. This was on the grounds that neither adhesive nor cohesive failure was seen ‘pure’, and the arbitrary cut was taken to try to assess the dominant behavior. Two specimens for each group were taken for further analysis of the fractured surfaces using Environmental Scanning Electron Microscope-Energy Dispersive Spectroscopy (ESEM-EDS) (Quanta 250 FEG SEM, FEI, Hillsboro, OR, USA). Low-vacuum imaging mode was used, with no need for a conductive specimen coating. Specimens were evaluated under 56× and 10,000× magnifications and analysed with energydispersive X-ray spectroscopy (EDS) at 43× magnification and 10 keV.
2.6.
Statistical analysis
Flexural strengths were subjected to two-way analysis of variance (2 × AoV), Resin × Surface Treatment, subsequently reducing to 1 × AoV on Surface Treatment, with checks for homogeneity of variance and normality (Shapiro–Wilk) (SigmaPlot 12.5, Systat Software, San Jose, CA, USA). Comparisons were then made using the Holm–Sidak method. Fracture-type distribution was examined by means of loglinear analysis of the three-way contingency table (3 × CT) [33], then by exact 2 (StatXact v.9, Cytel Software, Cambridge, MA, USA) on the then-indicated collapsed 2 × CT.
Please cite this article in press as: S¸anlı S, et al. Influence of surface treatment on the resin-bonding of zirconia. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.03.004
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Table 1 – Manufacturers’ information on products used. Product
Manufacturer
Panavia F 2.0a
Kuraray Medical, Tokyo, Japan
Composition description/mass%
Batch
A paste: BPEDMA/MDP/DMA B paste: * Ba-* B-Si-glass/silica-containing composite Glycerin gel
00037A 00020A
3 M/Espe Seefeld, Germany
Glass powder, silica, calcium hydroxide, pigment, substituted pyrimidine, peroxy compound, initiator Methacrylated, phosphoric esters, dimethacrylates, acetate, stabilizers, self-cure initiators, light-cure initiators
315751
Vita In-ceram YZ-40/19 Zirconia Block
Vita Zahnfabrik, Bad Säckingen, Germany
91–94%* ZrO2 , 4–6%* Y2 O, 2–4%* HfO2 ,
ARTICLE IN PRESS d e n t a l m a t e r i a l s x x x ( 2 0 1 5 ) xxx–xxx
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Influence of surface treatment on the resin-bonding of zirconia ˘ b , Erhan C¸ömlekoglu ˘ b, Seda S¸anlı a , Mine Dündar C¸ömlekoglu Mehmet Sonugelen b , Tijen Pamir c , B.W. Darvell d,∗ a
Ulukent Dental Clinic, Menemen, Izmir, Turkey Department of Prosthodontics, School of Dentistry, Ege University, Izmir, Turkey c Department of Restorative Dentistry, School of Dentistry, Ege University, Izmir, Turkey d Dental Materials Science, Faculty of Dentistry, Kuwait University, Kuwait b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. To compare the effect of various surface treatments on the bonding of luting resin
Received 29 September 2014
cements to zirconia under four-point bending.
Received in revised form
Methods. Bar specimens (n = 200) (2 mm × 5 mm × 25 mm) were prepared from zirconia blocks
8 January 2015
(VITA In-Ceram YZ, Vita Zahnfabrik) with the cementation surface (2 mm × 5 mm) of
Accepted 13 March 2015
groups of 40 treated in one of five ways: airborne particle abrasion with 50 m Al2 O3
Available online xxx
(GB), zirconia primer (Z-Prime Plus, Bisco) (Z), glaze ceramic (Crystall.Glaze spray, Ivoclar Vivadent) + hydrofluoric acid (GHF), fusion glass-ceramic (Crystall.Connect, Ivoclar Vivadent)
Keywords:
(CC), or left untreated as control (C). Within each treatment, bars were cleaned ultrasoni-
Zirconia surface treatment
cally for 15 min in ethanol and then deionized water before bonding in pairs with one of
Adhesive cementation
two luting resins: Panavia F 2.0, (Kuraray) (P); RelyX U-200 (3 M/Espe) (R), to form 10 test
Four-point bending test
specimens for each treatment and lute combination. Mechanical tests were performed and bond strengths (MPa) were subject, after log transformation, to analysis of variance, Shapiro–Wilk and Holm–Sidak tests; also log-linear contingency analysis of failure mode distribution; all with ˛ = 0.05. Fracture surfaces were examined under light and scanning electron microscopy. Results. While the effect of surface treatment was significant (p = 1.27 × 10−9 ), there was no detected effect due to resin (p = 0.829). All treatments except CC (30.1 MPa ×/÷ 1.44)* were significantly better than the untreated control (24.8 MPa ×/÷ 1.35) (p = 3.28 × 10−9 ). While the effect of GB – which gave the highest mean strength (50.5 MPa ×/÷ 1.29) – was not distinguishable from that of GHF (39.9 MPa ×/÷ 1.29) (p = 0.082), it was significantly better than treatment with either CC or Z (33.1 MPa ×/÷ 1.48) (p < 0.05). (* After log transformation for analysis and back; asymmetric error bounds as s.d. in log values.) Significance. The novel test method design, which has good discriminatory power, confirmed the value of gritblasting as a simple and effective treatment with low operator hazard. It gave the highest bond strengths regardless of the cement type. Glaze layer application followed
∗
Corresponding author. Tel.: +44 1225 81 06 71. E-mail address: [email protected] (B.W. Darvell).
http://dx.doi.org/10.1016/j.dental.2015.03.004 0109-5641/© 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: S¸anlı S, et al. Influence of surface treatment on the resin-bonding of zirconia. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.03.004
DENTAL-2527; No. of Pages 12
2
ARTICLE IN PRESS d e n t a l m a t e r i a l s x x x ( 2 0 1 5 ) xxx–xxx
by hydrofluoric acid-etching on zirconia before cementation might be viable for adhesive zirconia cementation, but represents a much greater hazard as well as having problems with thickness control. © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Zirconia, or rather yttria-partially stabilized tetragonal zirconia (YTZP), is increasingly used in dentistry in view of its remarkable strength [1,2]. However, a major impediment to its effective use is its lack of reactivity since it is a non-polar and more or less inert material which becomes very dense and homogeneous on sintering [3]. This means that all the usual means of bonding (in the broad chemical sense of established a covalent bond between adhesive material and the substrate), effective enough for other ceramic systems, do not work [3], and although phosphate-based systems have shown some improvement [3], nothing is yet considered reliable enough. This also includes the mechanical retention afforded by gritblasting, where particle size has to be limited to minimize subsurface damage and hence the risk of cracking [4]. Thus, it is of great interest to develop an effective means of bonding to zirconia to enable long-term retention. For this purpose, a great number of surface treatment methods have been tried, for example to roughen the surface with rotary systems, laser irradiation, or selective infiltration etching, or to modify the surface through silica coating, silane application, hydrofluoric acid etching after fused glass-ceramic application, or nanostructured alumina coating [3,5–7]. The nature of dentistry, in terms of economy, processing time, and the fact that all restorative devices are effectively one-off prototypes, means that the clinical and laboratory procedures to be used for zirconia-based polycrystalline ceramic restorations to achieve adhesive cementation should be practical and easily handled. We may note that while hydrofluoric acid (HF) etching is an easy – if hazardous – surface treatment for silicate-containing ceramics [8], some oxide ceramics, and in particular zirconia, which contain less than 15 mass% silica and little or no glass phase [9], cannot be so treated. However, the application of a fused glass-ceramic on the intaglio surfaces of zirconia restorations, which glaze is then etchable and so capable of adhesive cementation, has been described [5,10,11]. The effectiveness of this bonding depends on intimate contact and micromechanical key with grain boundary topography, rather than chemical interaction as has sometimes been suggested [12], there being no chemical reaction or elemental migration or diffusion at the interface [13]. Some studies suggest that compressive stresses arising from a difference in coefficient of thermal expansion (CTE) between a ceramic and zirconia have a positive correlation with the bond strength [14]. This is a general principle that should apply in the present context. Ordinary silane chemistry also may not improve the affinity of resin cements for zirconia since it is more stable then silica-based ceramics and cannot be hydrolysed easily [3]. Zirconate coupling agents have been introduced with the aim of improving the bonding of zirconia
to resin cements [3]. It has been reported that the phosphate groups of one of these primers, which is organophosphatecarboxylic acid monomer based, bond to metal oxides such as zirconia, while the organic moiety can be co-polymerized with the monomers of filled resins and therefore might be feasible as a surface treatment for zirconia [15]. The great variability in the data obtained from some studies of dental materials stems from a variety of reasons: inappropriate set-ups for the relevant clinical problem, unsuitable test methods, inappropriate specimen geometries, as well as several variables that cannot be standardized [16,17]. Bond strength is commonly treated in direct tension since brittle dental materials are more sensitive to tensile stresses than compression [18], but there are great difficulties in doing this with ceramics because of that brittleness: risk of introduction of surface flaws during the machining of the specimens, and parasitic stresses introduced in mounting – small misalignments are enough to be problematic [17,19]. Tensile strength as such is then often determined by a bending (or flexural) test [20], which is less sensitive to such problems, although alignment of the supports remains critical. Shear testing is associated with much greater difficulties [17,21]. Simple shear test designs, as commonly used in dental research, do not give a uniform stress at the interface, especially with elastic modulus mismatch, and with stress concentrations at contact points [22]. Flexural strength can be determined using three- and four-point loading. However, the two modes commonly yield different results [23]. Although the equations for the two approaches consider the differences in load application and stress distribution, this difference between results is explained by the volume of the test piece affected: in a threepoint test the effective volume of peak stress (or location for a critical flaw) is very small and lies under the center loading point, while in a four-point test the region between the inner span loading points (which may be half the outer span length) is uniformly stressed in pure bending such that the risk of a large critical flaw being present in the critical zone is much greater. However, if a bending test is to be used instead of direct tension for a bonding effectiveness assessment, logically the bonded surface must be central. However, in three-point bending this then requires great (and, routinely, essentially unobtainable) precision with the alignment of the surface directly under the central load roller, but if the ‘substrate-superstructure’ model is used, whether in a threeor four-point test, the specimen is then asymmetrical in modulus of elasticity and so in deflection, and thus the stress state is not accurately represented by the usual equations [24,25]. Furthermore, to place a load singularity on the adhesive layer is itself problematic. Despite this, such asymmetric systems have been used in four-point bending of zirconia-resin-dentin and zirconia-resin [24,26]. Thus, although it could be argued
Please cite this article in press as: S¸anlı S, et al. Influence of surface treatment on the resin-bonding of zirconia. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.03.004
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that concentrated loading at the interface (i.e. in tension on the lower surface) is relevant there would be practical difficulties. This is borne out perhaps by the large coefficients of variation found with this approach, some 20 ∼ 28% [24]. Elementary beam theory is based on the idea that the geometry of the system is unaffected under load. In addition, the shear stress in the specimen between the outer and inner support(s) contributes to the deflection, and if this is large it must be taken into account [20], especially for deflection calculations [27]. Even slight malalignment in the three-point case will thus superimpose a shear stress on the bonded region which will compromise the interpretation of the results, and this applies especially when the bonded region has a definite thickness (as it must), and especially with intermediary layers: the stress causing failure is unknown. In any case, a uniform stress state is required in order to identify the weakest link, and this cannot be attained in three-point bending. This does not apply in the four-point case. To control shear deformation, and thus without correction, a span to depth ratio (S/d) of 16 is considered acceptable for most materials, although some require S/d = 32–64 in four-point tests [27]. For dental ceramics, the recommended ratio is 10:1 or greater to provide consistent strength values [28]. Four-point bending is used for porous [29] materials such as spinel, zirconia and graphite, according to ASTM C1674–11, a standard flexural test method for advanced ceramics with engineered porosity (e.g. honeycomb cellular channels) at ambient temperatures. The goal of laboratory testing ought to be to provide interpretable data through a mode of loading relevant to the service conditions. Certainly, it is difficult to envisage a pure tensile stress state in any oro-dental system (likewise for pure shear, a fortiori). On the other hand, bending, especially at the pontic of a fixed dental prosthesis (FDP), is much more common, and occlusal loads there cause traction forces that have been reported to cause a lever effect on the terminal edges of the restorations [30] – rather similar to the stress state in the load axis plane of a flexural test specimen. This might therefore provide an appropriate model for simulating (or at least representing) intraoral loading conditions. Indeed, a flexural strength test is widely used and proposed for strength testing for ceramics [27], although the symmetrical four-point mode has not previously been used for dental ceramic-resin bond strength testing. In microtensile tests, since the resin, treated surfaces and zirconia have differing elastic moduli, Poisson ratios and strengths, the materials’ differing behaviors mutually interact and a pure, uniform stress field is unattainable [25]. This situation cannot be controlled. In the usual so-called ‘shear’ tests, uncontrolled and unmeasurable parasitic stresses occur since the bending of the material cannot be prevented despite all the precautions taken in test designs [17,21,31]. In contrast, three-point bending and microtensile tests are recommended for homogeneous materials, while four-point bending tests should be preferred for heterogeneous materials, where feasible [25]. The primary motivation for the present work was to be able to study the behavior of the interface between zirconia and resin cement or surface treatments (and, secondarily, between resin cement and surface treatment). The union between adhesive and tooth tissue was not of concern and
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thus could be eliminated from the test system. This suggested that a symmetrical zirconia-surface treatment-resin cementsurface treatment-zirconia test system could be used. Given then that all interfaces should be loaded identically, and also given the problems of direct tension and shear, four-point bending appears to provide all necessary conditions. Thus, with a view to identifying an effective procedure for bonding zirconia, and a suitable test method free of common problems, the effect of various surface treatments on the strength of the union to luting resins was investigated using a symmetrical specimen, bonding zirconia to zirconia, using four-point bending, enabling easier fabrication and simpler interpretation of results.
2.
Materials and methods
2.1.
Specimen preparation
Presintered zirconia blocks (Vita In-Ceram YZ-40/19; Vita Zahnfabrik, Bad Säckingen, Germany) were cut with a low-speed (400 rpm) diamond saw under water cooling (Isomet 1000; Buehler, Lake Bluff, IL, USA) to give bars with the dimensions of 31.25 mm × 6.25 mm × 2.50 mm (each aspect measured at three places with a digital caliper) using an acrylic jig. These were then sintered at 1530 ◦ C for 7.5 h in a furnace (MOS 160/1; Protherm, Ankara, Turkey) giving final dimensions of 25.0 mm × 5.0 mm × 2.0 mm after shrinkage. These dimensions were decided according to previous remarks on S/d ratio [27,28] for this test method [29], given the size of the zirconia blocks. The zirconia specimens were then ultrasonically cleaned (Sonorex RK102 Transistor; Bandelin, Walldorf, Germany) for 15 min in 96% ethanol (Smyras, Bornova, I˙zmir, Turkey) and 15 min in distilled water to ensure the absence of particulate debris immediately before surface treatment. Calculation indicated that 10 specimens per subgroup would give a power >90%.
2.2.
Surface treatments
The following treatments were used, with 20 pieces per group for n = 10 luted assemblies.
2.2.1.
Control (C)
No surface treatment.
2.2.2.
Gritblasting (GB)
Gritblasting (EasyBlast; Bego, Bremen, Germany) was performed normal to the luting surface with 50 m Al2 O3 (Shera Werkstoff Technologie, Hannover, Germany) for 13 s at a pressure of 2.8 bar from a distance of 10 mm [32].
2.2.3.
Glaze and hydrofluoric acid etching (GHF)
The luting surface received two coats of sprayed glaze (Crystall Glaze spray; Ivoclar Vivadent, Schaan, Liechtenstein) yielding a continuous, thin layer. Glaze firing was conducted with a ceramic furnace (Programat P90; Ivoclar Vivadent), following the manufacturer’s detailed program, but broadly this involved starting at 403 ◦ C, the temperature then being raised
Please cite this article in press as: S¸anlı S, et al. Influence of surface treatment on the resin-bonding of zirconia. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.03.004
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at 60 ◦ C/min to 725 ◦ C, which was held for 1 h before cooling. Glaze that extended beyond the cementation surface was removed, by grinding flat on 600-grit SiC paper under water, to avoid any supportive effect during the fracture test. The glazed surfaces were then treated at room temperature with 9.5% hydrofluoric acid gel (Porcelain Etch; Ultradent Products, South Jordan, UT, USA) for 60 s, rinsed with deionized water for 90 s. They were then neutralized with a slurry of neutralizing powder (CaCO3 , Na2 CO3 ) (IPS Ceramic Neutralizing Powder, Ivoclar Vivadent) for 5 min, washed thoroughly for 20 s with distilled water, and air dried [5].
2.2.4.
Fusion glass-ceramic (CC)
To mix the material (Crystall.Connect; Ivoclar Vivadent), the supplied closed capsule was lightly pressed onto a vibrating plate (Ivomix; Ivoclar Vivadent) for 10 s for agitation. The mixture was applied with a brush tip on the cementation surface, the specimen held against the vibrating plate for 5 s, and then allowed to dry for 60 s. After cleaning away the excess material with a clean brush tip, firing was conducted using a ceramic furnace (Programat P90; Ivoclar Vivadent), again following the manufacturer’s detailed program; broadly, this involved starting at 403 ◦ C, the temperature then being raised at 30 ◦ C/min to 820 ◦ C for 2 min, then to 840 ◦ C for 7 min before cooling. The prepared luting surface was then etched, washed, and dried as above.
2.2.5.
Zirconia primer (Z)
After cleaning the cementation surfaces by rinsing with deionized water and air drying, two coats of primer (Z-Prime Plus; Bisco, Schaumburg, IL, USA) were applied uniformly, wetting the bondable surface, and dried with an air syringe for 3–5 s to remove solvent, according to the manufacturer’s instructions.
2.3.
Luting
The zirconia bars were luted end-to-end on the prepared surfaces with one of two chemically distinct luting resins (Panavia F 2.0: Kuraray, Tokyo, Japan [P]; Rely-X U 200: 3 M ESPE, St. Paul, USA [R]) (Table 1). A custom-made stainless steel mold was used to standardize the luting material thickness to 0.10 ± 0.05 mm. Specimens were placed in the mold end to end having applied the cement to one end of each using a cotton pellet, the mold’s screw was tightened to approximate the pieces and decrease the cement gap thickness to the target value, as indicated by the separation of the mold faces. A 4 mm gap between the mold’s approximating faces was left to allow for the removal of excess cement. Prior to the cementation the length of each half-specimen (∼25 mm) was measured before placing in the mold. The gap between the mold’s edges was measured using a digital caliper to calculate the closure required. After closure, excess cement was removed and the mold face separation rechecked. For both resins, “catalyst” and “base” pastes in equal amounts were mixed according to the manufacturer’s instructions, applied to both luting surfaces of the zirconia bars, which were then brought together. Under magnification (loupe, 20×), excess cement was removed using microapplicators (Disposable micro applicators, fine, Premium Plus International, Hong Kong, China) to avoid dragging the resin
from the interface. For resin P, the material was then covered with an oxygen-inhibiting material (Oxyguard; Kuraray, Tokyo, Japan), and irradiated for 20 s each from the top and the bottom (LED Bluephase C5; Ivoclar Vivadent),. Resin R was similarly irradiated from top and bottom for 10 s each, to prevent movement, then left undisturbed for 5 min (25 ◦ C, 50–65% relative humidity) to complete self-curing.
2.4.
Four-point bending tests
Specimens were mounted with the luted interface centralized between the upper load points. The load was applied through rods of radius 2.0 mm at a crosshead speed of 0.5 mm/min on a computer-controlled universal testing machine (Autograph Model AG-5 kN; Shimadzu, Kyoto, Japan). The load at fracture was recorded. Four-point flexural strength ( 4 ) was calculated from: 4 =
3F(L − Li ) 2wh2
where F is the load at fracture, Li is the distance between the centers of the loading rollers (20.0 mm), L is the distance between the centers of the supporting rollers (40.0 mm), w is the specimen width, and h the mean thickness (2.0 mm) [29], giving S/d = 20.
2.5.
Fractographic analysis
After fracture, the surfaces of the specimens were evaluated under light microscopy (LM) (Eclipse ME600, Nikon, Melville, NY, USA) at 10× magnification. Failure was classified as: adhesive (a) – more than 85% between the framework and the cement; cohesive (c) – more than 85% debonding within the zirconia, lute, or both; and mixed (m) – other combination. This was on the grounds that neither adhesive nor cohesive failure was seen ‘pure’, and the arbitrary cut was taken to try to assess the dominant behavior. Two specimens for each group were taken for further analysis of the fractured surfaces using Environmental Scanning Electron Microscope-Energy Dispersive Spectroscopy (ESEM-EDS) (Quanta 250 FEG SEM, FEI, Hillsboro, OR, USA). Low-vacuum imaging mode was used, with no need for a conductive specimen coating. Specimens were evaluated under 56× and 10,000× magnifications and analysed with energydispersive X-ray spectroscopy (EDS) at 43× magnification and 10 keV.
2.6.
Statistical analysis
Flexural strengths were subjected to two-way analysis of variance (2 × AoV), Resin × Surface Treatment, subsequently reducing to 1 × AoV on Surface Treatment, with checks for homogeneity of variance and normality (Shapiro–Wilk) (SigmaPlot 12.5, Systat Software, San Jose, CA, USA). Comparisons were then made using the Holm–Sidak method. Fracture-type distribution was examined by means of loglinear analysis of the three-way contingency table (3 × CT) [33], then by exact 2 (StatXact v.9, Cytel Software, Cambridge, MA, USA) on the then-indicated collapsed 2 × CT.
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Table 1 – Manufacturers’ information on products used. Product
Manufacturer
Panavia F 2.0a
Kuraray Medical, Tokyo, Japan
Composition description/mass%
Batch
A paste: BPEDMA/MDP/DMA B paste: * Ba-* B-Si-glass/silica-containing composite Glycerin gel
00037A 00020A
3 M/Espe Seefeld, Germany
Glass powder, silica, calcium hydroxide, pigment, substituted pyrimidine, peroxy compound, initiator Methacrylated, phosphoric esters, dimethacrylates, acetate, stabilizers, self-cure initiators, light-cure initiators
315751
Vita In-ceram YZ-40/19 Zirconia Block
Vita Zahnfabrik, Bad Säckingen, Germany
91–94%* ZrO2 , 4–6%* Y2 O, 2–4%* HfO2 ,
