Australian Dental Journal

The official journal of the Australian Dental Association

Australian Dental Journal 2016; 61: 163–173 doi: 10.1111/adj.12365

An insight into current concepts and techniques in resin bonding to high strength ceramics R Luthra,* P Kaur† *Professor, Department of Prosthodontics, Swami Devi Dyal Hospital and Dental College, Barwala, Panchkula, Haryana, India. †Reader, Department of Prosthodontics, Swami Devi Dyal Hospital and Dental College, Barwala, Panchkula, Haryana, India.

ABSTRACT Background: Reliable bonding between high strength ceramics and resin composite cement is difficult to achieve because of their chemical inertness and lack of silica content. The aim of this review was to assess the current literature describing methods for resin bonding to ceramics with high flexural strength such as glass-infiltrated alumina and zirconia, densely sintered alumina and yttria-partially stabilized tetragonal zirconia polycrystalline ceramic (Y-TZP) with respect to bond strength and bond durability. Methods: Suitable peer reviewed publications in the English language were identified through searches performed in PubMed, Google Search and handsearches. The keywords or phrases used were ‘resin-ceramic bond’, ‘silane coupling agents’, ‘air particle abrasion’, ‘zirconia ceramic’ and ‘resin composite cements’. Studies from January 1989 to June 2015 were included. Results: The literature demonstrated that there are multiple techniques available for surface treatments but bond strength testing under different investigations have produced conflicting results. Conclusions: Within the scope of this review, there is no evidence to support a universal technique of ceramic surface treatment for adhesive cementation. A combination of chemical and mechanical treatments might be the recommended solution. The hydrolytic stability of the resin ceramic bond should be enhanced. Keywords: Air particle abrasion, resin-ceramic bond, resin composite cements, silane coupling agents, zirconia ceramic. Abbreviations and acronyms: AIN = aluminium nitride; APA = airborne particle abrasion; APF = acidulated phosphate fluoride; HIM = heat induced maturation; MTBS = microtensile bond strength testing; MVD = molecular vapor deposition; SIE = selective infiltration etching; TSC = tribochemical silicoating. (Accepted for publication 6 August 2015.)

INTRODUCTION In recent years, an increasing demand for high performance aesthetic restorations has led to the development of several new ceramics with high flexural strength such as glass-infiltrated alumina and zirconia, densely sintered alumina and yttria–partially stabilized tetragonal zirconia polycrystalline ceramic (Y-TZP).1–5 Clinical fracture of zirconia is rarely seen.6,7 Due to their high fracture resistance, these restorations can be cemented using conventional cements such as zinc phosphate and resin modified glass ionomer.8 However, resin cements offer the advantage of shadematching, marginal adaptation, high flexural strength and fracture resistance, especially in short clinical crowns and heavy occlusal forces.9–16 Resin bonding also helps to improve retention in CAD/CAM-milled ceramic restorations and to seal minor internal surface © 2016 Australian Dental Association

flaws created by acid-etching or airborne particle abrasion.9,17,18 A strong resin-ceramic bond relies on micromechanical interlocking and chemical bonding. Insufficient surface modification could affect the retention of ceramic.19,20 Conventional adhesive bonding techniques such as surface etching and silanization are well recognized for silica based or feldspathic ceramics, but establishing a strong and stable bond with high strength ceramics like alumina and zirconia has proved to be difficult as the materials are hard, acidresistant and silica free.21,22 The silica content of alumina ceramics is below 5 wt% and that of zirconia ceramics is below 1 wt%. The silica content of lithium-disilicate (IPS Empress 2) and feldspathic ceramics is approximately 50–60 wt%.22 Various researchers have investigated different surface pretreatments to optimize the surface of 163

R Luthra and P Kaur high strength ceramic materials but results have varied.13,23,24 The aim of this review was to assess the current literature for laboratory studies on resin bonding to high strength dental ceramics with respect to bond strength and bond durability. MATERIALS AND METHODS Suitable peer reviewed publications in the English language were identified through searches performed in PubMed, Google Search and handsearches. The keywords or phrases used were ‘resin-ceramic bond’, ‘silane coupling agents’, ‘air particle abrasion’, ‘zirconia ceramic’ and ‘resin composite cements’. Studies from January 1989 to June 2015 were included. Titles and abstracts were evaluated for appropriateness to fulfill the inclusion and exclusion criteria. Articles that did not focus exclusively on resin–ceramic bonding and properties of zirconia ceramics were excluded from further evaluation. Non-peer reviewed dental literature, abstracts and clinical reports were excluded from review. Of the retrieved articles, a total of 37 articles were selected on the resin bond to silica-based ceramics, and 78 articles on bonding to high strength dental ceramics, which included aluminium-oxide ceramics (16 articles) and zirconium-oxide ceramics (62 articles). An additional 27 references were included to provide supplementary information about the characteristics of resin bonding to ceramics. Common surface treatment options were airborne particle abrasion with aluminium oxide,22,23,25–44 diamond blasting with synthetic diamond particles,25 grinding,29,30 abrasion with diamond rotary instruments,35,41 application of fused glass micropearls,45 plasma spraying,45 tribochemical silicoating (TBS),21,22,26,32,34,37,46–58 Pyrosil-Pen technology,59 silica seed treatment,60 laser treatment,61 selective infiltration etching,39,62,63 alumina coating,64 acid-etching,19,21,28,32,65–86 silane coupling agents,13,19,20,58,66–68,74,84,87–100 zirconia primers,41,101–107 and luting agents containing adhesive phosphate monomer.20,34,39,47,52,55,56,58,75,81,108–126 Table 1 shows a chronological overview of some studies on resin bonding to high strength ceramics including the type of ceramic tested, surface treatment and cement, method of artificial ageing, method of testing and mean bond strength in each experiment. The effect of artificial ageing127–129 and testing methods130–139 has also been studied widely. This article presents a comprehensive review on various mechanical and chemical treatments currently being used for bonding to high strength ceramics. Mechanical methods Airborne particle abrasion (APA) may substantially increase the roughness and surface area of ceramic surfaces, enhance the potential for micromechanical 164

retention and increase the bond strength.27,32,33,42 APA is performed using 50 lm to 110 lm grain sized aluminium trioxide powder under 0.2 MPa pressure from a distance of 10–25 mm for 13–20 seconds until a white opaque colour appears. Hummel et al. stated ‘neither stable micromechanical retention nor stable chemical bonds could be achieved without sandblasting’.34 APA also cleans the surface of any contaminants or saliva which might prevent chemical bonding.28,40 On the other hand, few studies stated that APA did not change the surface microstructure of silica free ceramics and recommended alternate protocols to ensure adequate bonding.20,22,26,31 Borges found that APA of high purity alumina ceramic with 50 lm aluminium oxide caused flattening of the alumina crystals rather than creation of microretentive features.31 Few studies have expressed concerns about the potential long-term adverse effect of surface abrasion such as structural damage, creation of sharp crack tips, grain pullout and material loss, especially at the margins of the restorations.23,26,30,38,39 Zhang et al.23 found that strengths of sandblasted alumina and zirconia specimens showed significant reduction in both dynamic and cyclic tests, indicative of larger crack initiating flaws. These defects could further weaken the bond strength and compromise the fatigue strength of alumina and zirconia ceramics.38 In contrast, several studies found that APA increases the flexural strength of Y-TZP zirconia.29,35– 37,41,44 Low stresses developed during this process may cause a transformation of surface crystals from a tetragonal (t) to a monoclinic (m) phase, with consequent volume expansion and a compressive stress field around the crack tip, thus preventing its further propagation.35–37,41,44 Mild sandblasting (110 lm particle size and 0.2 MPa pressure) could be beneficial, whereas severe sandblasting (250 lm and 0.4 MPa) induces much larger damage.43 Tribochemical silicoating The ceramic surface is blasted with silica coated alumina particles using compressed air.21,26 The impact results in a partial coating of silica on the surface that is further primed by silanization, after which the restoration may be cemented using resin composite cement.49 The tribochemical effect has two aspects: micromechanical bonding to resin due to surface topography and promotion of a chemical bond between silica and resin via silane coupling agent. Several investigations have demonstrated higher bond strength after silicoating and silanation than that achieved by APA alone.22,26,32,34,37,46–50,53–57,90,95,135 Most of the studies which have used TBS along with © 2016 Australian Dental Association

© 2016 Australian Dental Association

Procera AllZirkon (Nobel Biocare)

InCeram Alumina, (Vita)

InCeram Zirconia (CEREC In-Lab, VITA) Procera AllCeram

Blatz et al.111 2004

Kim et al.22 2005

Bottino et al.53 2005

2005

Zirconia Procera

Zirconia Procera

Aboushelib et al.62 2008

Aboushelib et al.102 2009

Zirconia ZirCAD (Ivoclar Vivadent)

Zirconia (Y-TZP, Cercon)

Piascik et al.60 2009

Cavalcanti et al.61 2009

Zirconia (Cercon)

Y-TZP (Cercon, DeguDent)

Aboushelib et al.39 2007

2009

Zirconia (Cercon, Degudent)

Wolfart et al.118 2007

Oyague et al.

InCeram Zirconia (VITA) InCeram Zirconia (VITA)

Amaral et al.57 2006 Valandro et al.129 2007

121

ZrO2-TZP (Cercon Smart)

Luthy et al.56 2006

zirconium-oxide (Cercon, Degussa Dental)

zirconium-oxide (DCS)

2006

Kumbuloglu et al.114 2006

Atsu et al.

55

Derand et al.45 2005

Valandro et al.

54

Y-TZP Procera Zircon

Aluminium oxide (Procera AllCeram, Nobel Biocare)

2003

Blatz et al.109 2003

Janda et al.

59

glass infiltered aluminium oxide (InCeram, Vita) Zirconium oxide (Degussit)

Ceramic

Kern and Thompson47 1995

Study and year

Recommended surface treatment and cement

Air particle abrasion with 110-lm (Al2O3) + selective infiltration-etching + Panavia F 2.0 Selective-infiltration-etching + silane based zirconia primers + Panavia F 2.0 selective-infiltrationetching + silane based zirconia primers + Panavia F 2.0 Tribochemical silica coating + Clearfil Esthethic cement (Kuraray) Air particle abrasion with 50-lm (Al2O3) + SixO4 seed layer + silane + resin adhesive + resin luting cement (C&B Bisco) Air particle abrasion with 50-lm (Al2O3) + metal primer + BISGMA based resin cement (Calibra, Dentsply)

Air particle abrasion with 50-lm (Al2O3) + Panavia F

Tribochemical silica coating + silane + Panavia F Tribochemical silica coating + silane + Panavia F

Micropearls of low fusing porcelain + silane + Variolink II (Ivoclar Vivadent) Air particle abrasion with 125-lm (Al2O3) + Tribochemical silica coating + MDP–containing bonding/silane coupling agent mixture + Panavia F Air particle abrasion with 50-lm (Al2O3) + Tribochemical silica coating + Panavia F/Rely X unicem Tribochemical silica coating + Rely X unicem/Panavia21

Tribochemical silica coating + Silane + Panavia F

Air particle abrasion with 50-lm Al2O3 + Clearfil SE Bond/Porcelain Bond Activator (Kuraray) + Panavia F Silica coating + silane bonding agent/ primer and bonding resin + Z100Composite (3M ESPE) Tribochemical silica coating + silane + Panavia F

Tribochemical silica coating + BIS-GMA based resin cement PyroSil Pen (Flame treatment) + silane + BIS-GMA based resin cement APA with 50-lm Al2O3 + silane + Panavia

Table 1. Studies on resin bonding to high strength ceramics

24 hrs in water at 37 °C

90 days water storage at 37 °C 6 months water storage at 37 °C Room temp for 24 hrs

24 hours at room temp

48 hrs in water at 37 °C and 10 000 cycles thermocycling 7 days in water at 37 °C 300 days in water and 12 000 cycles thermocycling 150 days in water and 37 500 cycles thermocycling 1 month in water at 37 °C

24 hrs in water and 2000 cycles thermocycling

24 hrs in distilled water at 37 °C

150 days in water 37 500 cycles thermocycling 24 hr dry storage and 5000 cycles thermocycling 180 days in water and 12 000 cycles thermocycling 180 days in water, and 12 000 cycles thermocycling 72 hours in saline solution at 37 °C 7 days in distilled water at 37 °C. 7 days in distilled water at 37 °C Air storage for 1 hr

Method of artificial ageing

microshear

microtensile

microtensile

microtensile

microtensile

microtensile

tensile

microtensile microtensile

shear

shear

shear

shear

microtensile

microtensile

tensile

shear

shear

shear

tensile

Method of testing

(continued)

27.99

23.2

15.36

15-18

28-40.6

52.2

39.2

26.7 4.3

36.7-73.8

20.9

22.9

18.4

18.5

26.8

18.6

16.85

16.09

16

49.85

Mean bond strength (MPa)

Current concepts and techniques in resin bonding

165

166

TZ-3YB-E Zirconia (Tosoh, Tokyo, Japan)

Zirconia (Cercon Base)

Jevnikar et al.64 2010

Kitayama et al.98 2010

Zirconia (YZ-In Ceram YZ, VITA)

Zirconia (LAVA, 3M ESPE) Zirconia (LAVA, 3M ESPE) Zirconia (Cercon; Dentsply)

Zirconia (TZP BIOHIP; Metoxit AG)

deCastro et al.123 2012

Piascik et al.106 2012

Saryazdi et al.124 2014

2013

APA = air particle abrasion.

Chen et al.

107

Lung et al.105 2012

Matinlinna and Lassila104 2011

Zirconia (Lava Frame, 3M ESPE) Procera All Zircon

de Souza et al.122 2011

Zirconia (e.max ZirCAD, Ivoclar Vivadent)

Zirconium oxide (LAVA, 3M ESPE)

Magne et al.103 2010

Attia et al.58 2011

Y-TZP (ZirCAD, Ivoclar Vivadent)

Ceramic

Qeblawi et al.41 2010

Study and year

Table 1 continued

Tribochemical silica coating + silane + resin luting cement (Multilink Automix, Ivoclar Vivadent) Air particle abrasion with 50-lm (Al2O3) + zirconia primer (a mixture of organophosphate and carboxylic acid monomers) + resin luting cement (Duolink, Bisco) Air particle abrasion with 110-lm (Al2O3) + nanostructural alumina coating + phosphate ester containing resin luting cement (Rely X Unicem, 3M ESPE) Air particle abrasion with 110-lm (Al2O3) + phosphonic acid containing ceramic primers (AZ primer, Shofu) + resin luting cement (Resicem, Shofu) Tribochemical silica coating + universal primer containing a silane and a phosphate monomer (Monobond Plus, Ivoclar Vivadent) + resin luting cement (Multilink Automix, Ivoclar Vivadent) Primer containing MDP, VBATDT (Alloy Primer, Kuraray) + RelyX Unicem Tribochemical silica coating + silanization with silane monomer primers (glycidoxypropyltrimethoxysilane) + RelyX Unicem Tribochemical silica coating + Rely X Ceramic Primer + resin luting agent (Rely X ARC, 3M ESPE) Plasma Fluorination gas phase treatment for 5 mins. + Filtek Ultra Supreme (3M ESPE) Tribochemical silica coating + silane coupling agent + Rely X Unicem Air particle abrasion with 50-lm(Al2O3) + zirconia primer containing MDP and BisGMA (ZPrime Plus, Bisco) + resin composite cement (Duolink, Bisco) Air particle abrasion with 50-lm (Al2O3) + RelyX Unicem 2

Recommended surface treatment and cement

24 hrs in water at 37 °C + 5000 cycles thermocycling + dynamic axial loading

Water storage at 37 °C for 24 hrs 30 days in water and 6000 cycles of thermocycling Water storage at 37 °C for 24 hrs

60 days in water and 10 000 cycles thermocycling

6000 cycles of thermocycling

150 days in water at 37 °C

30 days in water at 37 °C and 7500 cycles thermocycling

24 hrs water storage

12 000 cycles thermocycling

90 days in an incubator at 100% humidity at 37 °C and 6000 thermal cycles 24 hrs in distilled water

Method of artificial ageing

tensile

shear

shear

shear

microtensile

shear

microtensile

tensile

tensile

shear

shear

shear

Method of testing

3.7

29.0

14.5

37.3

12.9

17.6

6.1

39.7

22.3

27.32

26.68

30.9

Mean bond strength (MPa)

R Luthra and P Kaur

© 2016 Australian Dental Association

Current concepts and techniques in resin bonding zirconia primers or phosphate monomer containing cements have shown bond strength value after artificial ageing (late bond strength) greater than 20 MPa (Table 1). Systems such as the Rocatecâ system (3M ESPE) consist of an APA pretreatment with Rocatec Pre powder (110 lm aluminium oxide) under 0.2 MPa pressure to clean the alumina/zirconium ceramic surface, then a silica coating with Rocatec Plus powder (110 lm aluminium oxide, coated with silicon dioxide) and finally, the application of silane. The CoJet System (3M ESPE) uses 30 lm aluminium oxide particles modified with salicylic acid. CoJet is used for clinical procedures, such as the intraoral repair of fractured metal-ceramic and all-ceramic restorations with resin composites. An oil-free air stream or ultrasonic cleaning in alcohol may be used to remove visible dust resulting from APA as loose surface particles might negatively influence adhesion.58 However, the cleaning method should not remove the silica coating layer from the ceramic surface. It is crucial to perform silica coating perpendicular to the surface to obtain the greatest benefit, as it is more difficult to achieve on the intaglio surface of a crown than on a flat ceramic specimen.52 Silicoating can also be achieved chairside by using PyroSil Penâ technology using flame treatment.59 Piascik et al. advocated an approach to surface functionalize zirconia with a SixOy ‘seed’ layer of thickness 2.6 nm by a molecular vapor deposition (MVD) of vapor phase mixture of water and tetrachlorosilane for 15 minutes in a vacuum chamber. The improved chemical reactivity of the silica seed treated surfaces was found to be superior to that of the tribochemical technique.60 While the TSC system significantly increased the bond strength for InCeram (Vita) and Procera AllCeram (Nobel Biocare) ceramics,48,51 it required the use of a resin composite containing an adhesive phosphate monomer for improved bond strength for zirconia ceramic.20,123 Cavalcanti et al. investigated the role of erbiumdoped yttrium aluminum garnet laser (Er:YAG) to enhance the bond strength of resin composites to ceramics.61 They used laser equipment emitting a 2.94 lm wavelength with a 1000 lm diameter straight-type contact probe perpendicular to the surface. The energy intensity was set low at 200 mJ. Er:YAG laser has the ability to remove particles by micro explosions and by vaporization, a process called ablation. The ceramic surface was irradiated for five seconds using a fine water spray. Results indicated that laser irradiation was not as effective in improving bond strength as air abrasion. SEM images showed that Er:YAG laser resulted in a smooth surface of Y-TZP plates, with some perceivable cracks. The mechanical properties of Y-TZP ceramics can be © 2016 Australian Dental Association

negatively affected by changes in temperature during laser treatment. Aboushelib et al. proposed selective infiltration etching (SIE), that uses principles of heat induced maturation (HIM) and grain boundary diffusion of molten glass, to selected areas of zirconia, providing nanomechanical retention.62 The bond strength of the HIM/SIE group was higher than APA treated specimens and not affected by artificial ageing.39 A similar technique was tested by atomic force microscopy and considered a promising treatment for conditioning zirconia.63 Chemical methods Chemical etching In feldspathic ceramics, hydrofluoric acid (HF acid 5–9.5%) applied for 60 seconds66,68,69,80,83 selectively dissolves the glassy components, producing a porous, irregular honeycomb-like surface,65,67,76 that provides more surface area and surface energy prior to combining with the silane solution.21,66,74,77,78,81,82 Various other chemicals have been used for etching of silica containing ceramics including orthophosphoric acid,28,72,73 sulphuric acid, nitric acid, ammonium hydrogen bifluoride71 and acidulated phosphate fluoride (APF);70 HF acid, being more aggressive, gave the highest bond strength.79,84–86 However, as the silica phase in ceramic is the only phase able to be etched by HF acid, it is inefficient in providing adequate retention in high strength silica free ceramics which cannot be etched.32,47,67,75 A silane coupling agent, 3-methacryloxypropyltrimethoxysilane (3-MPS), is often used prior to the application of an adhesive resin on the roughened ceramic surface as a standard practice of porcelain repair. Silane increases the substrate surface energy and improves surface wettability.13,49,58,68,87,93,95,96,99 Due to its bi-functional characteristics, it is capable of forming a siloxane network with the silica phase in ceramics on one side and copolymerizing with the organic matrix of the resin composite on the other, producing strong chemical bonds between composites and ceramics.19,74,84,88–94,98 Della Bona et al.93 found that the chemical adhesion produced by silane promoted higher mean bond strength values than the micromechanical retention produced by any etchant for the resin-ceramic systems. The traditional silane chemistry is not applicable to zirconia unless it is silicoated.45 Another clinical problem is the bond degradation over time in the oral environment.20,39,47,51,55,111,113,116,129 Kern and Wegner reported good initial bond strength of resin cement to APA treated zirconia. However, adding a silane did not improve the durability of the bond in 167

R Luthra and P Kaur water. Excessive thickness of silane (40 nm) can compromise its beneficial effect due to structural stratification in three layers. The innermost layer provides a strong siloxane bond, whereas the outermost and intermediate layers are only physically adsorbed. Heat treatment at 100 °C consolidates these three layers into a monolayer (30 nm) and eliminates silane byproducts like water or alcohol. Rinsing in hot water at 80 °C showed thinner silane films (14 nm).100 Heat treatment of the silane film improves its chemical reactivity and the resistance to hydrolysis. Derand et al. investigated plasma spray technique to deposit a siloxane coating (hexamethyldisiloxane) on zirconia.45 Bond strength with resin was improved but the exact mechanism of bond formation was unclear. Zirconia primers In recent years, manufacturers have developed several commercial zirconia primers (phosphate monomers) or silane primers.41,107 Aboushelib et al. used novel engineered zirconia primers in combination with selective infiltration etching as a surface pretreatment. These primers were 3-Acryloyloxypropyltrimethoxysilane, 3-Isocyanatopropyltriethoxysilane, 3-Styrylethyltrimethoxysilane and 3-methacryloyloxypropyltri methoxysilane.102 Presence of specific organofunctional groups in these primers may improve spatial compatibility and increase the reactivity of silane monomers. These primers initially showed high bond strength values but significant reduction in bond strength was seen after 90 days of water storage. Long-term bond stability requires developing more hydrophobic compounds. The bonding should rely on chemical interaction as well as a mechanically retentive surface of zirconia.102 Matinlinna et al. in a similar study found a significant increase in bond strength when these primers were applied to silica coated zirconia.104 Phosphate monomer 10-methacryloyloxydecyl dihydrogen phosphate (MDP), has a long carbonyl chain and plays a beneficial role in establishing a relatively hydrolytically stable chemical bond to zirconia.20,34,39,56,116 The application of an MDP containing bonding/silane coupling agent mixture (Clearfil SE Bond Primer and Clearfil Porcelain Bond Activator, Kuraray) to intaglio surfaces of Procera All Ceram alumina109 and Procera All Zirkon restorations abraded with airborne Al2O3 particles has shown high bond strength.58,111,119 MDP incorporated into the resin composite also showed high and durable bond strengths for air-abraded alumina and zirconia specimens,108,109,118 e.g. Panavia 21 (Kuraray-Noritake, Tokyo, Japan). Applying a 168

TSC and a silane coupling agent in combination with Panavia 21 could be a recommended option,47,121 whereas de Oyague stated that with a phosphate monomer containing luting system, other surface treatments like air abrasion or silica coating are not necessary.120 Various investigations have found cements like glass-ionomer cement110 or a 4-META containing adhesive resin (Superbond C&B, Sun Medical)52,75,112 produce a superior bonding to Y-TZP than Panavia 21. No inorganic filler is contained in Super-Bond C&B, as the primary ingredient is referred to as 4-META/PMMA-TBB (4 methacryloxyethyl-trimellit at-anhydrid/polymethylmethacrylate-tri-n-butylborane) resin. This cement has a low modulus of elasticity. The ductile resin cement thus functions as a shock absorber so that it can distribute forces during fracture testing on the tooth-cement-ceramic complex. In addition, Super-Bond C&B contains long flexible chains of high molecular weight, which tend to result in higher fracture toughness values when compared with highly cross-linked brittle materials contained in other composite cements. Plastic deformation delays the onset of brittle fracture, resulting in higher fracture toughness values. However, Hummel stated that water absorption of PMMA over long-term storage diminishes the bond strength of Superbond C&B.34 Another cement, RelyX Unicem (3M ESPE, Seefeld, Germany), a self-adhesive dual polymerizing phosphate ester containing resin composite, has shown greater bond strength than Panavia.56,81,112–115,122 The phosphate ester methacrylate monomer in RelyX Unicem’s composition was found to be more efficient for bonding to zirconia than the MDP monomer. Panavia has a high viscosity that results in a greater film thickness which compromises its adaptation to the zirconia surface. Saryazdi et al. stated that Panavia F and Rely X Unicem cements provided significantly higher retention than the Bis-GMA based resin cement. According to them, zirconia crown retention is dependent on the adhesive cement used and not on the internal surface treatment.124 However, Aboushelib et al. stressed that mechanical retention is important to gain any benefit from the MDP resin composite and cement alone will not be able to provide long-term bond strength.102 Cassuci et al.63 evaluated the effect of an experimental hot etching solution on the bonding potential of zirconia. The solution consisted of methanol, 37% HCl and ferric chloride, and was earlier used to etch the wings of Maryland bridges. They assumed that the solution may be somewhat beneficial for etching zirconium due to the metallic nature of pure zirconium. The results of their work showed that the solution improved surface roughness of zirconium through a corrosion-controlled process. © 2016 Australian Dental Association

Current concepts and techniques in resin bonding Metal primers, such as VBATDT (6-(N-(4vinylbenzyl)propylamino)-1,3,5-triazine-2,4-dithione; Kuraray-Noritake, Tokyo, Japan), Metaltite (MTU-6: 6-methacryloxyhexyl 2-thiouracil-5-carboxylate; Tokuyama Dental Corporation, Tokyo, Japan) and Metal Primer II (MEPS: thiophosphoric methacrylate; GC Corporation, Tokyo, Japan), capable of bonding to noble and base metals also have been tested to improve bond strength between Y-TZP and luting resin systems.34,61,101 The presence of phosphate monomer in these primers is the reason for the stability of the bond. Metal primers and air abrasion can have a synergistic effect on bonding to zirconia. A new light-polymerizable zirconia priming agent (Z-Prime Plus Bisco, Schaumburg, IL, USA) that is a mixture of organophosphate and carboxylic acid monomers has also showed a positive influence on the initial bond strength regardless of the luting agent used but the durability of the bond was not investigated.103,106 Kitayama et al. found that a new primer (AZ Primer, Shofu, Kyoto, Japan) containing a phosphonic acid monomer 6-MHPA (6-methacryloxyhexylphosphonoacetate), was effective in improving bonding performance.98 Lung et al. evaluated the application of three novel coupling agents: 2-hydroxyethylmethacrylate, itaconic acid and oleic acid to silicoated zirconia samples. These three coupling agents are cheap, have longer shelf life and do not require hydrolysis compared to silane coupling agents. However, the bond showed degradation after artificial ageing.105 Jevnikar proposed a non-invasive method where a nanostructured alumina coating was achieved by hydrolysis of aluminium nitride (AlN) powder to form cAlOOH (boehmite). Boehmite, when subjected to heat treatment, thermally decomposes to form the alumina coating.64 The coating had a high surface area, good wetting ability and achieved a micromechanical interlocking as it created nanosized interlamellar spaces, providing a highly retentive surface of Y-TZP ceramics for resin penetration.64 The coating thickness was found to be only 240 nm, so it did not alter the clinical fit of zirconia restorations. This can be classified as a chemical pretreatment method that increases the surface area. Plasma fluorinated zirconia has shown improved chemical bonding compared to zirconia primers by improving surface wetting.106 However, these methods require handling of very toxic and hazardous precursors like sulphur hexafluoride and tetrachlorosilane. Artificial ageing Besides establishing a strong resin-ceramic bond, maintaining this bond under functional conditions of fatigue, saliva and temperature changes for an acceptable © 2016 Australian Dental Association

period of time is a crucial aspect. Artificial ageing gives a measure of bond durability and is done either by water storage or thermocycling, with thermocycling having a greater impact than water storage at a constant temperature.128 A decrease in the resin-ceramic bond strength value after artificial ageing was observed for many commercial systems in several studies.20,39,47,51,55,95,111,113,116,129 However, several other studies did not subject their specimens to artificial ageing, or the ageing period was too short to simulate clinical conditions.22,39,45,53–55,57,60–62,98,103,106,107 Kumbuloglu et al.114 found that water storage for one week or 2000 cycles of thermocycling after water storage for 24 hours did not significantly affect the bonding properties of both Panavia F and Rely X Unicem cements to air abraded zirconia. In the literature, there is no consensus on a regimen for artificial ageing and the cycles are set arbitrarily, ranging from 100 to 50 000, which make it difficult to compare results. Gale and Darvel concluded that 10 000 cycles correspond to approximately one year of clinical function.127 Moreover, a meaningful test of ceramic bonding should involve cyclic loading, high numbers of low load chewing cycles and water storage for at least six months.42,112 All these investigations were performed under controlled laboratory conditions and no clinical trials were done to validate the results. Testing methods Many basic concepts of shear testing used in several investigations have been shown to be incorrect.42,138 Recently many authors have advocated microtensile bond strength testing (MTBS) where the load is applied perpendicular to the bonded interface and the specimen size is small.78,84,102,123,131,132,139 However, the MTBS test is a tedious and meticulous method which requires time and effort, especially during preparing and sectioning of the specimens, to avoid damaging the microbars.39,61,133 Furthermore, bond strength test findings should be combined with chemical analysis,117 fractographic analysis134–137 and cyclic loading.23,42,133 Future studies should also include a combined tooth/composite/ porcelain interface in the test complex.42,61,123,130,133 Kern emphasized the need for clinical evidence of successful bonding of ceramic restorations as a vast majority of the articles present laboratory research, while clinical trials are rare.125 He reviewed the clinical trials with resin bonded alumina and zirconia ceramic restorations that had limited mechanical retention and relied solely on adhesion. He concluded that air abrasion at a moderate pressure and using phosphate monomer containing luting resins provides long-term durable bonding to alumina and zirconia ceramic under humid and stressful oral conditions. 169

R Luthra and P Kaur CONCLUSIONS Adhesion between high strength ceramics and resin cements remains controversial compared with the high predictability of silica based ceramics and resin cements. To date, many different studies conducted on bond strength have not been able to provide a strong and durable bond, especially after thermal cycling or artificial ageing. The aim of ongoing research in this field is to achieve improved bond strength which can stand the test of time. Air abrasion of acid-resistant ceramics is important to improve bonding to resin. The use of tribochemical silica coating along with zirconia primers or phosphate monomer containing cements has shown a higher late bond strength values. The studies which used a combination of mechanical and chemical methods showed greater bond strength values than studies which used these methods alone. There are still some possibilities for improving bond strength and durability that need to be tested, including modern cements and adhesive primer materials. Further laboratory studies, as well as controlled clinical trials, are needed before clinical recommendations can be given. REFERENCES 1. Guazzato M, Albakry M, Ringer SP, Swain MV. Strength, fracture toughness and microstructure of a selection of allceramic materials. Part II. Zirconia-based dental ceramics. Dent Mater 2004;20:449–456. 2. Yilmaz H, Aydin C, Gul BE. Flexural strength and fracture toughness of dental core ceramics. J Prosthet Dent 2007;98:120–128. 3. Griggs JA. Recent advances in materials for all-ceramic restorations. Dent Clin North Am 2007;51:713–727. 4. Kelly JR, Denry I. Stabilized zirconia as a structural ceramic: an overview. Dent Mater 2008;24:289–298. 5. Denry I, Kelly JR. State of the art of zirconia for dental applications. Dent Mater 2008;24:299–307.

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