Short communication

Improved bonding of zirconia substructures toresin using a ‘‘glaze-on’’ technique

Peter Everson a, Owen Addison b, William M. Palin c, F.J. Trevor Burke d,*aGeneral Dental Practitioner, Leeds, UKbRestorative Dentistry, University of Birmingham, School of Dentistry, College of Medical and Dental Sciences, St. Chad’s Queensway,

Birmingham B4 6NN, UKcBiomaterials Unit, University of Birmingham, School of Dentistry, College of Medical and Dental Sciences, Birmingham B4 6NN, UKdPrimary Dental Care Research Group, University of Birmingham, School of Dentistry, College of Medical and Dental Sciences,

St. Chad’s Queensway, Birmingham B4 6NN, UK

j o u r n a l o f d e n t i s t r y 4 0 ( 2 0 1 2 ) 3 4 7 – 3 5 1

a r t i c l e i n f o

Article history:

Received 11 May 2011

Received in revised form

13 December 2011

Accepted 13 December 2011

Keywords:

Zirconia

Bonding

Ceramic

Resin luting cement

a b s t r a c t

Objective: To investigate the influence of applying thin intermediary coatings of acid-etch-

able glasses on the shear bond strength between a methacrylate resin based cement and an

yttria-stabilized zirconia dental ceramic substrate.

Methods: The upper and lower surfaces of 110 sintered yttria-stabilised tetragonal zirconia

polycrystalline disc-shaped specimens were polished using sequential grades of Silicon

Carbide, then air-abraded with 25 mm diameter alumina particles. Specimens were ran-

domly allocated to 11 groups (A–K) (n = 10), group A acting as control. The upper surface of

Group A specimens was subjected to a tribochemical coating regime (CoJet, 3M ESPE). Five

glazing ceramics were applied and fired according to the manufacturer’s recommended

firing regime. The glaze was etched with 10% HF acid and all specimens coated with a silane

primer. Two differing storage regimes were employed (wet storage vs thermocycling).

Shear bond strength testing specimens were created by cementing resin-based composite

cylinders to the centre of the prepared ceramic surface using Rely-X Unicem (3M ESPE)

resin based cement. Shear bond strength testing was performed and load at failure

recorded.

Results: A factorial analysis of variance at a 95% significance level demonstrated that all

glazing techniques resulted in a significant increase in the shear bond strength

compared with using the resin based cement alone (P < 0.01), with the different glazing

ceramics resulting in significant differences in mean shear bond strength (P = 0.008).

The differences were a function of the storage state (wet storage vs thermocycling

(P = 0.013)).

Conclusion: The glazing techniques used in the current investigation resulted in a signifi-

cantly enhanced shear bond stress to the resin based cement when compared with the

current ‘gold standard’ – tribochemical coating.

# 2012 Published by Elsevier Ltd.

Available online at www.sciencedirect.com

journal homepage: www.intl.elsevierhealth.com/journals/jden

* Corresponding author. Tel.: +44 121 237 2767; fax: +44 121 237 2768.E-mail address: f.j.t.burke@bham.ac.uk (F.J. Trevor Burke).

0300-5712/$ – see front matter # 2012 Published by Elsevier Ltd.doi:10.1016/j.jdent.2011.12.011

j o u r n a l o f d e n t i s t r y 4 0 ( 2 0 1 2 ) 3 4 7 – 3 5 1348

1. Introduction

Since the properties of zirconium dioxide (hitherto termed

Zirconia in this paper) were first considered by Garvie et al. in

1975, there has been considerable focus on the use of zirconia

in biomedical applications.1 In this respect, zirconia possesses

attractive properties for consideration as a dental restorative

material. It exists as a chemically stable, dense, opaque

material possessing a high flexural strength and a fracture

toughness exceeding all previous classes of commercially

available dental ceramic. Within dentistry, the advancement

of Computer Aided Design/Computer Aided Machining (CAD/

CAM) technologies have facilitated the manufacture of

zirconia prostheses and have allowed the manufacture of

restorations of ever increasing complexity and accuracy.2

Following the introduction of the Lava all-ceramic system in

2001 by 3M ESPE (Seefeld, Germany), in vivo clinical studies

have demonstrated promising clinical performance,3,4 and, in

the ensuing ten years, a wide variety of zirconia core materials

have been developed and are now commercially available. By

providing a genuine alternative to the metal in metal–ceramic

restorations, the recent improvements in technologies for

machining, veneering and bonding zirconia offer the oppor-

tunity to expand the perceived roles of the zirconia restora-

tions.

A key problem facing dental operators using highly crystal-

line ceramic restorations such as high purity alumina or

zirconia is the inability to etch and silanise the surface to

create a topography conducive to micromechanical and

chemical bonding to dental resin cements. Operators currently

may roughen the surfaces with particle air abrasion or rotary

instrumentation, employ phosphate monomer conditioners,

which bond weakly to ceramic oxides, or use tribochemical

techniques to enhance the resin bond to the ceramic substrate.5

The tribochemical techniques have been considered the ‘gold

standard’ by some manufacturers and involve impregnating

the ceramic surface with silica. This creates a chemically

modified surface onto which conventional silane-based pri-

mers will condense and bond.5 However, the bond strengths

achieved are considerably lower than those achievable to

conventional dental glass ceramics, where the ‘‘gold standard’’

may be considered to be the bond between the HF-etched

surface of a feldspathic ceramic veneer and phosphoric acid-

etched enamel via a resin-based luting material.6

The search for innovative processes to allow bonding

between resin based cements and highly crystalline (non-

etchable) ceramics is therefore one of the most studied topics

Table 1 – Glazing ceramics utilized in the study.

Ceramic material Manufacturer

Vitadur Alpha Vita Zahnfabrik

Ivoclar Emax Ceram Ivoclar Vivadent

Noritake Cerabien Noritake Ceramics

Vita VM9 Vita Zahnfabrik

Lava Ceram 3M ESPE

Lava (Y-TZP) 3M ESPE

in the field of Dental Materials research. Processes that have

been suggested include tribochemical techniques, use of

phosphate monomers, modification of zirconia surfaces to

include microporosity, precipitation of nano-sized alumina

crystals on the zirconia surfaces and selective infiltration

etching.5,7–10 The latter essentially is the application of a glaze

which is applied to the zirconia. During the fusion of the

veneering layer with the zirconia, the veneering layer

infiltrates the superficial layer of zirconia, leading to separa-

tion of the crystals. Subsequent dissolution of the glass

ceramic using 5% hydrofluoric acid solution in an ultrasonic

bath leads to a micromechanically retentive zirconia sur-

face,7,8 which has been considered to establish a strong and

durable bond of resin to zirconia-based materials.8

The current status of adhesive cementation of zirconia has

recently been the subject of a comprehensive review 11, so it is

not the intention of this work to add to this, but only to repeat

its conclusion, namely, ‘‘that although the science applied to

adhesive bonding with zirconia has improved, there is still

much to be learned to make this a predictable behaviour for

clinical use’’.11 The present work therefore suggests an

hypothesis that a simple, inexpensive and accessible labora-

tory process which is to ‘glaze-on’ a thin coating of a thermal

expansion-matched etchable ceramic onto the inner/‘fit’

surface of the zirconia restoration and subsequently etch this

layer with HF-acid in order to facilitate bonding to this layer.

Should this technique be successful, the role of zirconia may

be expanded to be used in minimally retentive situations,

including adhesive bridgework.11

2. Objectives

To investigate the impact of applying thin intermediary

coatings of acid-etchable glasses on the shear bond strength

between a methacrylate resin based cement and an Yttria-

stabilized zirconia dental ceramic substrate.

3. Materials and methods

One hundred and ten sintered yttria (-stabilised) tetragonal

zirconia polycrystalline (Y-TZP; LAVA) disc-shaped specimens

(12 mm diameter and 2 mm thickness) were supplied by 3M

ESPE (Seefeld, Germany). The upper and lower surfaces were

polished using sequential grades of Silicon Carbide abrasives

from P80 through P220, P320, P500, P800, P1200 to a P2000 grit

Thermal expansioncoefficient a (mm/mK),

mean (SD)

Glaze thickness(mm), mean (SD)

7.3 (0.1) 120 (20)

9.5 (0.2) 150 (30)

9.9 (0.3) 130 (20)

9.3 (0.1) 140 (10)

9.9 (0.1) 140 (20)

10.8 (0.1)

j o u r n a l o f d e n t i s t r y 4 0 ( 2 0 1 2 ) 3 4 7 – 3 5 1 349

for controlled time intervals. The specimen surfaces were then

air-abraded with 25 mm diameter alumina particles (Plas Dent,

West Midlands, UK) delivered perpendicular to the specimen

surface at a pressure of 5000 hPa from a distance of 20 mm for

5 s prior to thorough washing in distilled water.

Specimens were randomly allocated to 11 groups (A–K)

(n = 10) with group A acting as the control. The upper surface of

Group A specimens was subjected to a tribochemical coating

regime (CoJet, 3M ESPE), delivered according to the manu-

facturer’s recommendations, whereas all remaining groups

received a surface glaze.

Five glazing ceramics (Table 1), which possessed thermal

expansion coefficients matched for application to zirconia

substrates, were used in the current investigation. The

thickness of the LAVA specimens was determined prior to

coating using a screw gauge accurate to 10 mm (Moore and

Wright, Sheffield, UK). A thin slurry of each glazing ceramic was

formed by manipulating 0.035 g of ceramic frit with 0.2 ml of

distilled deionised water. The slurry was carefully transferred to

the LAVA surface with a clean brush, distributed evenly using a

vibrating technique, and allowed to air dry. Each glaze coating

was fired according to the manufacturer’s recommended firing

regime in a Vita Vacuumat 40 furnace (Vita Zahnfabrik, Bad

Sackingen, Germany) followed by slow cooling in air.

The thickness of the coated specimens was re-determined

prior to etching of the glaze with 10% hydrofluoric (HF) acid

(Fisher Scientific, Loughborough, UK) for 90 s. Following

etching, the specimens were repeatedly washed (10�) in

distilled water. All specimens were coated with a silane primer

(Rely-X Ceramic Primer, 3M ESPE), air dried and stored in a

desiccator until required for further testing.

The bond of the resin composite luting cement to the

modified zirconia surface was characterised using a shear

bond strength methodology. All specimens were coated with a

silane primer (Rely-X Ceramic Primer, 3M ESPE, St Paul, USA)

and allowed to air dry. Shear bond strength testing specimens

were created by cementing Z100 resin-based composite (3M

ESPE, St Paul, USA) cylinders (2.4 mm diameter and 4 mm

length) to the centre of the prepared ceramic surface with a

0

5

10

15

20

25

30

35

40

45

50

CoJet (A) Alpha (B/G) Nor itake (C /HGlaze

Shea

r Bon

d St

reng

th (M

Pa)

Wet storage T

Fig. 1 – Shear bond strength values for the materials under

controlled volume of mechanically mixed Rely-X Unicem (3M

ESPE) resin based cement, under a standard seating force, in a

silicone guide mould to minimise the cement ‘flash’. The

cement was light cured in accordance with the manufacturer’s

instructions.

Shear bond strength testing was performed on specimens

from Groups A–F following 24 h of water storage at 37 8C.

Specimens from Groups G–K were subjected to 1800 cycles of

thermal cycling between temperature extremes of 5 and 55 8C

with a 6 s dwell time at each temperature. Shear bond strength

was determined by loading the cylinder parallel to the ceramic

surface with a flat blade in a Bencor Multi-T alignment

apparatus (Danville Engineering Inc., San Ramon, USA) using a

universal tensile-testing instrument (Instron Model 5544,

Instron Ltd., Buckinghamshire, England) at a crosshead speed

of 1 mm per minute. The load at failure was recorded and the

shear stress calculated as a function of the loaded area. In

addition, the fracture surface of each debonded RBC specimen

was examined under a stereo-microscope (Wild M3C, Heer-

burg, Switzerland) at 20� magnification to determine whether

failure was cohesive or adhesive.

Scanning electron microscopy (SEM) with a JEOL JSM 5300

LV (JEOL Ltd., Akishima Tokyo, Japan) was used for the

examination of representative prepared ceramic surfaces and

cross-sections of the resin-coated ceramic specimens, follow-

ing deposition of a 2 mm gold layer.

Statistical analysis of the shear bond strength data was

performed using a factorial analysis of variance and post hoc

Tukey tests at a 95% significance level.

4. Results

A factorial analysis of variance at a 95% significance level

demonstrated that all glazing techniques resulted in a

significant increase in the shear bond strength compared

with using the resin based cement alone (P < 0.01).

The use of different glazing ceramics resulted in significant

differences in mean shear bond strength (P = 0.008) and the

) Emax (D /I) VM9 (E/J) La va Cer (F/K)Material

hermocycled

test in wet storage conditions and after thermocycling.

Fig. 2 – (a)–(d). Scanning electron micrographs of the Emax glazed fracture surface after bond strength testing (a) the etched

glazed surface demonstrating the microporosity induced by selective phase dissolution (b), the fracture surface

demonstrating residual glass on the zirconia surface (c) demonstrating cohesive failure within the glazing ceramic and (d)

the oblique fracture of the veneering ceramic.

j o u r n a l o f d e n t i s t r y 4 0 ( 2 0 1 2 ) 3 4 7 – 3 5 1350

differences were a function of the storage state (wet storage vs

thermocycling) (P = 0.013) (Fig. 1).

Post hoc Tukey tests demonstrated a significantly in-

creased shear bond strength when Noritake ceramic was used

when compared with the manufacturers recommended

ceramic, Lava Ceram (P = 0.049) and Vita VM9 (P = 0.014).

Scanning electron microscopy demonstrated that the

predominant failure modes were mixed, namely, cohesive

within the veneering ceramic and adhesive between the

veneering ceramic and the zirconia substrate.

All glazed surfaces demonstrated modification following

HF-acid etching. Vitadur-alpha, despite being microstructu-

rally largely amorphous, demonstrated etching patterns

consistent with the selective dissolution of pits and flaws.

The remaining veneering ceramics possessed microstructu-

rally mixed phases and demonstrated etching patterns

consistent with selective phase etching8 (Fig. 2a–d).

5. Discussion

There appears to be an increased demand from patients for

restorations of optimum aesthetics, so a reliable method for

the construction of all-ceramic crowns and bridges could

therefore be considered advantageous. In this respect, a

number of zirconia-based materials are now capable of being

stained with a variety of shades 12 prior to sintering, thereby

optimising the aesthetic quality of the completed restoration

in comparison with those formed in metal–ceramic types.

However, a reliable means of facilitating the bonding of

zirconia to tooth substance using resin-based luting materials,

in particular, the increasing popularity of self-adhesive resin

luting materials13, has proved elusive. Therefore, the use of

zirconia has been limited to its use in cavities/preparations,

which achieve resistance and retention form. Achieving a

reliable bond would allow zirconia-based restorations to be

used in, for example, resin-retained bridgework. Hence the

clinical application of the present work.

The statistically significant interaction between storage

state and mean shear bond strength is consistent with

observations of previous investigators. Although a detrimen-

tal effect of thermal fatigue on bond strength would be

expected from basic theory, the observations are thought to

reflect the modified compliance of the test specimen following

fatigue and are an acknowledged limitation of the testing

methodology. Although different glazing materials resulted in

differences in shear bond strength, there was no simple

relationship with thermal expansion coefficients. This is not

unexpected as the layering and sintering of thin coatings will

be influenced by numerous variables introduced by the

j o u r n a l o f d e n t i s t r y 4 0 ( 2 0 1 2 ) 3 4 7 – 3 5 1 351

operator and other materials variables, which have not been

explored within the constraints of this short investigation.

Furthermore, it would appear appropriate to use the same

methodology with a variety of zirconia core materials, since,

whilst this study used five veneering ceramics, only one core

material (Lava:3M ESPE) was employed.

The advantages of this technique are clear. The laboratory

technique is relatively simple in comparison to selective

infiltration, differing from it by not completely dissolving away

the veneering ceramic. It therefore requires no additional

materials or equipment and therefore is cheap and simple to

perform. Additionally, as veneering ceramics are silicacious, the

etched surface is also conducive to silane bonding (which

enhances the bond strength to resin based cements) using

conventional material formulations. The glazing techniques

used in the current investigation resulted in a significantly

enhanced shear bond stress to the resin based cement when

compared with the current ‘gold standard’ – tribochemical

coating.

However, the most important detracting issue in the

technique utilized in this work is the requirement to

accommodate space for the glaze itself. Although refinement

in ceramic frit size will inevitably allow for materials that can

be used to produce a glaze finish well below the 120 mm

thickness achieved in the current investigation, concerns

regarding accuracy of restoration fit may arise. CAD-CAM

restoration construction technology already has the capacity

to account for the glaze thickness but for the immediate future

this technique should be limited for adhesive bridges where

the geometry of retaining wings is simple and fit is unlikely to

be compromised.

Future work is required to examine the impact of glaze

thickness on shear bond strength and longer term fatigue

simulation would appear to be sensible to provide an

indication of likely clinical performance in the medium and

long term.

6. Conclusion

The glazing techniques used in the current investigation

resulted in a significantly enhanced shear bond stress to the

resin based cement when compared with tribochemical

coating.

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