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Short Communication

Bond strength of a dental leucite-based glass ceramicto a resin cement using different silane coupling agents

Tabassom Hooshmanda,n, Jukka P. Matinlinnab, Alireza Keshvadc,Solmaz Eskandariona, Fereshteh Zamania

aDepartment of Dental Biomaterials, School of Dentistry/Research Center for Science and Technology in Medicine, Tehran University of

Medical Sciences, Tehran, IranbDental Materials Science, Faculty of Dentistry, University of Hong Kong, Hong Kong, PR ChinacDepartment of Prosthodontics, School of Clinical Dentistry, Shahed University, Tehran, Iran

a r t i c l e i n f o

Article history:

Received 30 May 2012

Received in revised form

24 August 2012

Accepted 27 August 2012

Available online 1 September 2012

Keywords:

Bond strength

Leucite-based dental ceramic

Resin cement

Silane coupling agent

nt matter & 2012 Elsevie.1016/j.jmbbm.2012.08.02

hor. Tel.: þ98 21 88212484hoshmand@sina.tums.a

a b s t r a c t

Aim: To evaluate the effect of different types of novel silane coupling agents with two

concentrations on the micro-tensile bond strength of a dental glass ceramic with leucite

crystals to a dual-cured resin cement using an optimized method of silane application.

Methods: Leucite-reinforced feldspathic ceramic blocks were fabricated, wet ground and

cleansed. The bonding ceramic surfaces were treated with different organosilane solutions

as follows: Control silane: Monobond S; methacryloxypropyltrimethoxy silane and experi-

mental silanes with two concentrations (1.0 and 2.5 vol%): amino, isocyanate, styryl, and

acrylate silanes. The silane application method consisted of brush application, hot air

drying followed by rinsing with hot water and drying. Then a thin layer of an unfilled resin

and a dual-cured resin cement was light-cured on the ceramic surfaces. The resin–ceramic

blocks were stored in distilled water at 37 1C for 24 h and sectioned to produce beam

specimens (n¼17) with a 1.0 mm2 cross-sectional area. Specimens were then subjected to

thermocycling and tested in a micro-tensile tester device. Data were analyzed using

analysis of variance and Tamhane post-hoc test.

Results: The mean micro-tensile bond strength value for the styryl silane was significantly

higher (Po0.05) than the other types of silanes except for the Monobond S. The mean bond

strength values for isocyanate silanes were significantly lower than the other silanes tested

(Po0.05). No statistically significant difference in the bond strength between the 1.0 and

2.5 vol% of experimental silanes was observed (P40.05).

Conclusions: The micro-tensile bond strength of the leucite-based dental glass ceramic to a

resin cement was affected by the type of silane coupling agent and not by the concentra-

tion of silane solutions. The best bond strength overall was achieved by methacrylox-

ypropyltrimethoxysilane and experimental styryl silane solutions.

& 2012 Elsevier Ltd. All rights reserved.

r Ltd. All rights reserved.0

; fax: þ98 21 88081699.c.ir (T. Hooshmand).

j o u r n a l o f t h e m e c h a n i c a l b e h a v i o r o f b i o m e d i c a l m a t e r i a l s 1 7 ( 2 0 1 3 ) 3 2 7 – 3 3 2328

1. Introduction

Clinical performance of metal-free fixed-partial-dentures

(FPD) made of silica-based ceramics, either in the forms of

inlays, onlays or laminate veneers, substantially relies on the

adhesion of the resin cements to ceramic restorations and

tooth tissues. For the adhesive cementation procedures, the

silica-based ceramic surfaces are usually etched by hydro-

fluoric acid (HF) with or without gritblasting for mechanical

interlocking, and then silanated for chemical bonding. How-

ever, a major concern exists about the use of HF acid etching

due to its hazardous effects on health (Meldrum, 1999) and

possible deleterious effects on ceramic strength (Hussain

et al., 1979; Hooshmand et al., 2002). Still research continues

in introducing effective silane solutions to optimize the

chemical bond provided by silanes such that there would be

no need for the HF acid etching of the ceramic surfaces.

Silane coupling agents with the general formula R–Si–(OR0)3are capable of forming bonds with both inorganic and organic

surfaces. The organo-functional group (R) is chosen for

reactivity with the organic matrix and R is most frequently

a vinylic group,, while the hydrolyzable groups (R0) are

intermediates in the formation of silanol groups (Si–OH) for

bonding to mineral (e.g., ceramic) surfaces. They are usually

methoxy groups because they are kinetically favourbale for a

rapid hydrolysis during activation of the silane coupling

agent. The most commonly used silane in dental applications

is g-methacryloxypropyltrimethoxysilane (g-MPTS) which

can then be polymerized with the monomers of a resin

composite system.

Recent studies have shown that experimental silane mono-

mer primers can significantly increase the bond strengths

between resins and zirconia ceramic (Aboushelib et al., 2008,

2009; Matinlinna and Lassila, 2011). The rationale behind

applying experimental silane primers is that the presence

of specific organofunctional groups, such as an aromatic

reactive styryl, isocyanato (with the –N¼C¼O functionality),

glycidoxy (with epoxy ring), or acrylate, may improve the

spatial compatibility of the silane molecule (Matinlinna et al.,

2005a; Matinlinna et al., 2007; Heikkinen et al., 2009). This

steric improvement may also increase the reactivity of silane

monomers by enabling polymerization reactions between

methacrylate, acrylate and styryl groups and, on the other

hand, with methacrylate and phosphate ester groups in

resin-composite.

In our previous studies, the macro-tensile bond strength

and interfacial fracture toughness of smooth and roughened

leucite-based ceramic surfaces bonded to a luting resin using

an optimized silane treatment method was investigated

(Hooshmand et al., 2002; Moharamzadeh et al., 2008). It was

concluded that mechanical interlocking by gritblasting the

leucite ceramic surfaces could be sufficient with no need for

HF acid etching the ceramic surfaces when an appropriate

silane application procedure was used. A possible explana-

tion for these observations was that the silane application

procedure used resulted in the formation of a monolayer of

the silane coupling agent by washing away any unreacted

silane primer components, rather than the formation of

interphase layer as described by Ishida and Koenig (1980).

In addition, it has been found that the interfacial fracture

toughness for a lithium disilicate glass ceramic system was

affected by the surface treatment and type of luting agent

(Hooshmand et al., 2012). The optimized silane treatment

method was only capable of eliminating the need for HF acid

etching of ceramic surfaces bonded to one type of dual-cured

resin cements.

The purpose of this study was to evaluate the effect of

different types of novel silane coupling agents with two

concentrations on the micro-tensile bond strength of a dental

glass ceramic with leucite crystals to a dual-cured resin

cement using an optimized method of silane application.

2. Materials and methods

Eighteen leucite-reinforced feldspathic ceramic blocks

(Ceramco II, Dentsply) in dimensional of 6�6�6 mm3 were

fabricated according to the manufacturer’s instructions. The

ceramic blocks were ground on wet 400–800 grit SiC papers

and then cleansed in distilled water in an ultrasonic device

for 15 min. The experimental silane primers were used at

1.0% and 2.5% (v/v) in a standard solution of 95.0% (v/v)

ethanol and deionized water (Milli-Q purification system,

Millipore) that had been adjusted to pH 4.5 with 1 M acetic

acid. The solution was first allowed to stabilize for 24 h and

then silane monomer was added and allowed to activate for

1 h at room temperature. The chemical compositions of

silane coupling agents are shown in Fig. 1. The bonding

ceramic block surfaces were then treated with different

silane solutions (n¼2) as follows:

Group 1: (commercial as control): Monobond S; (Lot No.,

M01959; Ivoclar-Vivadent, Liechtenstein), 3-methacryloxy-

propyltrimethoxysilane (MPTS silane).

Group 2: Experimental 3-Acryloyloxypropyltrimethoxy-

silane (ACPS).

Group 3: Experimental 3-Isocyanatopropyltriethoxysilane

(ICS).

Group 4: Experimental Styrylethyltrimethoxysilane (STYRX).

Group 5: Experimental 3-(N-allylamino)propyltrimethoxy-

silane (ALAP).

Each experimental silane group was applied in two differ-

ent concentration of 1.0 and 2.5 percent by volume. The

silane application method consisted of brush application for

60 s, hot air drying at 5075 1C with a hair drier for 15 s

followed by rinsing with hot water for 15 s and drying again

with hot air for 30 s. Then a thin layer of an unfilled resin

(HelioBond, Lot No., J14889; Ivoclar-Vivadent, Liechtenstein)

was applied on the silane-treated ceramic surfaces and light-

cured for 20 s.

The ceramic blocks were then transferred to a custom-

made teflon mold in size of 6�6�12 mm3 and a dual-cured

resin cement (Variolink II, Lot No., K37003; Ivoclar-Vivadent)

was added and light-cured incrementally on the ceramic

surfaces using a halogen curing light at a light intensity of

800 mW/cm2 for 40 s. The specimens were also post-cured for

an additional of 40 s in different directions.

ResinCeramic

Fig. 2 – (a) Micro-tensile bond strength tester and (b)

schematic picture of microbar specimen fixation.

CH3

CH2=C-C-O-CH2CH2CH2-Si-OCH3

OCH3

OCH3

H2CO Si

OCH3

O

O

CH3

CH3O

OC2H5

OC2H5C2H5O

SiN

C

O

H2C Si

O

OO

CH3

CH3

CH3

H2C

N

Si

O

O

OCH3

CH3H3C

H

Fig. 1 – (a) methacryloxypropyltrimethoxysilane (MPTS); (b) 3-acryloyloxypropyltrimethoxysilane (ACPS);

(c) 3-isocyanatopropyltriethoxysilane (ICS); (d) styrylethyltrimethoxysilane (STYRX) and (e) 3-(N allylamino)

propyltrimethoxysilane (ALAP).

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The ceramic–resin cement blocks were kept in distilled water

at 37 1C for 24 h and then were cut using a slow speed diamond

saw (Isomet, Buehler, Lake Bluff, IL, USA) under water cooling.

The sectioning continued until 1 mm remained to keep the

specimen in a fixed position. The ceramic–resin cement block

was then rotated 901 and the procedure repeated. The peripheral

slices were disregarded in case the results could be influenced by

either excess or insufficient amount of resin cement at the

interface. A total of 17 bar specimens per group approximately

1 mm2 in cross section with bonded ceramic to resin cement

(6 mm in height each) were obtained.

The spemines were then subjected to thermocycling for 5000

cycles between 5 and 55 1C water baths (immersion time 20 s;

transfer time 20 s). Each microbar was glued with cyanoacrylate

and fixed in a micro-tensile tester device (Bisco micro-tensile

tester, USA) as shown in Fig. 2. The tensile load was applied at a

cross-head speed of 0.5 mm/min until fracture. The load at

failure in Newtons was recorded, and the fragments of the

specimen were carefully removed from the fixture with a scalpel

blade. The cross-sectional area at the site of fracture was

measured to calculate the bond strength at failure in MPa. The

debonded bar specimens were analyzed for mode of failures

with a stereomicroscope (Olympus, SZX 12, Tokyo, Japan).

Data were analyzed using two-way ANOVA to assess

possible differences and interaction between between type

of silane and silane concentration. One-way analysis of

variance and Tamhane test were used for post-hoc compar-

isons between experimental groups. The level of significance

was set at Po0.05 using statistical software (SPSS 11 for

Windows; SPSS Inc., Chicago, IL, USA).

3. Results

The description of micro-tensile bond strength data are

presented numerically in Table 1 and graphically in Fig. 3.

The interaction between silanes and silane concentration

was significant (Po0.0001), thus, one-way analysis of variance

and Tamhane test due to the non-homogeneity of variances

were used for post-hoc comparisons between experimental

groups.

The results showed that the highest mean micro-tensile

bond strength value was obtained for the 2.5% styryl silane

(STYRX) followed by Monobond S (MPTS silane). In addition,

Table 1 – Statistical description of micro-tensile bond strength data for different groups.

Mean (MPa) SDa SEb Statistical differencen

Monobond-S 26.56 2.88 0.69 a

1.0% ACPS 16.13 2.84 0.68 b

2.5% ACPS 15.90 3.05 0.74 b

1.0% ICS 11.14 3.02 0.73 c

2.5% ICS 12.54 2.32 0.56 c

1.0% STYRX 22.54 9.11 2.20 a, b and d

2.5% STYRX 31.80 9.30 2.25 a

1.0% ALAP 21.95 3.13 0.76 d

2.5% ALAP 21.91 3.07 0.74 d

a Standard deviation.b Standard error.n Identical letters indicate statistically no significant difference between groups (Po0.05).

Fig. 3 – Micro-tensile bond strengths data for different silane

solutions.

j o u r n a l o f t h e m e c h a n i c a l b e h a v i o r o f b i o m e d i c a l m a t e r i a l s 1 7 ( 2 0 1 3 ) 3 2 7 – 3 3 2330

the mean micro-tensile bond strength value for the 2.5%

styryl silane (STYRX) was significantly higher (Po0.05) than

the other types of silanes, except for the Monobond S and

1.0% styryl silanes (P40.05). The mean micro-tensile bond

strength value for the Monobond S (control) was significantly

higher (Po0.05) than the other types of silanes, except for the

styryl silanes (1.0% and 2.5% STYRX).

The lowest mean micro-tensile bond strength values were

obtained for the isocyanate silanes (1.0% and 2.5% ICS) which

were significantly lower than the other silanes tested

(Po0.05).

No statistically significant difference in the mean micro-

tensile bond strength between the 1.0 and 2.5 vol% of experi-

mental silanes was observed (P40.05).

The predominant mode of failure for all the groups tested

was interfacial adhesive fracture (410 out of 17 specimens).

The remaining failures were mixed ceramic–resin fracture.

4. Discussion

The degree of bond strength enhancement of resin to dental

ceramic mainly depends on the bond of silane to the ceramic

surface. In the present study, the bonding effectiveness of a

leucite-reinforced glass ceramic to a dual-cured resin cement

using different types of experimental silane solutions with

two concentrations were compared with that of a commercial

methacryloxypropyltrimethoxysilane. We performed the

micro-tensile bond strength test with small specimen size

of 1 mm2 because the validity of expressing bond strength in

terms of nominal (i.e., average) stress in macro-bond strength

tests has been questioned due to the heterogeneity of the

stress distribution at the bonded interface (Van Noort et al.,

1989, 1991). The need for new test methods to overcome

these limitations has led to the use of specimens with small

bonding areas (i.e., below 2 mm2), in the so-called micro-

tensile and micro-shear bond strength tests (Sano et al.,

1994). Smaller test specimens are ‘stronger’ than larger ones

due to the lower probability of having a critical sized defect

present and aligned in a crack opening orientation relative to

the applied load (Armstrong et al., 2010).

The silane treatment procedure used in the present study

was introduced previously (Hooshmand et al., 2002;

Moharamzadeh et al., 2008) using an optimized method for

ceramic–resin bonding such that a chemisorbed monolayer of

the silane coupling agent can be created with no need for HF

acid etching. We did not make any additional mechanical

interlocking by gritblasting or HF acid etching in order to

assess the unique impact of chemical adhesion by silane

coupling agents.

We may say that from this in vitro study it is quite clear

that different functional silanes have a significant impact on

the bond strength between resin composite and leucite-

reinforced dental ceramics. In our study, we used thermo-

cycling for artificial aging which is widely used as an accepted

laboratory method. We did not measure the initial bond

strength in this study because we aimed showing differences

in the micro-tensile bond strengths after thermocycling.

The literature demonstrate that different results in promoting

adhesion are obtained according to the type of silane used

(Hooshmand et al., 2004; Anagnostopoulos et al., 1993; Pratt

et al., 1989). Several silane coupling agent systems are available

and they are categorized into pre-hydrolyzed single liquid silane

primer and 2- or 3- liquid silane primers. Those supplied as a

single component system are previously hydrolyzed and the

silane content is usually about 1–5 vol%. For those supplied as 2

separate components, the hydrolysis occurs moments before

application and after mixing the two components.

j o u r n a l o f t h e m e c h a n i c a l b e h a v i o r o f b i o m e d i c a l m a t e r i a l s 1 7 ( 2 0 1 3 ) 3 2 7 – 3 3 2 331

Based on the statistical analysis, except for one experi-

mental 2.5 vol% styrylethyltrimethoxysilane (STYRX), other

silane primers resulted in lower micro-tensile bond strengths

than that of the control MPTS silane. It has been reported that

the use of styryl silane (STYRX) in experimental dimethacry-

late thermoset composites has produced the highest moduli

(Wilson and Antonucci, 2006). This was explained by the high

reactivity of STYRX. In the current study, a 2.5% conentration

of styryl silane produced 19.7% higher bond strengths, where

as the dilute 1.0% styryl-based primer produced 15% lower

bond strength than that of the control silane, although not

statistically different.

It has been reported that pre-acitivated dental silanes may

produce differences in adhesion promotion (Matinlinna et al.,

2006b). Monobond S is a single-phase and pre-activated silane

solution based on the 3-methacryloxyprophyltrimethoxy silane

molecules. It was selected as the control silane because it is

ready for immediate use as supplied by the manufacture. In

addition, Monobond S produced significantly higher bonding

than another dental silane product after artificial aging of the

resin bonded Ti specimens (Matinlinna et al., 2005b). The high

bonding efficacy observed for the Monobond S in this study is in

agreement with the results of our previous study (Hooshmand

et al., 2004) in which a pre-activated silane solution based on

g-MPTS did not deteriorate when stored up to one year.

The experimental 1.0 or 2.5 vol% 3-Isocyanatopropyltriethoxy-

silane (ICS) silane solutions did not improve adhesion between

the ceramic and resin cement in the present study. The lowest

micro-tensile bond strengths were obtained by ICS silanization

in both concentrations. This has been also confirmed by a

previous study (Matinlinna et al., 2004b). The effect of silane

concentration on the titanium surface treatment was investi-

gated in a study done by Matinlinna et al. (2008). They found that

a novel silane system with an optimal concentration of the

cross-linking silane of 1.0% as opposed to the 0.1%, 0.2%, 0.3%,

and 0.5% could produce significantly higher shear bond strength

between silica-coated titanium and a resin cement when com-

pared to a pre-activated silane product. On the contrary, it was

reported that 0.1 vol% experimental ICS silane primer enhanced

the resin to titanium bond strength significantly higher than that

of a 1.0 vol% ICS primer, a finding that supports the use of

relatively dilute primers (Matinlinna et al., 2005a). However,

silane solutions with greater concentrations in the present study

demonstrated comparable micro-tensile bond strength values

between the leucite-reinforced feldspathic ceramic and resin

cement with that of lower concentrations.

Surprisingly, both ALAP and ACPS silane primers exhibited

almost no significant improvement in the bond strength

regardless of the silane concentration. According to the

results obtained by another study (Matinlinna and Lassila,

2011), ACPS has produced significantly higher bond strengths

than experimental MPTS silane primer. On the other hand,

one study using 2.0 vol% experimental silanes on silica-

coated Ti substrate found that MPTS and ACPS did not differ

statistically from each other for their adhesion promotion

ability (Matinlinna et al., 2007). On the resin–zirconia bonding,

an in vitro study suggested that ACPS and MPTS perfomed

equally well in adhesion promotion (Matinlinna et al., 2006a).

It should be noted that results from these studies have been

obtained on different types of substrates, resin cements, and

test parameters than that of used in the present study. Thus,

comparison and interpretation of results are difficult.

It is also essential to note that ceramic surfaces and silica-

coated metal surfaces exhibit chemically and topologically

different surfaces, and are definitely not equivalent or com-

parable. It has been suggested that the surface nature of the

inorganic substrate or filler must be considered in the selec-

tion of a silane coupling agent (Matinlinna et al., 2004a; Shen

et al., 2004). Other factors to be considered include the type

and availability of surface hydroxyl groups (silanol vs

adsorbed water, hydrated ions, etc.), hydrolytic stability of

the oxane bond that forms, number of active hydroxyl groups

per unit area of substrate, surface reactivity and chemical/

physical properties of the silane and silanization conditions

(silane concentration, pH, temperature, type of catalysis,

method of silane application, etc.).

To our knowledge, there is no other experiment that has

evaluated the resin to leucite-reinforced glass ceramic bond

strength using the novel experimental silane primers in this

study, without HF acid pretratment. Finally, it should be noted

that there are some limitations to this study. The microbar

specimens were cut from very limited number of bonded

ceramic–resin blocks, it is possible that different results could

be obtained if the sample size was increased. These should be

considered when the data are compared because ceramic

testing is statistical in nature. Besides, the results obtained

from this in vitro study cannot necessarily be extrapolated to

the clinical situation because of the complex oral environ-

ment and further long-term storage time or clinical studies

on the bonding efficacy of these organosilanes are required.

5. Conclusions

The findings in this in vitro study may indicate that the

micro-tensile bond strength of a dental leucite-based glass

ceramic to a dual-cured resin cement is affected by the type

of silane coupling agent and not by the concentration (1 or

2.5 vol%) of silane solutions. The freshly made styryl silane

may be as effective as pre-activated methacryloxypropyltri-

methoxysilane in promoting bond strength.

Acknowledgments

This study was supported by Research Center for Science and

Technology in Medicine (Tehran University of Medical

Sciences). We thank Dr. M.J. Kharazifard for his invaluable

assistance in the statistical analysis.

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