Nagai and Kawamoto

Int Chin J Dent 2004; 4: 100-106.

Effect of hydrofluoric acid etching on bond strength of composite
luting agent to lithium disilicate ceramic material
Taro Nagai, DDS,a and Yoshikazu Kawamoto, DDS, PhDa,babDepartment of Crown and Bridge Prosthodontics, and Division of Advanced Dental Treatment, Dental
Research Center, Nihon University School of Dentistry, Tokyo, Japan

Purpose: The purpose of the current study was to evaluate the effect of surface preparation on bond strength of acomposite luting agent joined to a heat-pressed ceramic material.
Materials and Methods: Two sizes of disk specimens were made from a lithium disilicate-based ceramics (IPS
Empress 2) and their surfaces were separately prepared with three methods: etching with phosphoric acid (PE),etching with hydrofluoric acid (HF), and air-borne particle abrasion with alumina (AA). Each group was further
divided into two sub-groups: bonding with the Panavia F material (PF), and silane treatment followed bybonding with the Panavia F material. Shear testing was performed both before and after 20,000 thermocycles.
Results: Bond strength varied from 14.2 MPa to 46.6 MPa for the pre-thermocycling groups, whereas
post-thermocycling bond strength ranged from 0.3 MPa to 37.5 MPa.
Conclusion: Hydrofluoric acid etching effectively enhanced bond strength of the Panavia luting agent to theceramic material, regardless of the application of silane primer. (Int Chin J Dent 2004; 4: 100-106.)
Clinical Significance: Hydrofluoric acid etching roughened the IPS Empress 2 ceramic surface. The etchedsurface was suitable for adhesive bonding with the Panavia F composite luting agent.
Key Words: bonding, ceramics, composite, hydrofluoric acid, lithium disilicate.

Introduction
The IPS Empress glass-ceramics (Ivoclar Vivadent AG, Schaan, Liechtenstein) is a heat-pressed,
leucite-reinforced material designed for tooth-colored restorations.1,2 Mechanical properties of the material,
however, are not sufficient for use in fixed partial dentures (FPDs). The IPS Empress 2 (Ivoclar Vivadent AG)
glass-ceramics was introduced thereafter as an alternative material for single-unit restorations as well as for
3-unit FPDs limited to anterior and premolar areas.3 The definitive restoration, made of a lithium disilicate
framework ceramics and a fluoroapatite layering ceramics, offers clinical benefits in terms of machinability,
polishability, and reduced wear of opposing tooth structure.4
Bonding ceramics to tooth structure with composite luting agent increases fracture resistance of the tooth andthe restoration itself, and minimizes microleakage, which may be the determining factor in the success or failureof the restorative treatment.5 Microleakage causes marginal discoloration of restorations and thus failure ofrestorations. Comparative bond strength studies were performed for evaluation of the characteristics of bondingof luting systems to the ceramic material. Due to the fact that lithium disilicate is a new composition foradhesive bonding, proprietary surface preparation is necessary prior to bonding restorations. Although the
manufacturer recommends the use of a single-liquid silane primer for bonding the Empress 2 material, only
limited information is available about surface preparation for mechano-chemical bonding. The purpose of thecurrent study was to evaluate the effect of surface preparation on bonding of a dual-activated composite luting
agent to the Empress 2 ceramics.

Materials and Methods
A lithium disilicate-based ceramic material (IPS Empress 2) was selected as the substrate material.

A

composite luting material with or without silane primer was evaluated. The material, identified as Panavia F

100

Nagai and Kawamoto

Int Chin J Dent 2004; 4: 100-106.

(Kuraray Medical Co., Ltd., Tokyo, Japan, PF), is a dual-polymerizing two-paste luting composite that contains
10-methacryloyloxydecyl dihydrogen phosphate (MDP) as a functional monomer. The Clearfil Porcelain Bond
Activator material (Kuraray Medical Co., Ltd.) is a single liquid silane solution. The Clearfil Mega Bond Primer
material (Kuraray Medical Co. Ltd.) is a self-etching primer for dentin bonding. The manufacturer claims that
the Activator combined with the Mega Bond Primer is applicable, as a two-liquid primer (CP), for adhesive
bonding of ceramic materials. Two etchants (K-etchant, Kuraray Medical Co., Ltd.; HF Gel, GC Corp., Tokyo,
Japan) and 70 µm grain sized alumina (Hi-Aluminas, Shofu Inc., Kyoto, Japan) were used for ceramic surface
preparation. Details of the materials are summarized in Table 1.
Table 1. Materials assessed.
Material

Trade name

Lot number

Ceramics
Etchant

IPS Empress 2 Ingot 200
K-etchant (originally for enamel)
HF Gel (for ceramics)
Hi-Aluminas
Clearfil Porcelain Bond Activator
Clearfil Mega Bond Primer
Panavia F
A pasteB paste (Brown)

E41431
0307AA
090721
0703
00299A
00131A
00067A
00038A

Abrasive
Silane primer
Luting agent

Composition
57-80% SiO2, 11-19% Li2O, 0.1-14% La2O3, 0-14% K2O
40% Phosphoric acid
5% Hydrofluoric acid70 µm Aluminum oxide
γ-MPTS, Bis-GMA
MDP, HEMA, Water, Ethanol, etc.
MDP, Bis-GMA, Silica, Photo initiator, Accelerator
Bis-GMA, Barium glass, Sodium fluoride, MDP, BPO

MDP, 10-methacryloyloxydecyl dihydrogen phosphate; Bis-GMA, Adduct of bis-phenol A and glycidylmethacrylate; BPO,
Benzoyl peroxide; γ-MPTS, γ-methacryloxypropyl trimethoxysilane; HEMA, 2-hydroxyethyl methacrylate.

A total of 120 pairs of the Empress 2 ceramic disks (7 mm in diameter by 3 mm thickness; 5 mm in diameterby 2 mm thickness) were prepared according to the manufacturer’s specifications. The surface to be bonded was
ground with #2,000 silicon-carbide abrasive paper (New Maruto Lap ML-110, Maruto Instrument Co. Ltd.,Tokyo, Japan) and divided into three groups: 1) phosphoric acid etching (PE), 2) hydrofluoric acid etching (HF),
and 3) air-abrasion with alumina (AA) (n=40 pairs in each group). The phosphoric acid etchant was applied tothe ceramic surface for 60 s, rinsed with tap water, and air-dried. The hydrofluoric acid etchant was applied tothe ceramic surface for 20 s, rinsed with tap water, and air-dried. Air-borne particle abrasion was performed
with a particle abrader (Multiblaster, AWS Co. Ltd., Tokyo, Japan) for 10 s. The pressure was 0.3 MPa and the
distance of the orifice from the ceramic surface was approximately 20 mm. A piece of tape with a circular hole
3 mm in diameter was positioned on the 7-mm-diameter ceramic specimen to define the bond area and aconsistent 50 µm thickness of the luting material.
The disks were divided into six system groups (Fig. 1). Silane primer was not used in three of these six
systems and they were considered as controls (PF). For the remaining three groups, the two-liquid silane primer
was applied to both 7-mm- and 5-mm-diameter specimens with a sponge pellet and gently air-dried (CPPF). The
120 pairs of disks were bonded with the Panavia F material. Thirty minutes after bonding, the specimens were
immersed in 37˚C water for 24 hours. This state was defined as thermocycle 0. Six sets of ten pairs were
subjected to shear testing at thermocycle 0 and 24-hour bond strengths were determined. The remaining six setsof 10 paired specimens were subsequently placed in a thermocycling apparatus (Thermal Shock Tester TTS-1
LM, Thomas Kagaku Co. Ltd., Tokyo, Japan) and cycled between 5˚C and 55˚C water with a one-minute dwell
time per bath for 20,000 cycles. Post-thermocycling bond strengths were then determined. Before shear testing,each specimen was embedded in a steel mold and seated in an ISO/TR 11405 shear-testing jig. Bond strengths
were determined on a mechanical testing device (Type 5567, Instron Corp., Canton, MA, USA) at a crosshead101

Nagai and Kawamoto

Int Chin J Dent 2004; 4: 100-106.

speed of 1.0 mm per minute. For each condition, the average bond strength and standard deviation (SD) of 10
replications were calculated.
The values of each group were compared by analysis of variance (ANOVA). The three factors analyzed were
surface preparation, bonding system, and thermocycling. After the three-way ANOVA, two-way ANOVA,
one-way ANOVA, and post-hoc Tukey-Kramer tests were further performed with the value of statisticalsignificance set at 0.05.
After the shear testing, debonded surfaces were observed through an optical microscope (SZX9, Olympus
Corp., Tokyo, Japan) and failure mode was recorded as: A, adhesive failure at the luting agent-ceramic interface;C, cohesive failure inside the luting agent; and AC, combination of adhesive and cohesive failures. Selected
specimens were sputter-coated with osmium, and observed with a scanning electron microscope (S-4300,
Hitachi High-Technologies, Co., Ltd., Tokyo, Japan) operated at 15 kV.

Fig. 1. Procedures for specimen preparation.

Fig. 2. Shear testing results.

Results
Table 2 shows the ANOVA results for shear bond strengths.
Table 2. Analysis of variance results.
SourceDegree of freedom

Three-way ANOVA
Luting system (LS)
Surface preparation (SP)Thermocycling (TC)
LSxSP
LSxTC
SPxTC
LSxSPxTC
Residual
Two-way ANOVAs
PF
SP
TC
SPxTC
Residual
CPPF
SP
TC
SPxTC
Residual

Sum of square

Mean square

F-valuep-value

1
2
1
2
1
2
2
108

780.3
18,627.2
10,378.8
86.6
784.4
3,256.5
102.8
2,486.8780.3
9,313.6
10,378.8
43.3
784.4
1,628.2
51.3
23.0

33.9
404.5
450.7
1.9
34.1
70.7
2.2

0.00010.00010.00010.15730.00010.00010.11232
1
2
54

10,227.1
2,728.4
1,139.3
1,384.7

5,113.5
2,728.4
569.7
25.6

199.4
106.4
22.2

0.00010.00010.00012
1
2
54

8,486.8
8,434.8
2,219.9
1,102.1

4,243.4
8,434.8
1,110.0
20.4

207.9
413.3
54.4

0.00010.00010.0001

102

Nagai and Kawamoto

Int Chin J Dent 2004; 4: 100-106.

The table includes degree of freedom, sum of squares, mean squares, F-values, and p-values. Three-way
ANOVA indicated that the following interactions were significant (p<0.05); 1) luting system and thermocycling,and 2) surface preparation and thermocycling. Among the three factors, thermocycling showed the highest
F-value, followed by surface preparation, and cementing system. The bond strength results were analyzed by
two-way ANOVA. The results showed that interaction between the two factors were significant (p<0.05) for
both systems.
Fig. 2 demonstrates the bond strengths to the Empress 2 material before and after thermocycling. Table 3
shows the average bond strengths, standard deviations, and statistical categories. Bond strength varied from 46.6
MPa to 0.3 MPa. Surface preparations considerably affected bond strengths of both silanized and unsilanizedgroups. Specifically, the groups etched with the HF etchant exhibited greater post-thermocycling bond strengththan those treated with other methods. Bond strength of one group (etched with HF but unsilanized) did not
deteriorate after thermocycling.

No significant difference in post-thermocycling bond strength was foundbetween the silanized and unsilanized groups, when the ceramic surface was either alumina-blasted or etchedwith HF.
Table 3. Shear bond strengths in MPa and results of statistical analysis.
Preparation Thermocyles

Panavia F (PF)Mean
SD Category

Silane priming + Panavia F (CPPF)
Mean
SD Category
Difference between PF and CPPF

PE
HF
AA

0
0
0

14.2
40.2
30.8

2.9
5.9
5.2

C
A
B

23.6
46.6
45.5

2.4
5.2
6.7

G
E
E

Significant
Significant
Significant

PE
HF
AA

20,00020,00020,000

0.3
37.5
6.8

0.4
6.0
6.9

D
A
D

1.7
36.8
6.1

1.7
5.0
4.0

H
F
H

Significant
Not significant
Not significant

SD, Standard deviation; Category, Identical capital letters indicate that they are not statistically different (p>0.05).

Failure modes after shear testing are summarized in Table 4. Failure mode analysis revealed that PE-PF
groups showed type A adhesive failure for all specimens, whereas HF-CPPF groups exhibited type AC failure
for all specimens. The number of type A failures increased on the application of thermocycling for the AA-PF,
PE-CPPF, and AA-CPPF groups.
Table 4. Failure modes after the shear testing.
Preparation

Luting system

Failure mode

Thermocycle 0
A
CA C

20,000 cycles
A
CA C

PE
HF
AA

PF
PF
PF

10
1
6

0
9
4

0
0
0

10
1
10

0
9
0

0
0
0

PE
HF
AA

CPPF
CPPF
CPPF

3
0
2

7
10
8

0
0
0

9
0
9

1
10
1

0
0
0

A, Adhesive failure at the luting agent-ceramics interface; C, Cohesive failure inside the luting agent; CA, Combination ofcohesive and adhesive failures.

Fig. 3 shows scanning electron micrographs of ceramic disks after surface preparation including groundreference surface (Fig 3a). Phosphoric acid etching slightly roughened the ceramic surface (Fig. 3b), whereas

103

Nagai and Kawamoto

Int Chin J Dent 2004; 4: 100-106.

etching with hydrofluoric acid generated a splintery structure (Fig. 3c). An irregular relief pattern with loss of
surface structure was generated after air-borne particle abrasion with alumina (Fig. 3d). Fig. 4 shows thesectioned micrographs of the composite-ceramic interface. The composite luting agent penetrated into theundercut of the etched ceramic surface and the polymerized material appeared to be mechanically retained in theetched ceramic material (Fig. 4b).

3aGround3b

Etched with phosphoric acid3c Etched with hydrofluoric acid
3d Air-borne particle abraded with alumina
Fig. 3. Scanning electron micrographs of the Empress 2 ceramics after surface preparations.

4a Bonded to ground surface
4b Bonded to the surface etched with HF
Fig. 4. Sectional micrographs of the Empress 2 ceramics after bonding with the Panavia F material.

Discussion
Micromechanical retention is an important factor for durable bonding of resin-based luting agents to ceramics.
When the Empress 2 material was treated with phosphoric acid, the surface was not particularly etched (Fig. 3b)
and mechanical retention appeared to be insufficient. Bond strength after phosphoric acid etching was 23.6 MPa

104

Nagai and Kawamoto

Int Chin J Dent 2004; 4: 100-106.

or lower. This result suggests that phosphoric acid etching is inadequate to achieve mechanical retention
between the ceramic and luting agent.
When the Empress 2 was treated with hydrofluoric acid, an etched relief pattern composed of elongatedcrystals was clearly observed (Fig. 3c). According to a report concerning microstructure of the Empress 2material, the crystal phase is formed by elongated lithium disilicate (Li2Si2O5) and lithium orthophosphate(Li3PO4), both of which are surrounded by glass matrix.6 Hydrofluoric acid is able to remove the glass matrix
and lithium orthophosphate crystalline phase, thus creating irregularities within the lithium disilicate crystals.
The micrograph of the present study (Fig. 3c) suggests that hydrofluoric acid attacked both lithium
orthophosphate crystalline phase and glass matrix. This is confirmed by the fact that the etched surface displays
both attacked crystals and etched glass matrix. This irregularity is considered to be suitable for adhesive
bonding, and the speculation is supported by the shear testing results as well as the sectional micrograph (Fig.
4b). After HF etching, bond strengths greater than 30 MPa were recorded regardless of the use of the silane
agent as well as application of thermocycling.
Air-borne particle abrasion with alumina was also evaluated because this procedure is performed to remove
investment material after mold processing. As shown in Fig. 3d, alumina abrasion considerably roughened the
Empress 2 surface. However, reduction in bond strength after thermocycling was remarkable. This is probably
due to the lack of undercut on the abraded surface. Similar results have been reported for sintered porcelain.7,8Clinicians should therefore keep in mind that a particle-abraded surface is not mechanically retentive, although
the surface may appear more retentive than polished or glazed surfaces.
The effect of silane primer was not apparent in the current research. One of the problems associated with thetwo-liquid silane primer is that the Clearfil Mega Bond Primer material contains water and 2-hydroxyethyl
methacrylate (HEMA). Although these are indispensable for bonding dentin as a self-etching primer, they may
act as inhibitors of silane coupling as well as interlocking within the etched ceramic material. Silane agentsshould contain neither water nor HEMA for proper polymerization reaction, but should contain an acidic
monomer like MDP separated from the silane monomer. Also, an initiator should be added to the silane
coupling agent. The ineffectiveness of the current two-liquid silane agent is probably explained by the presenceof water that was originally included in the Mega Bond Primer liquid. Also, lack of an initiator in the two-liquidsilane agent may also be responsible for the undesirable curing and bonding characteristics.9
On the basis of the current experiments, it can be concluded that hydrofluoric acid etching effectively
enhanced bond strength of Panavia F luting agent to Empress 2 ceramic, regardless of the use of the two-liquidsilane primer.
Acknowledgment
This work was supported in part by a Grant-in-Aid for Young Scientists (B) 16791211 (2004) from the MEXT-Japan.

References
1. Lehner C, Studer S, Brodbeck U, Scharer P. Short-term results of IPS-Empress full-porcelain crowns. J Prosthodont 1997; 6:
20-30.
2. Krämer N, Frankenberger R. Clinical performance of bonded leucite-reinforced glass ceramic inlays and onlays after eight
years. Dent Mater 2004; in press.
3. Rosentritt M, Behr M, Handel G. Fixed partial dentures: all-ceramics, fibre-reinforced composites and experimental systems.
J Oral Rehabil 2003; 30, 873-7.
4. Sorensen JA, Cruz M, Mito WT, Raffeiner O, Meredith HR, Foser HP. A clinical investigation on three-unit fixed partial
dentures fabricated with a lithium disilicate glass-ceramic. Pract Periodontics Aesthet Dent 1998; 11: 95-106.
5. Sorensen JA, Kang SK, Avera SP. Porcelain-composite interface microleakage with various porcelain surface treatments.
Dent Mater 1991; 7: 118-23.
6. Höland W, Schweiger M, Frank M, Rheinberger V. A comparison of the microstructure and properties of the IPS Empress 2

105

Nagai and Kawamoto

Int Chin J Dent 2004; 4: 100-106.

and the IPS Empress glass-ceramics. J Biomed Mater Res 2000; 53: 297-303.
7. Bertolotti RL, Lacy AM, Watanabe LG. Adhesive monomers for porcelain repair. Int J Prosthodont 1989; 2: 483-9.
8. Kato H, Matsumura H, Atsuta M. Effect of etching and sandblasting on bond strength to sintered porcelain of unfilled resin. J
Oral Rehabil 2000; 27: 103-10.
9. Matsumura H, Kato H, Atsuta M. Shear bond strength to feldspathic porcelain of two luting cements in combination with threesurface treatments. J Prosthet Dent 1997; 78: 511-7.

Reprint request to:
Dr. Yoshikazu Kawamoto
Department of Crown and Bridge Prosthodontics, Nihon University School of Dentistry
1-8-13, Kanda-Surugadai, Chiyoda-Ku, Tokyo 101-8310, Japan
Fax: +81-3-3219-8351 E-mail: kawamoto-y@dent.nihon-u.ac.jp
Received November 5, 2004. Revised November 16, 2004. Accepted November 20, 2004.
Copyright ©2004 by the Editorial Council of the International Chinese Journal of Dentistry.

106