Posted at the Institutional Resources for Unique Collection and Academic Archives at Tokyo Dental College,

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TitleLuting of CAD/CAM ceramic inlays: direct composite

versus dual-cure luting cement

Author(s)

Alternative

Kameyama, A; Bonroy, K; Elsen, C; Lührs, AK;

Suyama, Y; Peumans, M; Van, Meerbeek; De Munck, J

JournalBio-medical materials and engineering, 25(3): 279-

288

URL http://hdl.handle.net/10130/3932

Right

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Original article

Luting of CAD/CAM ceramic inlays: direct composite versus dual-cure

luting cement

Atsushi Kameyama a,b, Kim Bonroy c, Caroline Elsen c, Anne-Katrin Lührs a,d,

Yuji Suyama a,e, Marleen Peumans a, Bart Van Meerbeek a and Jan De Munck c

a KU Leuven BIOMAT, Department of Oral Health Sciences, Biomedical Sciences Group,

KU Leuven (University of Leuven), Kapucijnenvoer 7, B-3000 Leuven, Belgium

b Division of General Dentistry, Department of Clinical Oral Health Science, Tokyo

Dental College, 2-9-18 Misaki-cho, Chiyoda-ku, Tokyo 101-0061, Japan

c Section of Operative Dentistry, School of Dentistry, Oral Pathology, and

Maxillo-facial Surgery, KU Leuven (University of Leuven), Kapucijnenvoer 7, B-3000

Leuven, Belgium

d Department of Conservative Dentistry, Periodontology and Preventive Dentistry, OE

7740, Hannover Medical School, Carl-Neuberg-Straße 1, 30625 Hannover, Germany

e Toranomon Hospital, Department of Dentistry, 2-2-2 Toranomon, Minato-ku, Tokyo

105-8470, Japan

Running short head: Luting of CAD/CAM ceramic inlays

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Corresponding author: Atsushi Kameyama, Division of General Dentistry, Department

of Clinical Oral Health Science, Tokyo Dental College, 2-9-18 Misaki-cho, Chiyoda-ku,

Tokyo 101-0061, Japan. Tel: +81-3-6380-9127; E-mail: kameyama@tdc.ac.jp

Abstract. The aim of this study was to investigate bonding effectiveness in direct

restorations. A two-step self-etch adhesive and a light-cure resin composite was

compared with luting with a conventional dual-cure resin cement and a two-step etch

and rinse adhesive. Class-I box-type cavities were prepared. Identical ceramic inlays

were designed and fabricated with a computer-aided design/computer-aided

manufacturing (CAD/CAM) device. The inlays were seated with Clearfil SE Bond/Clearfil

AP-X (Kuraray Medical) or ExciTE F DSC/Variolink II (Ivoclar Vivadent), each by two

operators (five teeth per group). The inlays were stored in water for one week at 37°C,

whereafter micro-tensile bond strength testing was conducted. The micro-tensile bond

strength of the direct composite was significantly higher than that from conventional

luting, and was independent of the operator (P < 0.0001). Pre-testing failures were only

observed with the conventional method. High-power light-curing of a direct composite

may be a viable alternative to luting lithium disilicate glass–ceramic CAD/CAM

restorations.

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Keywords: Micro-tensile bond strength, Ceramic inlay, Computer-aided

design/computer-aided manufacturing (CAD/CAM), Luting cement, Self-etch

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1. Introduction

The increasing demand for esthetic, tooth-colored restorations with reliable clinical

performance can be fulfilled partly by indirect computer-aided design/computer-aided

manufacturing (CAD/CAM)-restorations. This technology allows a ceramic restoration

to be prepared, designed and fabricated in a single appointment, which eliminates the

need for impression-taking and provisional restorations and prevents contamination of

adhesive surfaces by temporary cements [1].

Stable adhesion to CAD/CAM glass ceramics can be obtained using resin luting

cements to produce a combination of micro-mechanical interlocking and chemical

adhesion by silanization [2-4]. Adhesion to the tooth substrate is, however, more

challenging and weaker than that with the adhesive systems used in direct composite

restorations [5,6]. Moreover, a general distinction can be made between easy-to-use

but low-performing cements such as automix self-adhesive cements, and

high-performing cements that require multiple application steps [5,7]. Direct light-cure

restorative composites are perhaps considered more user-friendly because of the

prolonged processing time required. The higher filler content and lower initiator

concentration compared with dual-cure resin cements may be beneficial in terms of

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mechanical strength and the wear properties of the exposed margins [8].

The objective of this study was to assess bonding effectiveness in direct

restorations by comparing a gold-standard two-step self-etch adhesive and a light-cure

resin composite with luting with a conventional dual-cure resin cement and a two-step

etch and rinse adhesive. The null hypothesis tested in this study was that there was no

significant difference in bonding effectiveness between the two procedures in the

cementation of CAD/CAM ceramic blocks to Class-I cavity-bottom dentin.

2. Materials and Methods

2.1. Preparation of cavity and ceramic inlay

Twenty non-carious human third molars were harvested after extraction with

informed consent. The study protocol was approved by the Commission for Medical

Ethics of the University of Leuven. The specimens were stored in 0.5% chloramine T

solution at 4°C and used within 1 month of extraction. Each specimen was mounted on

a gypsum block to facilitate handling. A standard box-type Class-I cavity (approximately

4 × 4 mm) was prepared to a depth of 2.5 mm using a 1:5 high-speed motor hand-piece

(INTRAmatic 25CH, KaVo Dental GmbH, Biberach, Germany) with a tapered cylindrical

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medium-grit (100 µm) diamond bur (848 314 023, Komet, Lemgo, Germany) mounted

in a modified computer-controlled MicroSpecimen Former (University of Iowa, Iowa

City, IA, USA). A cavity of the same dimensions, but deeper (approximately 8 mm), was

similarly prepared in a poly(methyl methacrylate) block for the manufacture of inlays.

Before taking an optical impression, the prepared cavity was coated with a thin layer of

Vita CEREC Powder (CEREC Propellant, Vita Zahnfabrik, Bad Säckingen, Germany). A

standardized inlay with a convex occlusal surface was designed and fabricated with a

CEREC 3 CAD/CAM device (Sirona A.G., Bensheim, Germany). Twenty identical inlays

were cut from commercial lithium disilicate glass–ceramic blocks (IPS e.max CAD for

CEREC and inLab, HT A3/I12, Ivoclar Vivadent, Schaan, Liechtenstein; Batch # N32756).

These inlays were crystallized in a furnace (VITA Vacumat 40T, Vita Zahnfabrik)

according to the manufacturer’s instructions (10 min at 820°C and 7 min at 840°C).

2.2. Study design and bonding procedure (Fig. 1)

The prepared cavities and ceramic inlays were divided randomly into four

experimental groups: two inlay luting procedures (dual-cure resin cement versus

light-cure composite) were each performed by two operators in their final year of a

specialized clinical training program (Master-after-Master in Restorative Dentistry).

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All ceramic inlays were pretreated similarly. Immediately before bonding,

hydrofluoric acid (IPS Ceramic Etching gel, Ivoclar Vivadent) was applied for 60 s to all

cavity-facing surfaces, which were then rinsed thoroughly with water and air-dried.

Monobond Plus (Ivoclar Vivadent) was applied and left undisturbed for 60 s and then

air-dried. An unfilled adhesive resin (Heliobond, Ivoclar Vivadent) was applied but not

light-cured to improve resin infiltration into the etched pits in the ceramic surface [2,9].

The inlays were seated in accordance with the manufacturer’s instructions (Table

1). After seating, final light-curing was applied to the occlusal surface for a single cycle

of 40 s. The light had to travel 8 mm through the ceramic block before reaching the

surface being investigated. Light-curing was performed with a high-power light-emitting

diode (LED) light-curing unit (Bluephase 20i, Ivoclar Vivadent). Light intensity was

controlled using a Bluephase meter (Ivoclar Vivadent) to ensure a light output of at

least 2000 mW/cm2 [10].

2.3. Micro-tensile bond strength testing

The restored cavities were stored in water for 1 week at 37°C, and were then

sectioned perpendicular to the cavity bottom using a 300-µm diamond cut-off wheel

(M1D10, Struers A/S, Ballerup, Denmark) mounted in an automatic precision

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water-cooled cut-off machine (Accutom 50, Struers A/S) to obtain rectangular 1 ×

1-mm non-trimmed micro-specimens for micro-tensile bond strength (µTBS) testing.

The micro-specimens were kept moist until tested. They were attached to a notched

BIOMAT jig [11] with cyanoacrylate glue (Model Repair II Blue, Dentsply-Sankin,

Ohtawara, Japan) and stressed at a crosshead speed of 1 mm/min until failure in an LRX

testing device (Lloyd, Hampshire, UK) using a load cell of 100 N. After measuring the

exact dimensions of each fractured beam with a digital caliper (CD-15 CPX, Mitutoyo,

Tokyo, Japan), µTBS was measured (in MPa) by dividing the recorded force (in N) at the

time of fracture by bond area (in mm2). If a specimen failed before testing, a bond

strength of 0 MPa was used in statistical analyses. The number of pre-testing failures

was also noted. The data were evaluated statistically with a two-way analysis of

variance (ANOVA) and post hoc Tukey multiple comparison at a significance level of

0.05.

2.4. Failure analysis

The failure mode was determined at a 50 × magnification with a stereomicroscope

(Wild M5A, Heerbrugg, Switzerland). Representative specimens in each group were

selected for scanning electron microscopy (SEM) using standard specimen processing

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techniques, including fixation in 2.5% glutaraldehyde in cacodylate buffer solution,

dehydration in ascending concentrations of ethanol and chemical drying using

hexamethyldisilazane (HMDS). After being mounted on stubs, the specimens were

coated with a thin gold layer using an automatic sputter coater (JFC-1300, JEOL, Tokyo,

Japan) and they were then subjected to SEM (JSM-6610 LV, JEOL).

3. Results

The µTBS values and results of the multiple comparisons are presented in Table 2

and Fig. 2. The two-way ANOVA revealed significant differences in bonding

effectiveness between the two adhesive procedures (P < 0.0001).

The overall mean in the direct composite group (33.3 ± 12.0 MPa) was almost six

times higher than that in the luting cement group (5.7 ± 7.9 MPa). Both operators

performed equally (P = 0.6550), though not in every group, as a significant interaction

effect was recorded (P = 0.0013). Both operators obtained significantly higher bond

strength values with the direct composite compared with the dual-cure luting cement

(P < 0.0001). Pre-testing failures were only observed in the dual-cure cement group,

and 75% of the pre-testing failures were caused by a single operator (Table 2).

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Distribution of the failure mode is summarized in Table 2, and representative SEM

photomicrographs of the fractured surfaces after µTBS testing are shown in Figs 3 and

4. The type of failure varied considerably between the two procedures. With dual-cure

cement, more failures occurred at the resin–dentin interface and exposed the hybrid

layer over large parts of the fractured surface. The second predominant failure location

was the adhesive resin, which contained numerous small (< 1 µm) bubbles in almost all

specimens. A few specimens luted with dual-cure cement failed in the ceramic or at the

resin–ceramic interface. Specimens luted with direct composite almost never fractured

along the resin–dentin interface, and if the hybrid layer was exposed, it was only over a

very small area of the fractured surface. In the direct composite group, most failures

occurred within the resin composite, or at the resin–ceramic interface.

4. Discussion

The objective of this study was to compare the effectiveness of a direct light-cure

composite with that of a conventional dual-cure composite cement for luting of

CAD/CAM ceramic inlays in vitro. The statistical analysis revealed that, independently

of the operator, the bonding effectiveness of the alternative luting method was

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significantly higher than that of the conventional method. Therefore, the

null-hypothesis tested was rejected.

IPS e.max CAD is categorized as a lithium disilicate glass–ceramic. This ceramic has

high mechanical properties compared with conventional feldspathic or

leucite-reinforced glass ceramics [12,13]. We aimed to determine whether even

endo-crowns, a type of restoration where the crown and post are designed as a

“monobloc”, could be cemented with light-cure materials only. Therefore, a highly

translucent type of ceramic was chosen.

Variolink II luting cement is known for its high bonding effectiveness and superior

durability [2,14], even when exposed to long-term thermocycling [7,15]. The bond

strength values of Variolink II remained stable after thermocycling followed by storage

in water for 6 months [7]. A low bonding effectiveness resulted, especially when

compared with results obtained by the light-curing method. A previous study that

evaluated µTBS to flat surfaces showed that the direct composite performed better, but

Variolink yielded one of the best scores among conventional luting cements [16]. The

µTBS procedure in that earlier study was very similar to what we used and was

performed in the same laboratory. A number of possible explanations exist for the

observed differences: (1) the different cavity design, resulting c-factor (cavity versus flat

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surface; thick versus thin ceramic part) and reduced light transmission; (2) the different

adhesive systems applied (two-step versus three-step etch and rinse); (3) the different

ceramic (lithium disilicate glass–ceramic versus feldspathic ceramic); or (4) the different

light-curing units (conventional quartz–tungsten–halogen lamp versus high-power LED

lamp). Because the specimens failed predominantly at the resin–dentin interface (Table

2), the first two explanations appear to be the most plausible. When using Variolink II,

it is crucial that the light passes through the entire length of the IPS e.max CAD for

curing to occur. Earlier studies found that curing through 7-mm blocks reduced the

bonding performance significantly [16,17]. Two-step etch and rinse adhesives are more

technique sensitive, however, especially in more complex cavity configurations such as

those used in this study [18,19].

Box-shaped Class-I cavities were prepared to make adhesion to the bottom of the

cavity more difficult, thereby simulating clinical conditions. Water or resin tends to

pool in the corners of such cavities, making it impossible to standardize surface

moisture. Moreover, whereas the corners of the surface are too wet, the middle part of

the surface may be too dry [20]. It is often very difficult to remove water or resin from

such a narrow occlusal Class-I cavity, which may result in a pronounced water content

in the adhesive resin. This excess water cannot mix with the adhesive primer. When

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ethanol is evaporated by air drying, water droplets are formed in the adhesive resin

[21,22]. This was confirmed by the numerous small droplets observed by SEM in almost

all specimens in this group (Fig. 3d). Obviously this water excess does not contribute to

interfacial strength or stability [23]. Factors such as dentin moisture [24], seating

pressure [25,26] and even total application time [27] may differ between operators,

which may also explain the observed technique sensitivity when using conventional

dual-cure cement.

In contrast, when both operators used the alternative method (direct composite

and a two-step self-etch adhesive), significantly higher bond strength values were

obtained and no operator effect was observed. Since no chemical initiator is included in

Clearfil SE Bond or Clearfil AP-X, polymerization relies completely on the light

transmitted through the 8-mm ceramic, which corresponds approximately to the

thickness of a so-called endo-crown. To maximize light transmission, a powerful LED

curing device with an output of more than 2000 mW/cm2 was used. To mimic clinical

conditions, the occlusal surface was not flat and allowing for additional light scattering.

A thicker ceramic is known to reduce light intensity and other factors such as the

constitution of the ceramic (leucite-reinforced versus lithium–disilicate glass–ceramic)

and opacity [28-30]. This reduced light intensity affects the degree of conversion

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[31,32], curing depth and hardness [32,33].

Polymerization was performed through a thick ceramic layer. Therefore, apart from

a higher light intensity, an extended polymerization time of 40 s was also chosen. With

lithium–disilicate ceramic, in particular, a reduction in curing time and increase in

ceramic thickness can result in insufficient polymerization [28].

Limited research is available regarding the use of direct light-cure composites to

lute ceramic inlays. However, some in vivo studies compared both techniques with a

follow-up period of up to 12 years of clinical service [34-36]. One clinical study that

investigated Class-II ceramic restorations after 4 years found that a significant operator

effect resulted, but there was no influence from the luting material used (light-cure

ormocer versus high-viscosity dual-cure resin cement) [34]. Failure rates of 8% and 16%

were obtained after 8 and 12 years, respectively, in other in vivo examinations that

compared three different types of cement and one direct composite for luting IPS

Empress inlays [35,36]. After 4 years, a significantly greater excess of luting composite

was present when cement was used (Variolink Low; Ivoclar Vivadent) compared with

the direct composite (Tetric; Ivoclar Vivadent). This indicates that high-viscosity

materials are superior for use in daily clinical practice [35]. In addition to the high µTBS

values obtained, one earlier report noted that the amount of bulk fractures was

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significantly higher when the light-curing material was used than that for the low

viscosity resin cement over a 12-year clinical observation period [34]. We believe that

this is related to insufficient curing of the light-cure composite, which appeared less

important in this study, given the higher bond strengths in the direct composite group.

Recent innovations in light-curing units should also be taken into account; conventional

and relatively inefficient halogen units have been replaced by high-power, highly

efficient LED devices, as used in this study [37].

Besides the improved bond effectiveness observed in this in vitro study, the use of

a direct composite to lute ceramic inlays yields several clinical advantages.

Polymerization of the interface is instantaneous and on-demand because no chemical

curing is required. This results in extended working/seating time and facilitates the

luting of this type of restoration. If a light-cure composite is used with a self-etch

adhesive, the application procedure may be less technique-sensitive and therefore less

prone to application errors. It is also easier to remove composite excess when a viscous

direct composite resin rather than a low viscous material is used.

The higher viscosity of direct composites compared with conventional luting

cement may also be affected by pre-heating [38-40] or ultrasonic application [41] and

can therefore be adjusted based on clinical demand.

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5. Conclusions

The in vitro findings suggest that high-power LED light-curing (more than 2000

mW/cm2) of a direct composite offers a viable alternative to luting lithium disilicate

glass–ceramic CAD/CAM restorations in Class-1 cavities. Higher bond strength values

were obtained when the direct composite was light-cured. Furthermore, the luting

procedure was less technique-sensitive and clinically more appealing.

Author Disclosure Statement

The authors have no financial affiliation to any of the companies whose products

are included in this article.

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Figure Captions

Fig. 1 – Schematic of study set-up. (a) Standardized box-shaped Class-I cavity was

prepared at occlusal surface. (b) CAD/CAM-fabricated ceramic inlays were luted by

light-curing or dual-curing. (c) Resin–dentin micro-specimens (1 mm × 1 mm) were cut

with automated diamond saw and (d) stressed until failure.

Fig. 2 - Box whisker plots of µTBS (MPa). Groups showing no statistical difference are

connected by a horizontal line (Tukey multiple comparison, P > 0.05).

Fig. 3 - (a) SEM photomicrograph of fractured dentin-side surface in Group 1 (luting

composite, Operator A) showing mixed failure pattern. Circular scratches at cavity

bottom resulted from contact with tip of diamond bur during cavity preparation. (b)

Higher magnification of specimen in Fig. 3a; failure occurred in hybrid layer and

adhesive resin. Numerous droplets visible in adhesive resin. (c) Overview SEM

photomicrograph of fractured ceramic-side surface in Group 3 (luting composite,

Operator B). In contrast with Fig. 3a, failure occurred almost completely in adhesive

resin. (d) Higher magnification of specimen in Fig. 3c; typical accumulation of small

droplets (smaller than 500 nm) over entire surface. White rectangle shows origin of

detail presented (d).

Fig. 4 - (a) SEM photomicrograph of fractured dentin-side surface in Group 2 (direct

25

composite, Operator A) showing mixed failure pattern. Failure occurred within most of

the composite and at the composite/ceramic interface. (b) Higher magnification of

specimen in Fig. 4a, intersectional area with failure at level of hybrid layer and adhesive

resin. (c) SEM photomicrograph of fractured ceramic-side surface in Group 4 (direct

composite, Operator B). Failure occurred over most of the adhesive interface and in the

adhesive resin (d) Higher magnification of specimen in Fig. 4c; intersectional area with

failures between adhesive resin and adhesive interface. In contrast with Groups 1 and 3

(luting composite), no droplets were detected, and dentin surface was covered with

smear layer. White rectangle shows origin of detail presented (d).

26

Fig. 1

Fig. 2

27

Fig. 3

28

Fig. 4

1

Table 1 – Materials tested and application protocol

Group Product name

(manufacturer)

Components

[Batch No.]

Main ingredients Application protocol

Luting

composite

ExciTE F DSC /

Variolink II

(Ivoclar Vivadent,

Schaan,

Liechtenstein)

Total Etch

[N05612]

ExciTE F DSC

[N24042]

Base paste

[N23694]

Catalyst

[N22039]

37% phosphoric acid

Phosphonic acid acrylate, dimethacrylate, HEMA,

highly dispersed silicon dioxide, ethanol, catalysts,

stabilizers, fluoride, initiators

Bis-GMA, UDMA, TEGDMA, inorganic filler, ytterbium

trifluoride, initiators, stabilizers

Bis-GMA, UDMA, TEGDMA, inorganic filler, ytterbium

trifluoride, benzoyl peroxide, stabilizers

Etch cavity wall with Total Etch for 15 s and rinse;

apply ExciTE F DSC with a brush, on enamel and

dentin of cavity wall, and agitate for at least 10 s;

dry for 1–3 s; light-cure for 20 s; mix equal

amounts of base and catalyst for 10 s; apply the

mixture into cavity with a brush; insert inlay and

remove excess cement; light-cure for 40 s.

Direct

composite

Clearfil SE Bond /

Clearfil AP-X

(Kuraray Medical,

Kurashiki, Japan)

Primer

[00903B]

Bond

[01332A]

Clearfil AP-X

(PLT A3)

[000388]

MDP, HEMA, hydrophilic dimethacrylate,

photoinitiator (CQ, DEPT, others), water

MDP, bis-GMA, HEMA, hydrophilic dimethacrylate,

microfiller

Bis-GMA, TEGDMA, silanated barium glass filler,

silanated silica filler, silanated colloidal silica, CQ,

catalysts, accelerators, pigments, others

Apply primer and leave it in place for 20 s; gently

air dry; apply bond, thin with air pressure; light

cure for 10 s; apply Clearfil AP-X and insert inlay;

remove excess paste; light-cure for 40 s.

Bis-GMA: bisphenol-glycidyl methacrylate, HEMA: 2-hydroxyethylmethacrylate, MDP: 10-methacryloyloxydecyl dihydrogen phosphate, TEGDMA:

triethylene glycol dimethacrylate, UDMA: urethane dimethacrylate, CQ: dl-camphorquinone; DEPT: N,N-diethanol-p-toluidine

2

Table 2 –µTBS and failure analysis

Mean ± S.D.

(MPa)

PTF/n Failure analysis (%)*

Dentin Interface Composite Composite

/ ceramic

Ceramic

Luting composite – Operator A 9.0 ± 8.6b 7/21 0 45 30 9 15

Luting composite – Operator B 1.0 ± 4.2c 19/20 0 100 0 0 0

Direct composite – Operator A 29.7 ± 8.3a 0/17 0 14 37 49 0

Direct composite – Operator B 35.8 ± 13.6a 0/26 2 9 70 19 0

* Pre-testing failures were excluded from failure analysis.

µTBS: micro-tensile bond strength, PTF: number of pre-testing failures, n: total number of specimens, S.D.: standard deviation.

Means with same superscript letter were not significantly different (P > 0.05, post hoc Tukey multiple comparison test)