Title
Luting 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
Journal
Bio-medical materials and engineering, 25(3): 279288
URL
http://hdl.handle.net/10130/3932
Right
Posted at the Institutional Resources for Unique Collection and Academic Archives at Tokyo Dental College,
Available from http://ir.tdc.ac.jp/
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
1
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.
2
Keywords:
Micro-tensile
bond
strength,
Ceramic
inlay,
Computer-aided
design/computer-aided manufacturing (CAD/CAM), Luting cement, Self-etch
3
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
4
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
5
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).
6
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
7
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
8
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).
9
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
10
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
11
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
12
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
13
[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
14
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.
15
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.
16
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23
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
24
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).
25
Fig. 1
Fig. 2
26
Fig. 3
27
Fig. 4
28
Table 1 – Materials tested and application protocol
Group
Product name
Components
(manufacturer)
[Batch No.]
Luting
ExciTE F DSC /
composite
Variolink II
Total Etch
Main ingredients
Application protocol
37% phosphoric acid
Etch cavity wall with Total Etch for 15 s and rinse;
[N05612]
apply ExciTE F DSC with a brush, on enamel and
(Ivoclar Vivadent,
ExciTE F DSC
Phosphonic acid acrylate, dimethacrylate, HEMA,
dentin of cavity wall, and agitate for at least 10 s;
Schaan,
[N24042]
highly dispersed silicon dioxide, ethanol, catalysts,
dry for 1–3 s; light-cure for 20 s; mix equal
stabilizers, fluoride, initiators
amounts of base and catalyst for 10 s; apply the
Bis-GMA, UDMA, TEGDMA, inorganic filler, ytterbium
mixture into cavity with a brush; insert inlay and
Base paste
trifluoride, initiators, stabilizers
remove excess cement; light-cure for 40 s.
[N23694]
Bis-GMA, UDMA, TEGDMA, inorganic filler, ytterbium
Catalyst
trifluoride, benzoyl peroxide, stabilizers
Liechtenstein)
[N22039]
Direct
Clearfil SE Bond /
Primer
MDP,
composite
Clearfil AP-X
[00903B]
photoinitiator (CQ, DEPT, others), water
air dry; apply bond, thin with air pressure; light
(Kuraray
Bond
MDP, bis-GMA, HEMA, hydrophilic dimethacrylate,
cure for 10 s; apply Clearfil AP-X and insert inlay;
[01332A]
microfiller
remove excess paste; light-cure for 40 s.
Clearfil AP-X
Bis-GMA, TEGDMA, silanated barium glass filler,
(PLT A3)
silanated silica filler, silanated colloidal silica, CQ,
[000388]
catalysts, accelerators, pigments, others
Medical,
Kurashiki, Japan)
HEMA,
hydrophilic
dimethacrylate,
Apply primer and leave it in place for 20 s; gently
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
1
Table 2 –µTBS and failure analysis
Mean ± S.D.
PTF/n
(MPa)
Failure analysis (%)*
Dentin
Interface
Composite
Composite
Ceramic
/ ceramic
Luting composite – Operator A
9.0 ± 8.6b
Luting composite – Operator B
1.0 ± 4.2
c
Direct composite – Operator A
29.7 ± 8.3a
Direct composite – Operator B
35.8 ± 13.6
a
7/21
0
45
30
9
15
19/20
0
100
0
0
0
0/17
0
14
37
49
0
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)
2