Alle Rechte vorbehalten
Simplified self-etching and self-adhesive resin cements
have recently been marketed for luting inlays, onlays,
crowns, and posts. In comparison with multi-step systems,
self-etching, self-adhesive cements are claimed to be less
technique sensitive,26 with reduction in postoperative sen-
However, there are limitations to the use of simplified lut-
ing agents. The bond strength of the self-etching composite
cement Panavia F (Kuraray Medical, Tokyo, Japan) to dentin
appeared to be affected by water-induced interfacial
changes that occurred in relation to the permeability of the
acidic ED Primer, if the cement was not readily light cured.
These interfacial changes were prevented by coating the
primed dentin with an additional layer of light-cured hy-
drophobic resin that reduced the permeability of the adhe-
sive layer, thus improving the cement-dentin coupling.4
Microtensile Bond Strength and Interfacial Properties of
Self-etching and Self-adhesive Resin Cements Used to
Lute Composite Onlays Under Different Seating Forces
Cecilia Goraccia/Alvaro H. Curyb/Amerigo Cantoroc/Federica Papacchinib/Franklin R. Tayd/
Purpose: To evaluate strength and morphology of the interface created on enamel and dentin by Panavia F 2.0 (P, Ku-
raray), RelyX Unicem (RU, 3M ESPE), and Maxcem (M, Sybron-Kerr), applied under two standardized clinically realistic
Materials and Methods: Composite overlays (Paradigm MZ100, 3M ESPE) were luted on flat enamel or dentin sur-
faces of 48 extracted molars. During the initial 5-min self-curing period, a pressure of either 20 or 40 g/mm2was
maintained on the overlay. Curing was completed by light irradiation from the top of the overlay for 20 s. Microtensile
sticks and specimens for SEM observations were obtained from the luted teeth. Bond strength data from enamel and
dentin (MPa) were analyzed with separate two-way ANOVAs in order to assess the effect of cement type, luting pres-
sure, and their interactions. Tukey's test was used for post-hoc comparisons (_= 0.05).
Results: The bond strengths (MPa) to dentin were: P 20 g/mm27.5 ± 3.7, 40 g/mm210.9 ± 4.5; RU 20 g/mm26.8 ±
2.6, 40 g/mm214.5 ± 5.3; M 20 g/mm24.1 ± 1.8, 40 g/mm25.2 ± 1.6. The bond strengths (MPa) to enamel were:
P 20 g/mm225.2 ± 9.0, 40 g/mm230.7 ± 8.6; RU 20 g/mm210.7 ± 4.9, 40 g/mm211.1 ± 5; M 20 g/mm27.3 ±
3.1, 40 g/mm27.9 ± 3.2. Cement type, luting pressure, and the interaction of these two factors had significant influ-
ences on dentin bond strength, with RU and P outperforming M. RU and P significantly benefited from the increase in
luting pressure. In enamel, the type of cement was a significant factor for adhesion, with P yielding the highest and M
the lowest strength. The measured bond strengths corresponded with the morphological results.
Conclusion: Interfacial strength and adaptation of self-etching and self-adhesive dual-curing cements are enhanced
if a seating force greater than finger pressure is maintained throughout the initial self-curing period.
Keywords: self-etching resin cement, self-adhesive resin cement, seating force, microtensile bond strength, scanning
electron microscope, interface.
J Adhes Dent 2006; 8: 327-335. Submitted for publication: 22.12.05; accepted for publication: 03.04.06.
Vol 8, No 5, 2006 327
aAssistant Professor, Department of Dental Materials, University of Siena, Poli-
clinico “Le Scotte”, Siena, Italy.
bGraduate Student, Department of Dental Materials, University of Siena, Poli-
clinico “Le Scotte”, Siena, Italy.
cStudent, Department of Dental Materials, University of Siena, Policlinico “Le
Scotte”, Siena, Italy.
dProfessor, Department of Oral Biology and Maxillofacial Pathology, Medical
College of Georgia, Augusta, Georgia, USA.
eProfessor, Department of Dental Materials, University of Siena, Policlinico “Le
Scotte”, Siena, Italy.
Reprint requests: Cecilia Goracci, Research Center for Dental Health, 19 Pi-
azza Attias, 57125 Livorno, Italy. Tel. +39-0586-892-283, Fax +39-0586-898-
305. e-mail: firstname.lastname@example.org
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A limited ability to etch through clinically relevant smear lay-
ers into the underlying intact dentin, together with relatively
high viscosity and porosity were described for the self-ad-
hesive material RelyX Unicem (3M ESPE, Seefeld, Ger-
many).8,27 Furthermore, the bond strength of RelyX Unicem
to enamel was significantly reduced with the use of thermo-
cycling as a method to accelerate hydrolytic degradation.1
No information is available in the literature regarding the
bonding effectiveness of the recently introduced self-adhe-
sive resin cement Maxcem (Sybron-Kerr; Orange, CA, USA) to
enamel and dentin.
It has been suggested that because of the material's
thixotropic characteristics, the bonding of RelyX Unicem may
benefit from the application of seating pressure during the
luting procedure.8Moreover, a recent study indicated that the
application of a sustained seating pressure during the self-
curing of Panavia F 2.0 can limit the detrimental effect of ED
Primer permeability on dentin-cement coupling.6Although
the magnitude and duration of the pressure applied during
cementation of indirect restorations are parameters of clini-
cal relevance, these variables were not standardized in re-
cent studies on simplified resin cements.4,6,27 In particular,
the magnitude of the seating pressure has been over-
looked,2,23 or was generically described as “light pressure”,8
“finger pressure”,9,21 or rather defined as a function of the
desired cement film thickness.16 According to Black et al,3
forces routinely used for crown cementation lie within a range
of 20 to 60 N. A seating force as low as 10 N results in ade-
quate crown seating with reduced pulpward pressure. Twen-
ty-five N is considered a moderate, clinically realistic seating
force,5,13 whereas a force higher than 100 N may present a
risk of pulpal damage5and crown deformation.13
The present study was conducted with the aim of as-
sessing the strength and morphological aspects of the in-
terface established on enamel and dentin by a self-etching
and two self-adhesive dual-curing resin cements, Panavia
F2.0, RelyX Unicem, and Maxcem, respectively, when they
were used to lute composite onlays under two standardized,
clinically realistic seating pressures. The null hypothesis test-
ed was that there are no differences caused by the type of
luting cement and the seating pressure on the tooth-cement
interfacial morphology and microtensile strength.
MATERIALS AND METHODS
Bonding was performed on noncarious human third molars
extracted after informed consent had been obtained under
a protocol reviewed and approved by the institutional review
board of the University of Siena, Italy. The teeth were stored
in a 1% chloramine-T solution at 4°C and used within one
month following extraction. Prior to the bonding experi-
ments, the teeth were retrieved from the disinfectant solu-
tion and stored in distilled water, with four changes of the lat-
ter within 48 h to remove the disinfectant.
Tooth Preparation for Microtensile Bond Strength Tests
Flat, abraded enamel bonding surfaces were prepared from
the buccal or lingual surface of 30 teeth using wet 180-grit
silicon carbide (SiC) abrasive paper.14 Five teeth were ran-
domly chosen for each experimental group (ie, 3 cement
types and 2 seating pressures). They were rinsed with water
and kept moist until further processing.
Three molars per group were used for testing on dentin.
On each tooth, a deep coronal dentin substrate was exposed
by removing occlusal enamel and superficial dentin with a
slow-speed saw (Isomet, Buehler; Lake Bluff, IL, USA) under
water cooling. The exposed dentin surfaces were polished
with wet 180-grit SiC paper,14 rinsed copiously with water,
and kept moist until further processing.
The self-adhesive resin cements RelyX Unicem and Max-
cem, and the self-etching resin cement Panavia F 2.0 were
employed in the dual-curing mode to lute composite overlays
(Paradigm MZ100, 3M ESPE; St Paul, MN, USA) on the pre-
pared enamel and dentin substrates. The chemical compo-
sition of the tested cements is reported in Table 1.
The Paradigm MZ100 blocks were cut using the Isomet
saw under copious water cooling into 2-mm-thick blocks,
each with a surface area that was sufficient to cover the
bonding surface of the prepared teeth. The cut surface of
each composite block was ground with 180-grit SiC paper,
cleaned with ethanol, and air dried. The exposed dental sub-
strate was blot dried with cotton gauze for 5 s prior to the lut-
The luting cements were handled in strict accordance
with the manufacturers' instructions (see Table 2). In half of
the enamel specimens and half of the dentin specimens, a
2-mm-thick composite block was placed on the substrate un-
der a pressure of 20 g/mm2that was maintained for 5 min
at 37°C and 100% relative humidity (group A). The selection
of a seating pressure of 20 g/mm2was based on the mean
finger pressure applied on a 10-mm-diameter Paradigm
MZ100 block by twenty PhD students from the Department
of Dental Materials of the University of Siena. On the other
half of the enamel and dentin specimens, a pressure of 40
g/mm2was applied. The application of this seating pressure
on a 10-mm-diameter Paradigm block is equivalent to a force
of about 30 N, which may be considered as a moderate
crown seating force,5,13,20 and a light biting force.10,11,15, 25
The seating force was applied to the composite block by
means of a plunger that was loaded by a box filled with lead
pellets (Fig 1). The weight of the lead-pellet-filled box was ad-
justed based on the bonding surface area of the specimen,
so as to obtain the standard seating pressure of 20 or 40
g/mm2. The specimen maintained under pressure in the
loading device (Fig 1) was placed in an incubator, where it
was kept for 5 min at 37°C under 100% relative humidity
At the end of the 5-min self-curing period, the luting ce-
ment was light cured from the top of the 2-mm-thick com-
posite block for 20 s with a halogen light-curing unit
(XL3000, 3M ESPE) operating at 550 mW/cm2. Then, an-
other 2-mm-thick composite block was luted onto the first
one using RelyX Unicem in a self-curing mode, in order to en-
sure an adequate length for specimen handling during mi-
crotensile bond testing.
After a 24-h storage period at 37°C under 100% relative
humidity, each bonded tooth was sectioned occluso-gingi-
328 The Journal of Adhesive Dentistry
Goracci et al
Alle Rechte vorbehalten
Vol 8, No 5, 2006 329
Goracci et al
vally into serial slabs, using the Isomet saw under water
cooling at very low speed and with no additional pressure.
Each slab was then fixed on a glass slide with double-
sided sticky tape, and further sectioned into 0.9 x 0.9 mm
composite-dentin/composite-enamel sticks, according to
the non-trimming version of the microtensile test. The
specimens were stressed to failure under tension using a
universal testing machine (Controls; Milano, Italy) at a
crosshead speed of 1 mm/min.
Premature failures were excluded from the statistical calcu-
lations, as their inclusion would have affected the normality
of data distribution, thus precluding the use of parametric
tests for assessing the influence of each study variable and
of their interactions on the measured bond strengths.
Moreover, enamel and dentin data were treated sepa-
rately, as the pooled population of bond strength data from
both substrates was not normally distributed according to
the Kolmogorov-Smirnov test.
Having verified that the data from each substrate were
normally distributed (Kolmogorov-Smirnov test) and exhibit-
ed homogeneity of variances among groups (Levene test), a
two-way ANOVA was separately applied to the data from
each bonding substrate, with bond strength as the depen-
dent variable and cement type and seating pressure as fac-
tors. Tukey's test was applied for post-hoc comparisons. In
all the analyses, the level of significance was set at _= 0.05.
Two teeth from each experimental group were used for scan-
ning electron microscope observations of the bonded inter-
face. To observe dentin interfaces, after exposing a deep
dentin substrate by cutting with the Isomet saw, each tooth
was longitudinally split into halves, and each half had a 2-
mm-thick composite block luted under either 20 or 40
For the observation of enamel interfaces, each tooth was
longitudinally cut in a mesio-distal direction to obtain one
buccal and one lingual half. A flat surface was created on
each half by grinding the enamel with a 180-grit wet SiC pa-
per. A 2-mm-thick composite block was subsequently luted
under either 20 or 40 g/mm2of pressure. In all the luting
procedures, pressure was maintained for 5 min at 37°C and
100% relative humidity prior to light curing the cement
through the composite block for 20 s. The same light-curing
unit that was used for the preparation of specimens for bond
strength testing was employed in this part of the experi-
Panavia F2.0 ED Primer 2.0 Panavia F2.0
Primer A: ED Primer 2.0 Base paste: Hydrophobic aromatic and aliphatic
HEMA, MDP, 5-NMSA, dimethacrylate, sodium aromatic sulphinate,
water, accelerator (batch # 00186B) N,N-diethanol-p-toluidine, functionalized sodium fluoride,
Primer B: 5-NMSA, accelerator, water, silanized barium glass (batch # 00047A)
sodium benzene sulphinate Catalyst paste: MDP, hydrophobic aromatic and aliphatic
(batch # 00067B) photoinitiator, dibenzoyl peroxide
dimethacrylate, hydrophilic dimethacrylate, silanized silica,
(batch # 00006A)
(filler load 70.8%, particle size 2 μm)
RelyX Powder Liquid
Unicem Glass fillers, silica, calcium Methacrylated phosphoric esters,
(batch hydroxide, self-curing initiators, dimethacrylates, acetate, stabilizers,
# 70-201115642) pigments, light-curing initiators self-curing initiators, light-curing initiators
(filler load 72% wt, particle size < 9.5 μm)
Maxcem GPDM, co-monomers (mono-, di-, and tri-functional methacrylate monomers), proprietary self-curing redox acti-
(batch # 32857) vator, photoinitiator (CQ), stabilizer, barium glass fillers, fluoroaluminosilicate glass filler, fumed silica (filler
load 67% wt, particle size 3.6 μm)
Paradigm MZ100 Bis-GMA, TEG-DMA, zirconia silica filler (85% wt)
Table 1 Chemical composition and batch numbers of the tested materials
HEMA: 2-hydroxyethyl methacrylate; MDP 10-methacryloyloxydecyl dihydrogen phosphate; 5-NMSA: N-methacryloyl 5-aminosalicylic acid; bis-GMA: bis-
phenol A glycidyl dimethacrylate; TEG-DMA: triethylenglycol dimethacrylate; GPDM: glycerol dimethacrylate dihydrogen phosphate; CQ camphorquinone.
Alle Rechte vorbehalten
ment. A 2-mm-thick slab containing the coupled interface
was sectioned from each luted specimen and wet polished
with increasingly finer grits of SiC papers (Buehler, #600,
The interface was brought into relief by etching with 32%
silica-free phosphoric acid gel (Uni-Etch, Bisco; Schaumburg,
IL, USA), followed by brief deproteinization with a 2% sodium
hypochlorite solution for 60 s. After rinsing with water and
air drying for 10 s, an impression of the interface was taken
using a PVS (polyvinyl siloxane) impression material (Presi-
dent Jet Light, Coltène-Whaledent; Altstätten, Switzerland).
After separation of the impression material from the bond-
ed interface, positive replicas were obtained using an epoxy
resin (EpoxyCure Resin, Buehler). All replicas were mounted
on aluminum stubs, coated with a 15- to 20-nm-thick layer
of gold by means of the SC7620 Sputter Coater device (Po-
laron Range, Quorum Technologies; Newhaven, UK), and in-
spected with a scanning electron microscope (JSM-6060LV,
JEOL, Tokyo, Japan) at 500X magnification.
Dentin Bond Strength
Two-way ANOVA showed that the cement type, seating pres-
sure, and the interaction between these two factors signifi-
cantly influenced dentin bond strength (p < 0.05). Post-hoc
comparisons among the cements further revealed that the
bond strengths of RelyX Unicem and Panavia F 2.0 were
comparable and significantly higher than that of Maxcem.
When the interaction between the two factors was con-
sidered, RelyX Unicem and Panavia F 2.0 under 40 g/mm2
pressure achieved significantly higher bond strengths than
RelyX Unicem and Panavia F 2.0 under 20 g/mm2pressure,
and Maxcem under either pressure condition, which were all
comparable (Table 3).
Enamel Bond Strength Data
Two-way ANOVA revealed that the type of cement had a sig-
nificant influence on the enamel bond strength. Conversely,
seating pressure (p = 0.05) and the interaction between the
cement type and seating pressure (p = 0.09) did not signifi-
Goracci et al
330 The Journal of Adhesive Dentistry
Fig 1 The device used to apply a standardized seating pressure
to the luted tooth (T). On top of the plunger resting on the com-
posite overlay, a box filled with lead pellets (LP) was placed. The
weight of the lead-pellet-containing box could be adjusted based
on the specimen's bonding surface area, so as to obtain the stan-
dardized seating pressure. The specimen maintained under pres-
sure in the loading device was placed in an incubator at 37°C
and 100% relative humidity for 5 min.
Panavia F2.0 -Mix equal amounts of ED Primer -
2.0 liquids A and B. Apply the mix on
the bonding substrate with a brush and
leave it undisturbed for 30 s. Dry with a
gentle air flow.
-Mix equal amounts of base and catalyst For all materials:
for 20 s, apply the cement onto the primed Let the cement autocure for 5 min at 37°C
substrate. and 100% humidity under pressure.
(For Panavia F2.0, apply Oxiguard 2.0 along
RelyX Unicem -Activate the capsule for 2 s and mix it for the exposed margins while the specimen
10 s with Rotomix (3M ESPE). Is under pressure).
-Apply the cement onto the substrate. -Light cure through the composite overlay for 20 s
Maxcem -Mix base and catalyst through the automix
-Apply the cement onto the substrate.
Table 2 Handling of the luting agents
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Vol 8, No 5, 2006 331
Goracci et al
cantly affect enamel bond strength. Tukey's test showed
that the strength achieved on enamel by Panavia F 2.0 was
significantly higher than that of the other two cements. In ad-
dition, the strength of RelyX Unicem was significantly higher
than that of Maxcem (p < 0.05) (Table 4).
All the cements adequately coupled with the pre-cured com-
posite resin of the overlay (Figs 2 and 3).
The interfaces created by Maxcem in either bonding sub-
strate did not exhibit the microscopic signs of microme-
chanical bonding (Figs 2a and 3a). Rather, large tooth-ce-
ment interfacial discontinuities were frequently observed
(Fig 3a), and even under the heavier seating pressure, the
cement interacted only superficially with the dentin sub-
strate (Fig 2a). In dentin, smear plugs were retained, and no
hybrid layer was detectable (Fig 2a).
Cement Seating Premature No. of Mean SD Significance
pressure failures tested (MPa) p < 0.05
RelyX 20 g/mm25/52 (9.6%) 47 6.8 2.6 A, a
Unicem 40 g/mm23/46 (6.5%) 43 14.5 5.3 A, b
Maxcem 20 g/mm239/45 (86.6%) 6 4.1 1.8 B, a
40 g/mm234/46 (73.91%) 12 5.2 1.6 B, a
Panavia 20 g/mm226/53 (49%) 27 7.5 3.7 A, a
F2.0 40 g/mm213/52 (25%) 39 10.9 4.5 A, b
Different upper case letters indicate statistically significant differences among cements. Different lower case letters indicate
significant cement-pressure interactions in the dentin group.
Table 3 Frequency of pretest failures, number of tested specimens, and parametric statistics for the microtensile bond
strengths measured in dentin specimens
Fig 2a SEM micrograph of dentin specimens (500X, bar=50 μm.
CO=composite overlay, D=dentin). The interface developed when
the overlay was luted with Maxcem under 40 g/mm2of seating
force. Composite-dentin coupling seemed to rely on a very thin
layer of cement that appeared to only superficially interact with
dentin. Dentin tubules were still occluded by smear plugs, and no
signs of interaction with the dentin underlying the smear layer
could be detected.
Fig 2b SEM micrograph of dentin specimens (500X, bar=50 μm.
CO=composite overlay, RC=resin cement, D=dentin). Specimen
luted with Panavia F 2.0 under 20 g/mm2of pressure. A 1- to 2-
μm-thick hybridized smear layer was detectable, with short resin
Alle Rechte vorbehalten
Although RelyX Unicem exhibited better marginal conti-
nuity, no obvious dentin hybrid layer was seen (Fig 2c). In
specimens luted under finger pressure, the cement layer ap-
peared thick, densely filled with glass particles, and porous
(Fig 2c). The heavier seating pressure contributed to reduc-
ing cement thickness and porosity, although it did not in-
crease the resin penetration depth into dentin (Fig 2d). In
enamel as well, RelyX Unicem exhibited limited infiltration
ability, even in the presence of favorably cut enamel prisms
A 1- to 2-μm-thick hybridized smear layer with few and
short resin tags was detected in dentin and enamel speci-
mens luted with Panavia F 2.0 under both seating pressures
(Figs 2b, 3b).
In the present study, a relatively high number of premature-
ly failed specimens was recorded, particularly for teeth bond-
ed with Maxcem (Tables 3 and 4).
It is still debatable whether pretest failures should be ex-
cluded from the statistical calculations, included as zeros,19
or rather as greater than zero values by the assumption that
it must have taken a certain amount of stress to produce fail-
ure during sectioning.18,22 In this investigation, the decision
was made to exclude pretest failures from the statistical
analysis, although the authors are aware that basing calcu-
lations solely on specimens that survived preparation might
have biased the test toward an overestimation of the bond-
Goracci et al
332 The Journal of Adhesive Dentistry
Fig 2c SEM micrograph of dentin specimens (500X, bar=50 μm.
CO=composite overlay, RC=resin cement, D=dentin). Marginal
continuity was observed when luting was performed with RelyX
Unicem under 20 g/mm2. However, no obvious hybrid layer was
evident. The cement layer, about 25 μm thick, appeared densely
filled with glass particles and presented diffuse porosities.
Fig 2d SEM micrograph of dentin specimens (500X, bar=50 μm.
CO=composite overlay, RC=resin cement, D=dentin). When luting
with RelyX Unicem was performed under the heavier pressure (40
g/mm2), the cement layer was thinner and less porous. However,
pressure did not facilitate penetration into dentin, as the hybrid
layer was still extremely thin to nonexistent.
Cement Seating Premature # of tested Mean SD Significance
pressure failures specimens (MPa) p < 0.05
Enamel RelyX Unicem 20 g/mm210/32 (31.2%) 22 10.7 4.9 A
40 g/mm212/30 (40%) 18 11.1 5
Maxcem 20 g/mm231/55 (56.3%) 24 7.3 3.1 B
40 g/mm222/44 (50%) 22 7.9 3.2
Panavia F2.0 20 g/mm24/27 (14.8%) 23 25.2 9 C
40 g/mm23/22 (13.6%) 19 30.7 8.6
Different upper case letters indicate statistically significant differences among cements.
Table 4 Frequency of pretest failures, number of tested specimens, and parametric statistics for the microtensile bond
strengths measured in enamel specimens
Alle Rechte vorbehalten
ing potential.22 Nevertheless, pretest failure exclusion was
chosen for the purpose of preserving normality of data dis-
tributions as a condition to apply parametric tests (namely,
factorial ANOVAs), in order to assess the influence of each
variable and their interactions on the measured bond
strengths. Nevertheless, in agreement with Nikolaenko et
al,18 the number of premature failures was reported for each
group (Tables 3 and 4), as “an honest way to describe some
deficiencies”,18 and therefore as complementary informa-
tion for among-group comparisons.
In this study, cements were tested in dual-curing mode.
By allowing an initial self-curing period under temperature
and humidity conditions simulating the oral cavity, we fol-
lowed the instructions of RelyX Unicem and Maxcem manu-
facturers, who recommend letting the cement self-cure ini-
tially for 2 or 3 min before removing the excess cement.
A remarkable finding of this investigation was the rela-
tively poor bonding ability of Maxcem, which was supported
by both the microtensile data and SEM observations. Max-
cem expressed the lowest interfacial strengths and exhibit-
ed the highest number of premature failures (Tables 3 and
4), irrespective of whether it was applied to enamel or dentin
using either of the two seating pressures. Moreover, the mi-
croscopic features of the interfaces created by Maxcem on
the dental substrates were suggestive of a limited potential
for removal or modification of the smear layer and resin in-
filtration into the underlying dentin (Figs 2a and 3a).
Aside from the manufacturer's technical bulletin, no in-
formation is currently available on Maxcem's bonding mech-
anism. According to the manufacturer, the self-etching and
Vol 8, No 5, 2006 333
Goracci et al
Fig 3a SEM micrograph of enamel specimens (500X, bar=50 μm.
CO=composite overlay, RC=resin cement, E=enamel). Specimen
luted with Maxcem and a seating force of 20 g/mm2. The 35- to
40-μm-thick cement layer filled with glass particles appeared un-
able to interpenetrate the longitudinally exposed enamel prisms.
A large discontinuity existed between enamel and cement.
Fig 3b SEM micrograph of enamel specimens (500X, bar=50 μm.
CO=composite overlay, RC=resin cement, E=enamel). When
Panavia F 2.0 was applied on transversely cut enamel prisms
under the heavier seating force, a thin hybridized smear layer
was developed (arrows).
Fig 3c SEM micrograph of enamel specimens (500X, bar=50 μm.
CO=composite overlay, RC = resin cement, E = enamel). Speci-
men luted with RelyX Unicem under a 20 g/mm2pressure. The
resin cement did not deeply infiltrate the transversely cut enamel
prisms, and no hybrid layer was formed. The cement layer ap-
Alle Rechte vorbehalten
self-adhering properties of Maxcem are achieved through a
combination of several adhesive monomers, including glyc-
erol dimethacrylate dihydrogen phosphate (GPDM), which is
also used in other adhesive products from the same manu-
facturer, such as Optibond/Optibond FL, Optibond Solo plus,
and Solo plus Self-Etch. Furthermore, according to the man-
ufacturer, the inclusion of hydrophilic monomers in Maxcem
would provide the necessary wettability for adhesion to den-
tal substrates (Kerr Maxcem Technical Bulletin). Although
glycerol dimethacrylate esters of phosphoric acid have a
recognized ability to etch enamel and dentin,17 no informa-
tion is supplied on the initial pH of Maxcem, or on the way
acidic monomers are initially hydrolyzed, given that the ce-
ment is purportedly anhydrous prior to mixing. In addition,
as no chemical details are provided regarding the propri-
etary redox activator system used in Maxcem, it remains un-
certain whether the cement chemistry appropriately ad-
dresses the issue of a possible acid-base reaction between
the self-curing activator and acidic resin monomers.23
Our study suggests that the demineralizing potential of
Maxcem multifunctional monomers may have been insuffi-
cient to etch through clinically relevant smear layers that
rapidly buffered the monomers, with the result of a merely
superficial interaction of the cement with the dental sub-
strate. The increase in interfacial strength following luting
under increased pressure was also negligible for Maxcem in
either dental substrate (Table 4). In addition, the claim of a
“non-slumping” consistency for Maxcem (Kerr Maxcem Tech-
nical Bulletin) was disproved in our study by the microscop-
ic observation of an extremely thin cement layer in speci-
mens luted under the heavier seating force, with the dis-
placement of the luting cement from the interface as a re-
sult of the applied pressure (Fig 2a). Conversely, increases
in interfacial strength following application of a higher seat-
ing pressure were observed for RelyX Unicem and Panavia F
2.0. Such increases were statistically significant in dentin,
particularly for RelyX Unicem (Table 3).
Several phenomena may have accounted for the higher
bond strengths recorded for RelyX Unicem under the higher
seating pressure. First, although the manufacturer reports
the initial pH of the triturated luting cement as less than 2
(3M ESPE Product Profile), the high viscosity of the densely
filled RelyX Unicem could have hampered resin penetration
into the dentin substrate.8,27 By virtue of the RelyX Unicem's
thixotropic behavior, the viscosity of this material would be
reduced under a constant shear rate.8A higher seating pres-
sure could also have been effective in reducing the fre-
quency and extension of porosities that developed along the
bonding interfaces and within the luting agent (Figs 2c and
2d).8Furthermore, a closer adaptation between the adhe-
sive and the substrate could have optimized physical inter-
actions such as van der Waals forces, hydrogen bridges, and
charge transfers, which contribute cumulatively to micro-
mechanical retention and chemical bonding in the adhesion
A recent study reported that when a 98-N force was main-
tained on a composite overlay throughout the self-polymer-
ization of Panavia F 2.0, the cement bond strength to dentin
was increased, as the sustained pressure during cement
setting prevented fluid transudation from the underlying
dentin through the permeable ED Primer.6Water-induced in-
terfacial changes that unfavorably affect the coupling of
Panavia F to dentin may also be minimized by applying a
more hydrophobic adhesive layer on top of the self-etching
primer.4However, this resin coat may interfere with the fit of
We speculate that a similar phenomenon of water diffu-
sion along the dentin-cement interface may also have oc-
curred in our experiment during self-curing of Maxcem and
RelyX Unicem, as these resin cements are supposed to re-
tain their ability to attract water from the underlying dentin
until all the acidic resin monomers are saturated. The flow
of tubular fluid may indeed be even more conspicuous in vi-
vo under the physiological positive pulpal pressure.
It should be pointed out that for RelyX Unicem, an addi-
tional acid-base reaction is claimed to occur between the
phosphoric acid methacrylates and the basic ion-leachable
fillers. This subordinate cement reaction is expected to be
responsible for pH neutralization and for a hydrophobic shift
of the cured material. Furthermore, these neutralization re-
actions were purported to minimize hydrolytic degradation of
the set cement (3M ESPE Product Profile). Hydrolytic stabil-
ity and bond durability – relevant concerns with the use of
self-etching adhesive systems7– may also be inferred to in-
fluence the performance of self-etching and self-adhesive
The microscopic findings of this study were in agreement
with previous morphologic investigations8,12,27 in that RelyX
Unicem was unable to etch through thick smear layers into
the underlying intact dentin and establish a hybrid layer and
resin tags. The cement interaction with the dental substrate
indeed remained quite superficial (Fig 2c). Although the in-
terfacial morphology was not indicative of strong microme-
chanical retention, the bond strengths achieved by RelyX
Unicem on dentin under either seating pressure were com-
parable to those recorded for Panavia F 2.0. The similarity
in bond strengths between Panavia F 2.0 and RelyX Unicem
have previously been reported in shear and microtensile
tests to coronal1,8, and radicular12 dentin. However, for the
self-etching system ED Primer/Panavia F 2.0, the ability to
form a very thin hybridized smear layer has been observed
in previous investigations,1,8,12 and further confirmed in the
present study (Fig 2b).
With respect to enamel bonding, the most relevant finding
was the significantly higher bond strength achieved by ED
Primer/Panavia F 2.0 (Table 4). This is in agreement with the
results of previous studies.1,8 The difference may be ascribed
to the more effective etching of the primer in comparison with
the self-adhesive cements RelyX Unicem and Maxcem,
whose acidic potential may have been rapidly buffered by
enamel. Additionally, the lower viscosity of ED Primer may
have been advantageous for enamel wetting. Moreover, wa-
ter-induced changes at the enamel interface can be expect-
ed to be more limited, and therefore less critical, in relation
to the lower water content of enamel. This may partly explain
why the increase in enamel bond strength following the in-
crease in seating pressure was not as remarkable as that
seen in the dentin specimens (Tables 3 and 4).
Morphological observations revealed that ED Primer/
Panavia F 2.0 was able to develop a thin hybrid layer with
Goracci et al
334 The Journal of Adhesive Dentistry
Alle Rechte vorbehalten
short resin tags at the cement-enamel interface (Fig 3b). The
same could not be observed in the two self-adhesive resin
cements, the enamel interfaces of which exhibited evident
gaps (Figs 3a and 3c). The discontinuity could have been the
result of a very shallow interaction with enamel that yielded
to the polymerization shrinkage stress of the resin cement,
even in a highly compliant joint, such as those created in the
present study.16 As a further indication of the weakness of
the RelyX Unicem bond to enamel, Abo-Hamar et al1report-
ed that the enamel shear bond strength of RelyX Unicem
dropped significantly after 6000 thermocycles, whereas the
adhesion of Panavia F 2.0 was not significantly affected un-
der the same aging conditions.
In this study, pretreatment of composite blocks for bond-
ing was standardized, although it was known that each ce-
ment manufacturer had specific recommendations regard-
ing this procedure. This decision was made after preliminary
tests had demonstrated that adhesion at the cement-com-
posite joint was not an issue, as the microtensile sticks nev-
er failed at that interface. SEM observations also revealed a
good integration between resin cement and composite over-
lay with all luting agents under either seating pressure (Figs
2 and 3). Moreover, the two stacked 2-mm-thick composite
disks never failed along this interface.
With regard to the generalizability of the results to the clin-
ical situation, it should be considered that while cement may
extrude easily from the margins under the composite block
and allow for even seating pressure, in the presence of a
geometrically more complex inlay, onlay, or crown, hydraulic
effects may take place, leaving some areas under minimal
and some others under heavier pressure. In addition, the
compliance of the cavity design would be different in an in-
lay, an onlay, or a crown preparation.
Thus, within the limits of our experimental design, the null
hypothesis that the type of cement and seating pressure
have no significant influence on interfacial strength of the
resin cements to enamel and dentin has to be rejected. The
effect of seating pressure was more relevant in dentin and
for RelyX Unicem. This supposedly involves a reduction in ce-
ment viscosity and porosity, prevention of unfavorable water-
induced changes along the cement-dentin interface, and
promotion of physical adhesion. Conversely, Maxcem mani-
fested the least reliable bonding potential irrespective of the
seating pressure or the bonding substrate. Nevertheless, it
may be concluded that a seating pressure that is heavier
than finger pressure should be applied throughout the initial
self-curing period for enhancing the interfacial strength and
adaptation of Panavia F 2.0 and RelyX Unicem.
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Clinical relevance: Interfacial strength and adaptation of
self-etching and self-adhesive dual-curing resin cements
benefit from the application of a seating force heavier
than finger pressure during the initial self-curing period.
Vol 8, No 5, 2006 335
Goracci et al