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A Novel Technique for Class II Composite Restorations
with Self-adhesive Resin Cements
By
Mohammed Al-Saleh
A thesis submitted in conformity with the requirements for the degree of Master in Science
Graduate Department of Biomaterials University of Toronto
© Copyright by Mohammed Al-Saleh (2009)
ii
A Novel Technique for Class II Composite Restorations
with Self-adhesive Resin Cements
Mohammed Al-Saleh
Master of Science, 2009
Graduate Department of Biomaterials, Faculty of Dentistry
University of Toronto
Purpose: To determine microleakage and microtensile bond strength (µTBS) of
composite restorations bonded with self-adhesive resin-cements. Methods: Six groups of
molars were assigned to cements: RelyX-Unicem, Breeze, Monocem, PanaviaF-2.0,
Filtek-LS System, and Scotch-Bond-Multipurpose (adhesive). For microleakage, Class II
preparations were made. Cements were applied onto all cavity walls. Preparations were
restored, specimens themocycled and then immersed in red dye. Dye penetration was
assessed according to a 5-point scale. For µTBS test, 6 mm composite buildups were
made over tooth surfaces. Rectangular rods were cut and subjected to tensile force. Mean
µTBS and SDs were calculated. Results: RelyX-Unicem and Breeze showed low
microleakage, however, they had lower µTBS values. Filtek-LS System showed the least
microleakage and the highest µTBS with dentin. Conclusion: RelyX-Unicem, Breeze
and Filtek-LS System will improve marginal seal when used in subgingival Class II
composite restorations.
iii
ACKNOWLEDGMENTS
My sincere thanks go out to my supervisor and mentor Dr. Omar El-Mowafy who
has had a profound impact upon my academic development. Thank you for your support,
guidance, encouragement and friendship throughout my research.
I would like to thank my co-supervisor Dr. Laura Tam for her help with statistical
analysis and for her time spent reviewing this thesis. Her patience and guidance
throughout this project were greatly appreciated.
I would like to express my appreciation and gratitude to my advisory committee
members, Dr. Dorothy McComb and Dr. Aaron Fenton, for their significant input and
valuable instruction.
I would also like to thank 3M/ESPE, Pentron, Shofu and Kuraray for contributing
the materials for the study.
My deepest gratitude goes to my parents for their unfailing love and support.
They are the rock on which I stand.
Finally, I would like to extend my warmest gratitude to the love of my life my
wife Noura, who has been a constant source of love, patience and kindness. Her constant
love and support has kept me going throughout this project. I could not have
accomplished this without her.
iv
ABSTRACTS ii
ACKNOWLEDGMENTS iii
TABLE OF CONTENTS iv
LIST OF TABLES vii
LIST OF FIGURES viii
CHAPTER 1: INTRODUCTION AND LITERATURE REVIEW 1
1.1 Historical Background: Resin Composite Restorations 1
1.2 Potential Drawbacks of Class II Composite Restorations 4
1.2.1 Composite polymerization shrinkage and application problems 4
1.2.1.1 Silorane-based resin composite (Low-shrinkage composite) 11
1.2.2 Adhesion shortcomings 15
1.2.2.1 Total-etch adhesive system 15
1.2.2.2 Self-etch adhesive system 17
1.2.3 Postoperative hypersensitivity 19
1.2.4 Microleakage 21
1.3 Microleakage of Self-Adhesive Resin Cements 26
1.4 Microtensile Bond Strength (µTBS) 30
1.4.1 µTBS of self-adhesive resin cements 32
1.5 Statement of the Problem 35
1.6 Objectives 36
1.7 Null Hypothesis 36
CHAPTER 2: MATERIALS AND METHODS 37
2.1 Microleakage Testing 37
2.1.1 Pilot study 37
2.1.2 Main study 37
2.1.2.1 Specimen collection and storage 37
2.1.2.2 Specimen preparation 38
TABLE OF CONTENTS
v
2.1.2.3 Specimen grouping and restoration procedures 39
2.1.2.4 Thermocycling procedure 40
2.1.2.5 Microleakage Testing 41
2.1.2.6 Cement thickness 42
2.1.2.7 Data analysis 42
2.2 Microtensile Bond Strength Testing (µTBS) 43
2.2.1 Pilot Study 43
2.2.2 Main study 43
2.2.2.1 Specimen collection and storage 43
2.2.2.2 Specimen preparation 43
2.2.2.3 Specimen grouping and bonding procedures 44
2.2.2.4 Thermocycling procedure 45
2.2.2.5 Specimen preparation and µTBS testing 45
2.2.2.6 Evaluation of mode of failure 46
2.2.2.7 Scanning electron microscopy (SEM) 46
2.2.2.8 Data analysis 47
CHAPTER 3: RESULTS 55
3.1 Microleakage Test Results 55
3.2 µTBS Test Results 56
3.2.1. Mode of failure 57
CHAPTER 4: DISCUSSION 75
4.1 Effect of Study Methods 75
4.1.1 Effect of gamma irradiation 75
4.1.2 Effect of specimen preparation 76
4.1.3 Effect of water storage 77
4.1.4 Effect of thermal aging 77
4.1.5 Effect of using chemical dye for microleakage assessment 78
4.2 Effect of Material-related Factors 80
4.2.1 Effect of polymerization shrinkage on microleakage and bond strength 80
4.2.2 Effect of pH on microleakage and bond strength 82
4.2.2.1 Effect of pH on enamel tooth structure 82
vi
4.2.2.2 Effect of pH on dentin tooth structure 84
4.2.3 Effect of intermediate layer on microleakage and bond strength 87
4.2.4 Effect of hydrophobic layer on microleakage and bond strength 90
4.2.5 Effect of the self-adhesive cement composition on microleakage 93
and bond strength
4.2.6 Failure modes of µTBS test 95
4.3 Summary 97
4.4 Clinical Significance of the Study 99
4.5 Study Limitations 100
4.6 Future Studies 101
4.7 Conclusions 102
REFERENCE 104
vii
Table 1: Material composition of cements, adhesives and composites
as provided by the manufacturers. page 53
Table 2: Steps followed for materials application. page 54
Table 3: The range of cement thickness, modulus of elasticity and pH
of the materials used. page 63
Table 4: Distribution of the dentin side microleakage scores with group
means and SDs. page 64
Table 5: Distribution of the enamel side microleakage scores with group
means and SDs. page 65
Table 6: p-values (Mann-Whitney U-test) for the microleakage test groups. Page 66
Table 7: Means (MPa) and SDs of the µTBS of dentin and enamel subgroups. page 68
Table 8: p-values (Tukey’s t-test) for µTBS test subgroup. page 68
Table 9: Distribution of µTBS failure modes of the dentin subgroups. page 69
Table 10: Distribution of µTBS failure modes of the enamel subgroup. page 70
Table 11: Mean ranks and SDs of failure modes of dentin and enamel
subgroups. page 71
Table 12: p-values (Mann-Whitney U-test) for the mean ranks of the µTBS
failure modes. page 72
LIST OF TABLES
viii
1. Polymerization shrinkage caused by linear reduction of the reacted monomers in
methacrylate-based composites. (reproduced from 3M ESPE) page 5
2. Polymerization shrinkage stresses lead to bond failure at tooth-composite
interface. (reproduced from3M ESPE) page 6
3. Simple illustration of the chemical composition of the silorane-based composite.
(reproduced from 3M ESPE) page 14
4. The ring-shaped silorane monomers represent less polymerization shrinkage
than the methacrylates in composites. (reproduced from3M ESPE) page 14
5. Apical foramina of the teeth sealed with GI cement and roots sealed with
nail vanish to prevent dye penetration during microleakage testing. page 48
6. Teeth embedded in acrylic bases and crowns pumiced with rubber cups and slurry
of soft pumice. page 48
7. Preparation dimensions. page 48
8. Position of gingival seats. page 49
9. Matrixing. page 49
10. Radiometer showing light intensity of Demi LED unit. page 49
11. Occlusal and proximal views of a representative restored specimen sealed
with nail varnish. page 50
12. Representative specimen after immersion in red dye for 24 hours. page 50
13. Specimen sectioning. page 51
14. Extent of dye penetration scored according to five-point scale. page 51
15. Illustration scheme showing specimen preparation for µTBS test. page 52
16. Representative photographs of microleakage for RXU group. page 59
17. Representative photographs of microleakage for BRZ group. page 59
18. Representative photographs of microleakage for MON group. page 60
19. Representative photographs of microleakage for PAN group. page 60
20. Representative photographs of microleakage for FLS System group. page 61
LIST OF FIGURES
ix
21. Representative photographs of microleakage for SBMP group. page 61
22. Representative photographs showing the cement thicknesses at occlusal, axial and
gingival interfaces. page 62
23. Bar chart showing the % distribution of the microleakage scores for dentin
subgroup. page 64
24. Bar chart showing the % distribution of the microleakage scores for enamel
subgroup. page 65
25. Bar chart showing microleakage means and SDs of dentin and enamel
subgroups. page 66
26. Bar chart showing µTBS means and SDs of dentin subgroups. page 67
27. Bar chart showing µTBS means and SDs of enamel subgroups. page 67
28. Bar chart showing the % distribution of the different failure modes of dentin
subgroups. page 69
29. Bar chart showing the % distribution of the different failure modes of enamel
subgroups. page 70
30. Bar chart showing mean ranks and SDs of different failure modes of dentin and
enamel subgroups. page 71
31. SEM photograph for RXU cement. page 73
32. SEM photograph for BRZ cement. page 73
33. SEM photograph for PAN cement. page 73
34. SEM photograph for FLS System adhesive. page 74
35. SEM photograph for FLS System adhesive. page 74
36. SEM photograph for SBMP adhesive. page 74
37. SEM photograph for SBMP adhesive. page 74
1
INTRODUCTION AND LITERATURE REVIEW
1.1 Historical Background: Resin Composite Restorations
During the first half of the 20th
century, silicates were the only tooth-colored
aesthetic materials available for direct cavity restorations. Although silicates release
fluoride, they are no longer used for permanent teeth because the silicate material
becomes severely eroded within a few years. Acrylic resins, similar to those used for
custom impression trays and dentures (polymethacrylate [PMMA]), replaced the silicates
during the late 1940s and the early 1950s because of their more tooth-like appearance,
insolubility in oral fluids, ease of manipulation, and low cost. Unfortunately, these
acrylic resins also have relatively poor wear resistance and they shrink severely during
curing, which causes them to pull away from the cavity walls and produce leakage along
the margins. Their excessive thermal expansion and contraction causes further stress to
develop at the cavity margins when hot or cold beverages and foods are consumed.
These problems were reduced somewhat by the addition of quartz powder to form a
composite structure. Commonly used fillers have an extremely low thermal expansion
coefficient, approaching that of tooth structure. Incorporation of filler particles became a
practical means of reducing both curing contraction and thermal expansion. .
The early composites based on PMMA were not very successful, in part because
the filler particles simply reduced the volume of polymer resin but were not bonded
(coupled) to the resin. Defects therefore developed between the mechanically retained
2
particles and the surrounding resin, producing leakage, staining, and poor wear
resistance. A major advance was made when Dr. Ray L. Bowen (1962)1 of the American
Dental Association research unit at the National Bureau of Standards developed a new
type of resin composite material. Bowen’s main innovations were bisphenol glycidyl
methacrylate (BIS-GMA), a dimethacrylate resin, and an organic silane coupling agent to
form a bond between the filler particles and the resin matrix.
Patient demand for restorations that are highly aesthetic and affordable are factors
contributing to the choice of resin composite restorations. Because of their favourable
characteristics, resin composites are capable of providing an excellent balance of
performance features needed for use in the oral cavity. Ideally, these characteristics
include (1) biological compatibility, (2) physical properties, (3) ease of manipulation, (4)
aesthetic qualities, (5) relatively low cost, and (6) chemical stability in the mouth.2 As
well, the composites are free of metal and mercury.
The decline of amalgam use among clinicians and patients, however, began in the
early 1980s due to some inherent problems. For instance, amalgam’s tendency to corrode
and difficulty bonding to tooth structure, along with the necessity to remove sound tooth
structure for retention, are problematic.3 Also at issue for some people are its lack of
aesthetics and fears about potential mercury toxicity.4-6
As a result, the need for amalgam
alternatives has been an issue in the dental literature for several years. Amalgam has
been a public health concern in recent years in several countries and some clinicians
have advocated the replacement of metal restorations with mercury-free restorations such
as resin composite. In January of 2008, Norway’s government imposed a ban on the use
of mercury products, including amalgam restoration, due to environmental concerns.
3
This ban was denounced by Dr. Jones in 2008 7 due to the lack of scientific evidence. It
is important to know that Dr. Jones’s rationale makes sense to many clinicians in dental
practice. Practice should be guided by science, not by fear.
Resin-based composites were advocated as a possible alternative to amalgam
restorations because they were mercury-free and thermally nonconductive; further, they
matched the shade of natural teeth and easily bonded to tooth structure with the use of
adhesive systems. Resin-based composite systems are the material of choice for direct
aesthetic anterior restorations. These materials are gaining acceptance for restoration of
posterior occlusal areas and other high-stress-bearing sites. Early on, dentists who used
resin-based composites to restore posterior teeth experienced poor wear resistance,
difficulties in achieving good proximal contact and contour, polymerization shrinkage,
and poor dentin marginal adaptation.8, 9
More recently, the mean longevity of posterior
composites (seven years) is approaching that of amalgam (10 years).2 Resin composite
materials are also used in a variety of other dental applications, such as pit and fissure
sealants, bonding of ceramic veneers, and cementation of other fixed prostheses.
Recently, a new chemically modified composite was introduced into the market;
the previous composite’s methacrylate resin content was replaced with a silorane resin,
which uses a ring-shaped monomer instead of a linear monomer as found in the
methacrylate-based composites (Figures 1 and 4). The reaction between the ring-shaped
monomers is initiated by their opening and extending toward each other, which
technically results in low polymerization shrinkage. The reported amount of the
volumetric shrinkage of the silorane-based composite is <1%. Its efficacy on marginal
integrity, however, has not been explored.
4
1.2 Potential Drawbacks of Class II Composite Restorations
1.2.1 Composite polymerization shrinkage and application problems
Shrinkage occurs during polymerization as monomers are converted from an
aggregate of freely flowing molecules to a rigid assembly of cross-linked polymer
chains. Before polymerization, the monomers are held loosely together by van der Waal
forces at a spacing that produces minimum potential energy. As a polymer, the mer units
are connected by covalent bonds with a minimum potential energy spacing
approximately 20% less than that in the unreacted monomer. Upon the reaction among
monomer particles, a covalent bond is established resulting in substantial reduction in the
free volume which is translated into volumetric shrinkage (Figure 1).2 Shrinkage values
reported for BIS-GMA (5.2%) and TEGDMA (12.5%) are substantially higher than
those displayed by typical composites. Shrinkage is a direct function of the volume
fraction of polymer matrix in the composite, and therefore happens to a larger degree in
microfilled composites than in fine-particle composites or hybrids. Microfilled
composites typically show setting contractions of 2% to 4% while fine-particles and
hybrid composites show 1% to 1.7%.110,11, 12
Approximately 60% of the volume of hybrid
composites is occupied by filler particles while only 40% of the volume of microfilled
composites is occupied by filler particles.13
Similarly, low-viscosity (flowable)
composites present volumetric shrinkages up to 5%, in large part due to their reduced
inorganic content, which is typically below 50% by volume.14
5
Figure 1: When methacrylate monomers in the resin composites react to establish a
covalent bond, the distance between the two groups of atoms is reduced by their shifting
closer together in linear response, resulting in substantial reduction in the free volume
which is translated into volumetric shrinkage.2 (reproduced from 3M ESPE)
The magnitude of volumetric shrinkage experienced by a composite is
determined by a number of factors: filler volume fraction, the composition of the resin
matrix, elastic modulus and flow properties, rate and degree of conversion
(polymerization), the volume of the material to be polymerized, and the geometry of the
restoration.15
This shrinkage creates polymerization stresses as high as 18 MPa between
the composite and the tooth structure.1, 16
In turn, this curing shrinkage produces
unrelieved stresses in the resin when the point is reached at which the resin has gelled
and begins to harden. This stress is most destructive to the resin composite-tooth
interface; it may also induce mechanical stresses which can exceed the strength of any
bond between composite and dentin or enamel (Figure 2). The clinical effects of strain
6
are white lines at the bond interface and cracks in enamel adjacent to the margins.17, 18
Bond failure at the interface allows an influx of oral fluids and greatly contributes to the
possibility of postoperative hypersensitivity, marginal leakage or staining, and finally
secondary caries which may lead to pulpal damage. 1, 11, 16, 19
Figure 2: Polymerization shrinkage stresses applied on the tooth-composite interface. If the bond
to tooth structure is strong enough, tooth structure strain will occur,20, 21
while conversely, if the
bond between the composite and tooth structure is less than the force generated by the
polymerization shrinkage, marginal bond failure will occur.22, 23
(reproduced from 3M ESPE)
7
Efforts have been made to develop methods to lessen the polymerization
shrinkage in Class II composite restorations. These include reducing the ratio of bonded
to unbonded restoration surfaces (C factor),24
and strategic incremental placement
techniques to reduce residual stresses at the tooth-restoration interface.25
Adding the
composite in 2-mm increments and polymerizing each increment independently can
reduce the net effect of polymerization shrinkage. Net shrinkage is less because a smaller
volume of composite is allowed to shrink before successive additions.16
O’Brien
summarized the techniques to partially overcome the shrinkage problem associated with
resin restorations. Firstly, incremental addition and polymerization of thin composite
layers will minimize the total setting contraction; however, although this method does
result in lower stresses at the tooth-composite interface, the marginal gaps may still
occur. Secondly, a gradual curing process is applied by varying the light intensity during
curing exposure. The initial polymerization is done at a low intensity and then the final
aspect is cured at full light intensity. As a result, the absolute shrinkage is reduced and
the stresses on the interfacial adhesive are also reduced. The third approach involves the
preparation of a composite inlay either directly in the mouth or indirectly as a laboratory
procedure.1 The directed polymerization shrinkage technique was developed to help
direct polymerization shrinkage towards the tooth, rather than towards the center of the
composite mass. A transparent, cone-shaped light-tip was developed for use with the
light guide to reduce cervical contraction and gap formation in Class II composite
restorations by transmitting the curing light through the first composite increment in the
proximal box, while simultaneously maintaining pressure.26
8
Versluis et al 27
stated that the amount of polymerization contraction is influenced
more by the adhesion quality and the C-factor than by the position of the light source.
This is in line with many studies e.g.28-31
that have discussed reducing the polymerization
stress by influencing the stress development with soft start or pulse-delay light-
polymerization to prolong the setting of the gel-point. It appears that the different light-
polymerization techniques have only a limited influence on polymerization shrinkage
stress. Other authors completely deny the possibility of even minimally reducing
polymerization shrinkage strains with particular curing modes.17
Furthermore, attempts have been made to minimize polymerization shrinkage by
altering the filler load. Aw and Nicholls 32
showed a correlation between the filler
volume and shrinkage. They also came to the conclusion, however, that other factors
such as filler size and resin chemistry may also affect shrinkage. Braga et al 33
in a
systematic review stated that the resources currently available to reduce contraction
stress are somewhat limited. Nevertheless, based on scientific evidence, few aspects of
clinical interest can be observed:
1. Materials with high inorganic filler content and low volumetric shrinkage may result
in increased contraction stress at the bond interface.
2. Different light-polymerization methods do not necessarily lead to significant
reductions in contraction stress.
3. Application of an intermediate layer with low-modulus of elasticity may lead to
significant stress relief depending on its thickness and elastic modulus.33
9
Flowable composites are created by using the same small particle size of fillers of
traditional hybrid composites but with reduced filler content, resulting in reduced
viscosity.34
However, the low filler content caused some concern regarding inferior
mechanical properties and higher polymerization shrinkage in comparison to traditional
hybrid composites.14, 34, 35
Labella et al 14
found that various flowable composites
generally had a higher polymerization shrinkage volume, which ranged from 3.6% to
6%, while conventional hybrid composites had 1% to 1.7% volumetric shrinkage.
Increased volumetric shrinkage may indicate the potential for higher contraction stresses
at the interface. Flowable composites have a lower modulus of elasticity than their hybrid
predecessors. It has been postulated that low-modulus materials, when employed as
cavity liners, show stress-buffering capacity and may reduce contraction stresses at the
tooth restoration interface.36
It is generally suggested that the primary benefit of any low-
viscosity layer could be to act as a stress-absorbing layer between the hybrid layer and
the shrinking resin composite layer. If the walls of the cavity with an unfavourable C-
factor are coated with an elastic layer, the bulk contraction of the restoration can gain
some freedom of movement from the adhesive sides.
In their study, Chuang et al 37
evaluated the ability of various lining materials to
reduce cervical marginal microleakage and the internal voids within Class II composite
restorations. The flowable composite lining groups demonstrated either similar or more
cervical microleakage than did their non-flowable composite groups. The study indicated
that the use of flowable composite lining in Class II composite restorations failed to
achieve benefits in marginal quality but reduced internal voids in deep cavities. The
10
failure of flowable materials to improve the marginal quality may have been due to low
filler content as compared to the hybrid composite.
Tredwin et al 38
evaluated the gingival wall microleakage in packable and
microhybrid conventional composite restorations with and without a flowable composite
liner in Class II cavities. The conventional and packable resin composites tested were not
associated with differences in microleakage, while the microleakage scores were
significantly higher when a flowable liner was used with margins placed in dentin (root
cementum) than in enamel. The study concluded that gingival margins should be placed
in enamel. The microleakage scores did not support the use of flowable resin composites
in Class II resin composite restorations. A similar study 39
also proved the inability of
flowable composite to improve the marginal seal when utilized in Class II cavities.
Flowable composite, Vitrabond glass-ionomer base/liner and compomer were used in the
sandwich technique with the composite restorations. The study concluded that the glass-
ionomer liner on the cavosurface margin had significantly less microleakage. Although
the compomer-hybrid combination had low mean leakage scores, the wide range of
values led to unpredictable results. The flowable-hybrid combination and the packable
composite performed less favourably.
The results of the above studies thus bring into question the assumption that the
use of less viscous flowable liners results in less leakage around a resin composite filling.
Although such a lining might contribute to a more equal distribution of stress over the
adhesive interface, it may not be thick enough to provide sufficient strain capacity and
therefore, it does not appear to play an important role in relieving stress.40, 41
Using
flowable composites to reduce polymerization shrinkage stress is still being debated and
11
is not yet widely recommended. It appears that the use of a cured thin layer of flowable
composite does not produce significant stress reduction.42
1.2.1.1 Silorane-based resin composite (Low-shrinkage composite)
Since the 1940s, many technological developments have significantly improved
the clinical performance of dental resin composites. However, the common chemical
basis for all restorative composites has remained the radical polymerization of
methacrylates. Given the fact that different curing techniques had only little or no effect,
investigations were undertaken to explore new monomers that provided less shrinkage
and a lower modulus. These investigations have been ongoing for decades, and new
monomers like stereo-isomeric cyclics piro-ortho carbonates, which expand during
polymerization, or other cyclo-polymerizable monomers have been introduced.43, 44
Moreover, efforts have been made to reduce polymerization shrinkage by substituting
high shrinkage monomers such as TEGDMA with various new and experimental
comonomers that provide lower polymerization shrinkage,45
or by synthesizing other
new monomers.46
At the present time, the most promising technology for the reduction
of polymerization shrinkage is silorane technology.47
Ernst et al 48
examined the polymerization stress of different established
composite resins (Tetric Ceram, Vivadent; EsthetX, Surefil,Dentsply; Clearfil AP-X,
Clearfil Photo Posterior, Kuraray; Prodigy Condensable, Kerr; Filtek P60, 3M ESPE;
Solitaire 2, Heraeus-Kulzer) by means of a photo-elastic investigation and investigated
six new experimental composite resins, which had claims of less polymerization
shrinkage (InTen-S, Vivadent; K 112, K 051, Dentsply; Compox, Pluto, Hermes,
12
3M/ESPE). The study illustrated the advances in reducing polymerization shrinkage
strain by the use of new monomer compositions or modifications of the filler and
monomer ratios. Yamazaki et al 49
compared the microleakage of an experimental low-
shrinkage resin composite (Hermes, 3M ESPE), a nanofilled resin composite material
(Filtek Supreme, 3M ESPE) and a hybrid resin composite (Tetric Ceram, Ivoclar-
Vivadent) using a dye penetration method. The study showed that all restorative systems
had microleakage, regardless of the insertion technique and mechanical cycling load.
Incremental placement significantly reduced microleakage as compared to the bulk
technique, regardless of the restorative system used. Cyclic loading significantly affected
incrementally placed restorations, except for the Hermes system.
From a chemical stand point, new developments such as siloranes may offer an
interesting potential in the future of dental restorative materials. Siloranes (silicon-based
monomers with oxirane functional group) have been suggested as alternatives to
methacrylates as matrix resin components for dental composites because of their
hydrophobicity and lower polymerization shrinkage.47
The chemically modified silorane
monomer is composed of a combination of two chemical building blocks of siloxane and
oxirane (Figure 3). 3M ESPE produced a low-shrinkage Filtek LS restorative system,
which is based on the new ring-opening silorane chemistry, was introduced into the
market recently (Figure 4). The innovative resin matrix represents the major difference
between the Filtek LS restorative and conventional methacrylates. The initiating system
and the fillers have been adapted in order to provide the best performance of the new
technology. Like other silicon-containing monomers, siloranes can be extremely
hydrophobic, potentially making the oxirane groups inaccessible to attack by water or
13
water-soluble species. Eick et al 50
evaluated the stability of two siloranes (PH-SIL and
TET-SIL) and their 1:1 mixture (SIL-MIX) by measuring changes in the chemical
structure of the oxirane group in aqueous environments. The study concluded that the
oxirane functionality in siloranes was stable in aqueous solutions containing epoxide
hydrolase, porcine liver esterase or dilutes HCl, and attributed this to the lack of
solubility of the siloranes. The stability and insolubility of siloranes in aqueous
biological fluids enhance their potential as good candidate monomers for use in dental
composite materials.
A recent study by Buergers et al 51
compared the susceptibility of the silorane-
based composite (Filtek LS System) and four widely used conventional methacrylate-
based resin composites (Filtek Z250, Tetric Evo Ceram, Quixfil and Spectrum TPH) to
adherence of oral streptococci. Bacterial suspensions of S.mutans, S.oralis, S.sanguinis,
and S.gordonii were incubated with 15 test specimens for each composite material and
the bacterial adhesion was quantified with fluorescence dye and an automated multi-
detection reader. Results of the study showed that the lowest quantity of the adhering
streptococci was found on the novel silorane-based composite. The authors attributed this
to the lower surface roughness and the greater hydrophobicity of this new composite
material. Siloranes also exhibited good mechanical properties comparable to those of
clinically successful methacrylate-based composite materials.52
14
Figure 3: Simple illustration for the chemical composition of the silorane monomer, which is
composed of a combination of two chemical building blocks of siloxane and oxirane.
(reproduced from 3M ESPE)
Figure 4: Silorane- based composite has ring-shaped monomer particles instead of the linear ones
that found in the methacrylate-based composite. The reaction between the ring-shaped monomers
is initiated by their opining and extending toward each other, which theoretically results in low
polymerization shrinkage (<1%). (reproduced from 3M ESPE)
15
1.2.2 Adhesion shortcomings
1.2.2.1 Total-etch adhesive system
Contemporary resin-based composite restorations involve a degree of application
technique sensitivity that may compromise restoration longevity and marginal integrity.
Unpredictable postoperative sensitivity may appear if certain precautions are not
observed. The operator-dependant variables start with the application of dentin bonding
agent. The mechanism of adhesion to dentin for most of the current adhesive systems is
based on the hybridization of dentin and resin. In this process, dentin surfaces are etched
with acidic conditioners that remove the smear layer, open the dentinal tubules, partially
demineralize the outer dentin and leave a collagen meshwork. This meshwork allows the
adhesive resin to penetrate and provides an intermingled layer of collagen and resin, also
known as the reinforced zone, resin-infiltrated zone, or hybrid layer.53
Although the
hybrid layer represents an advance in dentinal bonding, dentin demineralization results in
a collagen fibril structure without mineral support. Maintaining a hydrated state of the
etched dentin surface theoretically keeps the collagen fibrils extended, allowing
hydrophilic adhesives to more readily access the microporous surface of the mineralized
tissue underneath.54
The etched dentin surfaces should not become over-dried, so that the
collagen collapses. Any collapse of the collagen matrix as a result of over-drying might
prevent monomers from penetrating deeper into dentin, which increases the risk of
adhesive failures.55
An incomplete infiltration of resin into demineralized dentin may
create porosities within the hybrid layer (nanoleakage),56
leaving exposed collagen at the
dentin–adhesive interface. This exposed collagen would be susceptible to degradation by
16
various exogenous substances, leading to possible premature failure of resin
restorations.57, 58
This failure could occur because of hydrolysis of the collagen fibrils, 59
and/or degradation of the polymerized resins.60
The dentin surface, on the other hand,
should not become too wet, because water limits the penetration and performance of
resins.61
A moist dentin bonding technique is difficult to standardize clinically, and is thus
sensitive to errors caused by inaccurate clinical handling.59
Hashimoto et al 62
evaluated the effect of errors commonly made (inadequate
solvent evaporation and over drying) in using total-etch adhesive on bond strength, fluid
movement and nanoleakage of resin-dentin bonds. Two total-etch adhesives were used
for bonding to dentin (Excite, Ivoclar Vivadent) and (OptiBond Solo Plus, Kerr). The
study showed that incomplete air-drying of the primer during bonding results in
increased residual water and other solvents within resin-dentin bonds. This water may
serve as a pathway for additional water movement. The water permeability of resin-
dentin bond made without complete evaporation of solvent was greater than that of dry
bonding. Longer air-drying times for the primers during wet bonding may help to
improve the bond strength and reduce nanoleakage for total-etch adhesives.
Despite advances in bonding 63
and composite resin materials, posterior
composite resins remain highly technique-sensitive.64
Christensen reviewed the
challenges of posterior resin-based composites and total-etch bonding.17, 65-68
He reported
that many practitioners use the well known total-etch concept on a routine basis and that
they have noticed several challenges with the concept. Among these are the following: it
is easy to dry tooth surfaces too much before applying the primer solutions; it is difficult
to apply the primer solution in coats sufficient to provide impregnation of the liquid into
17
the dentinal tubules; bonding solutions can too easily be blown thin. May be it wise to
consider other bonding materials and/or techniques to avoid any challenges that might
affect the bonding efficiency during etch and rise bonding in deep Class II cavities.
1.2.2.2 Self-etch adhesive system
Self-etch adhesives are an alternative approach to etch and rinse technique based
on the use of non-rinse acidic monomers that simultaneously condition and prime dentin.
The rinsing phase was eliminated, which not only lessens the clinical application time,
but also significantly reduces the technique-sensitivity or the risk of making errors during
application.69
There are two types of self-etch adhesives: mild and strong.70
Strong self-
etch adhesives have a very low pH (< 1) and exhibit a bonding mechanism and
interfacial ultra-morphology in dentin resembling that produced by etch-and-rinse
adhesives. Mild self-etch adhesives (pH of around 2) partially dissolve the dentin
surface, so that a substantial number of hydroxyapatite crystals remain within the hybrid
layer. Specific acid groups (carboxyl or phosphate) of functional monomers can then
chemically interact with this residual hydroxyapatite.71
This two-arm bonding
mechanism (i.e. micro-mechanical and chemical bonding) is believed to be advantageous
in terms of restoration durability. It has a micro-mechanical bonding component that may
in particular provide resistance to abrupt debonding stress. The chemical interaction may
result in bonds that better resist hydrolytic break-down and thus keep the restoration
margins sealed for a longer period. Miranda et al 72
evaluated the bonding effectiveness
of two types of self-etching adhesives (Prime & Bond NT and Prime & Bond 2.1,
Dentsply) with or without non-rinse conditioner and phosphoric acid. The regularity and
18
infiltration depth of adhesives in enamel was observed via Scanning Electronic
Microscopy (SEM). The study concluded that using Prime & Bond NT without previous
acid self-etching did not produce the micromechanical retention mechanism, and the
non-rinse conditioner presented results similar to phosphoric acid at 37% when
associated with Prime & Bond NT.
A 10-year clinical study, reported by Akimoto et al 73
evaluated the long-term
clinical performance of a self-etching adhesive system (Clearfil Liner Bond 2, Kuraray).
Different cavity designs (Class I, II, III, IV and V) were placed among 42 patients. The
restorations were evaluated in five categories: pulpal response, marginal integrity,
marginal discoloration, retention and secondary caries. Assessments were made at
baseline, immediately after placement, at 6-months, and at 1, 5, 7 and 10 years. The
study’s longitudinal clinical data demonstrated that the retention rate and pulpal response
of a self-etching adhesive system (Clearfil Liner Bond2) was excellent after 10-year.
Some marginal discoloration was evident; however, these changes were not severe, as
clinical conditions (due to recurrent decay) requiring replacements were not present.
According to the data obtained, Clearfil Liner Bond2 was considered to be acceptable for
the restoration of teeth as evidenced 10 years of clinical study.
Sauro, Pashley, Tay et al 74
evaluated the micro-permeability of several self-
etching and etch-and-rinse adhesive systems; these included a three-step adhesive
(Optibond FL, Kerr), a two-step silorane self-etching primer adhesive system (Filtek LS
System, 3M ESPE), a two-step total-etch adhesive (Scotch bond 1XT, 3M ESPE) and
two one-step self-etch adhesive systems (G-Bond, GC Corp; DC-Bond, Kuraray). The
dentin-adhesive interfaces were examined using a confocal scanning microscope and
19
micro-permeability was detected in all the adhesives. The study concluded that Filtek LS
(3M ESPE) and Optibond (Kerr) showed an adhesive layer that was free from water trees
and micro-permeability. The bond failure at the dentin-composite interface, which may
represent the pathway for hydrolytic and enzymatic degradation of dentin-composite
bonds over time, is strongly related to the degree of adhesive permeability.
Because of the hydrophilic nature, the self-etch adhesives act as a semi-
permeable membrane, diffuse water, and degrade faster than hydrophobic adhesives. In
general, their short-term effectiveness certainly compromises their long-term usefulness.
1.2.3 Postoperative hypersensitivity
An improper bonding technique and poorly-managed polymerization shrinkage
stresses are the main factors that could cause tooth sensitivity after restoration placement.
In the past few decades, there has been an increase in the frequency of replacing
amalgam restorations with direct composite restorations for aesthetic and other
reasons.75-77
The increased number of resin composite restorations placed in posterior
teeth has accordingly resulted in increased postoperative sensitivity concerns.78-80
Cavities should be filled with the least excess possible to minimize the occlusal
adjustment and finishing and polishing procedures. This fact should be emphasized,
because, in many cases, premature or exaggerated contacts are responsible for
postoperative sensitivity during mastication, as well as temperature variations.81, 82
Akpata and Behbehani 83
compared the postoperative sensitivity of posterior
composites lined with bonding systems that utilized either self-etch primer (SE Clearfill,
Kuraray) or a bonding system that utilizes phosphoric acid conditioner (One-step Plus,
20
Bisco). Class I cavities were prepared in 28 patients and lined with either adhesive, then
restored with hybrid composite. Postoperative sensitivity was assessed subjectively by
asking the patients to classify the pain into none, mild or severe, and objectively by
measuring the time it took the patient to feel cold sensation when an ice stick was
applied. The study showed that postoperative sensitivity was decreased significantly in
composite restorations lined with the self-etch primer compared with composite
restorations lined with total-etch system. The authors extrapolated that the etching with
phosphoric acid widens the dentinal tubules ends which may not be completely sealed by
adhesive resin. It has been shown that the self-etch primers produce a thin hybrid layer,
which is completely penetrated by the adhesive resin.84
This may partially explain the
higher postoperative sensitivity associated with etch and rinse adhesives compared to
self-etch adhesives. An interesting clinical study by Briso et al 85
evaluated the
postoperative sensitivity in posterior resin composite restorations. A total of 143 Class I
and 149 Class II restorations (MO/OD and MOD) were placed in patients ranging in age
from 30 to 50. A total-etch system (Prime & Bond NT, Dentsply/Caulk) and a micro-
hybrid resin composite (TPH Spectrum, Dentsply) were used to restore the cavities. The
patients were questioned after 24 hours and 7, 30 and 90 days postoperatively regarding
the presence of sensitivity, and the stimuli that triggered that sensitivity. Evaluation after
24 hours revealed significant differences in the postoperative sensitivity among the types
of cavity preparations: there was a higher frequency of sensitivity in Class II MOD
restorations (26%), followed by Class II MO/DO (15%) and Class I restorations (5%).
There was a decrease in the sensitivity for all groups after 7, 30 and 90 days.
21
It appears that the unsealed microporous zone of the acid-etched dentin allows
hydraulic dentinal fluid shift and penetration of microorganisms into the dentinal tubules,
which may lead to postoperative hypersensitivity. It has been also concluded that the
occurrence of sensitivity is related to the complexity of the design and the restorative
procedure.
1.2.4 Microleakage
Microleakage is defined as a clinically undetectable passage of bacteria, fluid,
molecules, or ions between the cavity wall and the restorative material,86
and often
manifests itself as postoperative hypersensitivity, the result of the hydrodynamic fluid
movement within the dentin tubules complex.86
The symptoms of microleakage range
from postoperative hypersensitivity or loss of the restoration due to bond failure, to
damage to vital dentin and pulp tissue which in some cases may be irreversible. It has
been established that many irritant properties previously associated with chemical action
of the restorative materials themselves are, in fact, related primarily to bacterial
microleakage.87, 88
Furthermore, the effects of microleakage include marginal
discoloration and secondary caries.89
These effects are due to the presence of bacteria,
their nutrients or hydrogen ions, originating from plaque on the surface and leaking into
the interfacial space.90
Bacterial marginal leakage has been implicated as an etiological
factor in recurrent caries and pulp irritation following the application of restorations.91, 92
Indeed, one of the main reasons for replacement of resin composite restorations is
secondary caries, which accounts for 40% to 70% of dentists’ stated reasons for doing
so.5 On the other hand, evidence is also growing that the relationship between marginal
22
deficiency, microleakage and secondary caries may not be as clear-cut as is widely
assumed.93
It has been well known for many years that conventional resin restorative
materials, and their bonding and application techniques do not provide a complete
marginal seal. Titley et al 94
argued that the main requirements of tooth-resin interface
bonding are impermeability to oral fluids, ability to seal dentinal tubules, protection of
the pulp, and longevity. Others
95-98 discussed the interactive significant effect of
restorative material and filling technique (incremental or bulk placement) versus the
effect of material alone on the microleakage. Idriss et al 99
investigated the correlation
between factors related to cavosurface marginal adaptation and microleakage in Class II
cavities restored with a light or chemical-activated resin composite. The study found that
microgaps were seen more with light-cured composites than with chemical-cured,
regardless of the placement technique. On the other hand, the microleakage assessment
showed that the groups of chemical-cured composites had better marginal adaptability
than the light-cured composites. Regardless of the choice of material and placement
technique, it seems that the tooth-composite bond is an important determining factor in
marginal seal and microleakage occurrence.
In Class II resin composite restorations where gingival margins extend below the
CEJ onto dentin, the durability of the gingival seal has been a prime concern. The
gingival portion has been thought to be the most common location for secondary
caries.100
Several factors contribute to the high incidence of recurrent caries in the
gingival area; these include improper placement technique by the clinician, plaque
accumulation due to patient difficulty in cleaning, and lack of patient compliance with
23
proper oral hygiene. In addition, the ability of resin composites to inhibit the progression
of caries has been shown to be less than that of glass-ionomer and amalgam.101
It is
therefore critical to achieve a seal on the gingival margin of Class II composite
restorations.
In a study Wibowo et al 39
evaluated the sealing ability of several Class II
restorations whose gingival margin was apical to CEJ. Preparations were restored with
several restorations: (1) total-etch adhesive (Scotch Bond Multi Purpose) and hybrid
composite (Z100) as control, (2) total-etch adhesive (Single-Bond) and Z100, (3) Single-
Bond and flowable composite (Wave), (4) Single-Bond and packable composite
(Surefil), (5) Single-Bond and compomer (F2000) as liner, then Z100, (6) Single-Bond
and glass-ionomer (Vitrebond) as liner/base, then Z100. (7) Same as the 6th
group except
the glass-ionomer was laminated with Z100 at the gingival margin. Computer imaging
software was used to determine the silver nitrate stain penetration. The study found that
the laminate technique of placing glass-ionomer on the gingival portion of the proximal
box 0.5 mm short of the gingival cavosurface margin, followed by the placement of resin
composite, was the technique of choice. Although Wibowo et al found the use of glass-
ionomer as a base in Class II composite restoration is advantageous, the marginal sealing
ability of the resin composite restoration when the cavosurface margins are placed at the
CEJ is not advantageous. Gladys, Van Meerbeek et al 102
compared the marginal sealing
ability of three types of composite restoration (microfine composite, ultrafine compact
filled composite and poly acid-modified resin-based composite), and two types of glass-
ionomer restorations (conventional glass-ionomers, resin-modified glass-ionomer). The
study showed resin-modified glass-ionomer performed better than the conventional
24
resin-based composites and conventional glass-ionomer. It seems that the resin content in
RMGI has significantly improved the marginal seal and decreased the microleakage, due
to its micro-mechanical interlocking adhesion character.
Gueders et al 103
evaluated the microleakage of composite restorations made with
four total-etch (Scotch Bond Multi Purpose, Optibond Solo Plus, Scotch Bond 1, and
Gluma Comfort Bond + Desensitizer) and three one-step self-etch adhesive systems
(Adper Prompt-L-Pop, Xeno III and iBond). Also, they evaluated the potential
improvement of marginal sealing in Class V cavities when flowable composite was
utilized as a cavity liner. The study concluded that Xeno III, a mild self-etch adhesive,
showed acceptable results, however, the authors reported that more clinical
investigations were required to confirm this performance. The study also found that the
addition of a thin layer of flowable composite gave no statistical improvement in the
majority of adhesives tested. Gueders’s study was in agreement with other study by
Pongprueksa et al 104
evaluated the effect of a filled-adhesive resin (Adper Single Bond)
and an unfilled-adhesive resin (Adper Single Bond) with and without a flowable
composite (Filtek Flow) as an elastic cavity liner on marginal leakage in Class V
composite restorations. The study found that the application of filled adhesive or
flowable composite had no influence on marginal leakage at both the enamel and dentin
margins; however, it had an influence on the µTBS to dentin of Class V restorations.
Flowable composites may be too stiff to be successfully used for this purpose.33
As per
the self-adhesive cements that have lower elastic modulus values, they should undergo
experimental investigation.
25
El-Mowafy et al 105
investigated the use of fiber inserts resin composite
restorations when gingival margins were on the root surface. Two types of fiber inserts
were used in Class II slot cavities: glass fiber (Ever Stick, Stick Tech) and polyethylene
(Ribbond-THM), with three bonding agents were employed: Scotch Bond Multipurpose
(3M ESPE), ClearfilSE Bond (Kuraray) and Xeno IV (Dentsply). Three-mm-long fiber
inserts were inserted into restorations at the gingival seat. The study showed that
microleakage was reduced in all groups that included fibers in their restorations. The
study concluded that glass-ceramic fiber inserts enhanced the quality of the marginal
seal, which resulted in a decrease in the overall volumetric polymerization contraction of
the composite. The fibers might also assist the initial increment of composite in resisting
pull-away from the margins toward the curing light.
Although the glass-ceramic fibers might reduce the polymerization shrinkage
stresses on tooth-composite interface by replacing a part of the composite; their
application is a technique sensitive and time consuming. In addition to that, other studies
reported little or no significant marginal seal improvement with the glass-fibers
insertion.106, 107
The thermal cycling effect on the leakage at the tooth-restoration interface was
found to have a minimal effect and sometimes to be inconclusive.39, 108
In contrast,
occlusal loading was found to significantly increase the microleakage at the tooth-
restoration interface.104
26
1.3 Microleakage of Self-Adhesive Resin Cements
Conventional luting cements and more recently, self-adhesive resin cements were
originally created and developed to be used in the cementation of indirect restorations
(inlays, onlays, crowns, bridges, posts) made of metal and metal-ceramics (PFM), all-
ceramics (Feldspathic), strengthened core ceramics (zirconium and aluminium) and
composite.2 In the past few years, there has been a growing interest in the use of self-etch
and self-adhesive resin cements because their use is less time-consuming and less
technique-sensitive.109-111
Reduced postoperative sensitivity and dentin permeability 112
are other potential properties of self-etch cements. The constituents of the self-etching
primer polymerize in the canals, combine with the debris in the canals, plug the canals,
and reduce or prevent postoperative sensitivity, although sometimes the sensitivity
remains.
The self-adhesive resin cements are composed of polyfunctional dimethacrylate-
based monomers, such as BIS-GMA and/or urethane dimethacrylate, and inorganic filler
of glass and silica. Therefore, their composition is analogous to that of resin composite
restoratives but with a lower filler loading. A study found that Panavia F resin cement
showed low solubility due to the composition of BIS-GMA based resin matrix and the
inorganic filler particles, which could resist acidic challenge in posterior restorations.113
Although the basic adhesion mechanism appears similar for all self-adhesive cements,
these materials are still relatively new, and detailed information on their composition and
adhesive properties is limited.114
27
In 1955, Buonocore showed that phosphoric acid-etching of the enamel created
microporosities at the enamel surface. The application of unfilled bonding resins to the
etched enamel can then form an enamel-composite interlocked hybrid layer.115
This bond
is still the gold standard for enamel bonding. The self-etching primers and adhesives
allow the omission of the separate etching step that may result in insufficient enamel-
composite bond formation due to the weak acidity of some mild self-etching systems.
This concern is more relevant when there is a large surface enamel-adhesive interface
area (e.g., laminate veneers). A separate step of acid-etching prior to the application of
the self-etching primer is suggested to overcome this drawback.
Self-etching primers and self-adhesive cements do not require a separate
conditioning step of the dentin, since their adhesion mechanisms are based on partial
retention of the smear layer. Although this feature should make them less susceptible to
moisture contamination, degradation of resin-dentin bonds may also be expected to occur
in self-etch systems, due to the presence of hydrophilic monomers and high solvent
concentrations in the adhesive blends.116
The smear layer, which is developed during
preparation, adheres to the dentin surface and hinders the resin diffusion to the
underlying dentin and resin tag formation. The contemporary resin-cements with the self-
etch primer demineralize the smear layer and incorporate it into the applied resin, which
slightly penetrates into the underlying dentin, hence creating a hybrid layer in which the
undissolved collagen fibers of the dentin and the remnants of the original smear layer are
incorporated.117
The recent one-step self-adhesive resin cements were developed with
multifunctional acidic contents to be applied on the prepared tooth surface with no
pretreatment. These cements depend upon the acidity of the resin matrix to condition the
28
tooth surface. Nevertheless, the weak acidity of the self-adhesive resin cements might
affect their bonding performance.
Self-adhesive resin cements can provide marginal adaptation at dentin with no
prior treatment, which is comparable to the established luting agents.118
However, these
cements do not perform as well when they are bonded to enamel. Therefore, selective
phosphoric acid treatment when bonding to enamel is advised.119
Piwowarczyk et al 120
evaluated the microleakage and marginal gaps in full cast crowns bonded with different
cementing agents. Crowns were made from a high-gold alloy with mesial and distal
margins were located in dentin. The specimens were divided into six groups of
cementing agents: zincphosphate cement (Harvard cement), conventional glass–ionomer
cement (Fuji I), resin-modified glass–ionomer cement (Fuji Plus), two groups with
standard resin cements (RelyX ARC, Panavia F), and a self-adhesive universal resin
cement (RelyX Unicem). Test specimens were evaluated for microleakage and marginal
gap, after they were placed in a silver nitrate solution, using a digital microscope camera.
The study reported that RelyX Unicem generally showed a minimal degree of
microleakage.
A recent study by Schenke et al 121
evaluated the marginal integrity of partial
ceramic crowns luted with self-adhesive resin cement (RelyX Unicem) and compared the
results with other conventional resin cements. Crown preparations were performed with
proximal margins placed 1 mm below the CEJ. Vita Mark II ceramic crowns were seated
on the preparations after surface treatment. Microleakage was assessed by evaluating
silver nitrate penetration on multiple tooth sections with an image analyzing system. The
study concluded that the self-adhesive resin cement (RelyX Unicem) can be preferably
29
recommended to reduce microleakage and gap formation at the dentin-restoration
interface. The previous two studies found the self-adhesive resin cements have the best
overall microleakage scores, they are also in agreement with other studies in the
literature 118, 122
that evaluated the bond strength and marginal adaptation of the self-
adhesive resin cements at the interface of ceramic crowns.
30
1.4 Microtensile Bond Strength (µTBS)
One of the most frequently used tests to screen adhesives effectiveness is the
µTBS. The reason for using this test is that the stronger the adhesion bound between
tooth and restorative material, the better it will resist stress imposed by resin
polymerization and oral function. “To assess long-term effectiveness, it is crucial that
one first determine the short-term bonding effectiveness of adhesives, these serve as base
line data”69
De Munck et al 123
evaluated the bonding effectiveness of three one-step self-
etch adhesives (AQ bond, Reactmer, Xeno CF bond), two two-step self-etch adhesives
(experimental ABF and Clearfil SE bond), one two-step total-etch adhesive (Prime &
Bond NT), and one three-step total-etch adhesive (OptiBond FL) as control. In dentin
groups, the occlusal third of the molars were removed using Isomet machine to prepare
the dentin surface for adhesive bonding; in the enamel groups the buccal or the lingual
surfaces were flattened with high speed diamond bur. The study concluded that the
µTBS of total-etch adhesives to enamel was significantly higher than that of the one-step
self-etch adhesives. Comparing the dentin µTBS results, the study found that the two-
step self-etch adhesives are nearly as effective as the total-etch adhesive. On the
contrary, the rather low µTBS of the one-step self-etch adhesives was explained by the
failure of these adhesives to optimally hybridize the smear layer that covered dentin.
Another point of view 124
suggested that applying two-layer of one-step self-adhesive can
nearly double the µTBS to dentin. A thick adhesive layer can probably act as a shock-
absorber at the tooth-composite interface. Coelho et al 125
evaluated the influence of
adhesive thicknesses on the µTBS values using the laboratory mechanical testing and the
31
finite element analysis (FEA). The study concluded that µTBS values were directly
proportional to the interfacial adhesive layer thickness for Clearfil SE (self-etch adhesive
system).
Pangsrisomboon et al 126
also evaluated the µTBS of three self-etching adhesive
systems with different degree of acidity (Clearfill SE Bond, One-Up Bond F, and
XenoIII). Assigned adhesive was applied on dentin surfaces and resin composite (Z250)
was built up to 6 mm height. The study showed that a higher acidity of a self-etching
adhesive may not relate to higher bond strength even if the high acidity was able to
completely dissolve the smear layer and smear plugs. It has been reported that there is no
correlation between the hybrid layer thickness and the bond strength.127
Ermis et al 109
evaluated the µTBS of several self-etch adhesives (Adper Prompt L-Pop, Clearfil S3
Bond, and Clearfil SE Bond) and a total-etch (Optibond FL) adhesive, when dentin was
prepared with three different grit size diamond burs (medium, fine, and extra-fine) to
produce smear layer with different thicknesses. It was concluded that different grit-sized
diamond burs did not affect the bond strength of the interface, except for the ultra-mild
one-step self-etch adhesive (Clearfil S3Bond), and the bonding effectiveness of total-
etch adhesives was hardly affected by the thickness of the smear layer.
Different techniques have been developed to measure µTBS.128
The variation in
the data reported in the literature is mainly dependent upon the different experimental
factors, for instance, the resin material type, generated stresses rate, sample size and
specimen preparation method.129
Therefore, the final test values cannot be used to
compare with, or draw conclusions from, data gathered in other studies.69
32
1.4.1 µTBS of self-adhesive resin cements
Different studies 111, 123, 126, 128, 130, 131
have examined the µTBS of the self-etching
resin cements and their bonding potential to dentin and enamel. Divergent reported
findings and values of the bond strength have been explained by variations in the test
methods and the inherent characteristics of dentin such as density of tubules, inorganic
content, moisture condition and surface treatment.132
De Munck et al 110
evaluated the
µTBS of new self-adhesive cement (RelyX Unicem) compared to self-etch primer resin
cement (Panavia F) to dentin and enamel, and evaluated the cements’ interaction with
tooth substrates. Conditioned tooth surface using phosphoric acid etchant prior to RelyX
Unicem application was also examined. Dentin and enamel flat surface were prepared,
and pre-cured resin composite blocks (Paradigm MZ100) were pressed on the cement
and were adapted on the teeth flat surfaces. Teeth were then sectioned perpendicular to
the bonding interface to obtain rectangular rods and µTBS was determined in MPa
results. The study concluded that RelyX Unicem had lower bond strength than Panavia F
with enamel and dentin surfaces. Finally, the best bonding effectiveness with this new
self-adhesive cement was obtained by selectively acid-etching enamel prior to luting.
However, the study did not illustrate whither pressing the resin cements against the tooth
surfaces during their application and light-curing would significantly improve their bond
strength.
Furthermore, Abo-Hamar et al 122
assessed the shear bond strength performance
of the self-adhesive cement (RelyX Unicem) to dentin and enamel compared to different
luting agents (Variolink II and Panavia F 2.0). Two increments of the luting cements
33
were applied on flat dentin surfaces. The shear bond strength was measured before and
after 6,000 thermocycles. The study found that the new self-adhesive cement (RelyX
Unicem) showed the best performance among all luting cements and the authors
recommended that it could be considered as an alternative to the conventional adhesive
systems for luting ceramic and metal-based restorations. The study also found that
thermocycling did not affect the bond strength of the tested luting cements when bonded
to dentin, whereas the effect was significant when bonded to enamel. Another study by
Piwowarczyk et al 133
who examined the bond strength of seven dual-cure resin luting
cements to human dentin in vitro after five months of storage in water plus 37,500
thermal cycles. The study found that the bond strength of the resin cements is
significantly decreased after being subjected to thermal cycling.
It has been reported that the light-polymerization of the dual-cure resin luting
cements resulted in higher bond strengths compared to the chemical-polymerization
alone.133
Thus, light-polymerization at the ceramic restorations margins can improve the
microleakage performance and the marginal seal and integrity. Arrais et al 134
also
reported that µTBS of the dual-polymerizing resin cements were significantly low when
the cements were left auto-polymerized compared to when they were light-polymerized.
The authors attributed that to the higher monomer conversion resulted from the light-
polymerization procedure.
Duarte et al 111
evaluated the µTBS of self-adhesive resin cement (RelyX
Unicem) when bonded to cervical enamel with and without phosphoric acid pre-
treatment. Two strong well-known self-etch (Multilink) and total-etch (RelyX ARC)
resin cements were used as control. The study showed that despite the low pH (2.1) of
34
RelyX Unicem the enamel demineralization obtained was only superficial. Significant
higher bond strength was obtained when the cervical enamel was etched with phosphoric
acid prior to RelyX Unicem application. In addition, Hikita et al 135
reported that µTBS
of RelyX Unicem to dentin was negatively influenced by the phosphoric acid pre-
treatment. Since contemporary self-adhesive resin cements do not require pre-treatment
for bonding, Monticelli et al 114
examined the acidity of different self-adhesive resin
cements (Panavia F 2.0, RelyX Unicem, Multilink, G-Cem, and Biscem) and reported,
with SEM micrographs, their diffusion into and reaction with the tooth dentin surface.
Generally, the self-adhesive cements were not able to demineralize/dissolve the
smear layer completely. Limited dentin demineralization and resin infiltration into the
underlying dentin were observed for self-adhesive cements. Although the interfacial
patterns of the self-adhesive cements were not comparable to those of total-etch luting
cement systems, the self-adhesive resin cements showed effective bond strength and
microleakage scores in many studies.110,111,118-122
35
1.5 Statement of the Problem
Evolving improvements associated with resin-based composite materials, dental
adhesives, filling techniques and light-curing have improved their predictability.
However, challenging problems still remain.
Self-adhesive resin cements have significant advantages for cementation of
indirect adhesively cemented restorations.135, 136
The use of these self-adhesive resin
cements at the interface of direct resin composite posterior restoratives to adhere to
dentin and to reduce microleakage has not yet been explored.
The recently introduced Filtek-LS low-shrinkage silorane composite is a unique
resin composite material that claims low-shrinkage material properties. Little
independent information is available regarding its bonding and microleakage
properties.51, 137
Major Questions
- Will self-adhesive resin cements provide an adequate marginal seal when utilized on
dentin to as an intermediate adhesive layer?
- Does the self-adhesive cement bond to tooth structures (enamel and dentin) effectively?
- Is the microtensile bond strength of self-adhesive resin cements with direct composite
restorations comparable to that found in indirect restorations?
- Does the silorane-based composite system (Filtek LS) reduce microleakage and
improve microtensile bond strength to dentin?
36
1.6 Objectives
1) To evaluate the microleakage of Class II direct composite restorations bonded
with self-adhesive resin cements when the gingival cavosurface margins are
located on enamel or on dentin.
2) To measure the microtensile bond strength of self-adhesive resin cements when
used to bond a direct composite restoration to enamel or dentin.
3) To evaluate the microleakage and the microtensile bond strength performances of
the newly introduced silorane-based low-shrinkage restorative system.
1.7 Null Hypotheses
1) There is no significant difference in the microleakage of a Class II resin
composite restoration between the self-adhesive resin cements and the
conventional total-etch adhesive system when the gingival cavosurface margins
are located on either enamel or dentin.
2) There is no significant difference in the microtensile bond strength between the
self-adhesive resin cements and the conventional total-etch adhesive system when
used to bond a direct composite restoration to enamel or dentin.
3) There is no significant difference in the microleakage and the microtensile bond
strength performances between the silorane-based low-shrinkage restorative
system used with its proprietary adhesive and the conventional methacrylate-
based composite restorative system used with a total-etch adhesive material.
37
MATERIALS AND METHODS
2.1 Microleakage Testing
2.1.1 Pilot study
The current study was preceded by a pilot study to detect any problems with the
proposed methods of testing. Following the completion of pilot study, 54 specimens were
employed for the main study.
2.1.2 Main study
2.1.2.1 Specimen collection and storage
Intact caries-free human molars were collected from the maxillofacial clinic at
the Faculty of Dentistry, University of Toronto, and stored in glass jars with distilled
water at 4º C until the experiment time, to preserve the dentin permeability.138
Specimens were selected according to specific criteria of size and dimensions. Selected
molars were sterilized with gamma radiation (Gammacell 220, Atomic Energy Ltd,
Mississauga, Canada) at the Department of Chemical Engineering and Applied
Chemistry, University of Toronto. Teeth were placed in a glass jar that was placed in a
cobalt chamber 5.5x8 inches, and exposed for 4 hours. The radiation dose rate was 0.3
kGy/h. This method of sterilization has been proved to not alter the tooth tissue
mechanical or the physical properties.139, 140
Teeth were cleaned with periodontal curettes
to remove the debris at the cervical area. Teeth were kept, wherever possible, in distilled
38
water during and between all experimental procedures, in order to preserve their
optimum mechanical and physical properties.
2.1.2.2 Specimen preparation
Apical foramina of the teeth were sealed with glass ionomer cement (GC Fuji I,
GC Corporation, Tokyo, Japan) and two layers of nail varnish were applied to the root
surfaces to prevent dye penetration during microleakage testing (Figure 5). Roots were
then embedded in chemically-cured acrylic resin bases (Ivolen, Ivoclar vivadent,
Liechtenstein, Germany), up to 2 mm apical to the cemento-enamel junction (CEJ), to
facilitate handling during test procedures. The teeth were then pumiced with a
prophylaxis rubber cup mounted on a low-speed rotary hand-piece with slurry of fine
pumice, and rinsed with water (Figure 6).
Class II mesio-occluso-distal (MOD) cavities were prepared. The outline of each
preparation was drawn with a pencil as a preliminary guide. The teeth with
undistinguished CEJ were excluded. The preparation dimensions were 4.0 mm wide
bucco-lingually measured from occlusal, and 1.5 - 2 mm deep axially (Figure 7).141
The
preparations’ gingival cavosurface outlines were located on dentin on one side (1.0 mm
below the CEJ), and on enamel at the other side (1.0 mm above CEJ) (Figure 8 a & b).
Tungsten carbide burs (#245, SS White, Great White Series, Lakewood, NJ, USA) were
used to carry out all preparations with a water-cooled high-speed air turbine hand-piece,
and a new bur was used every two cavity preparations to maintain cutting efficiency. All
line angles were prepared rounded. Each preparation’s dimensions was measured and
verified with a periodontal probe. One operator performed all preparations, while another
39
investigator checked them before restoration to ensure that they conformed to the
dimensions.
2.1.2.3 Specimen grouping and restoration procedures
Fifty-four teeth were divided into six groups (n=9) according to the adhesive
system that was used:
1) RXU (RelyX-Unicem, self-adhesive resin cement, 3M ESPE)
2) BRZ (Breeze, self-adhesive resin cement, Pentron Clinical)
3) MON (Monocem, self-adhesive resin cement, Shofu),
4) PAN (PanaviaF-2.0, resin cement with self-etch primer, Kuraray)
5) FLS System (newly introduced silorane-based composite with a proprietary self-etch
primer, Filtek LS, Low Shrink Posterior Restorative System, 3M ESPE)
6) SBMP (Scotch-Bond-Multipurpose, total-etch adhesive, 3M ESPE) as a control group.
The general composition of the adhesive luting agents provided by the manufacturers are
described in Table 1. Table 2 shows the list of adhesive luting agents with their
respective application procedures. Further, pH and modulus of elasticity of the tested
materials are reported in Table 3 in Chapter 3.
A universal metal matrix band/retainer (Tofflemire) was placed around each
prepared tooth and supported externally by applying low-fusing compound to maintain
adaptation of the band to the preparation’s margins. Each preparation was cleaned with
water spray and air-dried for 5 seconds. A thin layer of the predetermined adhesive or
cement assigned to each group was carefully applied on the entire preparation’s walls
and onto the cavosurface margins with a micro brush. A Demi LED (light-emitting-
diode) light polymerization unit (Kerr Corporation, Middleton, WI, USA, 1100-1200
40
mW/cm2) was used for light-curing the adhesive or the cement according to the
manufacturer’s instructions (Figures 9 and 10). A conventional methacrylate-based
hybrid resin composite (Filtek Z250 Universal Restorative, 3M ESPE) was incrementally
placed to restore all preparations, except in the FLS System group in which Filtek LS
(silorane-based low-shrink resin composite, 3M ESPE) was used. An approximately 1
mm –thick horizontal layer of composite was carefully adapted onto the preparation’s
gingival seat and light-cured for 20 seconds; a second 2 mm increment was added
diagonally on one side and light-cured for 20 seconds. Third, fourth and fifth increments,
filling up the remainder of the preparation, were placed and similarly light-cured.96
Great care was taken during insertion of the final resin composite increment in
order to keep the finishing to the minimum. Only the occlusal surfaces were then
finished with football-shaped multi-fluted carbide burs in a water-cooled high-speed
hand-piece (#023, H379UF, Ultrafine, Brasseler, Georgia, USA). Polishing followed
with aluminum oxide disks, used sequentially (Sof-lex LX pop-on, Paul, MN, 3M
ESPE). The restoration excess on the proximal sides was removed with a sharp hand-
scaler to simulate the clinical procedures. One operator performed all restorations, while
another investigator checked the restorations to ensure that they were free of defects.
2.1.2.4 Thermocycling procedure
After specimens were stored in distilled water at 37°C for 7 days, teeth were
subjected to artificial thermal ageing according to the ISO (The International
Organization for Standardization, ISO TR 11450 standard, 1994) recommendations.
Thermocycles were performed using a dwell time of 30 seconds in each bath and a
41
transfer time of 15 seconds between baths for 1,000 cycles between 5°C and 55°C.142
Then specimens underwent the microleakage testing procedures immediately after
thermocycling.
2.1.2.5 Microleakage testing
Two layers of black nail varnish were applied to the tooth surfaces to prevent dye
penetration, except for a 1 mm perimeter around tooth-restoration margins (Figure 11 a
& b). The teeth were then immersed in a 5% scarlet red fuchsine solution (Pararosanilin,
Imperial Chemical Industries) for 24 hours at 37°C.108, 143
After removal from the dye
solution, the teeth were rinsed with tap water for five minutes (Figure 12 a & b). Each
tooth was then sectioned mesio-distally with a low-speed micro-slicing machine (Isomet,
Buehler, Lake Buff, IL, USA) into two sections (Figure 13). All sections were scanned
into digital photos (300x300dpi, ScanMaker 9800XL, Microtech.Inc., CA, USA). The
section with the deepest dye penetration was selected to represent the tooth. The extent
of the die penetration was assessed according to a five-point scale (Figure 14):
0 = no leakage.
1 = leakage extending to the outer half of the gingival floor.
2 = leakage extending to the inner half of the gingival floor.
3 = leakage extending through the gingival wall up to 2/3 of the axial wall.
4 = leakage extending through the gingival wall up to the level of the pulpal floor.
Two independent examiners evaluated the extent of dye penetration for each selected
tooth section.
42
2.1.2.6 Cement thickness
The interfacial layer thickness at the gingival seat, the axial wall and the pulpal
floor were detected from five different areas of the interface under optical microscope at
60X magnification (SMZ800, Nikon Instruments Inc. NY, USA), and measured with
travelling spot insight camera (Model 3.2.0, Diagnostic Instruments Inc. MI, USA). The
range of the interfacial cement thicknesses is recorded in Table 3 and Figure 22 in
Chapter 3.
2.1.2.7 Data analysis
The microleakage data are quantitative (continuous) data. Descriptive analysis,
means and standard deviations were computed using SPSS (PC+ version 15 software,
Chicago, IL, USA). The data was analyzed using Kruskal-Wallis test (p≤ 0.05) at 95%
confidence level to detect the significant differences among the groups. Further analysis
with Mann-Whitney U-test was conducted for pair-wise comparisons among groups (p ≤
0.05) at 95% confidence level.
43
2.2 Microtensile Bond Strength Testing (µTBS)
2.2.1 Pilot study
The current study was preceded by a pilot study to detect any problems with the
proposed methods of testing. Following the completion of the pilot study, 36 specimens
were employed for the main study.
2.2.2 Main study
2.2.2.1 Specimen collection and storage
Teeth were collected, stored, sterilized and cleaned identically as described in
section 2.1.2.1
2.2.2.2 Specimen preparation
Teeth were pumiced with a prophylaxis rubber cup mounted on a low-speed
rotary hand-piece with slurry of fine pumice powder, and rinsed with an air/water
syringe. Twelve molars were employed and divided into two groups (n=6) according to
bonding substrate (dentin or enamel). Flat dentin and enamel surfaces were prepared to
measure the µTBS of the assigned adhesive or cement material. In the dentin group, a 3
mm-thick layer of occlusal enamel of three teeth was removed under running water with
the micro-slicing machine (Isomet, Buehler, Lake Bluff, IL). The exposed flat dentin
surfaces were wet-ground by means of a carbide bur (#245, SS White, Great White
Series, Lakewood, NJ, USA) to prepare a surface similar to cavity preparations in which
44
dentin is prepared by means of a bur. Only the mid-coronal dentin surface was used for
adhesive bonding in order to have all dentinal tubules perpendicularly oriented to the
bonding interface. In the enamel group, buccal enamel surfaces of three teeth were
flattened by a carbide bur parallel to long-axis of the teeth to standardize the orientation
of enamel prisms and similarly minimize the surface regional effects on the µTBS
results.144
2.2.2.3 Specimen grouping and bonding procedures
Thirty-six teeth were split into two groups (n=18) according to the bonded
substrate (dentin or enamel). Each group was divided into six groups (n=3) according to
the adhesive or the cement materials used for composite bonding as previously described
in section 2.1.2.3.
After surface preparation, a thin layer of the predetermined adhesive or cement
assigned to each group was carefully applied and cured with the Demi LED light
polymerization unit (Kerr Corporation, Middleton, WI, USA, 1100-1200 mW/cm2)
following the manufacturer’s instructions (Table 2). After completion of the bonding
procedures, 2 mm horizontal increments of composite were built up to a height of 6 mm
on the bonded surface with an approximate 6x6 mm cross sectional area, and each
increment was light cured for 20 seconds.125
The hybrid resin composite (Filtek Z250
Universal Restorative, 3M ESPE) was incrementally placed on the prepared surfaces of
all specimens, except the FLS System group, in which Filtek LS (silorane-based low
shrink posterior composite, 3M ESPE) was used. The specimens were then stored in
distilled water at 37°C for 7 days.
45
2.2.2.4 Thermocycling procedure
The teeth were subjected to artificial thermocycling using as dwell time of 30
seconds in each bath and a transfer time of 15 seconds between baths for 1,000 cycles
between 5°C and 55°C.142
As described in section 2.1.2.4
2.2.2.5 Specimen preparation and µTBS testing
Teeth were sectioned perpendicular to the adhesive-tooth interface into 1 mm-
thick slabs, under constant water cooling, using a thin (0.5 mm) diamond saw blade in a
low speed micro-slicing machine (Isomet, Buehler, Lake Bluff, IL). Slabs were fixed on
a plastic platform with sticky green compound and were serially cut to rectangular
specimens 1 mm2
in cross section, according to the “non-trimming” method of the µTBS
test (Figure 15). 109, 111, 125, 130, 134, 145
Specimens were inspected for the interface quality
(i.e., no voids or bubbles at the interface). Twelve specimens were randomly chosen
from each subgroup, and each specimen was measured with a digital caliper to confirm
dimensions. Then specimens were put in distilled water in their own individual vials and
were labeled by sample number. Specimens were tested on the same day they were
prepared for the µTBS testing. Cyanoacrylate adhesive and accelerator (Zapit, DVA,
Anaheim, CA, USA) were used to attach the microtensile specimens to opposing free-
sliding halves, which were designed to fit the µTBS Instron universal testing machine
(Bisco Inc. Schaumburg, IL). If any specimen was observed to have the cyanoacrylate
glue reach the interface, the specimen was discarded because the bond strength would
46
not have been represented correctly. Specimens were then stressed until fracture occurred
at crosshead speed of 1mm/min at room temperature, and were maintained moist
throughout the testing procedures. The force required to break each specimen was
recorded. µTBS was expressed in MPa, as derived from dividing the tensile force (N) at
the time of fracture by the bond area (mm2). After µTBS specimen testing, specimens
were kept in 30% alcohol at 4º C until observation for evaluation of mode of failure
under optical light microscope.
2.2.2.6 Evaluation of mode of failure
The two halves of each specimen were inspected by a single operator under a
stereomicroscope at 40x magnification. The appearance of interfacial failure (adhesive
layer on tooth, adhesive layer on composite surface and adhesive remnants on both sides)
was categorized as an adhesive failure mode. When adhesive failure was accompanied
by partial fracture of either one or both of the adherends, a category of mixed failure
mode was assigned. Finally, the third failure mode category was cohesive, when the
failure happened within dentin, enamel or composite and showed intact adhesive
interface. The category and location of the failures are demonstrated in Tables 9 and 10
in Chapter 3. For the purposes of statistical analysis, the premature, adhesive, mixed and
cohesive failure modes were assigned an ordinal-rank of 0, 1, 2 and 3 respectively.
2.2.2.7 Scanning electron microscopy (SEM)
For SEM analysis, one fractured specimen from each of the dentin subgroups,
already classified as adhesive failure, was allowed to dry overnight at 37º C with
47
ascending ethanol solutions. The two halves of the specimen were mounted, fracture face
up, on a 12 mm metal SEM stub using cyanoacrylate adhesive. The surfaces were then
sputter coated with gold (EMS-76M; Earnest F) and evaluated under a SEM at different
magnifications. Photographs were taken and stored digitally (Figures 31-37, Chapter 3)
2.2.2.8 Data analysis
The microtensile bond strength data are quantitative (continuous) data.
Descriptive analysis, mean and standard deviation of the µTBS were computed using
SPSS (PC+ version 15 software, Chicago, IL, USA). The microtensile bond strength was
analyzed using one-way analysis of variance (ANOVA) (p ≤ 0.05) at 95% confidence
level. The ANOVA was non-directional (i.e., two-tailed) which meant an effect in either
direction could be interpreted. Each variable was investigated for significance in means
using Tukey’s t-tests (p ≤ 0.05) at 95% confidence level. Specimens that were exposed to
premature failure were eliminated from the statistical analysis. Categorical rank-scaled
location of failure data were collected (Table 11 and figure 30, Chapter 3), and then
analyzed with Kruskal-Wallis test (p≤ 0.05) at 95% confidence level. Further analysis
with Mann-Whitney U-test was conducted for pair-wise comparisons among groups (p ≤
0.05) at 95% confidence level (Table 12, Chapter 3). Failure modes percentages are also
illustrated in Figure 28 and 29 in Chapter 3.
48
Figure 5: Teeth were cleaned with
periodontal curettes, apical foramina were
sealed with GI cement and roots were sealed
with nail vanish to prevent dye penetration
during microleakage testing.
Figure 6: Roots were embedded in acrylic
bases and crowns were pumiced with rubber
cups and slurry of soft pumice.
Figure 7: Preparation dimensions were 4mm
bucco-lingual measured from occlusal and 1.5 - 2
mm deep axially.
49
Figure 8: (a) gingival seat was placed on enamel (1 mm above CEJ); (b) gingival seat was placed
on root dentin (1 mm below CEJ).
Figure 9: Representative prepared specimen with
Tofflemire matrix that is secured with low-fusing
compound. All cavity surfaces were lined with
bonding adhesive or resin cement, then light-
cured before restoration with composite.
Figure 10: Demi LED light polymerization unit (pulses
between 1100-1200 mW/cm2).
50
Figure 11: Occlusal (a) and proximal (b) views of a representative restored specimen. Two layers
of black nail varnish were applied all over exposed surfaces but 1 mm short of the tooth-
restoration interface. (C) = composite, (T) = tooth structure, (V) = varnish seal.
Figures 12: Specimen after immersion in the red dye for 24h. (C) = composite, (T) = tooth
structure, (V) = varnish seal.
51
Figure 13: Teeth were sectioned mesio-distally with the
Isomet low-speed microslicing machine into two sections.
Figure 14: The extent of dye penetration was scored by
two independent observers according to a five-point
scale:
0 = no leakage
1= leakage extending to the outer half of the gingival seat
2= leakage extending to the inner half of the gingival seat
3 = leakage extending up to 2/3 of the axial wall
4 = leakage extending through the axial wall up to the
pulpal floor.
52
Figure 15: Illustration scheme showing specimen preparation for µTBS test. Preparation for
dentin subgroups was conducted from step B through E, and for enamel subgroups from step F
through I.
53
Table 1: Material composition of cements, adhesives and composites as provided by the
manufacturers.
Material (Manufacturer) Composition
RelyX Unicem
(3M ESPE, St Paul, USA)
Lot # 330621
Base paste: methacrylate monomers containing phosphoric acid groups,
methacrylate monomers, silanated fillers, initiator components, and
stabilizers.
Catalyst paste: methacrylate monomers, alkaline (basic) fillers, silanated
fillers, initiator components, stabilizers, and pigments.
Breeze
(Pentron Clinical,
Wallingford, USA)
Lot #165893
Mixture of BIS-GMA,UDMA,TEGDMA, HEMA& 4-MET resins, silane-
treated barium borosilicate glasses, silica with initiators, stabilizers and
UV absorber, organic and inorganic pigments, opacifiers, and aluminum
oxide.
Monocem
(Shofu Dental Co., San
Marcos,USA)
Lot #080118
Powder: di-, tri-, multifunctional acrylate resins, self-cure initiators,
light-cure initiators, and pigments.
Liquid: filler 60% and initiators.
PanaviaF 2.0
(Kuraray Medical Inc.,
Okayama, Japan)
Lot #61155
ED primer II: A: HEMA, 10-MDP, 5-NMSA, water, and accelerator.
B: 5-NMSA, water, and sodium benzene.
Paste A: 10-MDP, 5-NMSA, silica, dimethacrylate monomer, photo-
initiator, and accelerator.
Paste B: barium glass, sodium fluoride, dimethacrylate monomer, and
BPO.
Filtek LS System Adhesive (3M ESPE, Seefeld,
Germany)
Lot #20080415
Self-etch primer: phosphorylated methacrylates, vitrebond copolymer,
BIS-GMA, HEMA-Water, ethanol, silane-treated silica filler, initiators,
and stabilizers.
Bond: hydrophobic dimethacrylate, phosphorylated methacrylates,
TEGDMA, silane-treated silica filler, initiators, and stabilizers.
Scotch Bond Multi-Purpose
(3M ESPE, St Paul, USA)
Lot #20080516
Etchant: 35 % phosphoric acid.
Primer: vitrebond copolymer and HEMA-Water.
Bond: BIS-GMA, HEMA, and initiators.
Filtek Z250 (3M ESPE, St Paul, USA)
Lot #8EN
Methacrylate resin: BIS-GMA, UDMA, BIS-EMA.
Inorganic filler: zirconia/silica (60% by volume).
Particle size from 0.01-3.5 µm.
Filtek LS (3M ESPE, St Paul, USA)
Lot # 8AP
Silorane-resin; initiating system: camphorquinone, iodonium salt, electron
donor, stabilizers and pigments.
Inorganic fillers: quartz/yttrium fluoride (55% by volume).
Particle size from 0.1 – 2 µm.
Abbreviations: BIS-GMA (bisphenol a diglycidyl ether dimethacrylate), BIS-EMA (bisphenol a
polyethylene glycol diether-dimethacrylate), BPO (benzoyl peroxide), HEMA (hydroxyethyl
methacrylate), MET (methacryloxy ethyltrimellitic acid), MDP (methacryloyloxydecyl dihydrogen
phosphate), NMSA (n-methacryloyl-5-aminosalicylic acid), TEGDMA (tetraethyleneglycol
dimethacrylate), and UDMA (urethane dimethacrylate).
54
Table 2: Steps followed for materials application.
Materials
(Manufacturers)
Etchant Primer Adhesive Luting resin
cement
Resin filling
RelyX Unicem
(3M ESPE, St Paul,
USA)
------- ------ ------ Apply thin layer of
the self-adhesive
resin cement on all
cavity surfaces with
micro brush, Light-
cured (20s).
Incremental
application of
resin composite
restoration (Filtek
Z250).
Breeze
(Pentron Clinical,
Wallingford, USA)
------- ------ ------ Apply thin layer of
the self-adhesive
resin cement on all
cavity surfaces with
micro brush, Light-
cured (40s).
Incremental
application of
resin composite
restoration (Filtek
Z250).
Monocem
(Shofu Dental Co.,
San Marcos,USA)
------- ------ ------ Apply thin layer of
the self-adhesive
resin cement on all
cavity surfaces with
micro brush, Light-
cured (20s).
Incremental
application of
resin composite
restoration (Filtek
Z250).
Panavia F 2.0
(Kuraray Medical Inc,
Okayama, Japan)
------- Mix equal
amounts of A &
B ED primer II,
apply the mix,
and wait (30s),
gently air-dry.
--------- Mix equal amounts
of A & B pastes
(20s), apply thin
layer of the mixture,
Light-cured (20s).
Incremental
application of
resin composite
restoration (Filtek
Z250).
Filtek LS System
Adhesive (3M/ESPE, Seefeld,
Germany)
------- Apply primer
and massage
with brush (15s),
gentle air-blow,
and light-cure
(10s).
Apply
adhesive then
gentle air-
blow, and
light-cure
(10s).
-------- Incremental
application of
low shrinkage
resin composite
restoration (Filtek
LS).
Scotch Bond Multi-
Purpose (Control)
(3M ESPE, St Paul,
USA)
37%
phosphoric
acid etching
(15 s), water
rinse (15s), air-
dry (5s).
Apply primer
and air-dry (5s).
Apply
adhesive, air-
dry gently and
light-cure
(10s).
-------- Incremental
application of
resin composite
restoration (Filtek
Z250).
55
RESULTS
3.1 Microleakage Test Results
The microleakage scores at the gingival margins were collected from digital
photos. Figures 16 to 21 show images of representative tooth sections from each group.
Percentage distribution of the microleakage scores among the different materials at
dentin and enamel gingival margins are shown in bar charts (Figures 23 and 24). Means
and standard deviations of microleakage scores of all groups at dentin and enamel
gingival margins were collected and are shown in a bar chart (Figure 25) and in tables
(Tables 4 and 5). In general, the FLS System group showed no microleakage when the
gingival margins were located in dentin, and the lowest scores among all groups when
the gingival margins were located in enamel. The RXU and BRZ groups showed better
microleakage scores when gingival margins were in dentin than in enamel. The highest
microleakage scores were recorded with the MON group in both dentin and enamel
gingival margins. SBMP (control) showed relatively higher microleakage scores in the
dentin margins than in the enamel margins.
The descriptive analysis of the data showed that they were normally distributed.
Kruskal-Wallis test revealed significant difference among the groups in both enamel and
dentin interfaces (p < 0.001) at 95% confidence level. Mann-Whitney U-test showed a
significant difference between SBMP and (FLS System, RXU and BRZ) in dentin
gingival margins (p < 0.05). No significant differences were detected between SBMP
and PAN and MON when gingival margins were in dentin (p > 0.1). When gingival
56
margins were placed in enamel, Mann-Whitney U-test revealed no significant difference
between SBMP and (FLS System, RXU and BRZ) (p > 0.1). Significant differences were
detected, however, between SBMP and PAN, MON (p < 0.5). Table 6 shows the Mann-
Whitney U-test p-values of all groups. The first null hypothesis therefore was rejected;
there was a significant difference in the microleakage scores among the different dentin
and enamel groups in Class II preparations.
3.2 µTBS Test Results
After thermal cycling and during the preparation of the specimens, BRZ enamel
bonding subgroup specimens and both dentin and enamel MON subgroup specimens
debonded before they could be tested. A total of 98 rods was collected for µTBS testing,
data analysis and mode of failure evaluation. Representative pictures of modes of failure
were digitally photographed and stored on the computer. The means and standard
deviations data for µTBS, expressed in MPa, are shown in Table 7 and in Figures 26-27.
For dentin bonding subgroups, the higher µTBS values were obtained by FLS
System, followed by the SBMP, PAN, RXU and BRZ. Because the data were slightly
skewed, the descriptive analysis (histogram curves) revealed that the data were relatively
(marginally) normally distributed. Therefore, parametric (ANOVA) and non-parametric
(Kruskal-Wallis) analysis tests were conducted to evaluate the difference of the µTBS
values among the subgroups. The One-way ANOVA revealed that there was a significant
difference in µTBS among the dentin subgroups (p < 0.001) at the 95% confidence level.
Further analysis with Tukey’s t-test (pair-wise comparison) showed that there was
57
significant difference between FLS System subgroup and other subgroups and between
SBMP and other subgroups as well. No significant difference was found between FLS
System and SBMP (p = 0.999). Table 8 shows the p-values of the Tukey’s t-test for the
dentin bonding subgroups. Kruskal-Wallis test revealed that there was a significant
difference of the µTBS among the subgroups (p < 0.001) as well.
For enamel bonding subgroups, the higher µTBS values were obtained by SBMP,
followed by PAN, FLS System and RXU. Enamel bonding subgroup data were also
slightly skewed. The ANOVA test and Kruskal-Wallis test both were conducted to
evaluate the difference of the µTBS values among the subgroups. Both tests revealed that
there was a significant difference in µTBS among the dentin subgroups (p < 0.001) at the
95% confidence level. Further analysis with Tukey’s t-test (pair-wise comparison)
showed that there were significant differences among the subgroups except FLS System
and PAN (p = 0.189), and FLS System and RXU (p = 0.90). Table 8 shows the p-values
of the Tukey’s t-test for the enamel bonding subgroups.
3.2.1. Mode of failure
Most of the failures of the dentin bonding subgroups were either adhesive or
cohesive (failure of dentin or composite), and no specimens showed a mixed mode of
failure. In the enamel bonding subgroups, most of the failures were either adhesive or
mixed; only a few cohesive mode of failure were observed (Table 9 and 10). SEM
photographs represented the mode of failure of the subgroups (Figures 31 to 37).
Percentages of failure modes in each subgroup are illustrated in bar charts (Figures 28
and 29).
58
Analysis with Chi-square test could not be conducted because the values of more than
30% of the cells were less than five and none is zero.
The mean-rank values of the failure modes are presented in Figure 30 and Table
11. Kruskal-Wallis test revealed a significant difference among failure modes of both
dentin and enamel subgroups (p < 0.001). The results of Mann-Whitney U-test are
reported in Table 12.
59
Figure 16: Representative photographs of microleakage for RXU group. The enamel sides (E) of
both photographs show no microleakage at the tooth-composite interface (only the enamel tooth
structure is stained with the dye). The dentin side (D), photograph A shows microleakage (score
2), and photograph B shows no microleakage.
Figure 17: Representative photographs of microleakage for BRZ group. In photograph A, the
enamel side (E) shows microleakage (score 2), and the dentin side (D) shows no microleakage. In
photograph B, the enamel side (E) shows no microleakage, and the dentin side (D) shows
microleakage (score 1).
E D E D
E D E D
E D D E
60
Figure 18: Representative photographs of microleakage for the MON group. Photographs A and
B show microleakage extended through the axial wall up to the pulpal floor (score 4) at both the
enamel side (E) and the dentin side (D).
Figure 19: Representative photographs for the PAN group. In photograph A, the enamel side (E)
shows no microleakage at the tooth-composite interface, and the dentin side (D) shows
microleakage extended through the axial wall up to the pulpal floor (score 4). Photograph B
shows microleakage (score 4), at both the enamel sides (E) and the dentin (D).
D E E D
E D D E
61
Figure 20: Representative photographs for the FLS System group. Photographs A and B show no
microleakage at both the enamel side (E) and the dentin side (D).
Figure 21: Representative photographs for the SBMP group (control). Photographs A and B
show no microleakage at the enamel side (E). Photograph A shows microleakage (score 3) at the
dentin side (D), and photograph B shows microleakage (score 4) at the dentin side.
E D D E
E D D E
62
Figure 22: Representative photographs showing the cement thicknesses at the pulpal, axial
and gingival interfaces.
RXU
63
Table 3: The range of cement thickness, modulus of elasticity and pH of the materials used.
Material (Manufacturer)
Thickness
(µm)
Elastic
Modulus
(GPa) a
pHa
RelyX Unicem
(3M ESPE, St Paul, USA)
26.2 - 61.1 6.3 2.1
Breeze
(Pentron Clinical, Wallingford,
USA)
22.5 - 67.7 4.2 3.5
Monocem
(Shofu Dental Co., San
Marcos,USA)
27.9 - 73.5 2 2.2
PanaviaF 2.0
(Kuraray Medical Inc,
Okayama, Japan)
12.6 - 100 9.6 2.4
Filtek LS System Adhesive (3M ESPE, Seefeld, Germany)
36 - 37 -- 2.7
Scotch Bond Multi-Purpose
(3M ESPE, St Paul, USA)
-- -- 0.6
a. Data were collected from either the manufactures or studies in the literature.114, 146-149, 149, 150
64
Figure 23: Bar chart showing the % distribution of microleakage scores for all dentin subgroups.
Table 4: Distribution of the dentin side microleakage scores with group means and SDs.
0
10
20
30
40
50
60
70
80
90
100
FLS System BRZ RXU SBMP PAN MON
%
Bonding to dentin interface
0= No Leakage
1= Outer 1/2 of GF
2= Inner 1/2 of GF
3= Up to 2/3 of AW
4= Pulpal floor
Groups
Dentin microleakage scores
Mean
SD 0 1 2 3 4
FLS System 9 0 0 0 0 0 0
RXU 5 3 0 1 0 0.66 0.7
BRZ 4 4 1 0 0 0.66 1
SBMP 1 0 0 2 6 3.33 1.32
PAN 0 1 0 1 7 3.55 1.01
MON 0 0 0 0 9 4 0
65
Figure 24: Bar chart showing the % distribution of microleakage scores for all enamel subgroups.
Table 5: Distribution of the enamel side microleakage scores with group means and SDs.
0
10
20
30
40
50
60
70
80
90
100
FLS System SBMP BRZ RXU PAN MON
%
Bonding to enamel interface
0= No Leakage
1= Outer 1/2 of GF
2= Inner 1/2 of GF
3= Up to 2/3 of AW
4= Pulpal floor
Groups
Enamel microleakage scores
Mean
SD 0 1 2 3 4
FLS System 6 2 1 0 0 0.44 0.72
RXU 6 1 0 1 1 0.77 0.44
BRZ 2 7 0 0 0 1 0.5
SBMP 1 7 1 0 0 0.88 1.53
PAN 2 0 0 0 7 3.11 1.76
MON 0 0 0 0 9 4 0
66
Figure 25: Bar chart showing microleakage scores (means and SDs) of the dentin and enamel
subgroups.
Table 6: p-values (Mann-Whitney U-test) for the microleakage test groups.
a. Underlined values represent the enamel side groups while the non-underlined values are for the dentin side groups.
0
1
2
3
4
5
6
FLS System RXU BRZ SBMP PAN MON
Mea
n m
icro
leak
age
sco
re
Groups
Dentine Enamel
Groups p-values of all groupsa
FLS
System RXU BRZ SBMP PAN MON
FLS System -- 0.050 0.113 0.000 0.000 0.000
RXU 0.222 -- 0.796 0.002 0.000 0.000
BRZ 0.077 0.489 -- 0.002 0.000 0.000
SBMP 0.863 0.340 0.190 -- 0.730 0.258
PAN 0.011 0.031 0.040 0.024 -- 0.436
MON 0.000 0.000 0.000 0.000 0.436 --
67
Figure 26: Bar chart showing µTBS means (MPa) and SDs of dentin subgroups.
Figure 27: Bar chart showing µTBS means (MPa) and SDs of enamel subgroups.
0
5
10
15
20
25
30
SBMP FLS System PAN RXU BRZ
Mea
n o
f µ
TBS
valu
es
Dentin subgroups
0
5
10
15
20
25
30
35
SBMP FLS System PAN RXU
Mea
n o
f µ
TBS
valu
es
Enamel subgroups
68
Table 7: Means (MPa) and SDs of the µTBS of dentin and enamel subgroups. Specimens of
MON subgroups and BRZ enamel subgroup underwent premature failure.
Table 8: p-values (Tukey’s t-test) for the µTBS test subgroups.
a. Underlined values are for the enamel subgroups while the non-underlined values are for
the dentin subgroups.
Dentin Enamel
n Mean (MPa) SD n Mean (MPa) SD
SBMP 12 18.61 6.65 12 24.55 6.14
FLS System 12 19.14 8.70 12 8.57 3.30
PAN 12 11.57 4.72 12 12.06 3.86
RXU 10 6.69 3.30 9 4.13 1.35
BRZ 9 4.02 1.88 0 -- --
MON 0 -- -- 0 -- --
Groups p-values of all groups
a
FLS
System SBMP PAN RXU BRZ
FLS System -- 0.999 0.020 0.000 0.000
SBMP 0.000 -- 0.036 0.000 0.000
PAN 0.189 0.000 -- 0.299 0.037
RXU 0.090 0.000 0.001 -- 0.855
BRZ -- -- -- -- --
69
Figure 28: Bar chart showing the % distribution of different failure modes of dentin subgroups.
Table 9: Distribution of µTBS failure modes of the dentin subgroups.
Failure modes of dentin rods
Adhesive Cohesive Mixed
SBMP 7 4 (c)
1 (d) --
FLS System 2 (a/d)
4 (r) 6 (c) --
PAN 4 (a/c)
5 (r) 3 (c) --
RXU 3 (a/d)
5 (r) 2 (c) --
BRZ 9 (a/d) -- --
MON -- -- --
(a) adhesive; (c) composite; (d) dentin; (a/d) adhesive/dentin interface; (a/c) adhesive/composite
interface; (r) remnants on both sides.
0
10
20
30
40
50
60
70
80
90
100
FLS System
SBMP PAN RXU BRZ MON
%
Dentin subgroups
3= Cohesive
2= mixed
1= Adhesive
0= premature Failure
70
Figure 29: Bar chart showing the % distribution of different failure modes of enamel subgroups.
Table 10: Distribution of µTBS failure modes of the enamel subgroups.
Failure modes of enamel rods
Adhesive Cohesive Mixed
SBMP 5* 3 (c) 4
FLS System 2 (a/e)
7 (r) -- 3
PAN 5 (a/e)
2 (r) 1 (c) 4
RXU 4(a/e)
3 (r) 1 (c) 1
BRZ -- -- --
MON -- -- --
(*) could not be specified under the light microscope; (a) adhesive; (c) composite; (e) enamel;
(a/e) adhesive/enamel interface; (a/c) adhesive/composite interface; (r) remnants on both sides.
0
10
20
30
40
50
60
70
80
90
100
FLS System
SBMP PAN RXU BRZ MON
%
Enamel subgroups
3= Cohesive
2= mixed
1= Adheisve
0= premature failure
71
Figure 30: Bar chart showing mean ranks and SDs of different failure modes of dentin and
enamel subgroups.
Table 11: Mean ranks and SDs of different failure modes of dentin and enamel subgroups.
0
0.5
1
1.5
2
2.5
3
3.5
FLS System SBMP PAN RXU BRZ MON
Me
an r
ank
of
the
failu
re m
od
es
Subgroups
Dentine Enamel
Dentin Enamel
Mean (n=12) SD Mean (n=12) SD
FLS System 2 1.04 1.25 0.45
SBMP 1.8 1.02 1.8 0.83
PAN 1.5 0.9 1.6 0.79
RXU 1.2 0.93 1 0.85
BRZ 0.75 0.45 0 0
MON 0 0 0 0
72
Table 12: p-values (Mann-Whitney) for the mean ranks of the µTBS failure modes.
a. Underlined values are for the enamel subgroups while the non-underlined values are for the
dentin subgroups.
Groups p-values of all groups
a
FLS
System
SBMP PAN RXU BRZ MON
FLS System -- 0.688 0.216 0.048 0.003 0.000
SBMP 0.062 -- 0.397 0.092 0.005 0.000
PAN 0.297 0.432 -- 0.278 0.016 0.000
RXU 0.214 0.021 0.070 -- 0.278 0.000
BRZ 0.000 0.000 0.000 0.000 -- 0.000
MON 0.000 0.000 0.000 0.000 0.000 --
73
Figure 32: Breeze (Pentron) bonded to
bur cut dentin, SEM photomicrograph of
a fractured µTBS specimen viewed at
angle of 90⁰. The specimen failed 100%
adhesively, between the dentin and the
cement. The photomicrograph shows the
smear layer (SM). Magnification: x5000.
Figure 31: RelyX Unicem (3M ESPE)
bonded to bur cut dentin, SEM
photomicrograph of a fractured µTBS
specimen viewed at angle of 90⁰. The
specimen failed 100% adhesively,
between the dentin and the cement. The
photomicrograph shows cement remnants
over the composite surface (R).
Magnification: x5000.
Figure 33: Panavia F 2.0 (Kuraray) bonded to bur cut dentin, SEM photomicrograph of a
fractured µTBS specimen viewed at angle of 90⁰. The specimen failed 100% adhesively.
The photomicrograph showing the cement is packed with large filler particles (F).
Magnification: x5000.
F
R
SM
74
Figure 34: Filtek LS System (3M ESPE) bonded to
bur cut dentin, SEM photomicrograph of a fractured
µTBS specimen viewed at side angle. The specimen
had mixed failure between adhesive and composite.
The photomicrograph shows a bilayer band of
adhesive. (C) silorane composite; (B) FLS bond, it
is optimized for wetting and adhering to the
hydrophobic FLS composite (P) FLS self-etch
primer, it is rather hydrophilic to ensure proper
adhesion to dentin (D) dentin. Magnification: x2000.
Figure 35: Filtek LS System (3M
ESPE) bonded to bur cut dentin, SEM
photomicrograph of a fractured µTBS
specimen viewed at angle of 90⁰. The
specimen failed 100% adhesively. The
photomicrograph shows adhesive
layer filled with filler particles. (F)
filler particles. Magnification: x5000.
Figures 36 and 37: Scotch Bond Multi Purpose (3M ESPE) bonded to bur cut dentin, SEM
photomicrograph of a fractured µTBS specimen viewed at angle of 90⁰. The specimen failed 100%
adhesively. The photomicrograph showing a typical example of completely removed smear layer after
dentin surface conditioned with 35% phosphoric acid. Open dentin tubules and resin tags are
visible. Therefore, failure occurred either on top of dentin or through the hybrid layer. (D)
dentin surface; (DT) dentinal tubules; (RT) resin tag. Magnification: x5000.
DD
DDTT RRTT
C
B
P
D
F
75
DISCUSSION
This study evaluated the Class II gingival margin microleakage and the µTBS of
different self-adhesive resin cements with direct resin composite restoration. This study
attempted to replace the conventional restorative system with innovative ones, based
upon the common problems of the posterior subgingival Class II composite restorations,
by using resin cements to bond the resin composite to the tooth structure and by using a
new chemically-modified resin composite. During microleakage testing, RXU and BRZ
resin cements showed lower microleakage scores compared to the control adhesive
SBMP (total-etch) when bonding to dentin; while the FLS System showed microleakage-
free restorations when bonded to dentin. During the µTBS testing SBMP showed the
highest values among all subgroups when bonded to enamel while FLS System showed
the highest values when bonded to dentin. RXU and BRZ showed relatively low results
in µTBS test.
4.1 Effect of Study Methods
4.1.1 Effect of gamma irradiation
Extracted human molars were employed by the current study. It is of paramount
importance to note that extracted teeth are considered to be a potential biological hazard
and source of blood borne pathogens. Therefore, infectious agents associated with
extracted teeth need to be eliminated prior to dental research, with the minimal
76
alterations of the tooth structure’s physical and mechanical properties. Several
sterilization methods are used in dental research, including autoclaving, chemical heat or
dry heat sterilization. However, these sterilization methods have been proven to affect
the tooth structure.151
Gamma irradiation at the dosage used in this study has been shown
to sterilize non-carious teeth effectively without affecting the tooth structure
properties.139, 140
4.1.2 Effect of specimen preparation
The methodology of the current study was conducted to simulate the clinical
situation as closely as possible. In the current study, natural human teeth were employed
to measure and compare the microleakage and the bond strength of different materials
when bonded to natural tooth tissue. One of the shortcomings of using natural teeth for
bonding experiments is being subjected to dryness after their extraction. However, the
teeth were kept in water immediately after extraction and throughout the different test
procedures.
In the microleakage experiment, Class II preparations were prepared to measure
the microleakage at two gingival margins (enamel and dentin). Carbide burs were used to
prepare a surface with smear layer similar to the clinical situation. The preparations were
standardized in dimensions to minimize variability. The Class II preparation was used to
create a clinically-relevant C-factor. This contrasted the configuration in the µTBS
testing, where flat enamel and dentin surface were used. However, the exposed flat
dentin surfaces of the µTBS test specimens were wet-ground by means of a carbide bur.
77
4.1.3 Effect of water storage
Specimens of both microleakage and µTBS experiments were stored in distilled
water for 7 days prior to thermocycling. Laboratory studies have shown that individual
teeth have variable permeability and different solutions affect their permeability
differently.138, 152
It has been reported that different solutions such as 70% ethanol, 10%
formalin, distilled water and distilled water with thymol, do not affect the bond strength
of the dentin structure.138
To closely mimic the clinical situation, some studies have used
artificial saliva solutions for storage. The decrease in bond strength obtained with the
saliva solutions was found to be similar to that obtained with the pure water.153
Storage in water solutions is one of the common artificial aging techniques in
dental research. A decrease in bonding effectiveness due to water storage is attributed to
the degradation of the interfacial components (mainly resin and/or collagen) by
hydrolysis. The storage time period may vary from a few months 154
up to 4-5 years or
even longer.155
It has been reported that bond strengths may decrease significantly even
after relatively short storage periods like 3-6 months.156
The seven days of storage in
distilled water at room temperature, in the current study, is considered a brief period in
comparison to the life expectancy of the restorations. Therefore, a minimal or no effect
would be expected on the microleakage and µTBS experiment results.
4.1.4 Effect of thermal aging
The use of thermocycling may highlight a mismatch in the thermal coefficient of
expansion between the restoration and tooth structure, which would result in repeated
expansion-contraction stresses at the tooth-restoration interface. Studies reported that the
78
relative linear thermal expansion of the resin composite Filtek Z250 was 41.5 X10-6
/°C,157
and 11 X10-6
/°C and 17 X10-6
/°C for enamel and dentin respectively.158
The
difference in the coefficient of thermal expansion between the resin restorative material
and the tooth structure may induce stresses on teeth and restorative materials.159
Microgaps present at the tooth-restoration interface may expand slowly over many cycles
of such stress-inducing activity. The amount of microleakage would increase and the
bond strength would decrease over time in the complicated thermal environment of the
oral cavity. When thermal cycling is applied to specimens in which stresses similar to
that in the clinical situation (C-factor of Class II), the highest stress is obtained.69
The
ISO TR 11450 standard (1994) indicates that a thermocycling regimen comprised of 500
cycles in water between 5°C and 55°C is an appropriate artificial aging test.142
Crim et al
160 found no difference in dye penetration when the specimens were cycled between 100
and 1500 cycles. However, the hot water bath (55º C) may accelerate the hydrolysis of
the components of the interfacial material, the water absorption, and the extraction of the
breakdown collagen or poorly polymerized resin oligomers.161
In this study, all specimens were subjected to 1000 cycles between 5º C and 55º
C to measure the effectiveness of the tooth-restoration interfacial bond under closer
clinically-relevant protocol. Thus, it is possible that the microleakage and µTBS results
would have been even better had the specimens been not thermocycled.
4.1.5 Effect of using fuchsine dye for microleakage assessment
Many techniques have been utilized to evaluate the cavity-sealing properties both
in vivo and in vitro. In vitro microleakage evaluation studies have included the use of air
79
pressure, dyes, artificial caries techniques, bacteria, chemical tracers, radioactive
isotopes, neutron activation analysis, scanning electron microscopy, and electrical
conductivity. In addition to their contrasting color, the organic dyes do not chemically
react or cause any destruction to the specimens, and they are considered the oldest, most
successful, and most common methods of detecting microleakage in vitro. There has
been a wide variation in choice of the dye used, either as solutions or suspensions of
different particle size.
Some researchers believe that in vitro microleakage studies overestimate the
amount of leakage that actually happens in clinical situations.56, 162
The reported
molecular radii of silver nitrate, methylene blue and fuchsine red to be smaller at 0.5 um,
0.68 um, and 0.84 um, respectively, than the molecular radii of the bacterial endotoxins
and bacteria, responsible for pathosis and secondary caries, at 10 um and 100-500 um
respectively.143
However, the water molecule has a radius of 0.26 um which is small
enough to diffuse into the microporosities within the interfacial hybrid layer via marginal
microgaps, and may lead to hydrolysis of the exposed collagen fibers within the hybrid
zone.56
Furthermore, glucose particles, which are a bacterial nutrient source, are smaller
than the dye particles, and their diffusion permits the possibility of bacterial presence
and/or secondary caries development at the dentin-restoration interface.143
A one possible limitation of the microleakage experiment is that the results might
be influenced by the dye chemical composition (fuchsine) that is used in the current
study. However, utilizing different dyes like fuchsine red, methylene blue and silver
nitrate did not show any difference in the results obtained by different studies in the
literature.39, 99, 99, 102-105, 120, 121
80
4.2 Effect of Material-related Factors
4.2.1 Effect of polymerization shrinkage on microleakage and bond strength
To partially overcome the shrinkage stresses in the experiments of the current
study, 5 layers of the resin composite were applied, diagonally to restore the preparations
in the microleakage experiment, and horizontally on the prepared flat surfaces in the
µTBS experiment. The rationale for using the incremental technique is that minimal
shrinkage stresses occur within each increment, because there is a low cavity
configuration factor due to the large free surface that permits resin to flow during
polymerization. As additional layers are added, there is probably some compensation for
shrinkage and stress build-up in earlier layers.96, 99
Filtek Z250, a conventional hybrid
composite, and Filtek LS, a silorane-based composite were used in the current study. The
manufacturer, 3M ESPE, reported that volumetric polymerization shrinkage of Filtek
Z250 is 2%, and <1% for Filtek LS due to the different chemical nature of the Filtek LS.
Since silorane-based resins have been reported to produce minimal polymerization
shrinkage, 48, 50, 50-52
it seems that the incremental placement technique would help to
reduce the shrinkage stresses for the Filtek Z250 more significantly than for Filtek LS.
Effect of polymerization shrinkage on the microleakage results
When the forces of polymerization shrinkage exceed interfacial bond strength,
gaps between composite and cavity walls are created. Such gaps may permit
microleakage to occur. In dentin side margins, FLS System showed no leakage in
all specimens, while SBMP (control) showed significantly higher microleakage
81
scores (p < 0.001). In enamel side margins, FLS System and SBMP showed low
microleakage scores, and showed no significant difference between their
microleakage results (p = 0.86). The ring-shaped monomers of the Filtek LS
demonstrate a unique interaction during polymerization, which results in a
minimal volume contraction. The reduced contraction stress applied on the tooth-
restoration interface, might explain the absence of leakage at the dental gingival
margins, and the low microleakage at the enamel margins that were found with
FLS System group.
Effect of polymerization shrinkage on the µTBS results
In dentin subgroups, FLS System showed the highest bond strength result
(19.1(8.7) MPa), and SBMP showed a value of 18.6(6.7) MPa, no significant
difference was found between the two subgroups (p = 0.99). In enamel
subgroups, FLS System showed a lower bond strength result (8.6(3.3) MPa),
while SBMP showed a value of 24.6(6.1) MPa, a significant difference was found
between the two subgroups (p < 0.001). The result of FLS System dentin bond
strength is in line with the results reported by other study, 137
that µTBS of FLS
System were: 20.3(5.9). The difference in the enamel bond strength results is
attributed to the different study protocols.
The low-shrinkage composite restoration seems to have a greater effect on dentin
than on enamel tooth structures. FLS System significantly contributed to low
microleakage and high bond strength results; however, it did not seem to have as
positive effect on the bond strength to enamel. According to the results of the
current study, using the low-shrinkage composite is very promising.
82
4.2.2. Effect of pH on microleakage and bond strength
The demineralization of the tooth structure is influenced by several factors, such
as the type and concentration of the acid, etching time, pH, formulation of the cements
and acidic monomers and finally the buffer potential of the hydroxyapatite crystals. The
demineralized tooth surface allows the diffusion of the adhesive materials and resin tag
formation and then the hybrid layer formation.
4.2.2.1 Effect of pH on enamel tooth structure
The predictable performance of the total-etch adhesive at the enamel margin is
ascribed to the enamel’s perfect prismatic structure.115
The hydroxyapatite
crystals are well structured in enamel even after phosphoric acid etching. The
exposed crystals in enamel permit the production of a micromechanical interlock
and enable more intimate chemical interaction with the functional monomers, and
consequently help to prevent marginal leakage and improve the bond strength.163
Effect of pH on the enamel microleakage results
SBMP showed low microleakage results. Despite the higher pH of FLS System,
RXU and BRZ, their microleakage results were not significantly different than
SBMP (p ≥ 0.19). While PAN and MON showed significantly high microleakage
results compared to other groups (p < 0.05). Self-etch adhesives, with a pH above
2, produce a thin hybrid layer in comparison to total-etch systems,164
which may
explain the higher marginal microleakage. However, FLS System, RXU and BRZ
showed unexpected positive results, possibly due to other involved effective
factors such as the intermediate layer thickness and/or elasticity.
83
Effect of pH on the enamel µTBS results
SBMP showed the highest results (24.6(6.1) MPa), and it was significantly
different than PAN, FLS System and RXU (p < 0.05). BRZ and MON
prematurely failed due to their weak bond strength to enamel. The current study
results are in line with many studies.110, 111, 114, 135, 145
It was established in the
literature that there is significant correlation between the pH of the adhesive and
enamel bond strength, indicating that the bond strength tends to increase as the
acidity of the enamel conditioner is increased. On the other hand, Perdigao et al
145 found that the enamel bond strength of the new self-etching, self-priming
adhesive systems approaches the enamel bond strength of the total-etch
(phosphoric acid) adhesive systems, which are gradually replacing the
conventional total-etch systems.
Du Munck et al 110
reported that phosphoric acid treatment of the enamel surface
prior to RXU application increased the µTBS to the same level as that of PAN.
Additional layer application of the acidic primer, roughening the surface prior to
bonding, and conditioning the surface with phosphoric acid was proven to
improve the bond strength of the self-adhesive resin cements when bonded to
enamel.111, 135, 145
84
4.2.2.2 Effect of pH on dentin tooth structure
Bonding to dentin tooth structure is significantly affected by acid etching. The
collagen fibers in the dentin substrate collapse after phosphoric acid etching, and
result in an impaired interfacial bond. 163
Effect of pH on the dentin microleakage results
FLS System showed microleakage-free margins, RXU and BRZ showed low
microleakage results. SBMP, PAN and MON showed significantly higher
microleakage results compared to the other groups. The high pH values of FLS
System, RXU and BRZ seem to have a positive effect on the dentin marginal
leakage.
According to the manufacturer (3M ESPE), the bonding mechanism of RXU
relies more on chemical bonding than on micromechanical retention. The
phosphoric acid parts of the methacrylate monomer chelate the calcium ions of
the hydroxyapatite which is the promoting part of the chemical adhesion. The
RXU chemical-bond concept was also adopted by some authors during their
discussion of the reaction mechanism of this simplified self-adhesive cement.110,
111, 135 Moreover, Ibarra et al
119 found that the microleakage of RXU increased
significantly when the dentin surface was pre-treated with phosphoric acid. It has
been speculated that the pre-etching removes all of the buffering capacity of
dentin, interfering with its ability to raise the pH of the acidic resin as it sets,
thereby lowering its conversion. Using milder acidic adhesives to remove the
superficial loosely bonded smear layer could somewhat enhance adhesion and
therefore the marginal microleakage.114
85
Another explanation for the low microleakage results of the self-adhesive
cements is related to the less pronounced dentin demineralization. Consequently,
smear plugs occlude the orifices of the dentinal tubules, which are partially
infiltrated by resin. 165
These results were in line with in vitro investigations by
Behr et al, who evaluated the marginal adaptation of RXU at the dentin-
composite interface of all-ceramic crowns.118, 122
Also, it was hypothesized that
the residual hydroxyapatite within the hybrid layer may serve as a receptor for
additional intermolecular interaction with monomers of the mild self-etch
adhesive.70
In spite of the thin hybrid layer formed by self-etch systems, FLS
System showed excellent microleakage results comparable to or even higher than
the ones obtained by the total-etch adhesive. This is in line with other studies in
the literature.39, 103
Effect of pH on the dentin µTBS results
No significant difference in µTBS was found between FLS System and SBMP (p
= 0.99), self-adhesive resin cements PAN, RXU and BRZ showed better bond
strength with dentin than with enamel. Despite the initial low pH (2.1) of RXU,146
an intimate adaptation and only a slight superficial demineralization of the dentin
surface, but no hybrid layer or resin tag formation, were observed during SEM
morphological interface examination.110, 114
Furthermore, the direct light-
polymerization of the material together with rapid decrease of the acidity (6 pH in
5 minutes) 146
may lead to limited penetration and interaction with tooth
substrates. No evidence of surface demineralization and resin diffusion into
dentin has been found in any of several self-adhesive resin cements, including
86
RXU, when they were examined by SEM. Furthermore, complete dissolution of
the smear layer and/or hybridization at the micrometer level was not possible.114,
119, 166, 167 In general, this limited micro-mechanical retention of the self-adhesive
cements might be responsible for the relatively low µTBS to tooth structure
measured in the current study. It has been found that the mild-acidic (pH=2.4)
ED primer II/Panavia F2.0 produced minimal dentin demineralization, however
resin penetration was identified. The hydrophilic monomers (HEMA, 10-MDP, 5-
NMSA), with low molecular weight, may have selectively diffused into dentin,167
forming the hybridized complex.84
This might explain the higher bond strength of
PAN compared to the other self-adhesive resin cements. Furthermore, phosphoric
acid etching, prior to the application of RXU, has been shown to be detrimental to
effective dentin bonding due to the thick, weak and exposed collagen layer that
prevents the viscous cement to reach the deeper unaffected dentin. Using RXU
with no phosphoric acid pre-treatment, however, gave substantial higher bond
strength to dentin.110
Following the demineralization-adhesion concept, interaction between the tooth
structure and the adhesive material depends upon adsorption of the acid ions into
hydroxyapatite.168
The presence of smear layer at the prepared surface has been
recognized as the weak link in bonding of self-adhesive resin cements to tooth
structure.167
On the other hand, it has been reported that µTBS of the adhesive
material to tooth structure does not depend upon the thickness of the smear layer
or the thickness of the hybrid layer; rather, it depends upon the ability of the
bonding agent to wet the demineralized zone.169, 170
Pangsrisomboon 126
argued
87
that the higher acidity of the self-etching adhesives is able to completely dissolve
the smear layer, even the smear plugs; however, this was not related to higher
bond strength.
Effect of pH on µTBS of enamel versus dentin
Higher pH values may not sufficiently etch the enamel; this would explain the
poorer results for enamel than dentin for the self-etch materials, especially BRZ
which had the highest pH and for FLS System which showed the greatest relative
decrease in bond strength when comparing enamel versus dentin. It appears that
for FLS System, the advantage of low shrinkage could not overcome the
disadvantage of an insufficient enamel etch. SBMP (total-etch) had significantly
better enamel versus dentin results for microleakage. In conclusion, the results
suggest that a lower pH, such as that found in the total-etch material, is probably
essential for an enamel margin bonding and would likely benefit the enamel
margin when FLS System is used. In dentin substrate, the relation between the
pH values of the adhesive materials, and microleakage and bond strength results,
was found to be detrimental.
4.2.3 Effect of intermediate layer on microleakage and bond strength
An elastic intermediate layer or an application of a thicker adhesive layer
between the dentin and the resin composite surfaces will help to preserve the bond during
the polymerization contraction process.39, 125
The main purpose of applying a low-
stiffness intermediate material layer is to absorb part of the stress generated by the
composite polymerization shrinkage. For this reason, thicker adhesive layers of unfilled
88
adhesives, filled adhesives, and flowable composites have been proposed. The elastic
behavior of dentin-composite interfaces can easily be affected by the use of thin, thick,
filled and unfilled resin adhesives.171, 172
Elastic modulus represents the relative stiffness
of the material within the elastic range and can be determined from a stress-strain curve
by calculating the ratio of stress to strain. It was determined that the modulus of elasticity
of dentin is 14 to 20 GPa 1, 173
and that of the hybrid composite Filtek Z250 is 24 GPa.173
Van Meerbeek et al 41
confirmed the effectiveness of the flexible and low viscosity
intermediate layer on the marginal adaptation and retention of the composite restoration.
Understanding the mechanical properties of tooth substrate and restorative materials
could help to improve the interfacial adhesive layer. The modulus of elasticity values of
the self-adhesive resin cements, which were used in the current study, were collected
from studies in literature (Table 3).
Effect of intermediate layer on the microleakage results
RXU and BRZ have close modulus of elasticity values of 6.3 GPa and 4.2 GPa,
respectively. RXU and BRZ showed low microleakage results with dentin and
with enamel. PAN showed slightly higher modulus of elasticity value (9.6 GPa)
than the other materials, and it showed high microleakage results as well. Despite
the low modulus of elasticity of MON, its microleakage performance was very
poor. The chemical composition of MON cement, and possibly other factors,
could be the reason behind the poor performance of MON in the current study.
Using a finite element analysis, it was reported that the thicker the adhesive layer
the higher the elastic releasing effect, which provides a more uniform stress
distribution.174
Scholte and Davidson 175
showed that thicker adhesive layers are
89
related to lower stress at the tooth-restoration interface and better marginal
adaptation. On the other hand, a thick intermediate layer, with low solubility and
low physical properties, at the tooth-restoration interface will negatively affect
the restoration durability.39
FLS System has a thicker adhesive layer (36 µm) than
the control SBMP; this might explain the better microleakage performance of the
FLS System group. Also, RXU and BRZ, which have a similar intermediate layer
thickness range (22 µm -67 µm), have a similar microleakage results as well. It is
worth mentioning that, unlike indirect restorations, no pressure was applied on
the cements used in the current study during polymerization. The cement
viscosity most likely determined the cement layer thickness. Despite the cement
application with a micro-brush, which produced better adaptation and reduced
cement film thickness under direct resin restorations, wide variations in cement
thicknesses were detected. PAN has the widest range of intermediate layer
thickness (12 µm -100 µm) among the groups; it also showed high microleakage
results.
Effect of intermediate layer on the µTBS results
It has been reported that increasing the thickness with adding a second adhesive
layer, will increase the bond stability, and thus, improve the bond strength at the
dentin interface.176
Coelho et al 125
showed that the µTBS of the filled self-etch
adhesives increases when the adhesive thickness increases. FLS System showed
the highest µTBS when bonded to dentin (19(8.7) MPa). The relative thickness of
the FLS System bilayer filled adhesive (36-37µm) (Figures 34 and 35), may
90
allow self-alignment of the specimen during the tensile testing, showing minor
deviation correction and stress distribution, resulting in high µTBS results.
Since no pressure was applied during resin-cement application, it was difficult to
avoid the variability in cement thicknesses at the different cavity walls. Cement
pooling was also noticed at junction of the axial and gingival walls. This
variability might have affected the study assessment of the microleakage and the
bond strength. The microleakage results of the current study might indicate an
association between the elastic intermediate layer and the low microleakage.
However, no correlation between the elastic intermediate layer and the µTBS
results in the current study could be detected. Thus, conclusions should be drawn
with caution.
4.2.4 Effect of hydrophobic layer on microleakage and bond strength
Adhesive systems interact with the tooth structure using two different
approaches: either by completely removing the smear layer (total-etch technique) or by
modifying it (self-etch technique). In total-etch adhesive system enamel bonding requires
only a phosphoric acid-etch step followed by resin application. The primer application is
not necessary in enamel bonding; however, it does not negatively affect the bond
strength.115
SBMP showed better µTBS when bonded to enamel (24.6(6.1) MPa) than
dentin (18.6(6.7) MPa). SBMP adhesive contains 65% BIS-GMA and 35% HEMA.
HEMA-monomer is hydrophilic and readily mixes with water while BIS-GMA is mostly
hydrophobic, which means it forms resin globules in the presence of water. In the oral
cavity, water diffuses from the dentinal tubules toward the tooth-restoration interface.
91
Hydrophilic monomers (HEMA) of bonding adhesives (total-etch or self-etch) are
readily mixed with that water at the interface. Water dilutes the hydrophilic monomer,
and has the ability to interfere with the resin polymerization of the adhesive which can
reduce the conversion level of the adhesive system by 50%.177
Furthermore, the
simulated pulp pressure in Class V cavities was shown to induce more micro-
permeability and water absorption in total-etch adhesives than in self-etch adhesives.178
The current study samples were kept in distilled water for 7 days, and were subjected to
thermal cycling in water baths for 1000 cycles to mimic the clinical hydrolytic effect of
saliva on the bonding adhesives. That may explain the SBMP lower µTBS value than
FLS System at the dentin interface in the present study. SBMP has shown significant
water degradation in tooth-composite interface after four years in vivo water storage.155
The influence of dentinal tubular water after phosphoric acid etching might have reduced
the µTBS.179
Hashimoto et al 180
demonstrated that bond strength increases with each
hydrophobic adhesive layer coating up to four coats.
It has been reported that despite the thick hybrid layer that is formed by Adper
Prompt L-Pop (self-etch adhesive), the µTBS was extremely low. This behavior of the
one-step self-etch adhesives is attributed to the extra-hydrophilicity of these systems.109,
176 The FLS System adhesive is a two-step self-etch adhesive system. The self-etch
primer is gently applied over the smear layer that covers the dentin and agitated with
micro-brush for 5 min. Sauro et al 74
found that the FLS System adhesive was free of
water-trees and micro-permeability in comparison to total-etch and self-etch adhesive
systems, and they found that the FLS System adhesive is acidic enough (pH 2.7) to
demineralize the intertubular dentin to a depth of 1–1.5 µm. The thin film of the self-etch
92
primer, above the hybrid layer, is covered with a thick layer (36 µm) of a very
hydrophobic adhesive that resists any water diffusion to the tooth-restoration interface
(Figure 34). This primer is specially designed to convert the dentin surface from a wet
hydrophilic, collagenous surface, to a dry hydrophobic, sealed surface that can couple
with the silorane-filled adhesive.
The incorporation of high concentrations of hydrophilic and/or ionic resin
monomers in these self-adhesive cements results in an increase in the movement of the
water into dentin-adhesive interface.181
The residual water that remains within the
interface tends to polymerize together with self-adhesive resin;109, 176
and thus, water
absorption may decrease the mechanical properties of the polymer matrix by swelling
and reducing the frictional forces between the polymer chains, a process known as
“Plasticization”.182
How much water is absorbed by the cured resin cement and how
quickly and what changes in polymer matrix will result, will ultimately depend upon the
formulation of the cement itself and on the degree of polymerization.133
Solubility is also
related to the composition of the self-adhesive resin cement within the hybrid layer, as
higher solubility and lower bond strength are attributed to a lower concentration of
hydrophobic monomers.60
The significantly higher µTBS results of SBMP and FLS System to dentin, is
attributed to the hydrophobic overlaying adhesive layer. It appears that the hydrophobic
layer is important for the development of a strong and resistant bond to dentin. It is also
likely just as important for enamel bonding. Even though the FLS System adhesive had a
hydrophobic layer, it did not have a high bond strength result to enamel because of the
inadequate enamel etching.
93
4.2.5 Effect of the self-adhesive cement composition on microleakage and
bond strength
It has been suggested that using the glass-ionomer under resin composites can
compensate for the polymerization shrinkage of resin composites and enhance the
microleakage seal in Class II cavities. A minimal leakage was obtained using this
technique; however, the durability of the glass-ionomer at the gingival margin below the
CEJ was questionable.39, 125
The self-adhesive resin cements are composed of polyfunctional dimethacrylate-
based monomers, such as BIS-GMA and/or urethane dimethacrylate, and inorganic filler
of glass and silica (Figures 31- 33). Therefore, their composition is analogous to that of
resin composite restoratives but with a lower filler loading. Due to the resinous content,
self-adhesive resin cements are expected to show lower solubility in the oral
environment, despite the fact that they lose some of their contents when exposed to
water.113
RXU and BRZ, the self-adhesive resin cements, showed a relatively small
degree of leakage in dentin compared to the control SBMP adhesive. Upon contact with
the tooth surface, the negatively charged phosphoric acid groups of the methacrylate
monomers, which are contained in the RXU organic-matrix, bond to Ca2+
ions in the
tooth structure. Subsequently, the phosphoric acid groups are neutralized by the water
group and anchored at the tooth surface. The presence of multifunctional phosphoric
acid-modified methacrylate monomer seems to enable RXU to self-adhere to both
enamel and dentin, resulting in an effective tooth-cement interfacial seal. This reason in
particular and the low modulus of elasticity 119, 120
of RXU may explain its low
microleakage results. Other studies evaluated the microleakage of RXU using full
94
ceramic crowns, similar results were found when RXU was compared to conventional
total-etch luting cement.118, 119
More microleakage was observed at the enamel interface than at dentin interface
for the RXU and BRZ groups. This may suggest their insufficient ability to etch the
enamel tissue, and thus, the lack of formation of adequate micromechanical retention.
Pre-treating of the enamel surface with phosphoric acid-etching prior to the RXU
application has been proven to improve the enamel-cement bond strength.110, 111
Similarly, BRZ seems to perform better in microleakage on dentin than on enamel;
however, the differences were not statistically significant. The BIS-GMA-based resin
matrix and filler particles content of BRZ, may explain its low microleakage results and
acceptable marginal seal. The question as to what extent the acidity and/or the filler
content would influence the µTBS of RXU and BRZ cannot be answered based on their
microleakage results.
Further, the simplified self-etch ED primer II of the PAN system has hydrophilic
monomers (HEMA) which permit the transfusion of the dentinal tubule fluid into the
bonding area, and then affect the marginal seal of the tooth-cement interface.176
In
addition, the acid monomers remaining in the primer may possibly inhibit the chemical
curing of PAN.183
Sano et al 56
concluded that the impaired infiltration of the dental hard
tissue by the resin cement and the insufficient polymerization of the adhesive resin may
lead to water diffusion at the interfacial aspect of the hybrid layer. This might explain the
high microleakage results and the relatively low bond strength obtained by PAN.
MON was associated with the greatest microleakage at both dentin and enamel
sides, and showed premature failure of all specimens during bond strength testing.
95
Although the basic adhesion mechanism appears similar for all self-adhesive cements,
these materials are still relatively new, and detailed information on their composition and
adhesive properties is very limited. These poor results of MON cannot be explained by
the above factors of polymerization shrinkage, pH, elasticity, and layer thickness. The
composition of MON, most likely, plays a significant role in its poor results in the
current study.
4.2.6 Failure modes of µTBS test
The current study discussed the trend of failure modes of the tested specimens.
With regard to the total-etch adhesive system (SBMP), the failure modes at the dentin
bonding surface were mostly adhesive (58%), while cohesive failure occurred in 42% of
specimens, with no mixed failures. In contrast, at the enamel bonding surface the failure
modes were fairly distributed as 42% adhesive failure, 25% cohesive failure and 33%
mixed failure. It was not possible to identify the adhesive bond failure location under the
light-microscope for the SBMP group; however, SEM photographs demonstrated the
failure between the dentin and the adhesive (Figures 36 and 37).
Of interest was the fact that the highest percentage of composite-cohesive failures
(50 %) was noticed when the composite was bonded with the FLS System adhesive
system to the dentin surface; this is in match with the high µTBS results of FLS System
(19.1±8.7 MPa). Piwowarczyk 133
came to the conclusion that higher bond-strength
values increase cohesive failure rates, which accounts for the failure behavior of FLS
System in the present study. Cohesive failure indicates better bond integrity however; it
does not occur clinically rather, it is very much related to the test method. An adhesive
96
failure of FLS System, when bonded to dentin substrate, was photographed utilizing
SEM (Figure 34). The FLS System adhesive generally showed a trend of adhesive failure
modes (75%) at the enamel bonding side with adhesive remnants on both substrate sides.
The failure modes of self-adhesive resin cements were predominantly adhesive in
nature, particularly at the interface between the tooth and the cement. Adhesive failures
of RXU, PAN and BRZ were photographed utilizing SEM (Figures 31-33). Interestingly,
PAN and RXU showed almost similar percentages of adhesive failure mode at dentin
bonding surface (75% and 66% respectively); however, RXU showed 16% of premature
failure. This mode of failure of these resin cements might explain their bond strength
performance; therefore, PAN showed significantly higher µTBS (11. 6(4.7) MPa)
compared to RXU (6.7(3.3) MPa). Similarly, when bonded to enamel, both PAN and
RXU showed 58% adhesive failure mode; however, RXU showed 25% premature failure
specimens associated with low µTBS (4.1(1.4) MPa), while PAN showed µTBS of
12.6(3.7) MPa. As well, BRZ showed 75% of adhesive interface failure and 25% of pre-
mature failure associated with low µTBS (4.0(1.9) MPa) when bonded to dentin. Further,
all specimens of the BRZ group when bonded to enamel surface and all specimens of the
MON group when bonded to enamel and dentin surfaces underwent failure prematurely.
The fact that one operator who followed a standardized method conducted
specimen preparations of the current study, strongly suggests that the reported premature
failures should be ascribed to less effective bonding and not to manipulation errors.
97
4.3 Summary
In conclusion, microleakage and microtensile bond strength are two individual
parameters with different indications and no direct correlation. Microleakage assessment
studies have provided better information regarding marginal seal, microgaps at the tooth-
restoration interface and postoperative hypersensitivity. Microleakage has also been
considered as a test of the integrity of the resin-impregnated hybrid layer.184
In
comparison to bond strength, it may be more affected by factors such as aging, as
evidenced by restorations in vivo which exhibit staining as a result of microleakage over
time. Bond strength studies have been conducted to screen the adhesives. The point
behind the bond strength testing method is that the stronger the adhesion between the
tooth and the adhesive, the better it will resist stresses imposed by resin shrinkage and
oral functions. Therefore, bond strength test provides better information regarding the
resistance to debonding and the need for additional retentive features in the preparation.
It would be prudent to look at both parameters in order to assess the bond interface.
It is an innovative new concept to use self-adhesive resin cements instead of
conventional bonding adhesives to bond direct composite restorations. Although
intermediate elastic layers such as liners have been advocated, the use of cements in the
current study is different because the cements are extended out to the margins. Therefore,
cement solubility, smoothness, and thickness become critical factors. This usage of self-
adhesive resin cements shares similarities to using glass-ionomer or flowable resin
composites at the gingival margins of posterior composite restorations. Glass-ionomers,
although advantageous when used as a base in posterior restorations; have high solubility
in aqueous oral environment and their durability at the gingival margin is questionable.
98
Flowable resin composites have higher polymerization shrinkage and microleakage at the
gingival margins, and therefore, have not been shown to benefit the restoration marginal
quality. The application of self-etch resin cements is relatively simple and less time
consuming. In addition, self-adhesive resin cements have high mechanical properties due
to their filler content. Self-adhesive resin cements have shown favorable performance
and durability in indirect applications, and they are considered the material of choice for
the cementation of indirect resin restorations, where the shrinkage is limited and the
marginal adaptation is improved.
The direct resin restorations have a more complicated bonding environment.
Application-related and material properties-related factors significantly influence the
long-term bond durability. Self-etch adhesives have demonstrated high microleakage due
to their failure to resist the contraction stresses of the composite restorations. The high
concentration of the hydrophilic monomers in the self-etch adhesives made their
durability highly questionable. Incompatibility, due to differences in product
manufacturers, might also affect bond formation.
The FLS System addresses two major concerns in direct composite restorations.
It has low polymerization shrinkage due its chemically modified resin structure. It
utilizes an adhesive with a high concentration of silorane hydrophobic monomers. This
increases the resistance of the FLS System bonded interface to water hydrolysis. The
FLS System appears to be a promising restorative material, however, further
investigation is recommended.
99
4.4 Clinical Significance of the Study
The gingival margin of posterior Class II resin composite restorations is the most
common location for recurrent decay, and it is important for dentists to maximize bond
integrity and marginal seal at this location. For the in vitro studies, dental composite
restoration should be applied to simulate the real clinical situation as closely as possible.
Composite restorations should be ideally subjected to the same stresses, temperature
fluctuation, pH changes, mastication forces, and oral fluid and enzymes that clinical
restoration is subjected to during function in the oral cavity.
The current study evaluated a novel bonding procedure using resin cements for
the placement of posterior composite restorations, as well as a new restorative resin
composite material. The self-adhesive resin cements have the advantages of being, less
technique sensitive and less time consuming, which make them user friendly during the
clinical application. Moreover, their low modulus of elasticity and the relatively low
acidity, compared to the conventional total-etch bonding adhesives; render them
advantageous in terms of reducing the polymerization shrinkage stresses at the tooth-
composite interface and the postoperative hypersensitivity. Based on the results of the
current study, RXU and BRZ showed low microleakage at the gingival margins, and
despite of their general low bond strength results, their use in subgingival Class II
cavities can be suggested.
In Class II cavities, the conventional total-etch adhesives have shown high
microleakage, postoperative sensitivity, and low durability. The FLS System reduces the
negative effect of the composite shrinkage stresses and the adhesive application
100
technique sensitivity, and therefore, shows a potential advantage of using it clinically.
Further investigation is recommended to ascertain whether the low bond strength to
enamel would affect the durability and the effectiveness of the FLS System in the oral
cavity.
4.5 Study Limitations
In vitro investigations provide important information when evaluating
biomaterials; however, they have limitations and do not replace clinical studies. The
current study has some limitations that should be considered.
1) Using natural teeth is always associated with difficulty in standardization. Teeth
sizes are variable and this affects the preparation design and dimensions. Dentin
is heterogeneous, and differences in dentin depth, permeability, degree of
mineralization and tubule orientation would affect both microleakage and
microtensile bond results, Dentin variability would be further increased by
experimental conditions such as storage, which could affect its physical and
mechanical properties.
2) Standardization of composite increment thickness during preparation restoration
was difficult. As well, the composite build up over the flat tooth surface in the
µTBS testing was also difficult to standardize. However, all possible effort was
made to produce a standardized specimen.
3) In the microleakage test technique, the two-dimensional dye microleakage
assessment reflected a three-dimensional microleakage phenomenon.
101
4) In the µTBS test technique, a dumbbell-shaped specimen with a rounded or
cylindrical configuration in the bonded interface result in better stress distribution
than a bonded interface with sharp corners. A slight narrowing toward the testing
bonded interface uniformly concentrates and distributes stress in the region of
interest.130
1 mm2 rectangular rods with sharp corners were prepared in the
current study; the dumbbell-shape trimming machine could not be afforded
because of the budget limitations.
4.6 Future Studies
According to the current study findings, the following areas might need further
investigation:
The nature of bond between the tooth structure and the different adhesives.
The morphology and the composition of the hybrid layer, which is formed by
the self-adhesive resin cements and the self-etch Filtek LS System adhesive.
The mechanical and physical properties of the self-adhesive resin-cements,
including durability and solubility.
The mechanical and physical properties of Filtek LS System, the
polymerization shrinkage and the degree of conversion of the resin
composite.
The effect of mechanical load cycling on the marginal integrity and the
microleakage of the Filtek LS System.
102
4.7 Conclusions
Within the limits of this in-vitro study, it can be concluded that:
1) FLS System, RXU and BRZ improved the marginal seal at the tooth-composite
interface compared to the conventional resin composite with total-etch adhesives.
2) The self-adhesive resin cements cannot be considered as a consistent group of
cements. Using self-adhesive resin cements as bonding agents in Class II
composite restorations showed different degrees of microleakage and bond
strengths depending upon the specific material used. Self-adhesive resin cements
may not be the ideal materials for bonding direct resin restoration where a
considerable enamel surface area is present.
3) RXU and BRZ self-adhesive resin cements showed low microleakage results.
Lower microleakage was detected more in dentin than in enamel. MON and PAN
resin cements showed high microleakage results compared to other cements.
4) Materials could be ranked based upon their microleakage performance at the
dentin side from the best to the worst as follows: FLS System, RXU, BRZ,
SBMP, PAN, and MON. While at the enamel side, materials could be ranked as
follows: FLS System, RXU, SBMP, BRZ, PAN, and MON.
5) FLS System had the highest bond strength (19.2(8.7) MPa) when bonded to
dentin, followed by SBMP (18.6(6.7) MPa), PAN (11.6(4.7) MPa), RXU
(6.7(3.3) MPa), and BRZ (4(1.9) MPa). While MON cement specimens failed
prematurely during preparation.
103
6) SBMP had the highest bond strength (24.6(6.1) MPa) when bonded to enamel,
followed by, PAN (12.1(3.9) MPa), FLS System (8.6(3.3) MPa), and RXU
(4.1(1.4) MPa). While BRZ and MON cements’ specimens failed prematurely
during preparation.
7) The FLS System was unique with respect to its resin composite composition as
well its adhesive system. The current study was not able to distinguish whether
the low-shrinkage aspect of the composite or a superior bonding ability of the
adhesive played a primary role in the results. Further investigation should be
performed to study the properties of the resin composite material and the
proprietary adhesive system.
104
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