In the past decades, dentistry (and aesthetics in particular)
has undergone an evolution. Innovative dental materials have led to the
development of contemporary restorative systems and preparation designs that
require different cementation materials and methods. Conventional cements,
with acid-base reactions, are adequate for restorations that necessitate
mechanical and macromechanical retention and limited aesthetic expectations.
Resin cements, with polymerization reactions, have been formulated as
alternatives to conventional cements. Associated with the conditioning of
substrates, the use of resin cements is primarily based on micromechanical
interlocking and intended for aesthetic results. This allows the use of less
invasive preparations, as in partial metal-free restorations, since bond
strength is less dependent on the mechanical retention of the preparation
design and relies more on adhesion, which reaches values of 30 MPa to 40 MPa.
This bond strength is essential for porcelain laminate veneers and ceramic
inlays and onlays, where conventional cementation is contraindicated.
Considering the respective limitations of conventional and resin cements,
resin-modified glass-ionomer cements have been created in order to combine the
advantages of glass-ionomer cements and resin cements.
Full-coverage crowns, too, have acquired different features.
Prior to the introduction of adhesive luting agents and metal-free
restorations, conventional metal-ceramic crowns were almost the only option to
achieve aesthetics in anterior and posterior regions. These restorations
traditionally featured a metal collar at the cervical margin. Despite the use
of different preparation designs, the presence of this metal substructure has
somewhat limited natural light transmission in the cervical area.
The development of dental materials for all-ceramic
restorations has allowed clinicians to successfully treat many anterior cases,
yet there are some clinical situations where a metal substructure is still
discolored dentin substrates.
cast posts and cores.
all fixed partial dentures or splinted restorations.
with parafunctional habits.
The presence of metallic posts and cores has often been an
aesthetic concern in endodontically treated teeth, and root fractures have been
attributed to the corrosive by-products of these posts (Figures 1-2-3-4-5).1
Innovative materials (eg, zirconium and glass fiber posts) have been introduced
for use with either cast posts or posts and cores. While fiber posts have many mechanical
advantages due to their modulus of elasticity, they are used exclusively for
direct post and core placement. Zirconium posts can be layered with a pressed
ceramic core or a plastic material such as composite resin core for a direct
technique. Advantages of these modern posts are their aesthetic substrates and
ability to eliminate corrosive products that damage the root surfaces. Since
these posts are bonded to root dentin, less microleakage is also expected with
these post systems. Ideally, intraradicular cementation is performed with a
self-polymerizing resin cement.2
The treatment of the tooth substrate must also be
considered. While partial restorations rely on adhesion to enamel surfaces,
full-coverage crowns are cemented primarily to dentin surfaces. Enamel or
cementum may be present at the margins, but full-coverage crown preparations
will result in the exposure of more dentin tubules. Achieving an adequate seal
of the latter will help clinicians reduce postoperative sensitivity by minimizing
bacterial invasion and improving the biocompatibility of the restorations.
Penetration of primer and adhesive inside the collagen network seals dentin
tubules and creates the “hybrid layer.” While adhesion to dentin has been
studied for many years, it remains a challenging subject to researchers.
New techniques (eg, electroforming and capillarization) for
the laboratory fabrication of cast-free metallic frameworks have introduced
benefits that include pure gold copings with reduced thickness (0.2 mm) and
warm metallic shades.3,4 These characteristics allow natural results
to be achieved for metal-ceramic crowns, particularly when there is a need to
mask altered substrates. Another important feature of modern metal-ceramic
crowns is the vertical reduction of the metal framework (Figures 6-7-8-9),
which results in full-ceramic margins in the cervical region. When compared to
conventional metal-ceramic crowns, this reduced substructure — used in
conjunction with fluorescent margin ceramics — is advantageous due to improved
light transmission in the cervical third. While there was no aesthetic
interference of the cement in conventional metal-ceramic crowns, light
transmission with modern metal-ceramic frameworks, as for all-ceramic crowns,
dramatically changes the aesthetic requirements for dental cements.
When one considers that both all-ceramic crowns and advanced
metal-ceramic crowns have all-ceramic margins that contribute to the aesthetic
result, it is obvious that zinc-phosphate cements are obsolete in modern
aesthetic dentistry. This raises one important question: which cements are
suitable for cementation of contemporary porcelain crowns?
From a clinical perspective, successful cementation has the
following requirements (Table A):
compatible with the restorative material.
of postoperative sensitivity.
adhesion and an adequate dentin seal.
Table A: Clinical
Sequence for Crown Cementation
session) Complete crown preparation and reline provisional crown.
session) Before taking impression, condition tooth surface and hybridize
session) Properly treat internal crown surface (See Table C).
session) Apply luting agent to internal crown surface and to preparation
session) Insert crown with occlusal pressure.
session) Remove excess luting agent prior to final setting.
session) Apply glycerin gel to cervical margins of the crown
session) Remove excess of luting agent with a scalpel after its setting
Full-coverage crown preparations exhibit some characteristics
that can result in postoperative sensitivity, as the surface area of dentin
tubules exposed is much greater than those exposed in partial preparations.
Moreover, metal-free feldspathic ceramic-based restorations can also require
greater depth of preparation to achieve proper strength. Manufacturers,
however, are developing aluminum oxide and zirconia ceramic substructures to
achieve stronger mechanical resistance with less invasive (0.4 mm)
The first step for a biologically successful cementation is
to eliminate or reduce postoperative sensitivity. Once the preparation is
performed, pulp tissue is exposed, and a biological wound is created. Open
tubules allow the penetration of bacteria and other factors (eg, chemical
substances) that irritate the exposed pulp. Adequate chemical treatment of this
wound is required to minimize penetration of bacteria into dentin tubules and
disinfect them. This can be obtained by application of a 0.1% to 0.2%
Once the prepared tooth is disinfected, its surface must be
treated. At present, each prepared dentin surface should receive a seal to
prevent fluid movements and postoperative sensitivity. Since bonding became an
important step in modern dentistry, numerous investigations have been conducted
to improve comprehension of the involved processes and to more effectively
treat the surfaces to be bonded. Two surfaces must be considered: tooth and
restoration (Tables B and C). Enhancing mechanical retention will also improve
cementation and requires 1) the creation of microretentions on bonded surfaces
to augment surface energy and 2) bonding agents capable of penetrating even the
thinnest microretentions created.
Table B: Tooth
surface with chlorhexidine solution
rebasing provisional crown and before taking the impression, etch tooth
surface with 37% phosphoric acid for 10 to 15 seconds.
rinse to remove acid.
chlorhexidine solution and remove excessive moisture.
adhesive system of choice.
steps 1 and 3; reapply a thin layer of primer in case sensitivity is
present prior to final cementation.
(Continued from page 1 )
While bonding to enamel has been well established,5
dentin treatment is more challenging. Adhesion to dentin can be obtained
chemically, but it is primarily accomplished by mechanical means. Interlocking
will be obtained by resin penetration into dentin, and better surface
preparation results in more effective mechanical retention. Three diameters of
dentin tubules are essentially encountered clinically6:
branches: diameter ranges from 0.5 µm to 1 µm and localized peripherally.
branches: diameter of 0.3 µm to 0.7 µm and primarily localized at root
dentin or at coronal dentin where tubule/intertubular ratio is low.
branches: diameter ranges from 50 nm to 100 nm and located anywhere in
dentin but primarily at the crown.
Demineralization of dentin allows the penetration of resin
by widening these tubules and exposing the collagen matrix. Collagen itself is
not a very reactive surface,7,8 so adhesion is primarily obtained by
mechanical retention rather than chemical bonding.7,9
Macromechanical retention is formed by intratubular resin penetration, while
micromechanical retention is obtained by intertubular retention. The presence
of a smear layer will harm this interlock by creating a weaker link, so its
removal by etching is essential.7,8,10 This can be achieved through
the 10- to 15-second application of a 37% phosphoric acid that must be rinsed
thoroughly for complete removal. Once the smear layer is removed and the
surface is conditioned, resin will penetrate into widened tubules and the
intertubular dentin, which constitutes the hybrid layer.
Resin penetration is dependent on wettability of the
substrates and viscosity of adhesive.9 Enamel can be completely
dried to receive the resin, but dentin must be moist in order to avoid collagen
collapse and to promote hybridization; desiccation of dentin will harm the
formation of the hybrid layer. Collapsed collagen can prevent resin to flow into
these tubules.6 Blot drying with absorbent paper will remove
excessive moisture without desiccation, and chlorhexidine 0.1% to 0.2% can be
reapplied with a microbrush. Use of primers to chemically remove excess fluids
from the etched dentin surfaces will transform a hydrophilic into a hydrophobic
surface. Therefore, most primers require an acetone or ethanol to volatize all
the surface moisture, which allows penetration of the monomers into tubules,
their branches, and the collagen network.
Table C: Restorative
Internal Surface – Surface Treatment
Metal -- Sandblasting (50 µm to 250 µm Al2O3)
Metal (Optional) -- Chemical retention system
porcelain; leucite reinforced porcelain; lithium disilicate -- 9.6% flouridic acid and silanization
alumina – Sandblasting only
Machined alumina – Alcohol, sandblasting, and silanization
Zirconia to improve
wettability only – Silanization only
Proper diffusion of this monomer and correct formation of a
hybrid layer protects the cut dentin and prevents the damage of the pulp
tissue. Independently of the system chosen, one can assess clinically that a
hybrid layer has been formed by the shiny appearance of the dentin surface and
lack of sensitivity. Multiple- and one-step systems have been formulated in an
attempt to simplify these clinical procedures. If adhesion is easily obtained
on enamel, however, the dentin surface may require multiple applications of a
one-bottle system to achieve hybridization and reduce postoperative sensitivity.3
Hybridization should be performed when preparations and provisional
restorations are completed and prior to impressions. Over time, the clinician
can verify that the hybrid layer was removed if vital teeth become sensitive.
In the authors’ opinion, this procedure should be repeated only if sensitivity
is present. This means that the hybrid layer has been disturbed, which can
occur, for instance, with multiple try-in sessions. Unnecessarily repeating the
hybridization procedure could result in a thick seal, however, and prevent full
seating of the crown. If there is a need to rehybridize, one should clean the
surface, re-etch in the same manner, and use a fourth-generation adhesive
system. It is critical to obtain a very thin hybrid layer at this stage, as
thickness will increase the vertical discrepancy of the crown.10
If restorative surfaces can have their wettability improved
via surface treatment, metal substructures have to be sandblasted with 50 µm to
250 µm aluminum oxide in order to clean and augment microretention.11
Chemical retention to metal has been introduced in many systems to enhance the
resin-to-metal bond. These systems are indicated primarily for veneering with
resin. As with dentin surfaces, however, improving the wettability of metal
surfaces to be bonded will also enhance the interlocking of cements or luting
agents and contribute to better interfaces and reliable cementation.11
In all-ceramic crown restorations (feldspathic,
leucite-reinforced, lithium disilicate, slip-cast aluminum oxide, densely
sintered aluminum oxide, or zirconia), there are many possible surface
treatments, which include sandblasting, etching, and silanization.12
Leucite-reinforced and lithium disilicate ceramic are ideally conditioned with
10% hydrofluoric acid.11 When possible, etching is the most
effective surface treatment for increasing surface area and energy.12
These changes will create greater wettability and consequently a better
interlock with a resinous luting agent. While sandblasting is an effective
means of cleaning surfaces to be bonded and augmenting surface energy, it has little
effect on micromechanical retention and can increase the potential of
iatrogenic damage to thin margins. Sandblasting with 50µm aluminum oxide is the
only option for treating the surfaces of certain crowns,13 which
cannot be etched or silanated. Similarly machined aluminum oxide and zirconia
require internal surface cleaning with ethanol and sandblasting with 50µm
aluminum oxide particles prior to cementation or luting.11
Silanes are responsible for bonding to ceramic surfaces by
covalent and hydrogen bond formation. The ceramic surface is thus transformed
into a hydrophobic surface that prevents hydrolytic degradation. When a luting
agent is applied to this surface, its wettability is increased, and it is
transformed again into an organophilic surface. For complete silanization, it
is important that multiple layers be applied. Microscopic undercuts associated
with silane application result in reduced tensile stresses at the adhesive
joint and consequently less debonding.14
As previously stated for tooth surfaces, one could conclude
that greater bond strength can be achieved through treatment of the internal
aspect of the crowns. This promotes micromechanical retention and a better
chemical reaction among different surfaces (eg, metal, ceramics, and bonding
agents). It was also proven that the resistance of all-ceramic crowns is
greater when both dentin and ceramic are treated prior to bonding.14
A proper marginal seal is also desirable. For full-coverage
crown preparations, the margins can be placed on enamel, dentin, or cementum.
Lack of dentin tubules (as in cementum margins and peripheral dentin) and
changes in dentin structure (as found in sclerotic dentin) can complicate this
seal. Therefore, one can expect to encounter seals of differing quality
depending on which structure the margin is placed. Following the removal of
excess cement, a protective glycerin gel is applied to the cervical margins in
order to reduce oxygen inhibition to polymerization or water sorption.
Minimum film thickness is desirable to ensure the integrity
of the cement margin and the conservation of occlusion after cementation.
Although resin cements and resin-modified glass-ionomer cements have greater
resistance to solubility when compared to conventional cements, abrasion from a
toothbrush will cause washout of these margins. Guzman et al stated that the
larger the marginal gap, the more abrasion from a toothbrush will occur.15
Over time, the process of tooth brushing will cause horizontal discrepancy that
results in bacteria penetration, recurrent caries, and periodontal
inflammation.16-18 It is important to preserve occlusal adjustments
obtained prior to cementation. If cement thickness interferes with final
occlusion, undesirable adjustments will be necessary on glazed or mechanically
polished ceramic surfaces. Film thickness depends on various factors that
include margin design/adaptation, the amount of die spacing, and the type of
bonding and/or luting agent as well as the cementation technique itself.
The marginal adaptation of dental materials, which are
constantly improved by manufacturers,19-21 should obviously be
minimal. For ideal precision, crown spacing should ideally range from 25 µm to
40 µm and closely match the thickness of the cement. It has also been stated
that the use of varying types of die spacers and their variable layers on dies
will result in statistically different internal reliefs.22 In the
authors’ opinion, the internal relief of crowns designed via computer-aided
design/computer-assisted machining can thus be expected to be most reliable
Depending on quantity and diameter of the filler content and
the type of solvent in the bonding agent, great film thickness and marginal
discrepancy can be expected.10 Despite the availability of formulas
with minute filler particles, resin cements have the thickest layer due to the
prepolymerization of these particles. Ideally, one should expect that greater
filler content without prepolymerization would reduce film thickness, improve
cement resistance, and thus reduce washout.23
Glass-ionomer cements might appear to be thick, but their
film thickness will be comparable to that of zinc phosphate when cementation
occurs under pressure (ie, thixotropy).24,25 One concern with
glass-ionomer cements is the possibility of crack formation under all-ceramic
restorations due to water sorption and hygroscopic expansion.26
Recent formulations of resin-modified glass-ionomer cements have a wide range
of available shades, less expansion, and essentially eliminated concerns
regarding crack formation. Although resin cements have higher film thickness
when compared to zinc-phosphate and resin-modified glass-ionomer cements, they
are the only clinical option when an opacious cement is necessary to mask
discolored substrates. A chairside mixture of resin cements and intensive
colors allows the clinician to create a customized opacious cement that is well
adapted to the range of discoloration.
The aesthetic characteristics of contemporary dental cements
are of greatest clinical importance. The range of opacity is certainly more
critical than the range of chroma. When used in thin layers, differences in
chroma can hardly be distinguished, but variations in opacity can help to
conceal discolored substrates and create a transition to the crown. Considering
that aesthetic requirements for modern crowns have been updated, similar
efforts must be made to enhance the optical properties of dental cements.
Radiolucency is a requirement for dental cements and
all-ceramic crown restorations in particular. Ideally, the radiopacity of
modern cements should be comparable to the dentin in order to facilitate
diagnosis for recurrent caries. This is achieved with resin-modified
glass-ionomer cements (Figures 19-20-21-22-23-24).
Contemporary cements now have an active role in final
aesthetic results and must respect the aforementioned parameters in order to
achieve this goal. From conventional zinc-phosphate cements to modern
resin-modified glass-ionomer cements, many changes have been introduced to
improve cementation techniques — mechanically and aesthetically. Adhesion,
hybridization, biomechanical properties, aesthetic colors, and range of opacity
are important clinical improvements that can benefit all-ceramic and advanced
metal-ceramic crowns. Nevertheless, additional clinical experience and
research are necessary in order to more precisely assess the respective
indications and limitations of modified glass-ionomer and resin cements. It
would be of significant clinical interest to develop new cements that possess a
chemical bond with high-strength ceramics (eg, alumina and zirconia) and a
range of opacity.
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