New Perspectives on Dentin Adhesion
Differing Methods of Bonding
Ivo Krejci, DDS, PhD • Minos Stavridakis, DDS, MS
In recent years, adhesion has become fundamental
to restorative dentistry. Minimally invasive restorations,1 amalgam
alternatives,2-4 veneers,5-10 metal-free crowns,11-13
slot fixed partial dentures,14 and even posts15-18 rely
on adhesion. Nevertheless, adhesion to the tooth surface is always in
opposition to the polymerization shrinkage of the composite material.19,20
The negative effects of polymerization shrinkage (eg, marginal gaps or marginal
enamel fractures) become particularly pronounced in restorative systems with a
large volume of the shrinking composite and a small free surface area.21-24 In such a situation, volume loss caused by
polymerization shrinkage of the composite can hardly be compensated for by the
flow of the material from the free surface during the gel phase of
Free surfaces may be
found in instances where the restorative material does not adhere to the tooth
(ie, on the outer surface of the restoration), and inside the cavity if no
adhesion between the restoration and tooth is present in this area (Figures 1 and 2). The ratio between the free and bonded restoration surface is called the
configuration factor or the "C-factor."27 A high C-factor
makes it difficult to establish optimal adhesion. Even if this objective can be
established initially, a high C-factor leads in these instances to a preloaded
restorative system that may be prone to gap formation during loading. It has
been demonstrated that total bonding, creating the highest possible C-factor in
a given cavity configuration, is not always the most appropriate bonding
By using the total bonding technique, adhesion
is established to the entire surface of the cavity. As a consequence, no free
surface areas are present within the cavity. This type of adhesion is achieved
through the use of an enamel/dentin bonding agent and the omission of a base.
If the adhesion is stronger than the polymerization shrinkage stress and/or the
stresses under function, the interface between restoration and tooth remains
perfectly sealed (Figure 3). In certain cavity configurations, however,
shrinkage stresses may become higher than the bond strengths -- even of the
most potent adhesive systems. This results in partial delamination of the
adhesive system from the tooth surface. If the delamination occurs in the
marginal region, marginal gaps and/or enamel fractures are the consequences
(Figures 4-5-6-7). Clinically, they appear as "white margins,"
marginal discolorations, fissures, or even recurrent caries (Figure 8). If the
delamination occurs within the cavity or dentin that is in communication with
the pulp, dentin tubules become unprotected, which results in postoperative
sensitivity (Figure 9). As a rule, total bonding is well tolerated by
restorative systems with a small volume of shrinking composite, with a low
C-factor, and when combined with potent adhesive systems.
confines the adhesion of the restoration exclusively to the margins of the
preparation. This is irrespective of whether the margins are located in enamel
or dentin. This does not mean that adhesion is limited to a width of mere
microns at the margin of the restoration. The thickness of occlusal enamel is
approximately 1.2 mm, and this is the area where adhesion is established
(Figure 10). Selective bonding creates free surfaces within the cavity, thus
reducing the C-factor of the restoration. One concern with the selective
bonding technique is that dentin within the cavity remains unprotected, and is
thus prone to postoperative sensitivity. To resolve this concern, a liner or
base has to be introduced into the system. While this component has to seal
dentin, it must not adhere to the restorative material. For this purpose, a
chemically cured glass-ionomer cement (GIC) base may be used.29 It
can seal dentin and, if used as a build-up base, may reduce the volume of the
cavity, thus reducing the amount of shrinking composite (Figure 11).
Nevertheless, chemically cured GICs do not seal dentin perfectly, and they are
difficult to handle due to their potential for rapid desiccation and cracking.
Therefore, a light-cured GIC, if used as a build-up base (and particularly a
dentin adhesive system) or a liner, may be more advantageous. To prevent these
materials from adhering to the restorative composite, a thin layer of an
insulating agent, such as a polyurethane isocyanate varnish may be applied over
them (Figure 12). Following the application of these materials into the cavity,
its margins have to be refinished to remove the varnish layer. Thereafter, a
new bond is established on the freshly cut tooth surface in the marginal area.
In this way, the gap formation is confined to the interior of the cavity, and
the margins are perfectly adopted (Figure 13).
Selective bonding may
improve sealing and marginal adaptation of large restorations with a high
C-factor and those restored with an incremental technique. In addition, a
distinctive advantage of the selective bonding is that it creates two
independent penetration barriers within the restorative system. The first
barrier is the margin of the restoration. In the event this barrier fails, a
second barrier against penetration - formed by the adhesive base or the
adhesive liner - is located inside of the cavity.
Separate bonding extends
the concept of selective bonding by one step. In this case, the sealing of the
cavity and the restoration are completely separated. The cavity is sealed by an
adhesive system that does not adhere to or is insulated against the restorative
material (Figure 14). In this manner, a microgap is formed in dentin and enamel
along the entire interface between the sealed cavity and the restoration
(Figures 15 and 16). This is particularly useful in amalgam substitutes,4
where one or two composite layers and a simple polymerization regimen are
postulated for the restoration of large, box-shaped cavities. Under these parameters,
a gap-free restoration with contemporary materials remains impossible.30
If, however, such a rapid and simple restorative procedure is required (eg, for
economical reasons), separate bonding brings the separation into an area where
it is not detrimental to the tooth (Figure 15).
A variation of the
separate bonding technique is the secure bonding technique, where adhesion
between the restorative material and the adhesive system is not completely
eliminated, but it is weaker than the adhesion between the adhesive system and
the tooth. In such a system, if the applied stresses exceed the bond strength,
a partial separation will occur in the uncritical area between the adhesive
layer and the restoration instead of the biologically critical tooth/adhesive
interface, thus minimizing the risk of secondary caries for the tooth.
Taking the previously mentioned principles into
consideration, several clinical indications may be formulated. For amalgam
substitutes,4 the separate bonding technique may be the procedure of
choice. The advantage of this technique is a well-protected tooth structure
despite a simple restorative technique in conventionally prepared, large,
box-shaped cavities. The disadvantage is the presence of a microgap between the
restorative material and the adhesive system. It may become discolored, thus
compromising aesthetics and complicating clinical diagnosis of secondary
caries.31 In addition, despite the application of an adhesive
system, the restoration is nonadhesive and, as a consequence, requires
macroretentive cavity preparation. This is in agreement with amalgam
substitutes, however, as they target box-shaped, macroretentive cavities.30
Total bonding may be
indicated in all restorations with a small volume and/or a low C-factor and/or
in need of a large adhesive surface for retention. This is the case for fissure
sealings, preventive fissure sealings, small Class I and III composite
restorations, Class IV restorations, wedge-shaped Class Vs, veneers, and large
flat onlays. In these indications, total bonding results in optimal marginal
adaptation, retention, and sealing. In addition, it is the simplest adhesive
technique to perform.
Selective bonding is the
procedure of choice for large Class I and III restorations and for Class II
composite fillings, inlays, and small onlays. It is also indicated for direct
pulpal capping with dentin adhesives,32 as it prevents the
detachment of the adhesive system from the pulpal opening caused by
polymerization shrinkage of the restorative composite (Figure 17). Since it
diminishes the internal stresses within the restorative system, it might also
be the most effective solution for the restoration of cracked teeth. Used in
combination with indirect restorations, selective bonding has the advantage of
sealing the cavity dentin during provisionalization, thus avoiding
postoperative sensitivity and bacterial penetration. This allows for the use of
soft, resin-based provisional materials.
The practitioner should be aware that total
bonding is not the only way of working with an adhesive system. According to
the clinical situation, the best bonding technique should be selected to
achieve the best combination of sealing, marginal adaptation, retention, and
*Professor and Chairman, Department of
Cariology, Endodontics, and Pediatric Dentistry, University
of Geneva, Geneva, Switzerland.
†Visiting Professor, Department of Cariology,
Endodontics, and Pediatric Dentistry, University
of Geneva, Geneva, Switzerland.
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