Current trends in clinical restorative dentistry include an emphasis on self-etching adhesive systems. These systems were introduced in the late 1980s and have been further developed during the ensuing 20 years. This paper discusses scientific principles of self-etch adhesive systems, clinical considerations when using these systems, and provides an illustration of the clinical technique using an available self-etch system.
BACKGROUND
As compared to the earlier total-etch systems, the self-etching systems have enjoyed in-creased popularity. Total-etch systems use phosphoric acid as a surface conditioner. The acid is applied simultaneously to enamel and dentin prior to application of ambiphillic primers and adhesives, either in separate steps or combined in one mixture. Phosphoric acid demineralizes the hydroxyapatite in enamel, leaving a distinctive honeycomb-etch pattern, and removes the smear layer and demineralizes dentin to varying depths while opening dentinal tubules and exposing organic collagen fibers.1-3 Failure to support this collagen filigree with a proper level of moisture (ie, “wet-bonding”) and failure to fill the demineralized zone of the inorganic dentin completely with adhesive can adversely affect the resulting restoration. Excessive drying of the dentin can lead to collagen fibril collapse, which creates a variably impermeable layer that may prevent the diffusion of acrylic monomers into the demineralized layer.1
Furthermore, this incomplete diffusion of monomers results in incomplete sealing of the dentinal tubules that were opened during the etching process.1,3 Open but unsealed dentinal tubules permit dentinal fluid flow during thermal and tactile stresses, which in turn stimulates nerve fibers in the pulpal tissue. This stimulation is thought to be the primary source of postoperative sensitivity, and the patient experiences it when eating or drinking hot or cold foods or when simply biting on the restored tooth.4
Postoperative sensitivity of this nature is quite common and a source of patient complaints.
The newest adhesive systems are described as self-etching, since they employ the use of acidic monomers of acrylic that etch both enamel and dentin while simultaneously depositing resin into the demineralized zone. The primary advantage of self-etching systems is that they avoid the vulnerable period that occurs after traditional phosphoric acid etchant is rinsed from the tooth but prior to the subsequent application of acrylic monomers. At this time, exposed collagen must be supported by moisture.5,6 With self-etching systems, the acidic component is not rinsed from the tooth, thus eliminating this vulnerable period of collagen fibril collapse. Another advantage of these systems is that complete permeation of the acrylic monomers to the depth of the demineralized zone is guaranteed.5 With traditional total-etch systems, it is clinically possible to etch deeper than the subsequent primer/adhesive can penetrate, leaving a zone of unsupported, demineralized dentin that can weaken the adhesive layer and leave it vulnerable to hydrolysis and premature degradation.7 This phenomenon does not occur with self-etch systems, possibly enhancing their stability over time.7
TOOTH PREPARATION: ENAMEL
Although all modern self-etching primer/adhesives contain acidic monomers of varying concentration and pH, variability in the depth of resin tag formation in enamel exists.9-11 Studies have demonstrated that the acidic component of self-etching systems does not create enamel etch patterns on intact enamel seen with traditional phosphoric acid.12-16 Therefore some have questioned the long-term durability of these bonds in clinical situations.17
Various pretreatments of enamel surfaces have been suggested for self-etching systems,18-20 and several manufacturers recommend not applying self-etching adhesives to intact or nonabraded enamel.21 Intact enamel often demonstrates a low surface energy, aprismatic layer that is not conducive to sufficient mechanical etching by most self-etching systems.22,23 There is debate concerning how this instrumentation should be completed (air abrasion, use of a diamond bur, etc). Studies have shown that coarse diamond abrasion is sufficient to remove the aprismatic enamel layer and expose the enamel prisms to create micromechanical undercuts.24,25 This is dependent on the angle of the enamel prisms at the cavosurface margin, and practitioners should be mindful of this angulation when preparing teeth.
Milicich and others have studied typical posterior class I and class II preparations and determined that the use of beveling increases as the cavosurface margin moves closer to the cusp tip. This is based on the inclination of the enamel rods.26,27 Therefore, when enamel cavosurface margins are within the middle one third of the tooth, simple coarse diamond roughening without a bevel is sufficient. In larger preparations greater than one third of the diameter of the tooth where the enamel cavosurface approaches the terminus of a cuspal incline, deeper bevels are best accomplished by selective, round diamond burs. This type of beveling should be deep enough to permit an appropriate thickness of adhesive and composite, and not so shallow as to leave a thin, friable layer that is prone to chipping26,27 (Figures 1a to 1c).
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| Figures 1a to 1c. Because of the natural angulation of the enamel prisms that occurs when the preparation nears the cusp tip, deep enamel beveling is indicated. Coarse diamond burs expose the enamel prisms and create a surface suitable for self-etching systems. | |
In class V preparations, when all margins of the preparation are in a sufficient depth of enamel, a deep, long enamel bevel ranging from 1 to 2 mm should be prepared to permit the acidic component of the self-etching adhesive to come in contact with the abraded ends of enamel prisms rather than the sides of the enamel rods normally exposed with traditional amalgam-type preparations.29 The exact role of mechanical retention in class V preparations that are restored with self-etching systems is not well defined. It is reasonable to conclude that the use of selective micromechanical grooves and undercuts could have benefit, particularly in class V preparations where occlusal stress may be present and abfraction-type lesions are present.30-32
Several technical issues regarding the use of self-etching adhesive systems in class V restorations are addressed in the literature. The common occurrence of sclerotic or hypermineralized dentin, the lack of adequate cervical enamel, poor bond strengths of resin to exposed cementum margins, the role of occlusal stress, and the difficulty in creating and maintaining a dry operating field all support the potential benefit of incorporating mechanical retention into the preparation.28-36 It can be suggested that it is important to gain proper isolation of the defect via a rubber dam or atraumatic subgingival cord retraction. The use of an amalgam-type box preparation, with circumferential undercuts to include a gingival trough that is not prepared at the expense of any remaining cervical enamel, is also advised.29-36 When sclerotic dentin is present, a protocol is outlined in this paper (see Clinical Technique). For anterior teeth requiring class III or class IV restorations, a long, undulating enamel bevel extending at least 1 to 2 mm or beyond the cavosurface is suggested.28
TOOTH PREPARATION: DENTIN
The type of bur used to prepare dentin may affect the clinical performance of some self-etching adhesive systems. The advantages of using diamond burs to roughen enamel and create bevels in certain situations have been discussed. In contrast, it is im-portant to note that using a diamond bur to roughen healthy, nonsclerotic, noncaries-affected dentin provides no distinct advantage and possibly creates a problem with some self-etching systems.37 Studies have found that coarse diamond burs leave relatively thick smear layers as compared to carbide burs.38,39 While these smear layers are easily removed with total-etch systems, self-etching systems have difficulty penetrating thick smear layers, and consequently bond strengths will be diminished.37 Therefore, it is advisable to use traditional carbide burs to prepare dentin surfaces, while the relatively coarse diamonds are used to prepare bevels in enamel.
While the use of mechanical undercuts for retention has not been extensively studied in relation to any adhesive system, it can be argued that undercuts may provide benefit in low retentive preparations under high flexural stress. Clinical presentations such as class V restorations and certain interproximal “box” preparations where the restorations are under direct occlusal load and there is minimal surface area for bonding may be indications for mechanical undercut retention (Figures 2 and 3).
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| Figures 2 and 3. Internal retentive grooves may prove to be prudent additions to “slot-style” preparations where bonding surface is minimal and the occlusal load is high. In cases of abfraction where oblique forces lead to notches in the cervical area, long bevels and internal undercuts may improve retention. | |
Mechanical retention should be used sparingly and not at the expense of the axial wall or if it will compromise the strength of the remaining buttressing tooth structure. It is well accepted that modern adhesive restorative materials do not require mechanical undercuts as a primary source of adhesion.40,41 However, in certain high-stress-bearing areas, such as proximal box preparations, a less-than-favor-able enamel prism and dentinal tubule orientation may be present, and bond strength alone may not be adequate to withstand occlusal and functional forces.27,42,43 In such areas of high stress/low adhesion, judiciously placed retentive grooves could be beneficial.
THE DECISION TO ETCH
Some manufacturers of self-etching adhesives have proposed that an additional phos-phoric acid-etching step is needed when enamel is not instrumented.21 Studies have demonstrated that enamel bonds with some self-etching adhesives are improved by etching of the enamel with phosphoric acid.44-47 The honeycomb-etch pattern observed in enamel after phosphoric acid conditioning is more pronounced as compared to the pattern seen with self-etching systems. However, immediate shear bond strengths are comparable.8,47-49 Aside from bonding orthodontic brackets, repairing minor fractures or caries limited to enamel, and limited closure of a diastema, it is rare that general dentists will use self-etching systems on unprepared enamel. With only slight modifications to preparation design as outlined above when using self-etching adhesive systems, the cited studies do not support the need for an additional phosphoric acid-etching step for enamel.
Further, it is important to note that at least 2 potentially negative consequences to the routine use of phosphoric acid etching are present in conjunction with self-etching systems. The first and most obvious disadvantage is that by adding the phosphoric acid-etching step, the timesaving benefit of self-etching systems is eliminated.21 Adding a separate etching step with phosphoric acid, considering the time needed for rinsing and drying and then the subsequent self-etching primer/adhesive application, is equivalent to the number of steps needed for currently available total-etch systems. The second negative aspect of phos-phoric acid etching of enamel is that it can be difficult to prevent the phosphoric acid from coming in contact with dentin.
Some studies indicate that phosphoric acid etching of dentin for 15 seconds, either deliberately or by accidental contact while etching enamel, removes the smear layer and can demineralize hydroxyapatite beyond the point that can be penetrated by a self-etching primer/adhesive.1,50,51 The area void of resin infiltration between intact dentin and the hybrid layer is a weak link in the bond-ing system, and significantly reduces shear bond strength and tensile bond strength.51
Others suggest that this demineralized but unfilled area may prove to be the source of leakage as a result of the entrapment of moisture naturally present in dentin below this void area.52-54 The mechanism considered to be responsible for the destruction of resin/tooth bonds is the slow hydrolysis of the polymerized resin matrix.52 Therefore, it is not advisable to allow phosphoric acid to come in contact with dentin, either deliberately or accidentally, when using self-etching adhesive systems.
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| Figure 4. While self-etching adhesives work well on normal dentin, many manufacturers recommend a separate etching step when sclerotic dentin is present. |
SCLEROTIC DENTIN: SPECIAL CONSIDERATIONS
Whereas phosphoric acid conditioning of normal dentin is contraindicated with self-etching systems, clinicians are aware that unaffected dentin is not always present in many clinical situations. Sclerotic or hypermineralized dentin poses an obstacle for most self-etching systems and is commonly found under older, existing restorations.28 It is visibly identified as glassy in appearance, with little to no surface roughness, and is often stained by earlier carious activity or a long-standing, overlying amalgam restoration (Figure 4). Due to the high mineral content of this type of dentin, some studies recommend applying phosphoric acid to these affected dentin areas for 15 seconds and then rinsing prior to application of a self-etching system. This will help to ensure mechanical adaptation.46,55 It was suggested that hypermineralized or sclerotic dentin should be abraded with a diamond bur. While the relative amount of unaffected dentin in the area is an important determinant here, it should be considered that bond strengths of self-etching adhesives to sclerotic dentin that has not been etched with phosphoric acid or abraded are low.
CLINICAL TECHNIQUE
There are a number of self-etching adhesive systems available. For case illustration, i-Bond (Heraeus Kulzer) will be used as an example. It is important to emphasize that the manufacturer’s instructions for each specific system should be followed.
The use of i-Bond self-etching adhesive is relatively straightforward and involves just a few steps. However, attention to how and where it is applied and its dwell time on the prepared surfaces prior to curing must be closely followed to ensure optimum results.
The product is dispensed in either the unit-dose compule or a multiuse bottle. Both must be shaken prior to being dispensed. Phase separation with any primer/adhesive combination is possible, and shaking the compule or bottle just a few times allows the components to blend to the desired mixture prior to use56 (Figure 5).
Once the enamel and dentin surfaces have been prepared as previously outlined, the shaken compule is opened or the bottle is used to dispense the adhesive into a disposable dish. As with all adhesives containing volatile solvents, it is important to cap the bottle immediately to prevent the organic acetone solvent from evaporating and ambient moisture from entering.57
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| Figure 5. Heraeus Kulzer’s i-Bond must be shaken prior to being dispensed to prevent phase separation and ensure adequate mixture of the components. | Figure 6. With the unit dose, after mixing, the twist cap is removed, the capsule placed into the holding stand, and the microbrush is dipped and agitated. |
The microbrush that is provided is placed into the compule or well with an agitating motion to further stir the mixture, and a copious amount of adhesive is applied to the enamel surface (Figure 6). It is extremely important to extend the coat of i-Bond to—and slightly beyond—the diamond abraded enamel surface. Failure to extend a self-etching system properly onto the unprepared enamel surface and completely cover the prepared enamel surface may lead to an inadequate marginal seal and subsequent marginal staining (Figure 7).
After several passes over the enamel cavosurface, the microbrush is returned to the compule or well, and the brush is again saturated. A copious amount of adhesive is applied to the internal dentin aspects of the preparation to saturate the dentin walls and floor thoroughly (Figure 8). It has been suggested that agitation or scrubbing the microbrush onto the dentin surface may aid in the penetration of the acidic monomers through the smear layer and into dentin.58
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| Figure 7. When applying self-etching adhesives, it is important to saturate beyond the bevel at the cavosurface margin. Failure to do this may result in an incomplete seal and subsequent marginal staining. | Figure 8. After coating the margins of the preparation with adhesive, the brush is again dipped, and the internal cavity is fully saturated. Three consecutive coats are required. Slight scrubbing during this step may permit greater saturation. Allow a dwell time of 30 seconds (as specified by the manufacturer). |
Three consecutive coats of i-Bond are applied to all tooth surfaces to ensure a total dwell time of at least 30 seconds. This is extremely important.56 As with any dentin bonding system, whether total etch or self-etch, adequate time is required for the adhesive to fully penetrate the collagen filigree, pass into the dentin tubules, and reach micromechanical undercuts created by the etching. Fail-ure to allow sufficient dwell time is a common mistake. However, it could be one of the most important causes of adhesive failure and postoperative sensitivity.1 The complex chemical reactions that must run to completion, particularly with self-etching systems, take time, and practitioners should pay particular attention to the recommended dwell time.
No rinsing, drying, or curing between layers is recommended; i-Bond is a single-bottle adhesive, so there are no components to be mixed and no separate primer to be applied. This single product is used for the entire process.56
After a dwell time of 30 seconds, the remaining volatile acetone solvent must be evaporated. Failure to eliminate the solvent completely in any adhesive system adversely affects the bonding chemistry.56,58 With i-Bond, a gentle air stream (not a blast of air, but a gentle, continuous air stream) is passed over the adhesive layer. This air stream is continued for several seconds while the adhesive’s behavior is observed. Gentle rippling of the adhesive layer under the stream of air means that solvent is still present. Air is continually applied until rippling is no longer present and the adhesive appears still on the tooth surface. Depending on the thickness of the layers applied, this may take from 5 to 10 seconds56 (Figure 9).
Once the solvent has been volatilized, the adhesive is then light cured for 20 seconds if using a halogen light (Figure 10). The halogen light output must be monitored, and it is recommended to check the output routinely of all halogen lights with a curing radiometer on a monthly basis. The light-cure catalyst in i-Bond is camphorquinone (CQ) and therefore is also compatible with most plasma arc and LED curing lights. If these types of lights are used, curing time may be reduced after consulting the manufacturer.56,59
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| Figure 9. The solvent in all adhesives must be completely volatilized. With i-Bond, a gentle, consistent air stream is passed over the adhesive until all rippling has stopped. This indicates that the adhesive is ready to cure. | Figure 10. Light-cure the adhesive for 20 seconds with a halogen light. |
Once the self-etching adhesive is light cured, complete dentin tubule penetration and seal should be achieved, and approximately a 5 µm hybrid zone will be established. The dentin and enamel surface is then ready to receive a direct composite restoration.39 The completed posterior restoration using Venus composite resin (Heraeus Kulzer) is seen in Figure 11.
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| Figure 11. The completed posterior direct composite restoration. |
Another clinical case is demonstrated in Figures 12 to 16. Figure 12 shows a class IV composite that is stained and in need of replacement. Figure 13 illustrates the enamel bevel created with a coarse, flame-shaped diamond bur extending 3 to 4 mm beyond the cavity preparation. Figure 14 shows the application of i-Bond (3 consecutive coats) after shaking the unit-dose compule. Note that the adhesive is extended well onto and slightly beyond the diamond abraded surface to ensure a proper marginal seal. A 3-layered, 3-opacity direct composite restoration (Venus, Heraeus Kulzer, shades A2 Opaque, A2 Body, T1) is completed with internal brown and white Effect Color Tints (Heraeus Kulzer, Figures 15). The completed restoration is shown in Figure 16.
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| Figure 12. Preoperative view of a failing class IV composite to be replaced on tooth No. 9. | Figure 13. A 2- to 3-mm bevel was extended past the existing margins to permit roughening of the enamel sufficient to be etched by the self-etching system. |
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| Figure 14. Self-etch adhesive is applied with 3 consecutive coats to the prepared surface and slightly beyond. | |
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| Figure 15. Composite is applied in 3 opacities and shades along with color tints. | Figure 16. The computerized polychromatic, internally characterized restoration. |
CONCLUSION
Demand for simplified systems that avoid some of the complexities of total-etch systems has been answered by self-etching adhesive systems. By eliminating the separate etching step, certain vagaries of this step are no longer present, and the fear of collagen fibril collapse has been eliminated. With a simplified, 1-bottle system, the time needed to place an adhesive and the problem of postoperative sensitivity have been reduced. With proper enamel and dentin preparation, high bond strengths can be achieved; with proper placement of the adhesive well onto the prepared cavosurface margins, aesthetic restorations with optimum marginal seal can be realized.
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