Self-Etching Resin Adhesives

Howard E. Strassler, DMD

Howard E. Strassler, DMD

The Holy Grail for adhesion to enamel and dentin has been described as being a single component, no-mix adhesive that can be applied directly to enamel and dentin for the purpose of bonding any restorative material to tooth structure. While this product does not yet exist, the manufacturers and researchers are hard at work developing and evaluating improved bonding systems. The introduction of self-etch adhesive systems has been an important step in achieving this goal of an all-in-one bonding agent.

The idea of adhesive bonding to dentin was postulated more than 50 years ago as involving a potential chemical bond between the methacrylate group of resins to the collagen surface of dentin.¹ In 1955, Buonocore described a clinical technique that used a diluted phosphoric acid to etch the enamel surface and provide for retention of unfilled, self-cured acrylic resins.² The resin would mechanically lock to the microscopically roughened enamel surface, forming small "tags" as it flowed into the 10-µm to 40-µm deep enamel microporosities and then polymerized. The first clinical use of this technique was the placement of sealants.³ The combination of acid-etching enamel and adhesive composite resin restorations afforded the benefits of reduction or elimination of microleakage at the enamel margins, less discoloration at the margins, lower rates of recurrent caries, and improved retention of the restoration.^4,5

The effectiveness and success of the etched enamel/resin bond has been demonstrated in many reported clinical trials.⁶ Unlike enamel bonding, dentin bonding has seen an evolution in its viability. Effective dentin-bonding materials should fulfill the following goals:

• The material should be retentive to dentin at a clinically acceptable level, and should be able to withstand intraoral forces of occlusion and mastication.
• The bond should be instantaneous once the material has set.
• The material and technique must be biocompatible.
• The material should resist the forces of polymerization shrinkage of composite resins and the coefficient of thermal expansion and contraction to eliminate microleakage.
• The material should create a long-lasting bond to dentin.
• Postoperative sensitivity must be minimized or eliminated.

In 1956, the earliest research with dentin bonding focused on chemical adhesion of resins to the inorganic components of dentin. Buonocore and coworkers developed a methacrylate-based dentin adhesive that contained phosphate groups to attach to the calcium ions on the dentin surface.⁷ The basis of the bond was the presence of the dentin smear layer.⁸ While a weak bond was created, unfortunately it was a clinically unacceptable bond to dentin. This basis of a phosphate-calcium bond later became the third-generation phosphate-ester bonding systems. These bonding systems, eg, the original Scotchbond^™ (3M ESPE, St. Paul, MN) and BondLite (Sybron Dental Specialties, Inc, Orange, CA) among others, bonded to the calcium-rich dentin smear layer and to etched enamel. Their bond strengths to dentin were limited by the bond of the smear layer to the dentin. Unfortunately, the durability of the bond was impacted by hydrolysis that occurred over time to the phosphate/calcium bond.^9,10 These products had limited success and the search for a better adhesive to dentin continued.

At the same time, another research path for dentin bonding investigated the use of a total-etch approach, etching the enamel and dentin simultaneously.^11,12 At the time, there was concern that phosphoric acid placed on dentin would cause pulpal inflammation and necrosis.¹³ Jennings and Ranly demonstrated that the pulpal effect of phosphoric acid on dentin for 1 minute was minimal.¹⁴ Early results reported on dentin etching were disappointing because the adhesive resin used was the same unfilled, hydrophobic Bis-GMA bonding resin used for etched enamel.¹² The hydrophobic resin would not wet the moist, vital dentin and predictable adhesion could not be produced. The breakthrough in the total-etch approach was first described in the late 1970s by Fusayama and coworkers,¹⁵ Bertolotti,¹⁶ and Kanca.¹⁷ They demonstrated the success of the total-etch adhesive bond based on the addition of a hydrophilic monomer, usually hydroxyethyl methylmethacrylate to the primer and adhesive. This monomer allows the adhesive resin to penetrate the peritubular dentin and dentinal tubules.¹⁸

These concepts led to the development of multi-step adhesive bonding systems, which required the application of a primer and then an adhesive resin, in the late 1980s and early 1990s that used a total-etch technique with phosphoric acid. In the mid 1990s clinicians sought a simplified approach that used fewer steps for adhesive placement. Manufacturers responded with the introduction of single-bottle primer/adhesive total-etch bonding systems. With these two different classes of bonding systems came the classification and description of bonding systems based on generational timeline changes. Fourth-generation bonding systems referred to total-etch, multi-bottle (multi-step) systems and fifth-generation systems were total-etch, single-bottle bonding agents that contained both primer and adhesive. Both fourth- and fifth-generation products required a total-etch with phosphoric acid before adhesive placement.

Simplification of technique and a reduction in the number of steps was desired. It was obvious that the more steps that were required to bond a restoration, the greater the potential for inconsistency in the timing of application, rinsing, drying, rewetting dentin, and maintaining a controlled operative field during treatment. This inconsistency has an impact on the success of the bond and the durability of the restoration. Manufacturers responded by placing research efforts in the development of self-etching adhesive systems.¹

Many manufacturers have referred to self-etch adhesives as either sixth- or seventh-generation adhesive systems. In this author’s view, the description of adhesives based on a generational view can be confusing. Christensen described a classification system for bonding agents based on the components used to achieve adhesion to dentin and enamel.¹⁹ He divided adhesives into two main categories—total-etch (TE) and self-etch (SE). Within each category he then subdivided the classifications based on the number of reagents that were used for the adhesive technique. This classification system is listed in Table 1 with examples in each classification. Table 2 lists recommendations for the clinical applications of the two types of adhesive systems listed based on the clinical evidence.

Currently, the clinician has the choice between two different approaches for bonding that have different mechanisms in how they interact with the dentin smear layer: a TE approach or a SE approach.²⁰ The TE technique uses 30% to 40% phosphoric acid, which removes the dentin smear layer. The phosphoric acid is rinsed with water and dried from the dentin. The dentin is then rewetted with water, leaving a damp surface; an adhesive resin is then applied.

The introduction of a SE adhesive simplifies the bonding process. The SE approach does not require a separate etching step because the etchant is incorporated into the adhesive (either in a separate self-etching primer or in the adhesive itself). Also, a separate step of rewetting with water is eliminated because SE adhesives contain water and are never completely dried from the tooth. SE adhesives do not remove the smear layer but incorporate it into the adhesive. Their compositions are aqueous mixtures of acidic functional monomers, usually phosphoric acid esters, with a pH value higher than phosphoric acid (TE type) gels.²¹ It has been reported that the pH of Clearfil^® SE Bond (Kuraray America, Inc, New York, NY) is approximately 2.0, compared to a pH of 0.5 to 1.0 for typical phosphoric acid gels.²²

SELF-ETCHING ADHESIVES
As stated previously, there has always been concern for contamination and inconsistency with multiple-step bonding systems. In response to this concern, self-etching adhesive systems have been developed. Recent research has investigated self-etching adhesive systems. A chief complaint among practitioners with composite resin restorations has been the rate of postoperative sensitivity especially when using TE bonding after the placement of Class 1, 2, and 5 restorations. Several different studies evaluated postoperative sensitivity using both TE and SE adhesives.^19,23-26 The results of these studies demonstrated no difference in postoperative sensitivity between a TE and SE adhesive. In fact, the conclusion of one study stated that postoperative sensitivity may depend on the restorative technique and variability among operators rather than on the type of enamel-dentin adhesive used.¹⁹ This variability between operators can be minimized by simplifying the technique of adhesive placement with a SE bonding system.^27,28 Table 3 lists the advantages of the SE bonding systems.

Other research has compared SE systems to TE systems. Santini and coworkers investigated microleakage around Class 5 restorations bonded with TE and SE adhesives.²⁹ They concluded that SE systems were as reliable as TE systems. One area of inconsistency with TE bonding has been the bonding potential to desiccated dentin.^30,31 The inherent nature of SE adhesives is no-rinse, leaving the surface moist. This may contribute to minimizing postoperative sensitivity.²⁸ Finger and Tani investigated the effect of dentin wetness on bond reliability and found that the SE adhesives were unaffected by relative humidity of the dentin.³²

Some clinicians are concerned about bacterial contamination of cavity preparations and use cavity disinfectants before applying dental adhesives. The use of benzalkonium chloride and chlorhexidine gluconate has been found to have no detrimental effects on the sealing ability of no-rinse self-etching adhesives.³³ In some cases the self-etching adhesive acts as its own disinfectant. Both iBond^™ (Heraeus Kulzer, Inc, Armonk, NY) and Clearfil^® Protect Bond (Kuraray America, Inc) have data to support this claim. With the increased interest in tooth whitening and the availability of over-the-counter peroxide-based products, the clinician may not know if their patient is bleaching their teeth. Research supports waiting at least 1 week after bleaching before any restorative procedure with either an SE or TE adhesive to allow the enamel and dentin to recover from the bleaching procedure.³⁴ It is important to know whether or not your patients are using peroxide products before any bonding procedure.

There has also been concern about the bonding quality of SE adhesives to enamel. If enamel is left unprepared, it is resistant to etching and adhesion with most SE adhesives.^35-37 Bonding to unprepared enamel with orthodontic brackets using SE adhesives has been reported. One study demonstrated no difference in bracket retention between TE and SE³⁸ while two other studies had significantly more bond failures with the SE system.^39,40 Clearly, there are differences between SE systems when bonding to enamel. Multi-step SE systems appear to be more aggressive in etching enamel.^41-43 One study recommended doubling the conditioning time with a SE system to increase bond strength.⁴⁴ Clinical Research Associates evaluated and compared TE to SE adhesives.⁴⁵ They concluded that both adhesives have similar bond strengths to prepared enamel and dentin. When using any SE adhesive, it is recommended that enamel and dentin be prepared with either a rotary diamond or bur. Prepared enamel and dentin will have comparable bonding between TE and SE systems.^45-48 Some concern has been expressed that a thick dentin smear layer may interfere with bonding using an SE adhesive. Tani and Finger demonstrated effective bonding with SE adhesives to thick dentin smear layers.⁴⁹

What is the evidence in clinical trials for the use of self-etch adhesive systems? In recent years clinical trials using SE adhesives have been reported at both the International Association of Dental Research and American Association of Dental Research meetings. In many cases at these meetings, the clinical trials reported as abstracts with supporting documentation via oral presentations and/or poster presentations may not be submitted for publication in dental journals. A review of the last four meetings in Honolulu, Hawaii (2004); Baltimore, Maryland (2005); Orlando, Florida (2006) and Brisbane, Australia (2006) provided insight into a number of ongoing trials.

The clinical success of SE adhesives can be measured in a variety of ways. In the cases of routine Class 1 and Class 2 preparations, retention is not a primary evaluation criterion for a short-term study, but sensitivity and marginal staining are. For the restoration of noncarious cervical lesions (NCCL), retention and marginal staining are the benchmarks for success of an adhesive system. In a 10-year study with the SE adhesive Clearfil^® Liner Bond 2 (Kuraray America, Inc) Class 1 through Class 5 restorations were evaluated. At the 10-year recall, 90.9% of the restorations exhibited some marginal breakdown and 88.6% of the restorations had marginal staining. The retention rate of the restorations for those prepared cavity preparations and restorations with the SE was 100% at each recall period (50.5% at 10 years or the original placed restorations). This SE system was an acceptable adhesive for restoring prepared teeth.⁵⁰ Swift and coworkers compared a TE (Optibond^® Solo Plus, Kerr Corporation, Orange, CA) to a SE (Xeno^® III, DENTSPLY Caulk, Milford, DE) for sensitivity and found that for both adhesives, 23% of the treated teeth had short-duration sensitivity to biting pressure that resolved after 10 days.⁵¹ A 24-month clinical trial with a two-step SE (Clearfil Protect Bond) and a one-step SE (Xeno III) for NCCL demonstrated retention rates of over 96% with no postoperative sensitivity at the 2-year recall. Compared to 3% of the Clearfil Protect Bond, 12% of the Xeno III restorations had marginal staining.⁵² In an 18-month study with Clearfil SE Bond, enamel beveling or acid-etching did not improve the retention rate of NCCL restored lesions. The retention rate for the four groups tested was 100%.⁵³ A 5-year clinical evaluation of One-Up Bond F Plus (J. Morita USA, Inc, Irvine, CA) with 42.5% of the original restorations recalled showed a 92% retention rate, with 48% of the restorations demonstrating marginal staining.⁵⁴ Other studies have demonstrated similar results with SE systems to those previously cited.^55-63 Retention with SE adhesive systems is not a problem and there are minor rates of marginal staining. Figure 1A View Figure ; Figure 1B View Figure; Figure 2C DI200801_59x.jpg demonstrate two cases restored using SE adhesive systems.

An area of recent investigation has been the compatibility of TE and SE systems with composite resin cementation. There is contradictory evidence that some single-bottle adhesive systems do not bond well to self-cure and dual-cure composite resins because of the acidity of the single-bottle primer-adhesive. Studies demonstrating a decreased bond and other studies showing no effect have been reported.^64-66 Some recent studies evaluating SE systems and compatibility with dual-cure and self-cure composite resins have demonstrated some changes in chemistry that have resulted in composite resin–adhesive incompatibility.^67,68 This variance requires that the clinician review the manufacturer’s recommendations for use with self-cure and dual-cure composite resins.

CONCLUSION
Clinicians have seen multiple generations of adhesive systems in the last 20 years. Many of these bonding systems have required multiple steps to include etching with phosphoric acid, rinsing with an air-water spray, drying, rewetting the preparation, applying the primer, drying, applying the adhesive resin, and light-curing. Based on the current clinical evidence and the recommendations of manufacturers, SE adhesive systems can be used successfully for the restoration of Classes 1, 2, 3, and 5 preparations. SE adhesives provide adequate enamel etching to resist microleakage and marginal staining and adequate retention of both prepared teeth and NCCL Class 5 restorations.

With the introduction of clinically reliable self-etching bonding systems for use in the restoration of routine tooth preparations, the practitioner can place restorations in a more simplified manner. SE systems are different from the bonding systems previously used and the manufacturers’ recommendations must be followed to ensure clinical success.

DISCLOSURE
The author has received honorarium or grant/research support from 3M ESPE, Bisco, Den-Mat, DENTSPLY, GC America, Heraeus Kulzer, Pulpdent, and Ultradent.

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Figure 1A Class 2 carious lesion on the distal surface of the mandibular second premolar.		Figure 1B Class 2 cavity preparation.
Figure 1C Restoration using a self-etch adhesive with a micromatrix hybrid composite resin.		Figure 2A Class 3 carious lesion on the mesial surface of the maxillary lateral incisor.
Figure 2B Class 3 and lingual cavity preparations.		Figure 2C Restorations using a self-etch adhesive with a microfill hybrid composite resin.