Following the removal of old restorations and associated recurrent caries during dental restorative procedures, it is imperative to protect the dental pulp while reinforcing and strengthening the remaining tooth structure. For these and other important mechanical and biological reasons, bonded core buildups are essential to the long- term success of cast dental restorations. Central to this process is the proper conditioning of the dentin followed by the application of a bonding agent that provides good bond strength and other physical properties while simultaneously preventing pulpal pathology.
In this article, the author discusses new developments in adhesive technology, providing a rationale for the use of an antibacterial self-etching primer in place of any of the total-etch systems that utilize a 32% phosphoric acid etchant.
The total-etch technique, treating enamel and dentin simultaneously, employs 40% phosphoric acid etch-primer-bond products and is still in wide use. However, research shows that the decalcification caused by this acid creates issues with dentin treatment, such as postoperative sensitivity.
These total-etch systems work by opening the dentinal tubules, increasing dentinal permeability and decalcifying the intertubular and peritubular dentin.1 This total-etch process transforms the dentin from a hard, mineralized surface to a very soft, demineralized, collagen-rich surface that collapses when air-dried.2 Total-etching relies on the complete removal of the smear layer and smear plugs, thereby making the tubules available for resin tag formation. The etchant dissolves away the inorganic matrix and is rinsed off, leaving behind the fragile and supported collagen fibril network suspended in the rinse water. Air-drying the surface will collapse these tubules, decreasing the size of interfibrillar spaces that are necessary for resin uptake. This collapse results in a relatively impermeable organic film that prevents satisfactory infiltration of the bonding resin.3
Accordingly, a primer is necessary to support and to re-expand the collagen fibrils and hold them upright prior to applying the bonding agent. The technique requires the repeated application of 8 to 10 coats of primer not only to rewet the surface, but also to penetrate the full extent of the decalcified zone. As a practical matter, this is not always possible, and voids can develop where the acid has decalcified the dentin but where the primer has failed to penetrate.
Kanca4 discovered that leaving some moisture on the surface of etched dentin could result in markedly increased bond strengths.4,5 However, the issue of the correct amount of wetness is relevant, since an over-wet dentin surface can prevent the monomers from remaining dissolved in the solvents. They then can undergo phase changes that lead to low bond strengths due to the formation of water blisters at the dentin-adhesive interface, which results in incomplete hybridization.6 On the other hand, a dry surface results in the collapse of the collagen fibrils, a phenomenon that prevents the hydrophilic resin from forming a dense hybrid layer. Accordingly, the precise control of surface moisture is critical in these fourth-generation systems. Unfortunately, determining what is too wet and what is too dry is not only a subjective evaluation, but also a variable one, depending on the individual product used. Therefore, these fourth-generation total-etch products are considered to be technique sensitive, with clinical results that will vary markedly among testing laboratory findings and clinicians.2
SELF-ETCHING PRIMER/BOND SYSTEMS
The self-etching primers have been shown to treat the cut dentin surface in a more biologically acceptable way by eliminating the excessive decalcification that characterizes the total-etch systems. Rather than completely removing the smear layer as total-etch systems do, self-etch products work by infiltrating it and creating a thin but very strong hybrid layer with smear plugs intact. These systems reinforce rather than remove the smear layer, thus improving its adhesion to the underlying dentin.1
The first self-etching bonding system was called Clearfil Liner Bond 2 and was created by Kuraray Dental in 1993. The etching process was accomplished by using low pH primers and by chemical adhesion to tooth structure. In 1999, Kuraray Dental developed Clearfil SE Bond, a sixth-generation, 2-step primer and bond material. The Primer bottle contains a hydrophilic, acidic adhesive monomer called MDP that conditions the dentin without the potentially harmful effects of phosphoric acid. The Bond bottle contains a relatively hydrophobic, microfilled resin that copolymerizes with the primer following light activation.
With Clearfil SE Bond as well as other self-etching primers, there is complete support of the collagen fibrils during the entire process so that they remain upright and suitable for bonding. These self-etching primers are designed for use on dry dentin, a great advantage since the dentin is still completely mineralized prior to the application of the primer. From a clinician's standpoint, establishing a uniformly dry substrate is far easier and more predictable than creating one with uniformly optimal moisture content.
In addition, the self-etching properties of MDP are self-limiting; its pH reverts to neutral once it has penetrated the smear layer and bonded chemically to the calcium and the hydroxyapatite found in dentin. This property is of great benefit, since it eliminates the need for precise timing of the priming-etch step.
Following the application of the primer, the relatively hydrophobic, microfilled resin is applied and penetrates the primed dentin, copolymerizing with the primer to form a thin but very strong hybrid layer.
DISINFECTION OF THE PREPARED DENTIN SURFACE
Disinfection of cut dentin will go a long way toward a favorable pulpal response, especially in the realm of pulp capping. Brannstrom7 reported that since dental materials contract upon setting, the spaces that develop between the dentin and the restorative material are favorable for bacterial growth. His group showed that bacterial toxins were the main source of pulpal irritation in teeth restored with composite resins and zinc phosphate cement. Further, the Brannstrom group8 reported that microorganism growth on the walls of cavities that have been filled with composite resin is partly due to bacterial invasion from the oral cavity through the space between the restorative material and the cavity wall and partly due to continued growth of bacteria that were present in the cavity when the filling material was inserted. They concluded in another study that removal of grinding debris and the elimination of bacteria was advisable. They further recommended applying a liner to all dentin surfaces and avoiding contamination.9
A recent study by Cox, et al10 reported that restoring an exposed pulp with an antimicrobial adhesive and composite resin presented constant reproducible results. Dentin bridge formation was present directly adjacent to the restorative interface with no pulpal inflammation at long-term usage periods. Their data suggest that the new antimicrobial adhesive systems will provide a dynamic means to provide a longer-term clinical bacteriometic seal along the entire restoration interface.10
Thorough dentinal cleansing and disinfection has long been advocated by Strupp,11 who developed the use of successive scrubs of 4.0% chlorhexidine gluconate, Tubulicid Red (Global Dental Products), and sodium hypochlorite, each of which was followed with a thorough rinse prior to the initiation of the bonding process.11
A DISINFECTING, FLUORIDE-RELEASING, SELF-ETCHING SYSTEM
In 2004, Kuraray Dental created Clearfil Protect Bond, which combines the more biologically favorable self-etching system with MDPB, an antibacterial functional monomer contained in the primer. In addition, the hydrophobic bonding component of Clear-fil Protect Bond contains an encapsulated sodium fluoride molecule that releases fluoride into the dentin.
MDPB is a quaternary ammonium compound that disrupts bacterial cell membranes by imparting a positively charged pyridinium radical to the negatively charged cell membrane. This electrical imbalance causes the bacterial cell membranes to rupture, thus disinfecting the substrate without the need for multiple antibacterial scrubs. The MDPB also has a polymerization group that causes it to become polymerized along with the MDP by light-curing. Consequently, this monomer, like MDP, is also self-limiting in its action, avoiding the continuous dispersion of bactericide into the dentin.
Figure 1. Pretreatment.
Figure 2. Completed preparation.
Figure 3. Clearfil Protect Bond primer.
Figure 4. Clearfil Protect Bond resin.
Figure 5. Clearfil DC Core Automix into gingival seat.
Figure 6. Clearfil AP-X composite.
Figure 7. Completed restoration.
A 45-year-old woman came to the office wanting comprehensive dental care, starting with the decayed teeth in the lower right quadrant of her mouth. Radiographs were taken to rule out any pulpal pathology prior to restorative dentistry. The class II decay that was present in tooth No. 29 was moderate in size, so a bonded composite resin restoration was indicated. Tooth No. 28 exhibited a class V carious lesion for which a bonded restoration was indicated as well (Figure 1).
After satisfactory local anesthesia using Citanest (DENTSPLY Pharmaceutical), initial outline form and caries debridement was accomplished using a 330 bur on a high-speed handpiece. Decay detecting solution (Caries Detector [Kuraray Dental]) was applied and rinsed according to manufacturer's directions. Deeper decay was removed with a No. 4 round bur on the low-speed handpiece under continuous water irrigation from the 3-way syringe. Once the caries was completely eliminated from the preparation, the final form of the cavity preparations was accomplished using a flat-end diamond (F-62 [Pollard Dental Company]). Finally, the cavosurface margin of the preparation was slightly beveled using a large, round-end diamond (Big Bite No. 6 [Tanaka Dental Supply]). The prep was scrubbed with a cotton pellet under water irrigation and then rinsed (Figure 2).
The preparation was isolated in the usual manner with parotid pads (Dry Tips [Microcopy]) and cotton rolls. The bonding system employed in this case, Clearfil Protect Bond, is self-etching, with the addition of an antibacterial agent within the primer. The hydrophilic primer (Figure 3) was applied with a brush, and at the end of 20 seconds, the prep was dried with air. Then, the hydrophobic bonding resin (Figure 4) was brushed onto the prep, lightly aired, and then cured for 10 seconds with the curing light.
At this point, a standard Tofflemire matrix band was implemented in the usual manner to establish the proper distal contour, even in the absence of an adjacent tooth. Attention was first given to the gingival seat, where a small amount (less than 1.0 mm) of Clearfil DC Core Automix (Kuraray Dental) was syringed and cured (Fig-ure 5). A needle tube and plug system (Centrix) was used to place the flowable DC Core Automix precisely into the gingival seat of the prep. At this point, the first layer was polymerized with the curing light for 10 seconds. The remainder of the prep was restored (Figure 6) with Clearfil AP-X composite (Kuraray Dental).
The class V cervical lesion was treated in the same manner as the class II, except that the only restorative material needed was AP-X. Both restorations were contoured and polished using diamonds and Midget white points (Dedeco). Following satisfactory resolution of the chief complaint (Figure 7), the patient was rescheduled for a comprehensive workup to address periodontal as well as other restorative issues.
The shift toward using self-etching primers in place of total-etch systems represents an improvement for bonded restorations for many reasons. By eliminating the deleterious effects of the phosphoric acid on cut dentin, self-etching primers afford the practitioner a quicker, simpler, more predictable procedure for direct bonded restorations.
The self-etching system is further improved in Clearfil Protect Bond by its antibacterial activity during the priming process. The use of this product will disinfect the dentinal surface prior to bonding and further protect the pulp from potentially pathogenic bacteria.
The benefit of a simpler and easier clinical procedure is readily apparent to practicing clinicians. Phosphoric acid systems require, by design, a strictly timed etch step followed by a thorough rinse prior to application of dentin primer. The time interval necessary to accomplish these steps is further complicated when restoring multiple teeth and also by the need to change the parotid pads and cotton rolls during the procedure.
As a rule, the longer a procedure takes and the more steps involved, the greater the chance for operator-induced error. This problem increases when special mixing and timing steps occur. For example, the primer application from a single bottle such as Clearfil SE Bond is done more precisely than the primer application that requires the mixing of 2 separate components at the chair. The manufacturer is able to control the exact mixture much better at the factory than a doctor or an assistant can do in the middle of treatment.
While testing laboratories may report that the fourth-generation, total-etch systems produce higher bond strengths, these results may not necessarily be reflected in clinical practice. Further, the higher bond strength may not be clinically relevant, since the force that is working to disrupt the bond between the bonding resin and the underlying dentin is the force developed at the composite-dentin interface during polymerization shrinkage.
The total-etch systems generally showed good micro-leakage test results, according to Clinical Research Associates (CRA).12 This organization reported that while self-etching primers generally did not produce the same margin seals as total-etch systems, Clearfil SE Bond and Clearfil Liner Bond 2V were the noted exceptions, with microleakage scores comparable to the total-etch systems tested. In the same issue, CRA did a survey, reporting that 61.8% of the respondents who were using one of the available self-etch primers were using Clearfil SE Bond, results that correlate with this particular product's favorable testing data.
Clearfil Protect Bond, an antibacterial, self-etching primer, is very well-suited for use with direct bonded restorations. Its use eliminates the excessive decalcification from phosphoric acid frequently seen in total-etch systems by leaving the smear layer and smear plugs intact. Accordingly, postoperative sensitivity is reduced to nearly zero and the application technique is greatly simplified without sacrificing bond strength or efficacy. Further, its functional monomer, MDPB, exerts a self-limiting, antibacterial effect that reduces the incidence of pulpitis following restorative procedures.
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10. Cox CF, Imazato S, Stevenson III RG, et al. Pulp healing and dentin bridge formation adjacent to an antimicrobial adhesive: a long-term usage study. Presented at: International Symposium on Adhesive Dentistry; April 2002; Tokyo, Japan.
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12. Clinical Research Associates. Self-etch primer (SEP) adhesives update. CRA Newsletter. 2003;27:1-5.