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New Perspectives on Dentin Adhesion

Differing Methods of Bonding

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 polymerization.25,26

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 procedure.28  

 

Bonding Procedures

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.

Selective bonding 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.

Indications

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.

 

Conclusion

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 handling.

 

*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.

 

References

 

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