Quest for the Best Bonding Agent (2010)
Raymond L. Bertolotti DDS, PhD
(based on a presentation at the McLean Memorial Lecture, London, 2010)
In the Buonocore Memorial lecture in 2003, Van Meerbeek and colleagues stated
"…the data presented confirm that conventional three-step etch and rinse adhesives still perform most favorably and are most reliable in the long-term. Nevertheless, a self-etch approach may have the best future perspective"1. Now, considering the rapid progress of self-etching bonding agents, we might wonder if these statements are still true about 7 years later. Indeed, in a very recent paper2, Van Meerbeek states; " When bonding to enamel, an etch and rinse approach is definitely preferred, indicating that simple micro-mechanical interaction appears sufficient to achieve a durable bond to enamel. When bonding to dentin, a mild self-etch approach is superior, as it involves (like with glass-ionomers) additional ionic bonding with residual HAp. This additional primary chemical bonding definitely contributes to bond durability. Altogether, when bonding to both enamel and dentin, selective etching of enamel followed by the application of the 2-step self-etch adhesive to both enamel and dentin currently appears the best choice to effectively and durably bond to tooth tissue".
Self-etching primers have overtaken etch and rinse systems in the dental market in many regions of the world. In addition to the obvious simplification in clinical procedures, it appears that a major part of this trend toward self-etching primers is attributable to the well known fact that self-etching primers are far less prone to result in post-operative sensitivity than are etch and rinse systems. The most likely explanation is that self-etching primers do not remove smear plugs in the dentinal tubules, thereby precluding hydrodynamic fluid movement. Another compelling reason to use self-etching systems is the reported better dentin bond stability3. Current thinking is that the self-etching primers, unlike typical etch and rinse systems, do not demineralize dentin deeper than can be completely hybridized with infused primer and bonding agent4. Over-etched and unhybridized dentin is a source of nanoleakage, with ingress of water that is thought to cause hydrolytic breakdown of the demineralized layer, resulting in the bond instability typically observed with etch and rinse systems.
One concern is for the ability of self-etching systems to produce a strong and durable enamel bond. Due to major differences in acidity of the primers, the enamel bonding ability varies greatly among products. Clinical reports in the literature are sometimes contradictory for a particular product. However it is generally agreed that separate phosphoric acid etch and rinse of the enamel will improve the bond of most, if not all, self etching primers. An analogous improvement in dentin bond is not expected since the separate etching will tend to over etch the dentin.
"Immediate dentin sealing" 5,6 for indirect restorations is now becoming popular. As CAD-CAM (e.g. Cerec) restorations become more commonly used, a major factor in choice of bonding agent is the thickness of the bonding layer. For clinical efficiency, it is highly desirable to make the "optical impression" immediately after preparation. While milling, the preparation is cleaned and the bonding agent is applied to the tooth preparation and cured. The bonding agent must be sufficiently thin layer to preclude blocking the fit of the milled restoration. Curing of the bonding agent in advance of bonding the restorative material improves the tooth-to-bond interface5,6 . Applying the dentin seal even a few minutes before final bonding of the restoration will improve the interface significantly. The explanation is that a fully polymerized bonding agent does not easily pull away from the bonded surface when stressed by polymerization shrinkage of the luting composite.
With the above discussion in mind, here is a list of the properties that "dream bond" would have:
Work in self-etch or etch and rinse modes
High SE early enamel bond strength
High SE early dentin bond strength
SE ability on uncut enamel
Bond to metal/ceramic for repairs
No bond to metal matrix
Bond stability on dentin and enamel
No “transudation”
Form an acid-base resistant zone ahead of the hybrid layer
LC, SC, DC compatible
Minimal technique sensitivity
Not visible on x-ray as radiolucency (very thin or radiopaque)
Solvents easily evaporated
Shelf-life stable
Both direct composite and CAD-CAM friendly (very thin, dual-cure)
The last requirement for “dream bond”, that it be very thin and dual-cure, eliminates the majority of self-etching systems. Only two self-etching systems have been observed by the author to be sufficiently thin as to not interfere with fit of CAD-CAM restorations, assuming a fit tolerance of 100 microns. These are Bond Force (Tokuyama) and Prelude SE (Danville). Most other bonds observed are too thick to meet this requirement (e.g. Clearfil SE Bond, (Kuraray)). Evaluation of bond thickness in the corners of preparations, rather than on a flat surface (as manufactures usually do), is thought to be more clinically relevant to actual tooth preparations. Prelude SE is a dual-cure bonding agent whereas Bond Force is a light-cure only bonding agent. Application of one-bottle Bond Force is easier than Prelude SE but the light cure only capability it may limit its use for indirect restorations.
Several popular bonding agents were evaluated by confocal laser scanning microscopy. This microscope does not require dehydration so it eliminates artifacts produced by dehydration. Images are presented below:
Bond Force (Tokuyama)
Prelude SE (Danville)
Clearfil SE Bond (Kuraray)
SE Bond, Prelude SE and Bond Force all appear to make an excellent adaptation to the tooth preparation. The high thickness of SE Bond, especially in the pooled corners, makes it too thick for CAD-CAM restorations when immediate dentin sealing is done after making the optical impression. On the positive side, many researchers prefer the high thickness since it is thought to act as an elastic buffer zone. On the negative side, radiolucency of thick bonds is a consideration. The trade-offs of bond thickness on clinical results are yet to be fully resolved.
Some other bonds observed suffer from a variety of problems. For example, Xeno IV and AdheseOne show poor adhesion and incomplete sealing in the corners of a tooth preparation. Solvent removal appears to be a problem with Clearfil S3. Phase separation appears to be a problem with G Bond. Also the acetone solvent in G Bond evaporates very fast and the bonding resin becomes very sticky, preventing application of a thin layer. It is worth noting that the poor performers were all simplified, “one bottle” bonds. Bond Force is the only one bottle bond observed by the author to perform well.
Clearfil S3 (Kuraray) product C
Acknowledgement: The author thanks Dr. Shigehisa Inokoshi for specimen preparation and Mr. Matsushige at Tokuyama for the excellent images.
Transudation7, that is permeation of pulpal fluids through the hybrid layer, is a problem common to all of the one bottle self-etching systems. In contrast, the two layer systems do not exhibit transudation, due to the sealing ability of the water-free, hydrophobic second layer. While transudation can be sufficiently managed, it appears that two layer systems would be less technique sensitive.
Transudation of the bond layer of a one component bond (left image) and no transudation of a two layer bond (right image). Acknowledgement: The author thanks Dr. Franklin Tay for these images.
Recent research8 has shown that self-etching primers can produce a protective acid-base resistant zone in dentin, below the hybrid layer. This layer is not found with etch and rinse systems. This is yet another good reason to use self-etching primers.
Bond Force and SE Bond appear to be excellent choices for light cured, direct composite restorations. However, for “universal” use a good case can be made for selecting Prelude SE considering it features a remarkably thin bond layer and universal compatibility (LC, SC, DC). With the exception of metal and ceramic bonding for repairs, Prelude SE satisfies all of the requirements for "dream bond" as outlined above. (Prelude’s minimal bond to metal allows placement of metal matrices prior to restorative procedures.) It may be used as a self-etching system or as an etch and rinse system, omitting use of the self-etching primer and instead using phosphoric acid etchant. The bond strength and the bond to tooth interface are about the same for both techniques.
In summary, current thinking is that the technique simplification, improved dentin bond durability, lack of post-operative sensitivity and excellent clinical results make a good case for using the best self-etching primer systems rather than etch and rinse systems. Adaptation to cavity preparations can be remarkable different among products. Supplemental etching of enamel areas generally enhances results but may be impossible in certain cavity preparations without exposing dentin to undesirable separate etching.
References.
1. Buonocore Memorial Lecture. Adhesion to Enamel and Dentin: Current Status and Future Challenges. Van Meerbeek B, et al., Oper Dent 2003; 28,215-35.
2. Relationship between bond-strength tests and clinical outcomes. Van Meerbeek B et al. Dent Mater 2010; 26:100-121.
3. . Seven-year dentin bond strengths of a total- and self-etch system. Burrow MF et al. Eur J of Oral Sci 2005;113:265-270.
4., Influence of Additional Acid Etch Treatment on Resin Cement Dentin Infiltration. Walker et al., J Prosth 2000; 9:77-81.
5. The effect of a “Resin Coating” on the interfacial adaptation of composite inlays. Jayasooriya PR et al., Oper Dent 2003; 28-35.
6. Immediate dentin sealing supports delayed restoration placement. Magne P et al., J Prost Dent 2007;98:166-174.
7. Conversion of one-step to two-step self-etch adhesives for improved efficacy and extended application. King NM, et al., Am Journal Dent 2005;18:126-34.
8. Assessment of the nanostructure of acid–base resistant zone by the application of all-in-one adhesive systems: Super dentin formation. Nikaido T, et al., Bio-Medical Materials and Engineering 2009:19(2-3)163-171.