Bonding resin to tooth structure is still confusing

During my encounters with practicing dentists during speaking events, one of the most confusing topics, and one on which I receive constant questions, is the best way to bond resin to dentin. Thankfully, bonding to enamel is more fully understood and, obviously, more clinically successful.

I can understand the confusion. Although we have several dental materials scientists working in the Clinicians Report Foundation’s laboratories, confusion is still present. Testing methodology differs in various studies, and the differing bonding values are frustrating. Enamel bonds remain about the same after a period of time in tests while dentin bonds often depreciate. Yet when originally bonding resin to dentin, the bond values are higher on dentin than on enamel. Let’s see if I can help answer the dentin bonding question.

In this article, I will attempt to clarify the dentin bonding question based on available evidence from the dental literature, research from our own laboratories and my personal observations when speaking to dentists globally and working in study clubs.

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The following information will be provided in the article:

• The types of bonding agents

• Differences among the types

• Clinical significance of the differences among the types

• Suggested clinical techniques and products

Identifiable generations or types of bonding agents

This way of organizing bonding agents is thought to be out of date by some, but it traces the history of bonding agents well and is readily understandable. There are currently seven, maybe even eight, “generations” or types of dentin bonding agents.

The first three types, introduced up to 50 years ago, are now of only historical significance. They had minimal bond but were a good start.

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The fourth-generation bonding agents were introduced in the early 1990s, and the fifth generation was introduced in the mid-1990s. These two generations included the first products in the total-etch concept, now called “etch and rinse” by some. Using the etch-and-rinse or total-etch technique, all of the tooth structure, enamel and dentin are etched, removing the smear layer completely, then washed, primed with a second “wetting” constituent and sealed with an unfilled bonding resin.

In the fifth type, the prime and bond constituents were placed together. The fourth and fifth generations are still considered to be the so-called “gold standard” for tooth-bonding by many researchers because the techniques removed all of the debris (smear layer) on the tooth, and, when done correctly, sealed and desensitized the tooth.

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If used correctly, this etch and rinse was excellent. An old but very successful representative three-constituent product many dentists will remember is 3M ESPEScotchbond Multi-Purpose. An older example of a fifth-time two-bottle product with combined primer and bond is DENTSPLY Caulk Prime & Bond® NT™. Both of these products are still currently used by some dentists.

Advantages and disadvantages of the etch-and-rinse technique

The challenge with the etch-and-rinse concept is that since the dentin canals are opened and the debris in the canals removed by the acid etch, the acid, primer and unfilled bond have to be used meticulously or postoperative tooth sensitivity can occur. Our research has shown two times the post-operative tooth sensitivity in typical, busy practices using the etch-and-rinse concept vs. the self-etch concept. However, I can verify that if the technique is done well with the etch-and-rinse procedure, there is minimal or no post-operative tooth sensitivity.

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What are the positive aspects of the etch-and-rinse technique?

Research shows the bond to enamel produced by the etch-and-rinse technique can be better than the bond produced by some of the self-etch bonding products we will discuss later. Is this greater bond clinically significant?

That is a moot point. For generations, dentists have successfully placed long-lasting restorations without any bond and with significant microleakage (amalgam, silicate cement and zinc-phosphate cement). In my opinion, the greater bond to enamel produced by etch-and-rinse products over self-etch products is probably not clinically significant in mechanically retentive tooth preparations, such as Class I and II restorations.

If you want to use the etch-and-rinse materials - as many dentists do - please carry out the clinical procedure meticulously, and you will have optimum success.

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Self-etching products

The sixth and seventh types of bonding agents are the products used by many U.S. dentists today. These are the so-called self-etching products. The sixth type has two bottles-an acidic primer and wetting agent and an unfilled bonding agent. Some of the bonding agents have a relatively thick film of about 40 microns (the approximate thickness of a human hair).

The seventh and eighth types of bonding agents are usually contained in one bottle with all of the constituents placed in one application on the tooth preparation. Some of these are now known as universal bonding agents. When using universal bonds, they may or may not remove the smear layer from the dentin. They are universal because they may be used with all three bonding concepts, etch-and-rinse, selective-etch and self-etch. Popular examples are (by alphabetical order) Adhere Universal (Ivoclar Vivadent), Clearfil Universal Bond (Kuraray), Prime & Bond Elect (DENTSPLY Caulk), Scotchbond Universal (3M ESPE) and many others. When used as a self-etch application, they impregnate and seal the remaining smear layer with the ingredients of the bonding systems and etch the enamel to various degrees.

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Why have many dentists selected self-etching types of bonding agents?

The previously described etch-and-rinse systems are excellent if used properly; however, the unpredictable, frustrating and patient-upsetting post-operative tooth sensitivity frequently produced by etch-and-rinse systems led many to change to self-etch systems. Research from many locations, especially dental schools, has produced varying reports on self-etch products. Generally, they report decreased post-operative tooth sensitivity and similar, measurable bonds to dentin comparable to those produced using the etch-and-rinse systems.

The only potentially negative characteristic of self-etch products is the previously mentioned slightly reduced bond to enamel for some of the brands. This reduced bond to enamel has not discouraged dentists from enthusiastically using them. The reported overwhelming clinical success and acceptance of self-etching bonding agents overshadows their alleged weakness reported in some of the in-vitro research literature.

In summary, both etch-and-rinse and self-etch bonding agents have been reported to produce similar bonds to dentin although some brands of self-etch products have received reports of somewhat reduced bonds to cut and uncut self-etched enamel.

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Dentin vs. enamel bonding

Is the bond to dentin reported for etch-and-rinse or self-etch products a long-term one as has been proven to be the case with enamel? When summarizing available research reports and combining them with the clinical observations of practicing dentists, enamel bonding is strong when done properly and it remains indefinitely while dentin bonding is less predictable and reliable and reduces over time.

Any dentist who has removed ceramic veneers bonded to dentin knows that usually ceramic can easily be picked off the dentin, but it must be laboriously cut from the enamel. Bonds to dentin are often equal to or greater than bonds to enamel on extracted teeth in research labs.

Advertisements and research reports that show these in-vitro studies are actually misleading to clinicians.

I am pleased to have significant tooth desensitization provided by dentin bonding agents, but I consider the less predictable dentin bond provided by these agents to be a bonus when present, not an expectation.

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Bonding and desensitizing procedures

The following successful bonding and desensitizing procedures are listed in order of etch-and-rinse then self-etch. All of these techniques have been reported in dental literature as successful clinical procedures when done correctly:

Comments about these techniques:

• The popular glutaraldehyde/HEMA solutions, such as G5 (Clinician’s Choice); GLUMA (Heraeus Kulzer); Glu/Sense™ (Centrix) or MicroPrime G (Danville), will be noted as glutaraldehyde in the descriptions that follow. After significant scientific investigation by Clinicians Report Foundation, I prefer to use glutaraldehyde application in all of these techniques. Our research shows that two one-minute applications of glutaraldehyde provide optimum disinfection and desensitization. You may disagree with my conclusions and either exclude the glutaraldehyde completely from the techniques or substitute it with another desensitizer/disinfectant of your choice.

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• Some research projects show that placement of two coats of bond are better in producing bond and reducing sensitivity than one coat. Below, “bond” means two coats.

• Etch-and-rinse or total-etch indicates etching enamel and dentin with a single solution followed by primer and then bond or prime and bond together (the fourth or fifth types of bonding agents).

• Self-etch indicates use of one of the sixth, seventh or eighth types of bonding agents.

• Selective-etch: Many dentists, including myself, prefer to etch just the enamel using a gel acid, thus leaving the dentin “plugs” relatively unaltered

• Primers and bonds may be in separate containers or distributed combined in one container.

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Techniques for dentin and enamel bonding

Accomplished properly, any of the following concepts will provide successful, predictable clinical results. Select whichever technique is preferable for your patients:

1. Etch-and-rinse (total-etch): Tooth preparation, etch and rinse entire preparation, glutaraldehyde, suction only-do not wash off, primer, bond, composite restorative-either incrementally placed or in realistic bulk quantities

2. Etch-and-rinse (total-etch): Tooth preparation, etch and rinse, glutaraldehyde, primer, bond, thin flowable resin layer, composite restorative

3. Etch-and-rinse (total-etch): Tooth preparation, etch and rinse, glutaraldehyde, bioactive material on deepest dentin surfaces (3M ESPE VitreBond™, GC Fuji LINING LC or BISCO Theracal), second slight application of glutaraldehyde, primer, bond, composite restorative

4. Self-etch: Tooth preparation, glutaraldehyde, self-etch primer, bond, composite restorative

5. Self-etch: Tooth preparation, glutaraldehyde, self-etch primer, bond, thin layer of flowable resin, composite restorative

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6. Self-etch: Tooth preparation, glutaraldehyde, bioactive material on deepest dentin surfaces (3M ESPE Vitrebond,GC Lining LC or BISCO Theracal), self-etch primer, bond, composite

7. Selective-etch: Tooth preparation, etch external enamel margins with gel resin and rinse rapidly, glutaraldehyde, self-etch primer, bond, composite restorative

8. Selective-etch: Tooth preparation, etch external enamel margins with gel resin and rinse rapidly, glutaraldehyde, self-etch primer, bond, thin layer of flowable resin, composite restorative

9. Selective-etch: Tooth preparation, etch external enamel margins with gel resin and rinse rapidly, glutaraldehyde, bioactive material on deepest dentin surfaces (3M ESPE VitreBond or GC Fuji LINING LC or BISCO Theracal), second application of glutaraldehyde, self-etch primer, bond, composite restorative

In summary, all of the preceding techniques, when accomplished properly, were reported to be successful, according to various dental publications; however, after combining this research with formal and informal surveys of practitioners, I prefer technique No. 9 for busy practices with the known challenge of predictably treating multiple patients. This technique has proven clinical predictability and success. If you prefer an etch-and-rinse technique, I prefer technique No. 3 on the previous list.

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