Bonding agents or adhesives, as the name implies, are products that allow us to adhere various types of dental materials to enamel, dentin, and even other dental materials. Unlike the old hydrophobic materials used to bond to enamel, virtually all of the current bonding agents are typically hydrophilic to some extent to be compatible with dentin. However, it is this need to be hydrophilic that has caused most of the problems when it comes to bonding. Therefore, success or failure with these products depends as much on how they are applied as which product you choose to use.
Types of Adhesives
There are many ways to categorize bonding agents. One popular method uses the "generations" nomenclature. But, unless you are a dental historian, there is really no value in using the "generations" system. Who really cares if an adhesive is a "fifth generation" or "sixth generation" or "seventh generation"? Who is counting anyway?
Another way to separate these products is by the number of components. This system became popular (and has been used by us in the past) during the years that the single-component products were being introduced. We still use this nomenclature to describe the products, but not for categorization.
We used to categorize these products as to whether they could be used for all procedures or just those for light-cured, direct procedures. These latter products were not effective when self-cured composites and/or indirect restorative procedures were used. However, even these products can usually be transformed into all-purpose by using various chemical activators. Therefore, we have dropped this separation.
This brings us to the categorization we have chosen to use: etch & rinse (previously known as total-etch), self-etch, and universal. This category covers the self-etch products.
What makes these products self-etch
Instead of using phosphoric acid to prepare the tooth surface for bonding, these products utilize an acidic monomer. In two-component products, this is typically the job of the primer. In single-component adhesives, the bonding agent itself assumes this function. One advantage to this substitution is that the acidic monomer does not require rinsing. This is a big advantage, especially when the rubber dam is not being used.
But the primary reason for the growth of self-etchers is that they seem to cause less sensitivity. This is evidently due to the fact that since etching and priming are being done concurrently, it is not possible to etch deeper than the primer can penetrate.
However, it does not mean a restoration was done properly just because a patient does not have any sensitivity. Self-etchers can work well in Class I, II, and V situations, but the application procedures of these materials can greatly influence their performance maybe even more so compared to etch & rinse.
And their ability to bond to self-cured composites and unprepared enamel is typically below that of the etch & rinse products. Indeed, some self-etchers recommend etching the enamel with phosphoric acid, especially if the enamel is unprepared. One evaluator reported marginal enamel staining as early as the 6-month recall when a self-etcher was used, which is significantly worse than reported for etch & rinse products.
Components and Composition
Most self-etch bonding agents have one or two components, although a few have three. Those with two components typically have the following:
Applied first, this is the component that etches and primes at the same time. It typically contains a solvent (making it hydrophilic), and usually (but not always) does not have any light-curing capabilities. This component is typically premixed and packaged in one bottle, syringe, or unidose, but some primers may have two components that require mixing.
Applied after the primer. Can enhance adhesion by its wetting agent function or it can also have hydrophilic qualities and thus can actually contribute to enhancing the hybrid layer (see below). On the other hand, another theory is that the bonding resin should be hydrophobic to prevent formation of water trees (see below). If its companion primer is one component, the bonding resin may have two components that need to be mixed when it needs self-cure capability. Conversely, if the primer is in two components and needs to be mixed, the bonding resin may only need to be one component. And, in the products that only have a single-component, the primer and bonding resin are mixed together in one bottle.
Some adhesives add different types of fillers to the normally unfilled product for the following reasons:
1. Fluoride release if the filler glass has this capacity. While this is a noble goal, it is highly unlikely that there will be enough fluoride released by the adhesive to be of clinical value.
2. Lower modulus of elasticity to impart added flexibility to a restoration done with a stiff material such as a universal (nanohybrid) composite. Materials with more elasticity, such as microfills, may be better retained in Class V lesions due to their higher flexibility, compared to hybrids. The flexibility could also help to reduce the stress at the interface between the restoration and tooth caused by polymerization shrinkage. In other words, the elasticity of the adhesive could absorb within itself some of the curing stresses. By reducing this stress at the tooth-restoration interface, the restoration could be expected to have better retention. While this theory sounds feasible, it has yet to be proven.
3. Filler could actually reinforce and strengthen the hybrid layer.
4. Filler could help the adhesive cover the dentin surface in one application rather than multiple ones.
Our tests have shown that it is virtually impossible to visualize a bonding agent on an x-ray image when the bonding agent is placed in the very thin layer (about 10 microns) typical of the current products on the market. Therefore, we suggest ignoring radiopacity claims when choosing a bonding agent.
While working time limitations are well-known with light-sensitive restorative materials, this issue has not been investigated thoroughly when it comes to bonding agents. And, although some bonding agents have light-protected dispensing wells as part of their intro kits, this is not universal with all products.
However, based on lab tests and clinical experiences, we have found working times of various bonding agents can fluctuate greatly. Therefore, to overcome working time problems, please follow these rules:
1. Dispense your bonding agent immediately before using it.
2. Use a light filter on your headlight or dental unit light.
3. If you apply a bonding agent on multiple preparations simultaneously, even the ambient light in your treatment room may begin to polymerize highly reactive materials prematurely, possibly before you have had a chance to evaporate the solvent. Therefore, using a light filter shield positioned over the intraoral treatment area to restrict as much as possible the ambient light.
Enamel vs. Dentin
While dentin is the substrate that has garnered most of the interest, enamel is still the most reliable tooth tissue for bonding. Although some initial dentin bond strengths may even exceed those of enamel, the long-term bonding stability favors enamel. In addition, microleakage testing usually shows significantly lower scores on enamel compared to those on dentin/cementum.
The main reason bonding to enamel is more reliable is that it is primarily inorganic. When you use an etch & rinse product, etch-dry-bond is a straightforward procedure. Etching creates micromechanical retention by selectively demineralizing the surface. Once dried, low viscosity resin penetrates into the microscopic spaces in its surface that were created by the etching process. After curing the resin, it tenaciously adheres to the enamel. However, as noted previously, some self-etch primers may not be aggressive enough to etch enamel, especially if it is unprepared, compared to phosphoric acid.
Unlike enamel, dentin is primarily organic, mainly collagen and water, making it a much different substrate for bonding. It is also in direct contact with the pulp, which tells us that whatever we do to dentin must be done in a way that it does not injure the pulp.
Most efforts to bond to dentin have focused on both mechanical and theoretical chemical adhesion. The early adhesives tried to bond to the smear layer, which is basically the layer of preparation debris that is difficult to remove with conventional cleaning procedures, but, paradoxically, is not firmly attached to the dentin for bonding purposes. This means that trying to bond to the smear layer has been unreliable and not very successful.
While the phosphoric acid of etch & rinse products usually removes the smear layer, the primers in self-etch products presumably dissolve and penetrate through the smear layer but do not remove it. With self-etch products, the smear layer actually becomes part of the hybrid layer.
Another issue of dentin bonding is the stability of the bond. This topic rarely arises with enamel, but the question is: Do dentin bonds get weaker or start to hydrolyze over time? There is evidence that does, indeed, suggest this is happening, but its effect on the longevity of restorations is uncertain.
Bond Strength Tests
These are the most common tests done on bonding agents. They can be done relatively quickly and are especially valuable when trying to distinguish between technique variations. Many manufacturers tout the superiority of their adhesives based on these scores. Indeed, there have even been conferences devoted to "battle of the bonds", which are sort of like bond strength Olympiads!
The most common yardstick to judge whether a specific bonding agent has an adequate bond strength is to compare it to an enamel control. Since bonding to frosty, dry, etched enamel using a conventional unfilled resin has been validated with over 30 years of clinical use, equivalent dentin values should be an indication that the product will perform satisfactorily, at least initially.
In the RRL, we periodically test bond strength to dry, frosty enamel using an unfilled, hydrophobic resin (Heliobond). While consistency is our goal, there have been variations in this bond strength test over the years based on the temperature and humidity in the lab, as well as differences in human extracted teeth. The range of these bond strengths have been from 20MPa to 27MPa. Therefore, bond strengths for adhesives that produce similar values should be clinically acceptable.
However, the clinical significance of bond strength tests (and just about all in vitro tests) continues to be debated. No one really knows how much bond strength is enough. It is also unknown whether bonding agents with bond strengths, which exceed that of the enamel control, have any clinical advantages over those with equivalent values.
Therefore, we continue to use bond strength tests as screening devices. For example, when we first discovered clinically that self-cured core materials were debonding when a single-component adhesive was used, we tested this combination in the lab and confirmed that there is a basic incompatibility between most one-component adhesives and self-cured composites. This also applies to some self-etch bonding agents with two components.
Spotting trends and discovering uses of these materials that just do not work are really the main purposes of these tests. Consequently, while bond strengths falling below that of the enamel control may cause some justifiable concern, we do not use these values as the sole determinant in ranking the products, unless the scores are significantly lower than the average in the group.
Our usual testing apparatus (developed by Ultradent) employs a mold that produces a small cylinder of composite about 2.2mm in diameter and 2.0mm in height. While this is adequate for testing bonding agents in the light-cured mode using typical 2.0mm increments of composite, it does not permit testing when bulk filling a deep preparation for a core build-up. To solve this problem, we developed molds that are wider (3.8mm in diameter) to allow easier insertion of the restorative material and in various depths (2mm, 3mm, 4mm, and 5mm). These deeper and wider molds are manufactured by 3M.
All bond strength testing in our lab is done on extracted teeth that have been embedded in acrylic and then flattened. While flattened teeth really do not simulate cavity preparations, this protocol is essential to conduct these tests. The only exception to this "flat tooth" rule is when unprepared enamel is tested. In this case, we select the extracted teeth that are as flat as nature would allow, but the variables of some surface curvature and irregularities are unavoidable. Consequently, we do not perform bond strength tests on unprepared enamel as a rule. Although dentin is the substrate that we are most often testing, all bonding agents are also tested on enamel.
Immediate vs. Delayed Testing
Since we usually begin to stress a restoration with finishing procedures as soon as it is light cured, it is important to know this early bond strength. Consequently, we test each bonding agent as soon as light curing the restorative material is completed. This is our most common test.
Typically used for self-cured products such as core materials and cements to simulate the usual waiting time for these materials to chemically polymerize in the mouth. Some testing of dual-cured materials is also done at this time, but not routinely since we have found only small differences in bond strength between immediate and five minutes. The time starts after the mold is filled and the specimen is placed in an incubator set at 37°C/95% humidity in an effort to simulate the oral environment if a patient is allowed to close during the polymerization period. (While the intraoral temperature and humidity may not perfectly match these parameters in all cases, it should be reasonably close.) Teeth used for these tests are typically warmed to body temperature prior to testing.
Most common test reported by manufacturers. It may produce higher values compared to those recorded immediately. In addition, many research facilities do not have their equipment conveniently arranged for immediate testing, so the 24 hour test is done due to logistical considerations. We use this time delay typically only for dual-cured and self-cured products such as core materials and cements, the latter of which is consistent with the typical advice given to patients to be careful with what they eat for the first 24 hours after luting a restoration. However, some light-cured products are additionally tested at this time interval, the results of which can then be compared to those achieved immediately.
Moisture Status of Preparation
Prior to applying a self-etch primer, you need to know the optimal moisture status of the preparation. Our tests show that this can be one of the most critical steps during the adhesive process. However, the directions from many manufacturers either omit this description entirely or it is covered inadequately. Therefore, we test all bonding agents using three protocols:
Air syringe is used to produce forceful air for 2-3 seconds for a small preparation or longer until the cavity is completely void of visible moisture, but not totally desiccated.
Blot with a lint-free, dry 2x2 gauze, mini-sponge, or virtually any absorbent product that will not leave a residue on the tooth surface. The result should be a tooth surface with no visible moisture, but appears to have either a matte or slightly shiny appearance (depending on the character of the dentin and type of instrument used to prepare it).
This protocol will leave necessary moisture in the dentin. Clinically, you would have to cut the 2x2 gauze (if used) into small sections or use other products such as an applicator tip or mini-sponge to siphon off the excess moisture in small areas. For large areas, such as a complete crown preparation, draping an intact 2x2 over the preparation may be more effective and expeditious.
Use the same type of blotting material described under Moist and dip it into distilled water. Then blot this wet material with a dry 2x2 to create a "damp" blotting material. Use this "damp" material to blot the cavity preparation, leaving tooth surface with a definite sheen of water on it, but be sure all puddles of water are removed.
Note: To create the moist or wet status, many manufacturers will tell you to use your air syringe. Typical instructions will tell you to "dry lightly for 2-3 seconds" or some variation thereof. Our tests show that using air to create moist or wet options is not effective. From a clinical perspective, it is even more difficult. For example, if you are trying to "dry lightly" an MOD preparation, you may merely be blowing the excess water from one proximal box to another. Therefore, the blotting techniques we have developed have proven to be much more effective and easier to calibrate.
In addition, when using the moist or wet option, you should apply the adhesive or primer immediately after blotting the tooth. If you wait even 15 seconds (especially if you are using the rubber dam), some of the moisture could evaporate, which could affect the performance of the adhesive.
Method and Time for Applying the Primer/Adhesive
The manner by which you apply a primer or adhesive can vary from the simple "let sit" of some manufacturers to literally scrubbing it into the surface of the preparation. "Let sit" may be effective in the laboratory on flat test teeth, but it rarely works with the complex surfaces found in many preparations, especially vertical ones. "Scrubbing" is a relative term and the force by which it is accomplished is limited by the application instrument. In other words, the applicator tips that are usually supplied with bonding agents are rarely stiff enough to allow vigorous "scrubbing".
"Gentle agitation", which refers to applying the primer/adhesive and then basically moving it around on the preparation surface to ensure that there is optimal contact in all areas, is in-between "scrubbing" and "let sit". "Rubbing" is basically the same as "scrubbing", but it is presumably accomplished in a less aggressive manner, although from a clinical perspective, trying to differentiate between "rubbing" and "scrubbing" is virtually impossible.
Application time can also vary the effectiveness. Our tests found the best method and optimal time to apply the bonding agent. Be aware that the size of the preparation will affect the application time. While the stated times are application for small Class V preparations, you should extend the times for large Class IIs. In addition, do not start your timer until the preparation is thoroughly wet with primer or adhesive.
Method of Evaporating the Solvent
The monomers in the primers or in bonding agents themselves are typically dissolved in solvents (usually ethanol, acetone, and/or water), which carry these monomers into the demineralized surface of the preparation. Indeed, in self-etching primers, these monomers are acidic and are virtually etching and priming simultaneously. In other words, the primers are first demineralizing the dentin surface so they can penetrate into it and then are reinforcing it through hybridization.
The solvents also tend to be "water-chasing." This property refers to the solvent being attracted to the moisture content of the dentin. As the solvent "chases the water", it literally brings the active monomers with it. Once this job is done, however, the solvent needs to be evaporated, since it can actually interfere with adhesion if it remains in the adhesive.
Part of the reason for the interference is the dilution factor. In other words, solvents literally water-down the adhesive. But even more important is that the hydrophilic nature of a solvent can prevent the bonding of the hydrophobic materials such as composites and cements subsequently being placed.
After application of an adhesive or primer, the excess is usually first removed by suctioning, being careful to allow a continuous coating to remain on the tooth. If you do not suction the excess, there is a tendency to just blow the primer or adhesive from one part of the cavity to another section or onto the soft tissue, which can cause untoward reactions that are typically self-limiting but may provoke undesirable questions from the patient. After suctioning the excess, the solvent needs to be evaporated using air.
The typical method is to place the tip of the air syringe about 1.0cm from the tooth and direct a gentle stream of air (like a breeze) at the preparation surface. Some adhesives, however, require a more forceful air flow, not only to evaporate the solvent, but also to keep the film thickness as minimal as possible so the layer of adhesive does not interfere with the complete seating of an indirect restoration.
When using forceful air, be careful not to blow the primer or adhesive completely off the tooth. This can usually be done by starting the evaporation procedure with gentle air and proceed to forceful. We have determined the optimal technique for each adhesive.
The result of this evaporation effort should be a layer of primer that appears dry (you cannot move it around with the air syringe) and the preparation surface appears shiny. This air-evaporation effort may take five or more seconds, depending on the size of the cavity.
If the dentin surface is not shiny, it may not be sealed. Therefore, it seems prudent to apply additional adhesive and repeat the evaporation process. Typically, the reapplication is merely applying the material and immediately beginning the evaporation process as you do not need to wait as you may have when you applied it initially. If, after the second application, the dentin surface is still not shiny, we recommend applying the adhesive one final time.
Light Curing the Primer/Adhesive
If this is the final component of the bonding agent (such as when you are using a single-component product), you would typically cure it at this time. The conventional curing time has always been 20 seconds. However, this may be overkill for some adhesives. We tested the optimal curing time for each adhesive. But be aware that even these times may need to be altered based on the distance from the tip of the light to the tooth surface. For some bonding agents used in the dual-cured or self-cured mode, in which an activator is mixed with the main component of the kit, you may need to wait until the restorative material or indirect restoration is placed to cure the adhesive or allow it to cure concurrently along with the dual-cured or self-cured composite or cement.
Method of Applying the Bonding Resin
As this component does not typically contain any solvents and, indeed, is usually primarily hydrophobic, you do not have to apply air other than to thin out the layer. While this will work, overaggressive use of the air can lead to overthinning, which, in turn, can affect the performance of the bonding agent. You can alternately remove the excess by using the same applicator, but blotting it with a 2x2 each time you remove more excess. This is known as the "brush on, brush off" technique and works well.
Component(s) to use when bonding a dual-cured core or an indirect restoration such as an onlay
It has been thought that, if you are using a dual-cured core material to build up a tooth or dual-cured cement to lute a thick, indirect restoration, you also need to use a dual-cured adhesive (two components) for compatibility and just in case the light does not penetrate to the tooth surface. However, if you can light cure all the way through the core or to the bottom of the indirect restoration (especially if you position the tip of the curing light on the facial, lingual, and occlusal surfaces and use a high-powered light), then a single-component, light-cured adhesive or the light-cured version of a multi-component product may be quite satisfactory. In addition, a light-cured cement may also work instead of a dual-cured version for the indirect restoration.
Component(s) to use when bonding a self-cured core or an indirect, metal-based restoration such as a ceramometal crown
Similar to the dual-cured core test, this test is done by using the self-cured version of the adhesive (if it has one). The test simulates a direct, self-cured core build-up or indirect restoration cemented with a self-cured resin cement if you waited five minutes after placement to allow the material to chemically-cure.
Miscellaneous Adhesive Issues
Effect of Moisture on Restorative Materials
If a restoration fractures, not only do we have to bond to the remaining section of the restoration, but also to any exposed tooth surface. If dentin is exposed, you would typically want to use an adhesive that will bond securely to the restorative material and to the tooth itself. If your typical bonding agent requires you to leave the tooth moist or wet for optimal bond strength, then it may be difficult to leave the tooth moist or wet but dry the fractured area of the hydrophobic restoration.
We tested the effect of moisture on bond strength to several restorative materials. Our tests confirm that composites and ceramic materials should be repaired dry. Therefore, the use of an adhesive that bonds to dry enamel and dentin reasonably well would be the product of choice.
Effect of Silane on Tooth Structure
When a repair is done, silane is typically applied to the fractured surface of a ceramic restoration and may also be effective on certain composites. The silane would be applied prior to applying the bonding agent on the tooth and restorative material. However, silane is a liquid and, during its application on the fractured surface of a restoration, some of it may flow onto the adjacent tooth surface that also needs to be repaired. With this in mind, it would be important to know if silane could have an effect on the bond of the repair material to dentin.
Our test results show that silane can interfere with the bond of the composite to dentin. Therefore, when applying silane, make every attempt to confine its application just to the fractured restorative material. This confinement will not be possible if you merely douse the tooth with silane. But if you apply it with a fine brush, keeping it off the tooth is certainly a possibility.
With the increased concern about contaminated water lines, products are being added to self-contained water reservoirs on dental units. However, some of these products have not been tested properly as to their effects on adhesion. Our own testing of one product showed bond strengths decreased 22% compared to the untreated control. Therefore, we strongly suggest not adding any disinfectant to your water reservoir without having reliable data on its effect on issues such as adhesion.
Even when the dentin is shiny and appears to be totally sealed, bonding agents that do not employ a final layer of a hydrophobic bonding resin may be subjected to a phenomenon called the "semi-permeable membrane effect". Discovered by Editorial Team Member Dr. Franklin Tay, this effect is characterized by a porous hybrid layer, which allows pulpal fluid to diffuse onto the dentin surface and potentially interfere with the bond between the hydrophobic restorative material and hydrophilic adhesive. This diffusion effect simulates branches on a tree, thus the term "water trees" was created to describe it.
However, while this effect has been shown to exist clinically primarily with etch & rinse products, it remains to be seen what its long-term sequelae may be. Nevertheless, using a multi-component adhesive that includes a final layer of a hydrophobic bonding resin will presumably prevent this problem from occurring and is one reason that the multi-component adhesives are making somewhat of a comeback.
Hybridizing the Dentin at the Preparation Appointment
There is a trend to create a hybrid layer at the preparation appointment for an indirect restoration. This is also known as "IDS (immediate dentin sealing)". The purposes of the hybrid layer in this instance are to protect the pulp by sealing the dentin surface, minimize or eliminate sensitivity between the preparation and seating appointments, and allow seating of the definitive restoration without having to anesthetize the tooth. While these are noble goals, there are also some downsides to this procedure.
In the first place, the hybridizing procedure is done before taking the impression. Depending on the type of adhesive used to create the hybrid layer, the sharp margins of the preparation could be less distinct and more difficult for the lab to see. Second, applying an adhesive after preparing a tooth especially for full coverage may be difficult due to the challenges of adequately isolating the preparation.
Third, the adhesive could bond with the provisional cement if a resin version is used. Fourth, bonding to this precured adhesive layer may not be as adhesive as starting with fresh tooth structure. And, finally, unless you are using a hydrophobic layer over your hydrophilic adhesive, this presumably sealed dentin surface may be susceptible to the formation of water trees. Therefore, this procedure may not produce all of its presumed benefits and may not be worth the effort it takes.
Shaking the bottle/Stirring the Unidose
Many bonding agents suffer from phase separations of their components in bottles and unidose vessels. This means that most bottles should be shaken before dispensing and the contents of unidose vessels should be stirred before applying to the preparation. Some bottles even have a small ball inside to act as a shaker to help mix the components. When in doubt, shake or stir.
In each product commentary, we have listed the application techniques that we have found to be most effective and practical for clinical use. In some instances, these procedures may vary from the manufacturers directions. However, we feel strongly that our techniques will give you the best chance for success and will minimize failures.
Please see the Introduction to Etch & Rinse Bonding Agents for a step-by-step photographic application procedure. While all the steps will not obviously apply to self-etch bonding agents, many of them have utility with both types of adhesives.