DR. PHILLIPS What factors should be considered in the choice of an adhesive?
MR. GANGE The choice of adhesive depends on the individual needs and specific preferences of each doctor. A light-cured adhesive is recommended for unlimited working time. A no-mix adhesive is quick, reduces waste, and lessens peripheral flash. A two-paste mix system provides consistent bond strength with reduced technique sensitivity. A dual-cure system affords the operator the benefits of a light cure and the complete, consistent cure of a mix adhesive. One point to consider is that all the systems, when applied correctly, provide enough strength to bond metal, ceramic, or plastic brackets molar to molar. Therefore, the deciding factors should be which system best fits the office structure, and the speed and accuracy of the operator applying the brackets.
DR. PHILLIPS How does a no-mix adhesive lessen flash?
MR. GANGE With a no-mix adhesive, the flash that extrudes from the periphery of the bracket base contacts the primer (the catalyst) only from one side (the tooth). Therefore, the majority of the paste will not totally polymerize, making cleanup quicker and easier. A whole arch of brackets can be placed, and then the operator can go back to clean the flash from the first bracket. But a light-cured adhesive would provide the easiest, least stressful flash cleanup.
DR. PHILLIPS How does filler particle size influence bond strength?
MR. GANGE With the introduction of resins filled with smaller particles, the particle size of the filler in adhesives is not as relevant now as the technique for applying the adhesive to the bracket base. The finest mesh used on metal brackets is 100 gauge, which can accommodate up to a 155-micron particle size of filler. The largest filler utilized in any orthodontic adhesive is about 45 microns.
DR. PHILLIPS How does bracket base size influence bond strength?
MR. GANGE Bracket base size and bond strength are in direct proportion. Obviously, the bicuspid and molar brackets produce the highest bond failure rate because they are subjected to the highest level of force. A larger bracket base is desirable on these teeth to increase the surface area of the bonded interface. For esthetic value, the base size of brackets has been reduced recently, and the mesh size has become finer.
Regardless of base size, contouring the bracket to the anatomy of the tooth to be bonded will increase the bond strength. The better the fit of the bracket, the more uniform thinness of adhesive resin layer is achieved with better stress distribution.
When paste is applied to a metal bracket, it should be buttered into the base with a small instrument. If a syringe delivery system is employed, the applicator tip should be used to work the paste into the base, ensuring mechanical retention. The majority of bonding pastes are made thick enough to prevent bracket flotation after placement. Bonding adhesives are low-film-thickness materials that produce higher values in a thin layer, which reduces voids.1
DR. PHILLIPS What is an adequate bond strength? Does it vary from tooth to tooth?
MR. GANGE Dr. Reynolds showed the average amount of force applied through mastication on anterior brackets is 5 MPa, whereas on posterior brackets it is 20 MPa.2 Accordingly, for uniform results throughout the mouth, the adhesive should withstand forces in the 20 MPa range.
DR. PHILLIPS Should orthodontists be selecting different adhesives for different bracket locations?
MR. GANGE Technically, an orthodontist could bond successfully using a weaker adhesive on anteriors and a stronger adhesive on posteriors, but this is not practical or necessary. The majority of adhesives available today, when correctly integrated with proper tooth-preparation techniques, will provide strengths in the 20 MPa range, considerably reducing the risk of any significant enamel damage upon removal.
DR. PHILLIPS How much do the shape and age of the tooth surface itself influence bonding?
MR. GANGE There are no studies to support the contention that there is a clinically significant difference between etching old vs. young enamel. Proper tooth preparation is imperative regardless of the age or shape of the enamel. The majority of bond failures could be eliminated if the enamel were properly prophied, isolated, etched, rinsed, and dried prior to cementation.
DR. PHILLIPS What are the ideal preparation steps to insure maximum adhesion?
MR. GANGE Preparing a tooth for bonding involves a mechanical procedure (prophy) and a chemical procedure (acid etching). First, any calculus should be scaled from the enamel. Then the prophy should be performed with a watery slurry of pumice. The teeth to be bonded should be thoroughly cleaned and rinsed prior to acid etching. The enamel should be air-dried and properly isolated. Then the etchant should be applied to the enamel with a dabbing motion. The acid should not be rubbed onto the enamel, as this may damage the enamel rods and reduce the surface area to be bonded. After etching for 15 to 30 seconds, each tooth should be rinsed for 10 seconds with a water-air spray (or for 15 seconds if a gel etchant is used). One should not etch for more than 90 seconds, because an insoluble layer of calcium phosphate salt may form on the surface and impair resin penetration.
The purpose of a thorough rinse is to remove soluble reaction product from the enamel, whose surface energy or clinical adhesiveness is now raised by the chemical conditioning. Each tooth should be desiccated with air free of oil, water, or other contamination. Saliva contamination of the enamel from this point on requires only re-etching. The preparation procedure chemically cleans the enamel, although the result cannot be seen with the naked eye, except for a frosty appearance. If the operator deviates from the prescribed method, there is no way to determine visually whether the surface is acceptable for bonding.
DR. PHILLIPS There have been studies that seemed to indicate that a weaker etching solution and less etching time than traditionally used would be adequate. What is the least etching strength and time needed for an adequate bond?
MR. GANGE Recently, at the University of Michigan, we tested several concentrations of phosphoric acid and found that a 37% concentration was the most consistent.3 Fifteen-second exposure with 37% concentration of phosphoric acid on normal permanent enamel is enough to produce the depth required for optimum bond strength (20 MPa).
There is no significant increase in adhesion when etching longer. Moreover, etching for longer than 90 seconds will actually decrease adhesion, as I mentioned, due to the dissolution of the enamel rods and the formation of an insoluble calcium phosphate reaction product. Even with longer etch times up to 90 seconds, aprismatic, hypocalcified, and fluorosed enamel may provide inconsistent results. These situations should be treated with a coupling agent such as Enhance Adhesion Booster prior to bonding to ensure consistent results.
DR. PHILLIPS How do these coupling agents work?
MR. GANGE On the enamel side, the chemical components in Enhance are of such low viscosity that they completely penetrate the microporosities of the enamel. Enhance thoroughly wets the enamel surface, which is a chemical prerequisite for good adhesion. Some unfilled resins or bonding pastes are of such high viscosity that they can't adequately penetrate the microporosities of the enamel surface.
On the bracket side, the polymerization shrinkage of the composite material affects adhesion to enamel. Enhance contains NTG-GMA, a powerful accelerator of self-curing composite. When a bracket is bonded to an Enhance-wetted tooth, the enamel surface is wetted properly and the polymerization shrinkage is reduced, resulting in increased adhesion. Long-term in vivo results in areas where fluorosed enamel is prevalent show that Enhance dramatically reduces bond failure. It has made it possible to successfully bond, rather than band, fluorosed teeth.
DR. PHILLIPS Do you note any increase in fluorosis in the United States?
MR. GANGE We have not done any formal surveys, but when asked, almost all orthodontists--regardless of their geographic location-- have stated that they have patients with fluorosed enamel. Ten years ago, that situation was not prevalent. I believe that due to the large amount of fluoride ingested by patients from birth--from vitamins, water supplies, toothpaste, and fluoride treatments--the incidence of fluorosis has definitely increased.
DR. PHILLIPS Can we increase the adhesion of composites to brackets and eliminate the mesh screen by microetching the bracket bases?
MR. GANGE Microetching of the mesh base of brackets or the interior of bands can significantly increase the retention between the adhesive and the bracket or band. The manufacturers recognize this benefit, which is the reason several bracket bases and band interiors are now factory-microetched. A study we recently performed shows an increase in adhesion of 33% with microetched metal brackets and of 400% with microetched bands. As far as eliminating the mesh screen, the retention to metal is primarily mechanical. Any base design must incorporate a mechanical locking device for the adhesive to permeate prior to polymerization.
DR. PHILLIPS You mentioned several advantages of light curing. What other benefits do light-cured adhesives offer orthodontists?
MR. GANGE Light-cured adhesives provide two advantages over chemical systems: unlimited working time in an ambient-light-controlled environment, and immediate arch ligation. The common misconception about light-cured adhesives is that they produce higher bond strengths. While they have improved physical and mechanical properties, their adhesion to enamel is not greater than that of self-curing composites. However, compared to their chemically cured counterparts, light-cured systems have fewer operator steps and can result in faster, more consistent results.
Where a light-cured system is particularly advantageous is when an enamel-protective sealant is desired. Most self-curing sealants do not polymerize adequately in a thin layer due to oxygen inhibition, and are rinsed off immediately from the exposed enamel surrounding the bracket base. However, Maximum Cure is a fluoride-releasing, self-curing sealant that will polymerize in the presence of oxygen and has been shown to be an effective enamel sealant.4,5
Light Bond Filled Sealant is a fluoride-releasing, light-cured sealant that contains a small-particle glass filler to reduce abrasion from toothbrushing. Oxygen inhibition is not a factor because it is light-cured. Thus, a thin protective layer can be applied to the entire labial surface prior to the placement of the bracket. Several clinicians have reported this sealant beneficial in controlling decalcification in patients with poor hygiene.
DR. PHILLIPS Do quartz-filled sealants protect the enamel better than the non-filled sealants, and do they last throughout treatment?
MR. GANGE In vivo reports reveal that the effectiveness of Maximum Cure and light-cured filled sealants are difficult to gauge from patient to patient. Light Bond will outperform Maximum Cure 2 to 1 in terms of resistance to toothbrushing, but both were found on the enamel of patients with poor brushing habits at the conclusion of treatment.5 Another study showed that Maximum Cure increased bond strength when used with a two-paste mix system.6
DR. PHILLIPS Do light-cured composites polymerize more quickly and completely than the self-curing types?
MR. GANGE When properly cured from a close, direct angle, light-cured composites polymerize within 20 to 30 seconds, which allows immediate archwire ligation. There is a small degree of post-cure that occurs after the light is removed from the area, but not enough to significantly increase the bond strength. However, it is important to note that light energy falls off with the reverse of the square of the distance. The closer the light director is placed to the adhesive or sealant, the quicker and better the cure. That is why we always recommend curing a metal bracket from the incisal edge with the light directed parallel to the long axis of the tooth and as close to the adhesive pad as possible. An additional curing should be done from either the mesial, the distal, or the gingival margin, aiming the light between the bracket base and the enamel. Light-cured adhesive cures quickly and efficiently with a minimal amount of post-cure when the light is positioned directly on the labial surface of a ceramic or plastic bracket, because the light source is close to the adhesive and unobstructed. It is also of utmost importance to regularly monitor the energy output of curing lights with a radiometer. Reduction in output of the light energy can significantly affect the degree of polymerization .
Self-curing composites, conversely, should be allowed to fully polymerize for five minutes after initial set before active archwires are engaged.
DR. PHILLIPS Does indirect bonding produce weaker or stronger bond strengths than direct bonding?
MR. GANGE Indirect bonding does not produce any higher or lower bond strength compared to direct bonding. However, clinicians such as Drs. Frank Shamy, Jack Hickham, and Thomas Creekmore have demonstrated a reduction in bond failures with indirect bonding.7-9 I believe this can be attributed to a couple of factors: better conformation of each bracket to the anatomy of each tooth, and the reduction of chairtime during bracket placement, which reduces the risk of saliva contamination, especially on posterior teeth. It is important to remember, though, that an indirect bonding tray is difficult to fabricate, and if it is not fabricated properly, a space can form between the bracket base and enamel, resulting in a weak or failed bond.
DR. PHILLIPS What role do glass ionomer cements have in orthodontics?
MR. GANGE Glass ionomers offer increased compressive strength (or less washout) and increased adhesion to enamel over zinc phosphate cements, due to the presence of polyacrylic acid. They are also more forgiving in a slightly wet environment because they are hydrophilic. We recently developed a fluoride-releasing, two-paste glass ionomer cement, Band Lok, that provides higher compressive and tensile strengths than a conventional powder/liquid, water-based glass ionomer cement.10 Band Lok has also proven to have better chemical adhesion to metal and a more consistent mix viscosity.
DR. PHILLIPS What benefits should orthodontists expect from the new light-cured glass ionomer cements?
MR. GANGE Light-cured glass ionomers have more desirable physical properties than standard chemically cured glass ionomers. However, they are still water-based cements, as opposed to the new generation of resin-based glass ionomers, which produce significantly greater values.
DR. PHILLIPS What can orthodontists do to insure better adhesion of plastic brackets?
MR. GANGE The plastic brackets being marketed today are far superior in design and materials to their earlier counterparts. As a result, the technique is more involved. For instance, when using a mix or light-cured adhesive system, the bracket base should be primed with a plastic conditioner and allowed to dry. Then the sealant is applied to the bracket base and the enamel surface before the adhesive paste is put on. This insures chemical bonding between the adhesive and the bracket base.
Our no-mix adhesive, Rely-a-Bond for Plastic, contains an additional ingredient to promote chemical adhesion to plastic. Otherwise, one should prime the bracket base with plastic conditioner, allow it to dry, and then apply the no-mix primer to the base and enamel and the paste to the bracket.
DR. PHILLIPS Bonding to porcelain seems to be inconsistent at best. What is the most dependable method?
MR. GANGE With the new and varied types of porcelain used today in veneers and crowns, the most consistent bonding procedure is as follows:
1. Lightly roughen the porcelain surface with a green stone, or abrade it by microetching with fine aluminum oxide.
2. If a rubber dam is not available, place a barrier gel on the gingiva adjacent to the crown, and carefully protect the tissue with cotton rolls.
3. Place an 8% hydrofluoric acid on the porcelain surface, and allow it to remain for three minutes.
4. Wipe the bulk of the acid off with a cotton roll. Carefully and thoroughly rinse the surface into suction.
5. Air-dry the surface, then apply a generous coat of a silane treatment such as Reliance Porcelain Conditioner. It is important to insure hydrolysis of the silane, and therefore acid conditioning of the porcelain is a prerequisite.
6. After 60 seconds, apply your bonding sealant or primer in a thin layer, and place the bracket.
7. To restore the luster of the porcelain after debonding, simply polish the surface with a diamond polishing paste such as Restore in a prophy cup.
DR. PHILLIPS Do shear strength, tensile strength, and crushing strength of composites have any clinical significance to orthodontists?
MR. GANGE The most representative measure of force for orthodontic bonding is resistance to shear or peel shear forces. Orthodontic adhesives must attain an acceptable diametral tensile strength to resist cohesive failure. We should keep in mind at all times that orthodontic adhesives, unlike restorative materials, must be removed from the enamel--generally within two years--without damaging it. Excessively strong materials, therefore, are not practical and could pose a risk for bonding orthodontic brackets.
Wear resistance is a practical requirement for bonding fixed lingual retainers. Adequate adhesion to enamel, porcelain, and composite should be the measuring stick for an orthodontic adhesive.
DR. PHILLIPS When investigators examining bond strength flatten the enamel surface, are the results valid?
MR. GANGE There has been no evidence to suggest that grinding or flattening the enamel has any impact on the adhesion factor in bond tests.11 As long as the bracket base fits the approximate anatomy of the tooth, grinding the enamel will not alter the results.
DR. PHILLIPS With so many variables involved in bonding, how does one evaluate a published study? To what extent can in vitro studies be extrapolated to in vivo surmises?
MR. GANGE In vitro studies allow us to develop materials with ideal physical properties and test their performance under ideal conditions. Unfortunately, the orthodontist does not encounter such conditions in the workplace. Placing a bracket on a lower second bicuspid of a squirming, salivating youngster cannot be equated with placing a bracket on a composite surface in a stress-free laboratory environment. In vitro studies provide very important data concerning the physical and mechanical properties of a material, but the final evaluation can only be provided by in vivo clinical assessment.
A two-year clinical test of a bonding adhesive, noting failure sites and rate of occurrence, would be much more valuable than a shear bond strength test in a laboratory. Many bonding systems are technique-sensitive--in other words, too much paste, too much primer, or manipulation of the material into the gel period can greatly affect the resulting strength. We test all substances in vitro as a screening mechanism, but our final decision to market a product is based solely on the in vivo performance of the material in actual patients.
DR. PHILLIPS For testing purposes, is there a difference between human and bovine enamel?
MR. GANGE Because of supply and demand, several studies have been performed with bovine enamel as a replacement for human teeth. The morphology of bovine enamel is different in that the surface is not as easily wetted after etching. Consequently, bovine enamel will produce bond strengths approximately 25% lower than human enamel. In vitro comparisons should always be performed on the same surfaces--bovine vs. bovine or human vs. human.
DR. PHILLIPS Does the use of fluoride prophylaxis pastes diminish the bond strength of composites?
MR. GANGE Recent studies indicate that the use of a fluoride prophylaxis paste or a fluoride treatment prior to bonding does not diminish the bond strength to etched enamel.12
DR. PHILLIPS What is the effect of fluoride-releasing composites on bond strength?
MR. GANGE There are two types of fluoride-releasing composites: organic (hydrogen fluoride and boron trifluoride) and inorganic (sodium). The organic ones work by means of an ion exchange with the saliva and thus do not weaken the resin matrix. An HF or BF3 ion leaches from the composite and is replaced by an OH ion from the oral environment.13 Inorganic fluoride-releasing composites do not replace the fluoride lost. This increases the rate of water absorption and, consequently, weakens the resin matrix. Since orthodontic adhesives are in the mouth for only two years, this weakening generally has no effect on bracket bond strength.
DR. PHILLIPS Do fluoride-releasing composites prevent caries or decalcification?
MR. GANGE The sole means of determining this is through extensive in vivo testing. Fluoride composites release a large amount of their fluoride in the first few days in the mouth. After this initial burst, the amount decreases significantly and levels off to a low release of a few parts per million. There are theories that the burst effect will help expedite the remineralization of the enamel around the periphery of the bracket base. This is of tremendous benefit in itself. Acid etching removes the fluoride-rich surface layer of enamel, and it is believed that the resulting surface is prone to bacterial build-up and possible decalcification. We know that fluoride is beneficial in reducing caries formation. For patients with poor hygiene, the introduction of fluoride into the oral environment has a positive effect, but to what degree has not been proven.
DR. PHILLIPS How deeply do composite tags penetrate into etched enamel?
MR. GANGE Dr. Silverstone reported that etching with 37% phosphoric acid for 60 seconds will result in a depth of etch of about 10 microns, with an additional 20 microns of histological change.14 Therefore, the composite tags could penetrate to a depth of 30 microns. Shorter etching times would significantly reduce this amount, but bond strength is determined by how well and how thoroughly the surface area of the enamel is wetted, not by how deeply the composite tags penetrate the enamel. Again, proper surface wetting can only be achieved on properly prepared enamel.
DR. PHILLIPS How significant is the amount of enamel lost in debonding?
MR. GANGE In three published studies, the range of enamel loss was from 5 to 55 microns when a highly filled adhesive was bonded to etched enamel.15-17 The discrepancy in the amount of enamel loss can be traced to the aggressiveness of the rotary instrument used to remove the adhesive.
Furthermore, these studies were undertaken on teeth that were acid-etched for 90 seconds with 37% phosphoric acid. The current average etching time is 15 to 30 seconds. Assuming an aggressive bur were used to remove all the adhesive, the maximum amount of enamel lost would be 10 microns. The point of concern, however, is the removal of the fluoride-rich surface layer, and how quickly that protective layer is restored. Etched enamel is believed to take 72 hours to remineralize. Poor hygiene during that time could lead to decalcification.
DR. PHILLIPS What is the optimum debond from the point of view of fracture site, enamel and tooth integrity, and patient comfort?
MR. GANGE To maintain tooth integrity, the ideal failure site in debonding is between the bracket base and the composite. While this will increase the duration and difficulty of cleanup, it will reduce the risk of enamel damage. The composite can be removed with a finishing bur and the enamel polished with points, wheels, or discs. With most metal brackets, when the base is squeezed, it will cause a mixed cohesive fracture within the adhesive, leaving composite on the enamel and in the mesh.
DR. PHILLIPS What are the advantages of thermal debonding?
MR. GANGE Thermal debonding is designed to soften the adhesive.18 This causes a weakening between bracket and adhesive, resulting in easier bracket removal with no enamel damage. Residual adhesive must still be removed mechanically from the enamel, but when compared to pinching or squeezing, the risk of enamel fracture is virtually eliminated.
DR. PHILLIPS Are there any new developments in research that promise to change or improve bonding of orthodontic brackets?
MR. GANGE Orthodontic bonding changes dynamically as we speak, and we strive to improve adhesion to all surfaces, as well as to better protect the enamel. Since acid-etching removes the fluoride-rich surface layer of enamel, our goal is to eliminate etching, which we believe would reduce decalcification and save time. This can't be done yet, however, without adversely affecting bond strength.
DR. PHILLIPS Thank you for your insights into this complex subject.
