The bonding of orthodontic appliances has progressed from weak, messy, odiferous powder/liquid methyl methacrylate cements1 and brittle plastic brackets to sturdy, esthetic appliances bonded with strong, durable adhesives. Orthodontic bonding has experienced more advances in the last ten years than during the previous two and a half decades. Wet field adhesives, enamel protective sealants, atypical enamel adhesives and self-etching primers are a few of the product developments that have made placing appliances more dependable. Orthodontic appliances are bonded with chemical, light and dual cure systems that can be equally successful if their inherent variables are controlled.
Bonding success or failure starts with the tooth surface. The proper conditioning of the surface of the tooth, whether it is enamel, composite, porcelain or metal, is a variable that must be controlled when bonding a bracket with adhesive. A surface that is not conditioned properly or one that becomes contaminated cannot produce satisfactory bond strength regardless of adhesive type. Worldwide light cure systems are slightly more popular for full fixed appliance replacement than chemical cure and gaining more market share each year. Let’s take a closer look at the adhesive systems available.
The two type of chemical cure systems used to bond attachments are no mix and two paste mix. The no mix system has two components: a single liquid primer (catalyst) and a single paste. The primer is applied to the etched, dried enamel and the bracket base. The paste then is applied to the bracket base and the bracket is placed on the tooth. The primer serves as the catalyst for the paste and therein rests the variability with this system. Even though it is termed “no mix,” this system actually mixes the paste and primer directly on the tooth. Therefore, to mix the paste and primer properly and achieve maximum strength, the bracket base must fit the tooth surface flush. A thin as opposed to a thick coat of primer must be applied to the enamel and the bracket base.2 The primed/pasted bracket is placed on the tooth and pressed to place when in the desired position. To summarize, the variables with a no mix system are:
1) Bracket base must fit the tooth surface flush;
2) Thin coat of primer on tooth surface;
3) Thin coat of primer on the bracket base;
4) Thin layer of paste on the bracket base.
No-Mix Systems
Rely.a.Bond (Reliance Orthodontic Products, Inc.)
Right On (T.P. Orthodontics)
System I (Ormco)
Unite (3M Unitek)
The mix adhesive, the most popular of the chemical cure systems, requires the mixing and application of two liquids (bonding resin) to the enamel and the mixing and application of two pastes to the bracket base. As long as the operator thoroughly mixes the two pastes and does not place brackets after the working time has expired, maximum strength can be achieved consistently. The mix system has the fewest number of variables of any adhesive type. With all chemical cure systems, archwires should not be ligated for at least five minutes, preferably ten minutes, after the placement of the last bracket.
Two Paste Mix Systems
Phase II (Reliance Orthodontic Products, Inc.)
Concise (3M Unitek)
Light cure composites have become very popular for bracket placement because they offer the following benefits:
1) Unlimited working time to position bracket and clean up flash;
2) Immediate archwire placement;
3) More efficient utilization of staff.
Light cure composites polymerize due to a reaction between the catalyst in the adhesive and the photon emitted by the light-curing source. The wavelength of the photon, measured in nanometers, must coincide with the catalyst utilized in the material. The strength or intensity of the light source, which determines the length of curing time, is measured in mW/cm2. The higher the light intensity, the faster the cure.
The biggest advancements in light cure bonding have been with the curing lights and not the composites. The evolution has been from bulky, corded halogen curing lamps to lightweight, portable, LED lights.3 The LED (light emitting diode) has a diode rather than a bulb that emits photons in the same manner as a halogen lamp that reacts with the catalyst in the composite. The diode usually has a useful life of 10,000 hours as opposed to a halogen bulb with a life of approximately 50 hours.
All curing lamps, whether halogen or LED, have a common deficiency: They emit photons in a divergent manner, which means the light director should be placed as close to the bracket base or the composite target as possible. The key to quick polymerization resulting in maximum bond strength is to bombard the adhesive with as many photons as possible. This topic leads us to a discussion of the variables that determine bond strength when using a light cure system.
1) The light must be of a proper intensity to cure the composite.
2) The light must emit a photon that will cure the specific catalyst properly within the composite. Most cements cure in the blue spectrum (440-480 nanometers).
3) The light director must be placed as close to the bracket base or composite base as possible.
4) The light must maintain its intensity when illuminated for a period of time while curing multiple brackets.
5) The amount of curing time is determined by the intensity of the light. 10,000 units of energy are required for maximum bond strength. The formula to achieve this is as follows: Intensity of light (mW/cm2) multiplied by length of curing time (seconds) equals 10,000. Therefore, if your light has an intensity of 1,000 mW/cm2, you must cure 10 seconds per bracket.
L.E.D. Curing Lights
L.E.D. Demetron (Ormco)
TuTu (G&H Wire Products, Inc.)
Unitek L.E.D. (Unitek)
Vivadent Blue Phase (Vivadent)
NM 468 (Reliance)
Innovations from a materials standpoint have been the development of light activated color change adhesives, which aid in removal of peripheral flash at the time of bracket placement. Recently, a fluorescing adhesive paste was introduced that discloses flash at bracket placement and adhesive remnants left after debonding and cleanup. The latest development in reducing decalcification is Pro Seal, a fluoride light cure filled sealant with a proprietary catalyst that sets the resin without an oxygen inhibited layer. This complete polymerization prevents oral fluid absorption and reduces toothbrush abrasion. Consequently, this sealant can protect the enamel of poor hygiene patients for an extended period of time. The catalyst in Pro Seal is activated in the purple spectrum, 390-430 nanometers. Halogen bulb lights emit high intensity in the purple and blue range. Most LED’s focus intensity in the blue spectrum only.
Light Cure Systems
Transbond (3M Unitek)
Light Bond (Reliance Orthodontic Products, Inc.)
Enlight (Ormco)
Eagle No Drift (American Orthodontics)
Blue Gloo (Ormco)
Pad Lock (Reliance Orthodontic Products, Inc.)
Enamel Protective Sealants
Pro Seal (Reliance Orthodontic Products, Inc.)
The Dual Cure system, the least popular choice for placing brackets, is a two-part system (chemical cure) with a light activator. The benefit to this system is that the operator generally has additional working time (four minutes) to place appliances and clean up flash, then immediately harden the composite by light curing to prevent appliance movement and protection from contamination. The chemical cure mechanism will continue to cure after exposure to the light, ensuring a complete polymerization. The variables with the dual cure system are as follows:
1) Operator cannot place or move appliances after the chemical cure working time has expired.
2) At some point, the mixed composite should be light cured or additional time allotted for the chemical cure to adequately set.
Dual Cure Systems
Phase II Dual Cure (Reliance Orthodontic Products, Inc.)
BONDING… SUCCESS OR FAILURE?
The answer to this question starts with the tooth surface. Now let’s examine the various tooth surfaces and their proper conditioning for success.
Enamel
Enamel preparation is accomplished by one of two methods: traditional phosphoric acid etching or self-etching primers. Both systems require a clean tooth surface to be successful, which means a through prophylaxis, preferably with a rotary instrument and medium pumice. The traditional phosphoric acid etching is performed with 37% concentration, available in a liquid or gel consistency. The recommended etch time for this concentration of phosphoric acid is a minimum of 15 seconds and a maximum of 90 seconds per tooth. There is no strength advantage to etching more than 15 seconds, but a definite reduction in strength occurs if the acid remains on the tooth surface more than 90 seconds. Over-etching causes dissolution of the enamel rods and the formation of an insoluble calcium phosphate salt crystal. Phosphoric acid etched enamel then must be rinsed thoroughly (five seconds per tooth liquid etch, ten seconds per tooth gel etch) and dried before the application of the bonding resin.
Recently, self-etching primers, a combination of etchant and bonding resin, have gained popularity because they eliminate the rinsing step and subsequent application of the bonding resin. Contrary to the application of phosphoric acid, which is dabbed on the enamel, the self-etching primer mixture is scrubbed onto the surface for five seconds and then dried.4 After drying, the enamel should appear resin shiny, not wet shiny, which could indicate the presence of water, a component of self-etching primers that could weaken the bond if not removed. Self-etching primers have several clinical applications in today’s orthodontic practice such as preparing the enamel surface for:
1) rebonds;
2) bonding ceramic brackets, where enamel damage during debonding is a concern;
3) short term bonded acrylic appliances (RPE maxillary part of Herbst appliances);
4) wet field bonding with a powder/ liquid glass ionomer cement;
5) initial appliance placement.
Self-etching primers are hydrophilic, which means that they will adhere to a slightly wet surface. However, if they become contaminated after application, they must be reapplied.
Self Etching Primers
Transbond Plus (3M Unitek)
Reliance SEP (Reliance Orthodontic Products, Inc.)
Atypical Enamel Surfaces
Enamel surfaces that are fluorosed, hypocalcified or deciduous produce inconsistent results when conditioned in the conventional manner. Hydrophilic Universal Bonding Resins have solved the problem of bonding to these surfaces. Assure, Enhance and Ortho Solo are bonding resins that have been successful in restoring bond strength to these atypical enamel surfaces.5 Multiple coats of Assure, Enhance or Ortho Solo are applied to phosphoric acid etched enamel with a light drying of the surface after the last coat. The appliance then can be placed with any chemical or light cure system.
The treatment of adult patients poses a different problem in bonding appliances to dentin and bleached enamel. Conventional or self-etching primer preparation of these surfaces alone does not produce acceptable bond strength levels. Consistent maximum bonding strength can be achieved in the following manner:
Dentin
1) Prophy, rinse, 30-second phosphoric acid etch, rinse and lightly dry. The dentin should be left moist. Do not desiccate.
2) Apply four coats of Assure Universal Bonding Resin, and lightly dry after the fourth coat.
3) Apply bracket with chemical or light cure paste to the conditioned dentin surface.
Bleached Enamel
It has been shown that patients who bleach their enamel regularly should not have appliances placed for at least two weeks after the last bleaching application. F.R. Tay et al, from the University of Hong Kong reported in a 2002 study, that compromised bonding to peroxide bleached enamel can be reversed with sodium ascorbate before bonding The recommended technique for consistent results when attempting to bond to bleached enamel is as follows:
1) Prophy the enamel with medium pumice, rinse and dry.
2) Prophy the enamel a second time with a slurry of water and a crushed vitamin C tablet. Allow the slurry to remain on the surface for 1 minute. The vitamin C will neutralize the effects of the residual peroxide on the enamel. Rinse and dry thoroughly.
3) Acid etch with phosphoric acid for 30 seconds, rinse and dry thoroughly.
4) Apply four coats of Assure Universal Bonding Resin or Ortho Solo; lightly drying after the fourth coat.
5) Place bracket with paste on the conditioned enamel.
Hydrophilic Bonding Resins
Assure Universal Bonding Resin (Reliance Orthodontic Products, Inc.)
Ortho Solo (Ormco)
M.I.P (3M Unitek)
Enhance (Reliance Orthodontic Products, Inc.)
Composite restorations
The composite restoration must be roughened mechanically with a fine diamond or microetcher, rinsed and dried. Then the surface is conditioned with a plastic conditioner or bonding enhancer followed by a bonding resin or a universal bonding resin that will bond to composite. The surface then is ready to accept the bracket with paste.
Composite Surface Conditioners
Reliance Plastic Conditioner (Reliance Orthodontic Products, Inc.)
Ormco Plastic Conditioner (Ormco)
Assure Universal Bonding Resin (Reliance Orthodontic Products, Inc.)
Enhance Adhesion Booster (Reliance Orthodontic Products, Inc.)
Porcelain crowns
Porcelain crowns and veneers are being fabricated with increased amounts of alumina oxide and zirconium to increase strength. The most reliable procedure for any type of porcelain bonding is to sandblast the surface, thus breaking the glaze. The glass and porcelain then are etched for four minutes with hydrofluoric acid, rinsed and dried thoroughly.6 Placement of a rubber dam or a gingival barrier gel is strongly recommended to protect adjacent soft tissue from the hydrofluoric acid. The etchant then is rinsed into suction and the surface dried. Porcelain conditioner (silane) is applied in a very thin layer and allowed to dry before applying a bonding resin that will bond to metal such as Assure in the event there is a high content of alumina oxide. Next, the bracket with paste is applied to the conditioned surface.
Porcelain Etchants
Porc Etch (Reliance Orthodontic Products, Inc.)
Ultra Dent Porcelain Etch and Silane (Ultra Dent)
Reliance Porcelain Conditioner (Reliance Orthodontic Products, Inc.)
Ormco Porcelain Conditioner (Ormco)
Metal (gold, amalgam, stainless steel)
Micro-etching (sand blasting) of any metal surface in the mouth is imperative to be successful.6 Sand blasted metal produces twice the strength of medium bur roughened enamel. The mechanically prepared metal is treated chemically with either a 4meta metal primer, bonding enhancer that bonds to metal followed by a bonding resin, or a universal bonding resin that bonds to metal. At this point, the prepared metal surface is ready to accept a bracket with paste. If a light cure composite is used, it is recommended to cure longer than if bonding to enamel, as the light reflection of the metal is not as effective.
Metal Primers
Assure Universal Bonding Resin (Reliance Orthodontic Products, Inc.)
Reliance Metal Primer (Reliance Orthodontic Products, Inc.)
Enhance Adhesion Booster (Reliance Orthodontic Products, Inc.)
Occlusal buildup
One of the most effective ways of creating a bite-plate effect in deep overbite patients is the placement of composite on the occlusal surface of the posterior teeth. The adhesive is generally placed on the occlusal surface of the first molars. The adhesive can be gradually reduced in height as the bite opens. If the first molars have been
restored, bonding may be difficult, in which case the second bicuspids as well as the second molars may be built up. The bonding material must have a wear factor similar to enamel and should color contrast the enamel to expedite removal. These same materials may also be used to facilitate placement of lower incisor brackets, in deep overbite cases, by bonding stops or incisal ledges on the lingual of the maxillary incisors.
Occlusal Build-up Materials
Herculite (Kerr Company)
Ultra Band Lok Blue (Reliance Orthodontic Products, Inc.)
Flow Tain (Reliance Orthodontic Products, Inc.)
Bonding large acrylic appliances
In the early 1980s, McNamara and Howe7,8 developed the acrylic splint Herbst appliance in an attempt to replace the breakage prone banded Herbst.8 Since then, several different acrylic appliances have been utilized for orthodontic procedures, including the bonded acrylic splint expander. The maxillary part of the acrylic splint Herbst appliance can be either bonded or removable, while it is recommended that the mandibular part of the appliance always is removable.
These appliances are fabricated out of 2.5 – 3.00mm thick splint Biocryl. The occlusal surfaces are not etched, making removal manageable. When bonding a large acrylic appliance, the buccal and lingual enamel surfaces are etched as is the distal surface of the last molar, but the occlusal surfaces are not, facilitating appliance removal later. Any deciduous enamel must be conditioned with Assure or Enhance before applying the appliance. The acrylic is first conditioned with a plastic conditioner, and then bonded with a thin viscosity, extended working time bonding agent. The two types of bonding materials that satisfy the criteria are a two paste chemical cure mix specifically formulated for bonding large acrylic appliances (e.g., Excel) and a light cure resin (Light Bond). Placement of the bonding paste in the buccal, lingual and occlusal of the appliance eliminates any voids that can cause decalcification.
Large Acrylic Appliance Adhesives
Excel Regular (Reliance Orthodontic Products, Inc.)
Ultra Band Lok (Reliance Orthodontic Products, Inc.)
Opti Band (Ormco)
Band Lok (Reliance Orthodontic Products, Inc.)
Transbond LR (3M Unitek)
Direct or indirect placement of brackets
Precision positioning of orthodontic brackets for proper clinical height and angulation is essential for effective treatment. This placement protocol usually is accomplished in a direct manner, one bracket at a time which can be inaccurate and stressful.
Indirect bonding of orthodontic attachments offers the operator the quick, accurate placement of a whole arch of brackets at one time via a tray delivery system. Silverman and Cohen, Royce Thomas,9 Elliott Moskowitz, Frank Shamy and Larry White have all been innovators and developers of the indirect method over the past 30 years. At some point in their career, virtually every orthodontist has experimented (or will experiment) with indirect bonding. Some are successful, and some not. At this time about 15% of all brackets are placed indirectly.
The initial indirect technique, termed clean base, bonds brackets to the stone model with a water-soluble cement, then transfers the brackets into the mouth with a chemical cure bonding paste. The only drawback to this technique is the paste flash around each bracket base that is difficult to remove and later can trap plaque. The more recent custom base technique requires the brackets to be bonded to a separating medium treated stone model with a self-cure, light-cure10 or heat-cure composite11 that is then polymerized. The brackets, with a custom pad of hardened composite formed to the anatomy of each tooth, are transferred to the mouth with a two part chemical cure bonding resin. The use of a bonding resin eliminates the problem of peripheral flash. If the custom pad does not fit flush with the tooth surface flush, however, bond strength will be compromised.
Recently, a flowable microfil, light cure composite has been used as the bonding medium to transfer the custom pad brackets into the mouth. The flowable material is a lightly filled paste (50-60%) that can be dispensed precisely from a needle tip, providing the strength of a paste to fill any void between the custom pad and the tooth surface. The paste is light cured through a clear tray, generally for a longer period of time, because the light source is recessed from the composite.
Custom base indirect bonding only is as successful as the fit between the custom pad and the enamel. Direct placement of brackets can be tedious, time consuming and often frustrating; however, it still is the preferred manner of placing brackets because it is reliable. Why? Mainly because there are fewer variables to achieve maximum strength, one of which is proper bracket adaptation to the tooth surface, minimizing the amount of composite underneath the bracket base. Orthodontic adhesives are low film thickness adhesives that have a higher strength in a thin layer than a thick layer. Direct or indirect technique, chemical cure or light cure adhesive, whichever is chosen, can be successful in achieving maximum bond strength, if the technique is correct.
Indirect Bonding Custom Pad Adhesives
Transbond (3M Unitek)
Light Bond (Reliance Orthodontic Products, Inc.)
Enlight (Ormco)
BluGloo (Ormco)
Eagle No Drift (American Orthodontics)
Therma Cure (Reliance Orthodontic Products, Inc.)
Indirect Transfer Bonding Resins
Custom I.Q. (Reliance Orthodontic Products, Inc.)
Sondhi Rapid-Set (3M Unitek)
Maximum Cure (Reliance Orthodontic Products, Inc.)
Wet field bonding
At some point, every practitioner will have to bond attachments in a wet field. Generally there are two types of wet field encountered, excessive wetness, as seen in bonding lower second molars, or a smear layer, about the amount of saliva present if the cheek were to touch the buccal surface of the tooth. The most hydrophilic material available is the powder/liquid glass ionomer cement. The most practical powder liquid cement is Fuji Ortho LC because it is dual cure and can withstand force applied five minutes after activation with a curing light. Phosphoric acid etching or the use of a self-etching primer before application will improve its bond strength.12
For a small amount of moisture or saliva, there are several hydrophilic bonding resins available that are very dependable. Assure, Enhance Ortho Solo and MIP are the most popular in the orthodontic industry. They all require acid etching of the enamel. A very important fact to remember is the following: If a hydrophilic resin is applied to a surface in the mouth and then becomes contaminated by saliva, the bond strength will be compromised. To restore the bond strength, another layer of hydrophilic resin must be applied.
Wet Field Cements
Fuji Ortho L.C. (G.C. America)
Assure Universal Bonding Resin (Reliance Orthodontic Products, Inc.)
Ortho Solo (Ormco)
M.I.P. (3M Unitek)
SUMMARY
The bonding materials today that the orthodontist has at his or her disposal are ideal to bond metal, ceramic and plastic appliances to all surfaces in the mouth. Proper technique in handling these materials and conditioning of these surfaces is mandatory. No adhesive is foolproof. The offices that experience the least amount of bond failure are the offices with the best technique. Chemical cure or light cure, direct or indirect-they all can produce desired results if handled properly.
Mr. Gange is president, of Reliance Orthodontic Products, Inc.
E-mail: paulgropi@aol.com
REFERENCES
1.Newman, G.V.: Concept and commentary: Current status of bonding attachments, J. Clin. Orthod. 7:425-449, 1973
2.Weiss, A.: Reducing bond failures with a no-mix adhesive, J. Clin. Orthod. 19:139-141,1985.
3.Krug, A.Y. and Canley, R.S.: Shear bond strengths using an indirect technique with different light sources, J. Clin. Orthod. 39:485-487, 2005.
4.Barkmeier, W.W; Shaffer, S.E.; and Gwinnett, A.T.: Effects of 15 sec vs. 60 sec enamel acid conditioning on adhesion and morphology, Oper. Dent. 11:111-116, 1986.
5.Wang, W.N.; Yeh, C.L.; Fang, B.D.; Sun, K.T.; and Arvystas, M.G.: Effect of H3P04 concentration on bond strength, Angle Orthod. 64:377-382, 1994.
6.Dootz, E.: Comparative bond strengths with reduced cancen trations of phosphoric acid etch (unpublished data), University of Michigan, Ann Arbor, 1992.
7.Miller, R.A.: Laboratory and clinical evaluation of a self-etch ing primer, J. Clin. Orthod. 35:42-45, 2001.
8.Goel, S. and Patil, V.: Effect of an adhesion booster on bond failure rates: A clinical study, J. Clin. Orthod. 39:360, 2005.
9.Lai, S.C.; Tay, F.R.; Cheung, G.S.; Mak, Y.F.; Carvalho, R.M.; Wei, S.H.; Tofedano, M.; Osorio, R.; and Pashley, D.H.: Re versal of compromised bonding in bleached enamel, J. Dent. Res. 81:477-481,2002.
10.ZeChrisson, B.U. and Buyukyilmaz, T.: Recent advances in bonding to gold, amalgam, and porcelain, J. Ciin. 4rthod. 27:661-675,1993.
11.Cohen, S.M.; Marulli, R.; Binder, R.E.; and Vaidyanathan, TX: Shear bond strengths of chemically and light cured resinmodified ionomers, J. Clin. Orthod. 32:423-426,1998.
12.McNamara, J.A. Jr.: Fabrication of the acrylic splint Herbst appliance, Am. J. Orthod. 94:10-18, 1988.
