Light cure composites polymerize due to the catalyst in the adhesive absorbing 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/cm 2 . The higher the light intensity, the faster the cure. Composite Curing Lights The biggest advancements in light cure bonding have been with the curing lights and not the compos-ites. The evolution has been from bulky, corded halo-gen curing lamps to lightweight, portable LED lights. 3 The LED (light emitting diode) utilizes 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 curing life of 5,000 to 10,000 hours as opposed to a halogen bulb with a life of approximately 75 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 light source should maintain its intensity as multiple brackets are cured. If the light intensity dissipated by more than 10% during the curing process, bond strength can be compromised unless the curing time is increased. Multiply your light intensity by curing time to equal 10,000 (e.g. 1,000mw X 10 seconds). Points to remem-ber with a light cure system: ᕡ The light director must be placed as close to the bracket base or composite base as possible. ᕢ Position the light to shine between the bracket base and the tooth surface. ᕣ To determine hoe long to cure each bracket with your light, multiply your light intensity by curing time to equal10,000 (eg. 10,000Mw X 10 seconds). Decalcification 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. Decalcification is a problem for any doctor placing fixed appliances. 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 with-out reapplication during treatment. Currently, there are 12 university studies documenting the benefits of Pro Seal. Bonding…success or failure? The answer to this question starts with the tooth surface. Now let’s exam-ine the various tooth surfaces and their proper condi-tioning for success. 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 etch-ing 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 phos-phate 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 appli-cation 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. The most common technique error with a self-etch-ing primer is not drying the surface thoroughly after scrubbing. Self-etching primers, which can only be used with light cure composite, have several clinical applica-tions 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 bond-ing with a powder/liquid glass ionomer cement, or 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. Atypical Enamel Surfaces Enamel surfaces that are fluorosed, hypocalcified or deciduous produce inconsistent results when condi-tioned in the conventional manner. Hydrophilic univer-sal 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 One coat of Assure ® , Enhance ® or Ortho Solo ® is applied to phosphoric acid etched enamel and stroked over several times to insure complete penetration into the porous surface. The tooth is then lightly air-dried to evaporate the solvent before the appliance 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 www.orthodontics.com May/June 2011 17 Enamel Enamel preparation is accomplished by one of two methods: traditional phosphoric acid etching or self-
Journal of the American Orthodontic Society May-June 2011: Page 17
