Acid EtchingEtching of dental surface with anacid to remove the smear layer andopen enamel tubules, increaseretention of resin sealant, andpromote mechanical retention.
Development A concept of etching enamel surfaces with phosphoric acid, first proposed by Buonocore in1955 to increase the bond strength between the composite resin and etched enamel.
Mechanism of Action The irregular enamel surface created by dissolving hydroxyapatite crystals permits penetration of the ﬂuid adhesive components and this penetration provides micromechanical retention. Acid etching of enamel appears to improve retention by selectively eroding certain hydroxyapatite formations and facilitating penetration by the development of resin tags.
Types of Acid EtchingAccording to Silverstone et al there are 5 types of acid etching pattern:1. Type 1: preferential dissolution of prism cores, resulting in honey comb like appearance.2. Type II: preferential dissolution of prism peripheries, giving cobblestone like appearance.3. Type III: a mixture of type I and II pattern.4. Type VI: pitted enamel surfaces as well as structures that look like unfinished puzzle, maps or networks.5. Type V: flat, smooth surface
Acid etching removes approximately 10 µm of enamel surface and creates a morphologically porous layer (5 µm to 50 µm deep). The low-viscosity fluid resin contacts the surface and is attracted to the interior of these microporosities created by capillary attraction. Resin tags are formed into microporosities of conditioned enamel that after adequate polymerization, provide a resistant, long- lasting bond by micromechanical interlocking with this tissue.
Effects of Important Variables onBond StrengthDifferences among Teeth Differences in bond strength are generally not observed among different teeth. Likewise, no differences in bond strength are observed between lingual and buccal surfaces.
Effects of Fluoride Teeth with a higher concentration of fluoride are more resistant to acid etching than normal teeth and may require an extended etching time. Bond strengths to a group of severely and moderately fluorotic teeth, were about 40 % lower than bond strengths to normal teeth, Group of mildly to moderately fluorotic teeth from young adults showed similar bond strengths when compared to normal teeth.
Type and Concentration ofAcid In restorative dentistry the highest possible bond strength to tooth structure is desirable. In contrast, the orthodontic bond strength must be sufficient to retain the brackets but low enough to allow easy clean-up of adhesive when the case is completed and the brackets are removed.
Factors which are affecting acidetching of enamel include: 1. Type of the acid 2. Concentration of the acid 3. The time of etching. Etching with 10 % or 37 % phosphoric acid produces the highest bond strengths to enamel. The use of 10 % maleic acid for etching results in a lower bond strength No etching yields a very low bond strength. No differences in bond strengths are observed when enamel is etched with phosphoric acid ranging in concentration from 2 % to 37 %.
Duration of Etching No differences in bond strength are detected between 15-second and 60- second etching with 37 % phosphoric acid; however, shorter etching times cause less enamel damage on debonding. Decreasing etching time between 30 and 10 seconds does not affect bond strength(11 Mpa) or location of failure site Whereas etching for 0 or 5 seconds reduces bond strength (less than 3 MPa)
Etching vs. Not Etching Resin composite does not bond well to unetched enamel However, hybrid ionomer orthodontic cements have bond strengths to moist, unetched enamel ranging from 8 to 25 MPa. Hybrid ionomer orthodontic cements presently bond better to moist, unetched enamel than to sandblasted metal brackets. Once their bond strength to metal brackets is improved, these cements could be used in a non-acid-
Comparison of relative properties of cementsand adhesives used for orthodontic bondingand banding Cement / Adhesive Bond Strength to Bond Strength to Enamel Metal Brackets Resin composite High Medium – high Hybrid ionomer Medium – high Low-medium
Use of Pumice with or withoutFluoride Pumice is used to clean the enamel surface before acid etching and bonding. However, bond strength appears to be unaffected whether pumice is used or not. Use of a fluoridated pumice or paste with varying fluoride concentrations also does NOT affect bond strength or location of bond failures.
Iatrogenic Effects ofEtching1. Fracture and cracking of enamel upon debonding.2. Increased surface porosity - possible staining.3. Loss of acquired fluoride in outer 10 µm of enamel surface.4. Loss of enamel during etching about 10 ±20 µm of enamel.5. Resin tags retained in enamel - possible discoloration of resin.6. Rougher surface if over-etched.
Crystal-Growing Solutions A proposed alternative to etching enamel for retention of an adhesive is to grow crystals on the enamel surface, this technique is called crystal bonding. Potential advantages of crystal bonding include: 1. Easier debonding 2. Less residual adhesive left on the tooth 3. Less damage to enamel. Crystal bonding involves application to enamel of a poly (acrylic acid) solution containing sulfate ions, which causes growth of calcium sulfate dihydrate crystals on the enamel surface. These crystals in turn retain the adhesive. Since crystal bonding produces bond strengths of 60±80 % of the bond strength obtained with acid etching, it is not yet considered a practical technique.
Acidic Primers(Self EtchingPrimer) Another alternative to etching enamel with phosphoric acid is to use an acidic primer of the type used to bond restorative composites to enamel and dentin. Although these primers are expensive, comparable bond strengths are found.
Air Abrasion Air abrasion, also referred to as micro- etching, is a technique in which particles of aluminum oxide are propelled against the surface of enamel by high air pressure, causing abrasion of the surface. Bond strengths to air-abraded enamel are only about 50 % of those to acid- etched enamel. Air abrasion could be an alternative to pumicing the teeth before etching.
Laser Etching The application of laser energy to an enamel surface causes localized melting and ablation. Removal of enamel (etching) results primarily from the micro-explosion of entrapped water in the enamel and some melting of the hydroxyapatite crystals. Laser etching of enamel by a neodymium-yttrium-aluminum garnet (Nd:YAG) laser typically produces lower bond strengths than does acid etching.
Moisture-Resistant Primers An alternative to bonding to dry enamel is to apply a moisture-resistant primer to etched enamel that has been contaminated with moisture or saliva. This type of primer is a hydrophilic methacrylate monomer that will wet enamel contaminated with saliva or moisture. The bond strength of a resin composite adhesive applied to enamel primed with the moisture-resistant primer is similar to that of resin composite adhesive applied to etched, dry enamel.
Chlorhexidine Chlorhexidine can be applied on the teeth and over orthodontic appliances during treatment to reduce bacterial colonization. Bond strength is not affected if the chlorhexidine is applied after bonding has been completed or as a prophylactic paste on enamel before etching. Bond strength is reduced if the chlorhexidine is applied as a layer on etched enamel or on the sealant before the adhesive is applied. A chlorhexidine-containing primer did not significantly affect bond strength.
Bleaching Teeth recently have significantly lower bond strengths to resin composites. The bleach produces oxygen, which inhibits free radical polymerization of resin composites. Research has shown that orthodontic brackets can be placed after use of carbamide peroxide bleaching with no adverse effect on bond strength.
References1. http://www.angle.org/doi/pdf/10.2319/120605- 426R.12. http://www.forp.usp.br/bdj/Bdj9(1)/t0191/t0191. html3. http://www.angle.org/doi/pdf/10.2319/120605- 426R.14. http://www.danielathys.com.br/downloads/enam el_acid_etching_-_a_review.pdf5. http://www.jaypeejournals.com/eJournals/Show Text.aspx?ID=3128&Type=FREE&TYP=TOP&I N=_eJournals/images/JPLOGO.gif&IID=241&is PDF=YES6. Orthodontic Material Scientific and Clinical Aspects, William A. Brantley and Theodore Eliades (2001).