VARIATIONS IN ACID ETCHING METHODOLOGIES
Currently phosphoric acid is the acid of choice, but it is possible that other acidic etching agents such as pyruvic acid may be used in the future. A controversial issue, however, is the optimal concentration of phosphoric acid. The most widely used concentrations of phosphoric acid used in clinical practice exceed 30% phosphoric acid. This is partly based on the findings the phase diagram of the phosphoric acid +/- calcium hydroxide +/- water ternary system. They demonstrated that the application of phosphoric acid solutions greater than 27% phosphoric acid resulted in the formation of monocalcium phosphate monohydrate.
While dicalcium phosphate dihydrate was formed with phosphoric acid concentrations less than 27% phosphoric acid. The former product is readily soluble and would be completely washed away in the clinical situation, while the latter product is less soluble. The reaction products, if not completely removed after the etching procedure, may interfere with the bonding of composite resins to etched enamel surfaces.
The effect of phosphoric acid concentration on the tensile bond strength of a conventional composite resin to enamel surfaces etched with 10, 20, 30, 40, 50, 60, and 70% phosphoric acid was determined. The tensile bond strength to enamel surfaces etched with 70% phosphoric acid was significantly lower than the bond strengths recorded to enamel surfaces etched with other phosphoric acid concentrations.
The application of a phosphoric acid etching solution to freshly cut dentin may elicit a pulpal response. To prevent the flow of phosphoric acid applied to the enamel walls of preparations to the freshly exposed dentin at the floors of the preparations phosphoric acid gels were recently introduced. The objective was to confine the acid-etching agent to the intended site of application. It is recommended that the etching agent should be applied to the enamel surface using a dabbing action as opposed to rubbing. Another issue that has not been resolved is the optimal duration of etching with phosphoric acid.
It is surprising that some authors recommend that the etchant should remain on the tooth surface for at least 60seconds to develop an appropriate etched pattern. The etch duration is of particular importance in acid etching enamel prior to the direct bonding of orthodontic attachments, as it is practically impossible to confine the bonding site. Fluoride is not evenly distributed in enamel but allows a negative exponential distribution with fluoride concentration being in the surface enamel.
The loss of fluoride rich enamel surface during prolonged etching may make the adjacent enamel more susceptible to enamel decalcification during orthodontic treatment. The reaction products that are formed on the enamel surface after phosphoric acid etching should be removed completely, as incomplete removal may interfere with bond strength. Etched surface should be washed for at least 15 secs to remove the reaction products.
The tooth to be restored should be isolated with a rubber dam to prevent saliva contamination prior to acid etching and the placement of composite resin. It is generally recommended that saliva contaminated etched enamel should be washed and retched. O’Brien and others showed, however that it was not necessary to re-etch an enamel surface contaminated briefly with saliva, as a thorough washing of such a surface did not have a detrimental effect on bond strength.
Buonocore M (1955) introduced a simple conservative technique for bonding restorative resins to enamel. He placed a drop of self-curing acrylic resin on the labial enamel surface of upper central incisor of ten subjects. One surface was treated prior to resin placement with 85% phosphoric acid for 30 seconds. He noted that the acid conditioning of the enamel resulted in on uncontioned control surfaces lasted less than 12 hours. After three decades of laboratory and clinical research, Buonocore’s method is widely adopted and has added a new and exciting technical dimension to the practice of dentistry.
Gwinnett, Matsui and Buonocore (1969) suggested that formation of resin tags was the primary attachment mechanism of resin to phosphoric acid. Acid etching removes about 10 microns of the enamel surface and creates a porous layer ranging from 5-50 microns deep.
When a low viscosity resin is applied, it flows into the micro porosities and channels to this layer and polymerizes to form a micro mechanical bond with the enamel.
Fusayama et al (1979) introduced an etching technique for both the enamel and the dentin cavity wall using 37% phosphoric acid followed by a dentin-bonding agent containing methacryloxyethyl hydrogen phenyl phosphate (phenyl-P). This improved bond strength greater extent and dentinal etching has become fairly common practice in Japan. However, the concept of total etching only recently has gained acceptance in the United States.
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