Fluorine depth profiles in human enamel after different acid etching times

This study was designed to monitor fluorine concentration changes in the surface layer of enamel subjected to acid etching for orthodontic direct bonding. Prior to etching, the area described by the reference pins was partitioned into quadrants and initial fluorine concentration profiles in the surface 4 μm were recorded using a non-destructive nuclear resonance technique. Measurements of enamel surface location were made, relative to the reference pins, prior to etching and after 15,30, 60 and 120 seconds exposure to Superbond gel etchant. Each quadrant was then re-profiled for fluorine concentration. The measuring technique, therefore, provided information regarding etching depth (surface loss) and fluorine change in each of the quadrants.

Although considerable variations in surface fluorine profiles were noted between teeth and within the same tooth, depletion of fluorine commenced with as little as 15 seconds of etching and plateaued off at between 0.1% to 0.2% after 120 seconds. Loss of enamel during the etching procedures also varied between teeth and between test quadrants but indicated minimal difference for 15, 30 or 60 seconds of etch (ranging from an average of 1.1 μm to an average of 7.4μm). After 120 seconds of etching, enamel loss was significantly greater (averaging as much as 16.5 μm in one tooth).

Etching times beyond 60 seconds eliminated the protective surface fluorine and risked significant loss of the enamel surface, indicating a 30-second to 60-second etch to be a good compromise for most clinical bonding situations.