Effects of phosphoric acid concentration on depth of etch and shear bond strength of orthodontic brackets to bovine enamel

Kim, Soo Cheol; Lee, Ki Soo

Abstract

Previous study had shown the diversities in the propriety for optimal bond strength on the concentration of the etchant. The aim of present study in vitro was to evaluate and compare the shear bond strength of orthodontic brackets to enamel and to measure the depth of etch on the phosphoric acid concentrations. A hundred and seventy six extracted bovine lower centrals were ground to yield flat surfaces and etched by the concentration 0%, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% and 85% of phosphoric acid respectively during 60 seconds. The shear bond strength of orthodontic brackets, the depth of etch and surface roughness of the enamel were measured, and scanning electron microscopic observations on the etched enamel surfaces were carried out. The data obtained from the very experiments were processed and statistically analyzed and evaluated. The gradual increase in the depth of etch to enamel as the accretion of the concentration of the phosphoric acid up to 40-50% and decline henceforth were manifested. The surface roughness showed no correlation with the depth of etch, yet moderate correlation with the shear bond strength of brackets. Scanning electron microscopic investigation revealed that morphological patterns of the etched enamel surfaces for 5% to 40% of concentrations were even and homogenous, and those for 50% as well as 60% exhibited the overetched and unhomogenous. The shear bond strengths from 10% to 60% of concentration showed no statistically significant differences. It was suggested that the shear bond strengths at 5% and 70% were sufficient to tolerate the force levels of the ordinary orthodontic treatment notwithstanding to be significantly lower than those from 10% to 60% phosphoric acid solution.