Effects of various methods of chlorhexidine application on shear bond strength.

Chlorhexidine has been applied on the teeth and over orthodontic appliances during treatment in order to reduce bacterial colonization. The purpose of this study was to determine whether the application of chlorhexidine with or without a sealant, to the etched enamel will affect the shear bond strength and the bracket/adhesive failure modes of orthodontic brackets. A total of 132 extracted third molars were cleaned and then divided into 6 groups of at least 20 teeth each. The teeth were etched with a 37% phosphoric acid gel and a sealant was applied in 5 of the groups. Stainless steel orthodontic brackets were then bonded with the Transbond XT bonding system. The chlorhexidine was applied to the teeth either as a prophylactic paste or as a varnish under the following conditions: (1) as a varnish over the bracket and tooth surfaces after the bonding procedure was completed; (2) as a prophylactic paste containing 0.12% chlorhexidine over the intact enamel i.e., before etching; (3) mixed with the sealant and applied to the etched enamel before bonding, (4) as a varnish applied to the etched enamel with no sealant used; (5) as a varnish over the light cured sealant; and (6) as a varnish over the sealant before the sealant was light cured. All teeth were mounted in phenolic rings and stored in deionized water at 37 degrees C for 48 hours. A Zwick Universal Testing Machine was used to determine shear bond strengths. The residual adhesive on the enamel surface was evaluated with the Adhesive Remnant Index. The analysis of variance and chi square tests were used to compare the six groups. Significance was predetermined at the p < or = 0.05. The findings in this study indicated that shear bond strength was not significantly affected when chlorhexidine was applied; (1) over the bracket and tooth surfaces after the bonding procedure was completed (mean = 12.3 +/- 3.1 MPa), (2) as a prophylactic paste over the unetched enamel surface before the bonding procedure is initiated (mean = 9.6 +/- 2.3 MPa), and (3) when the varnish was premixed with the sealant and applied on the etched enamel surface (mean 11.8 +/- 2.1 MPa). On the other hand, in all the experimental groups in which the chlorhexidine varnish was applied as a layer on the etched enamel surface or over the sealant, shear bond strength values and bracket failure rates were of a magnitude that made them clinically unacceptable.