A Technique to Prevent Surface Contamination of Etched Enamel

I t has been recommended that a thin layer of primer be applied to the entire etched enamel surface before bonding to seal the enamel, protect it from decay, and provide maximum bond strength.1-3

If the etched surface becomes contaminated by oral fluids before bonding, however, the bond is likely to fail at the resin/enamel interface.4 Contamination can occur during application of the primer if saliva or sulcular fluid is unintentionally spread by the applicator brush.5 This is most likely to occur when bonding to surgically exposed palatal canines or to teeth with short clinical crowns, where the working area and visibility are limited.

In such cases, it may be more effective to apply the primer directly to the adhesive resin on the back of the orthodontic attachment. The question is whether the primer will penetrate into the etched enamel enough to ensure an adequate bond. The purpose of this study was to compare the bond strength achieved with this modified technique to that attained with conventional bonding.

Materials and Methods

Forty-four human premolars extracted for orthodontic treatment were collected for this study. The teeth were stored in .9% normal saline solution prior to bonding with the Transbond orthodontic adhesive system. Each tooth was cleaned with pumice and a rubber cup and rinsed, air-dried, and acid-etched according to the manufacturer's instructions. Ultratrimm standard edgewise brackets were then bonded using either the conventional or the modified technique.

The teeth were randomly divided into two groups of 22. In the control group, a thin film of primer was applied directly to the etched enamel surface. Adhesive resin was applied to the bracket, which was placed on the tooth and light-cured for 40 seconds. In the experimental group, the adhesive resin was first applied to the bracket, and a thin coating of primer was brushed over the adhesive. The bracket was then placed on the tooth and light-cured for 40 seconds.

The teeth were stored in .9% normal saline solution for one week before debonding. Each sample was mounted in plaster, using a dental surveyor with an analyzing rod to ensure proper alignment of the bracket. Shear bond strengths were measured with an Instron Universal Testing Machine at .02mm/second. The statistical significance of the difference between the bonding methods was evaluated with a t-test.

Results

The mean shear bond strengths were 9.8 MPa for the conventional technique and 10.0 MPa for the experimental technique (Fig. 1). The difference was not statistically significant (p > .20).

Only three of the 44 teeth--two in the control group and one in the experimental group--debonded at the resin/enamel interface. The remainder of the samples debonded at the bracket/resin interface.

Discussion

Applying the primer over the adhesive resin on the bracket base appeared to have no adverse effect on bond strength, compared to applying the primer directly to the etched enamel as recommended by the manufacturer. In fact, the mean bond strengths were nearly identical. Almost none of the bond failures in either group occurred at the enamel surface, suggesting that the weakest point in the bonding process is the bracket/resin interface.

The technique proposed here would be especially useful when it is difficult to apply primer directly to the tooth because of poor visibility or limited exposure. Such situations include bonding to an exposed palatal canine or to a posterior tooth with a short clinical crown, where proximity to the gingiva increases the likelihood of contamination from oral fluids.