A Comparison of Three Debonding Techniques Employing Two Different Cements

Abstract

The theory and practice of bonding orthodontic brackets to enamel has become the accepted standard. However, regardless of the adhesive of choice, much controversy exists regarding bond strength values and testing protocols. Most bond strength testing has been done in either shear/shear-peel or tension. Some studies have used shear and tension and very few have used shear, tension and torsion. Some authors contend there is no difference in the stress required to produce bond failure by either tensile or shear test models. However, it has been shown that stress is not distributed uniformly during loading and each mode of strength testing produces unique stress patterns. Additionally, since in the oral cavity brackets are subject to shear, tensile and torsion forces, it seems logical that a complete picture of bond strength could not be formulated without all three test methods. Confounding the issue is the fact that adhesive research is being performed in non-standardized manners making it impossible to compare results among different researchers. Despite the vast amount of information presented in articles, this has resulted in a lack of consensus regarding clinical bond strength values. The purpose of this investigation was to evaluate the three debonding techniques (shear-peal, tension, torsion) using stainless steel brackets and two different bonding agents (traditional resin cement vs. resin reinforced glass ionomer). The hypotheses of this investigation were (1) the relative shear-peal, tensile and torsional bond strengths will show consistent results and (2) cement type will have a significant effect on the bond strengths. One hundred sixty-two bovine incisors were randomly assigned to 6 groups of 27 specimens per group. Teeth were bonded with either a resin composite adhesive or a resin reinforced glass ionomer cement following manufacturers' instructions. Bonding was performed under controlled temperature and humidity (71 °F± 2° and 56% RH± 5%). In addition, specimens were bonded utilizing a bonding jig that held the thickness of the adhesive constant at 0.006 inches. All groups were tested to failure using the MTS Bionix machine in shear, tension and torsion. The results showed that the resin composite had a significantly higher load at failure in shear and torque than the resin-modified glass ionomer. However, in tension, no significant difference was found between the two cements. Additionally, analysis of relative strength indicated a difference between shear strength and tension suggesting that testing mode influences bond strength values. It is the conclusion of this study that the load at failure for resin composite and resin-modified glass ionomer are not consistent and depend on the loading mode.