Relative Orthodontic Bracket Bond Strength Measured in Three Testing Modalities

Abstract

This project represents a continuation of work in which computer models, primarily FEM (Finite Element Model) stress analyses, and associated experiments, have been conducted to compare the strength measurement results obtained with three testing modalities (tension, shear-peel and torque). Simply stated, we have examined the following question: If a manufacturer, using tensile debonding loads, claims that product Y is twice as strong as product X, will the same results be obtained by a laboratory that used shear-peel or torsion testing? Madding tested three types of orthodontic bracket cements (System 1 +, a chemically cured resin (Res); Ketac-Fil, a self-cured glass ionomer (CC), and PhotacBond, a light-cured glass ionomer (LC)) in three loading modalities: tension, shear-peel, and torsion. Contrary to expectations, the results demonstrated that the relative bond strengths of the orthodontic cements were independent of the loading methods. One of the variables that may have contributed to these contradictory results is adhesive thickness. The aim of this study is to measure the relative bond strengths of three types of cements while maintaining the same adhesive thicknesses. Hypothesis: The relative bond strengths of three cements will be the same, regardless of the testing modality, if the bond thicknesses are held constant. Three commercially available systems were utilized: System 1 + (Res), Ketac-Fil (CC), and Photac-Bond (LC). One hundred and sixty two standard twin edgewise central incisor brackets were flattened with approximately 200 N per bracket using self-centering plates on an MTS Bionix machine. A total of 54 bovine incisor teeth were randomly divided into three groups (A, B, C). Each group of 18 teeth was bonded and rebounded with a single cement, and further divided into three subgroups of 6 (Al, A2, A3). Each specimen within all subgroups was initially "virgin" and subsequently recycled between different testing modalities (tensile, shear-peel and torque). A special bonding jig aided bracket placement and ensured parallelism between bracket base and enamel surface, while the thickness of the cement was held at 0. 152 mm. The debonding procedure in all three testing modalities was done on an MTS Bionix machine. The Adhesive Remnant Index (ARI) was determined stereomicroscopically for each specimen post debond. The trend of the results indicate that the relative bond strengths in tension and torsion was LC> Res> CC, whereas in shear-peel, Res> LC> CC. These results are contrary to the hypothesis and are significant for CC and LC, and between CC and Res in all testing modalities. There was only a weak correlation between ARI and bond strength for LC (r value= 0.59) and CC (r value= -0.48) in the shear-peel testing modality. Additionally, a weak correlation was found for LC (r value= 0.54) in the torsional testing modality. There were no correlations between bond strength and ARI for any of the three adhesives in the other testing modalities. Statistically, testing in torsion gave LC> Res> CC, but shear-peel and tension showed LC= Res> CC. In practical terms, the manufacturer of LC could legitimately claim that its product is stronger than Res. However, that torsion-based claim could not be made based on tension. Conclusion: The relative bond strengths of the three orthodontic cements are different and are influenced by the testing modality when the bond thickness is held constant.