The Effect of Cement Thickness on the Tensile and Torsional Bond Strengths of Orthodontic Brackets

Abstract

Many investigations have been conducted to determine the bond strength of orthodontic adhesives subjected to particular debonding loads. The majority of these studies have failed to consider the effect of adhesive thickness. According to previous studies and clinical experience, the optimal bond strength is obtained with the minimum cement thickness. However, preliminary Finite Element Model (FEM) analysis has indicated that maximum stresses are reduced with increasing cement layer thickness, indicating that the bond strength should increase. The purpose of this study was to evaluate the effect of cement thickness on the tensile and torsional bond strengths of a light-cured composite resin used to bond metal orthodontic brackets. Extracted bovine incisors were bonded in three thicknesses up to 0.5 mm using a bonding jig and metal shim spacers of known thicknesses. After being stored in deionized water at 370C for 48 hours, the samples were tested to failure by either a tensile or torque load applied using the MTS Bionix Testing System. The three adhesive thicknesses were compared for differences in tensile and torsional bond strengths using analysis of variance models suitable for a crossover design. The models included terms for adhesive thickness, day, specimen, and order in which adhesives were used. Multiple comparisons were made using Tukey's multiple comparisons method at a 95 percent overall confidence level. Debonding characteristics were evaluated using the Adhesive Remnant Index (ARI). The results of tensile debonding showed no significant differences due to the day of testing (p=0.97) or to the order that adhesive thicknesses were bonded to the specimens (p=0.38). The thin adhesive thickness had significantly higher mean tensile bond strength than the medium (p=0.002) or thick (p=0.001). Medium and thick were not significantly different (p=0.97). The results in torsion debonding showed no significant differences due to day (p=0.81) or the order in which adhesive thicknesses were tested (p=0.30). The thick layer had significantly lower mean torsional bond strength than the medium or thin (p<0.001). Medium and thin layers were not significantly different in torsion (p=0.24). The results failed to support the FEM predictions; the tensile and torsional bond strengths decreased as the light-cured composite resin thickness increased under a metal bracket. But, interestingly, in tension, the significant difference occurred between thin and medium; while in torsion, it took place between thick and medium.